Electromagnetic Testing

Electromagnetic Testing 

Study Guide Electromagnetic Testing 

My ASNT Level III 

Pre-Exam Preparatory 

Self Study Notes 

17th April 2015 

Charlie Chong/ Fion Zhang

E&P Applications 


E&P Applications 


http://independent.academia.edu/CharlieChong1 

http://www.yumpu.com/zh/browse/user/charliechong 

http://issuu.com/charlieccchong 



Fion Zhang at Shanghai 

17th April 2015 

http://meilishouxihu.blog.163.com/ 


乱七八糟 – 随看随记 


乱七八糟 – 随看随记 



Charlie Chong/ Fion Zhang 

http://greekhouseoffonts.com/


http://www.naturalreaders.com


http://www.naturalreaders.cn/

IVONA TTS Capable. 


http://www.naturalreaders.com/

Chapter 1 

Principles of Eddy Current Testing 


EDDY CURRENT an Overview 

Description of Eddy Current Detectors 

Coil configurations 

Appropriate coil selection is the most important part of solving an eddy current application, no instrument can 

achieve much if it doesn’t get the right signals from the probe. 

Coil designs can be split into three main groups: 

1. Surface probes used mostly with the probe axis normal to the surface, in addition to the basic ‘pancake’ 

coil this includes pencil probes and special-purpose surface probes such as those used inside a fastener 

hole. 

2. Encircling coils are normally used for in-line inspection of round products, The product to be tested is 

inserted though a circular coil. 

3. ID probes are normally used for in-service inspection of heat exchangers. The probe is inserted into the 

tube. Normally ID probes are wound with the coil axis along the centre of the tube. 

Absolute probes 

These categories are not exhaustive and there are obviously overlaps, for example between non-circumferential wound ID probes 

and internal surface probes. To this point we have only discussed eddy current probes consisting of a single coil. These are 

commonly used in many applications and are commonly known as absolute probes because they give an ‘absolute’ value of the 

condition at the test point. Absolute probes are very good for metal sorting and detection of cracks in many situations, however 

they are sensitive also to material variations, temperature changes etc. 

Differential’ probe 

Another commonly used probe type is the ‘differential’ probe this has two sensing elements looking at different areas of the 

material being tested. The instrument responds to the difference between the eddy current conditions at the two points. Differential 

probes are particularly good for detection of small defects, and are relatively unaffected by lift-off (although the sensitivity is 

reduced in just the same way), temperature changes and external interference. (assuming the instrument circuitry operates in a 

"balanced“ configuration) 


http://www.eng.morgan.edu/~hubert/IEGR470/eddycurrent.html

Note the characteristic "figure of eight for differential probe" response as first one probe element, then the other, 

move over the defect. In general the closer the element spacing the wider the "loop" in the signal. Lift-off should 

be cancelled out assuming that the probe is perfectly balanced, but there will still be a "wobble" response as 

the probe is moved and tilted slightly. 

Reflection or driver pick-up probes have a primary winding driven from the oscillator and one or more 

sensor windings connected to the measurement circuit. Depending on the configuration of the sensor windings 

reflection probes may give response equivalent to either an absolute or differential probe. The two coils 

(differential or absolute plus balancing coil) form the ‘legs’ of a bridge. When the bridge is balanced the 

measured voltage will be zero. Any change in the condition of either coil will result in an unbalanced bridge, the 

degree of imbalance corresponds to the change in coil impedance. 

The diagram shows a typical response from a 

differential probe. 

Driver pick-up: As can be seen the essential 

elements are the same for a driver pick-up 

configuration as for a bridge, the necessary 

changes can be achieved by simple switching 

or probe connection changes 



Tangential Probe 



Orthogonal Probe 



Electromagnetic Testing Advantages 

The following characteristics of the method can be used to advantage : 

• it can be used without making physical contact with the product ; 

• it does not need a coupling medium such as water ; 

• it is capable of being used at high throughput speeds. 


EN 12084 : 2001

Factors Affecting Eddy Current Responses 

The basic parameters which influence the measured quantity are all of the 

following properties of the product to be tested, alone or in combination : 

• the conductivity of the material ; 

• the magnetic permeability of the material ; (magnetic factor) 

• the size and geometry of the product to be tested ; (magnetic factor) 

• the geometry between the eddy current probe and the product to be tested. 

(magnetic factor) 


EN 12084 : 2001

Factors Affecting Eddy Current Response 

Material conductivity 

The conductivity of a material has a very direct effect on the eddy current flow: the greater the conductivity of a 

material the greater the flow of eddy currents on the surface. Conductivity is often measured by an eddy current 

technique, and inferences can then be drawn about the different factors affecting conductivity, such as material 

composition, heat treatment, work hardening etc. 

Permeability 

This may be described as the ease with which a material can be magnetised. For non-ferrous metals such as 

copper, brass, aluminum etc., and for austenitic stainless steels the permeability is the same as that of ‘free 

space’, i.e. the relative permeability (μ r 

) is one. For ferrous metals however the value of μ r 

may be several 

hundred, and this has a very significant influence on the eddy current response, in addition it is not uncommon 

for the permeability to vary greatly within a metal part due to localised stresses, heating effects etc. 

Frequency 

As we will discuss, eddy current response is greatly affected by the test frequency chosen, fortunately this is 

one property we can control. 

Geometry 

In a real part, for example one which is not flat or of infinite size, geometrical features such as curvature, edges, 

grooves etc. will exist and will effect the eddy current response. Test techniques must recognise this, for 

example in testing an edge for cracks the probe will normally be moved along parallel to the edge so that small 

changes may be easily seen. Where the material thickness is less than the effective depth of penetration (see 

below) this will also effect the eddy current response 



Proximity / Lift-off 

The closer a probe coil is to the surface the greater will be the effect on that coil. This has two main effects: 

The "lift-off" signal as the probe is moved on and off the surface. A reduction in sensitivity as the coil to product 

spacing increases. 

Depth of penetration 

The eddy current density, and thus the strength of the response from a flaw, is greatest on the surface of the 

metal being tested and declines with depth. It is mathematically convenient to define the "standard depth of 

penetration" where the eddy current is 1/e (37%) of its surface value. The standard depth of penetration in mm 

is given by the formula: 

Where: 

δ is standard depth in mm 

ρ is resistivity in μΩ.cm 

f is frequency in Hz 

μ r 

is relative permeability 



from this it can be seen that depth of penetration: 

1. Decreases with an increase in frequency 

2. Decreases with an increase in conductivity 

3. Decreases with an increase in permeability: this can be very significant penetration into ferrous materials at 

practical frequencies is very small. 

δ 

δ 

The graph above shows the effect of frequency on standard depth of penetration. 

It is also common to talk about the "effective depth of penetration" usually defined as three times the standard 

depth, where eddy current density has fallen to around 3% (5%?) of its surface value. This is the depth at 

which there is considered to be no influence on the eddy current field. 



The Impedance Plane 

Eddy current responses of a single coil may be conveniently described by reference to the "impedance plane". 

This is a graphical representation of the complex probe impedance where the abscissa (X value) represents the 

resistance and the ordinate (Y value) represents the inductive reactance. 



http://www.eng.morgan.edu/~hubert/IEGR470/eddycurrent.html 

The Impedance Plane 


Note that, while the general form of the impedance plane remains the same, the details are unique for a 

particular probe and frequency. The display of a typical CRT eddy current instrument represents a ‘window’ into 

the impedance plane, which can be rotated and "zoomed" to suit the needs of the application. For example in 

the above impedance plane diagram a rotated detail of the "probe on aluminum" area would appear as below: 

This shows the display when moving over a series of simulated cracks of varying depths. Note that in the 

example shown both the amplitude and the phase of response from the different sized cracks varies. 

Reliability 

Eddy currents are often generated in transformers and lead to power losses. To combat this, thin, laminated 

strips of metal are used in the construction of power transformers, rather than making the transformer out of 

one solid piece of metal. Insulating glue, which confines the eddy currents to the strips, separates the thin strips. 

This reduces the eddy currents, thus reducing the power loss. Beside that, Eddy-Current Detectors are very 

reliable as far as their industrial usage. They are so reliable that nuclear plants are using robots to the tests, 

instead of risking real human beings. 



Robotic 


Robotic 


Measurement Techniques (EN!) 

a) Absolute measurement. 

The measurement of the deviation from a fixed reference point. The reference point is defined by a calibration 

procedure and can be generated by a reference voltage or coi l. This technique can be used for sorting the 

product into classes based on physical properties such as hardness, dimensions or chemical composition. It 

can also be used for the identification of continuous or gradually changing discontinuities. 

b) Comparative measurement. 

The subtraction of two measurements, one of which is taken as a reference. This technique is normally used to 

sort the product into classes. 

c) Differential measurement. 

The subtraction of two measurements made at a constant distance between the measurement locations and on 

the same scanning path. This measurement technique reduces the background noise due to slow variations 

in the product to be tested. (?) 

d) Double differential measurement. 

The subtraction of two differential measurements. This measurement technique provides high-pass filtering of a 

differential measurement independent of the relative speed between the probe and the product to be tested. 

e) Pseudo differential measurements 

The subtraction of two measurements made at a constant distance between the measurement locations. 


EN 12084 : 2001

Historical Background 

Before discussing the principles of eddy current testing, it seems appropriate to briefly discuss the concept of 

magnetism and electromagnetism that serve as the foundation for this study. In the period from 1775 to 1900, 

scientific experimenters Andre Marie Ampere, Françios Arago, Charles Augustin coulomb, Michael Faraday, 

Lord William Thomson Kelvin, James Clerk Maxwell and Hans Christian Oersted had investigated and 

cataloged most of what is known about magnetism and electromagnetism. Arago discovered that the oscillation 

of a magnet was rapidly damped when a nonmagnetic conductor disk was placed near the magnet. He also 

observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a 

varying magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a 

secondary magnetic field in the disk that affected the magnet. Arago's simple model is a basis for many 

automobile speedometers used today. This experiment can be modeled as shown in Figure 1.1. 


http://pegna.vialattea.net/2Arago_Disk.htm

Figure 1.1 Arago’s Experiment 


Arago’s Disk Experiment 

Arago discovered that the oscillation of a magnet was rapidly damped when a nonmagnetic conducting disk was placed near the 

magnet. He also observed that by rotating the disk, the magnet was attracted to the disk. In effect, Arago had introduced a varying 

magnetic field into the metallic disk causing eddy currents to flow in the disk. This produced a secondary magnetic field in the disk 

that affected the magnet. Arago's simple model is a basis for many automobile speedometers used today. 

■ https://www.youtube.com/embed/sChcqdkcLGE 


https://www.youtube.com/watch?v=sChcqdkcLGE

Oersted discovered the presence of a magnetic field around a current carrying conductor and 

observed magnetic field developed in a perpendicular plane to the direction of current flow in a 

wire. Ampere observed that equal and opposite currents flowing in adjacent conductors cancelled 

this magnetic effect. Ampere's observation is used in differential coil applications and to 

manufacture non inductive precision resistor. Faraday's first experiments investigated induced 

currents by the relative motion of magnet and a coil (Figure 1.2). Faraday's major contribution 

was the discovery of electromagnetic induction. His work can be summarized by the example 

shown in Figure 1.3. 

A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a 

voltmeter is near by. When switch S is closed it produces a current in coil A in the direction 

shown (a). A momentary current is also induced in coil in direction (b) opposite to the current 

flow in coil A. If S is now opened, a momentary current will appear in coil B having the direction 

of (c). In each case current flows in coil B only while the current in coil A is changing. 


Figure 1.2: Induced current with coil and magnet 


Figure 1.3: Induced current electromagnetic technique 

A coil "A" is connected to a battery through a switch, "S", A second coil, B, connected to a voltmeter is near by. 

When switch S is closed it produces a current in coil A in the direction shown (a). A momentary current is also 

induced in coil in direction (b) opposite to the current flow in coil A. If S is now opened, a momentary current 

will appear in coil B having the direction of (c). In each case current flows in coil B only while the current in coil 

A is changing. 


Electromagnetic induction is the production of an electromotive force across a conductor when it is 

exposed to a varying magnetic field. It is described mathematically by Faraday's law of induction, named after Michael Faraday 

who is generally credited with the discovery of induction in 1831. 

Electromagnetic induction was first discovered by Michael Faraday, who made his discovery public in 1831. It was discovered 

independently by Joseph Henry in 1832. 

In Faraday's first experimental demonstration (August 29, 1831), he wrapped two wires around opposite sides of an iron ring or 

"torus" (an arrangement similar to a modern toroidal transformer). Based on his assessment of recently discovered properties of 

electromagnets, he expected that when current started to flow in one wire, a sort of wave would travel through the ring and cause 

some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other 

wire to a battery. Indeed, he saw a transient current (which he called a "wave of electricity") when he connected the wire to the 

battery, and another when he disconnected it. This induction was due to the change in magnetic flux that occurred when the 

battery was connected and disconnected. Within two months, Faraday found several other manifestations of electromagnetic 

induction. For example, he saw transient currents when he quickly slid a bar magnet in and out of a coil of wires, and he 

generated a steady (DC) current by rotating a copper disk near the bar magnet with a sliding electrical lead ("Faraday's disk"). 

Faraday explained electromagnetic induction using a concept he called lines of force. However, scientists at the time widely 

rejected his theoretical ideas, mainly because they were not formulated mathematically. An exception was Maxwell, who used 

Faraday's ideas as the basis of his quantitative electromagnetic theory. In Maxwell's model, the time varying aspect of 

electromagnetic induction is expressed as a differential equation which Oliver Heaviside referred to as Faraday's law even though 

it is slightly different from Faraday's original formulation and does not describe motional EMF. Heaviside's version (see Maxwell– 

Faraday equation below) is the form recognized today in the group of equations known as Maxwell's equations. 

Heinrich Lenz formulated the law named after him in 1834, to describe the "flux through the circuit". Lenz's law gives the direction 

of the induced EMF and current resulting from electromagnetic induction (elaborated upon in the examples below). 

Following the understanding brought by these laws, many kinds of device employing magnetic induction have been invented. 


http://en.wikipedia.org/wiki/Electromagnetic_induction




http://en.wikipedia.org/wiki/Homopolar_generator


http://en.wikipedia.org/wiki/Homopolar_generator

Faraday's Law - Any change in the magnetic environment of a coil of wire will 

cause a voltage (emf) to be "induced" in the coil. No matter how the change is 

produced, the voltage will be generated. The change could be produced by 

changing the magnetic field strength, moving a magnet toward or away from 

the coil, moving the coil into or out of the magnetic field, rotating the coil 

relative to the magnet, etc. Faraday's law is a fundamental relationship which 

comes from Maxwell's equations. It serves as a summary of the ways a 

voltage (or emf) may be generated by a changing magnetic environment. The 

induced emf in a coil is equal to the negative of the rate of change of 

magnetic flux times the number of turns in the coil. It involves the interaction 

of charge with magnetic field. 


http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html

The law of physics describing the process of electromagnetic induction is known as Faraday's law of induction 

and the most widespread version of this law states that the induced electromotive force in any closed circuit is 

equal to the rate of change of the magnetic flux enclosed by the circuit. Or mathematically, 

ε = dф B 

/ dt 

where ε (epsilon) is the electromotive force (EMF) and ΦB (Φ= BA) is the magnetic flux. The direction of the 

electromotive force is given by Lenz's law. This version of Faraday's law strictly holds only when the closed 

circuit is a loop of infinitely thin wire, and is invalid in some other circumstances. A different version, the 

Maxwell–Faraday equation (discussed below), is valid in all circumstances. For a tightly wound coil of wire, 

composed of N identical turns, each with the same magnetic flux going through them, the resulting EMF is 

given by 

ε = -N dф B 

/ dt 

Faraday's law of induction makes use of the magnetic flux ΦB through a hypothetical surface Σ whose 

boundary is a wire loop. Since the wire loop may be moving, we write Σ(t) for the surface. The magnetic flux is 

defined by a surface integral: 

фB = ∫ Σ(t) B(r,t)∙dA 

where dA is an element of surface area of the moving surface Σ(t), B is the magnetic field, and B·dA is a vector 

dot product (the infinitesimal amount of magnetic flux). In more visual terms, the magnetic flux through the wire 

loop is proportional to the number of magnetic flux lines that pass through the loop. 







Lenz's Law 

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced 

emf is such that it produces a current whose magnetic field opposes the change which produces it. The 

induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the 

examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the 

induced field acts in the direction of the applied field to try to keep it constant. 



Magnetic Force 

The magnetic field B is defined from the Lorentz Force Law, and specifically from the magnetic force on a moving charge: 

The implications of this expression include: 

1. The force is perpendicular to both the velocity v of the charge q and the magnetic field B. 

2. The magnitude of the force is F = q∙v∙B sin θ where θ is the angle < 180 degrees between the velocity and the magnetic field. 

This implies that the magnetic force on a stationary charge or a charge moving parallel to the magnetic field is zero. 

3. The direction of the force is given by the right hand rule. The force relationship above is in the form of a vector product. 

When the magnetic force relationship is applied to a current-carrying wire, the right-hand rule may be used to determine the 

direction of force on the wire. From the force relationship above it can be deduced that the units of magnetic field are Newton 

seconds /(Coulomb meter) or Newtons per Ampere meter. This unit is named the Tesla. It is a large unit, and the smaller unit 

Gauss is used for small fields like the Earth's magnetic field. A Tesla is 10,000 Gauss. The Earth's magnetic field at the surface is 

on the order of half a Gauss 


Lorentz force 

In physics, particularly electromagnetism, the Lorentz force is the combination of electric and magnetic force on 

a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of 

an electric field E and a magnetic field B, then it will experience a force 

F = - q∙ [ E + (v x B) ] 

(in SI units). Variations on this basic formula describe the magnetic force on a current-carrying wire (sometimes 

called Laplace force), the electromotive force in a wire loop moving through a magnetic field (an aspect of 

Faraday's law of induction), and the force on a charged particle which might be traveling near the speed of light 

(relativistic form of the Lorentz force). 

The first derivation of the Lorentz force is commonly attributed to Oliver Heaviside in 1889, although other 

historians suggest an earlier origin in an 1865 paper by James Clerk Maxwell. Hendrik Lorentz derived it a few 

years after Heaviside. 


http://en.wikipedia.org/wiki/Lorentz_force

Generation of Eddy Currents 

When a conductor is place in the area 

influence by the primary field, eddy current is 

induced in the conductor, see Fig. 1.4. 

Following Lenz’s law, the induced eddy 

current IE will produce a secondary field ф E 

that oppose the ф P . The magnitude of ф E is 

proportional to IE. 

The test objet, conductor B’s characteristic 

like, material conductivity, permeability and 

geometry will affect the IE, this in turn cause 

variation in ф E . The variation in ф E is reflected 

in conductor CA by ф E influences on ф p . The 

variations are recorded in media like meter, 

CRT, digital read out or chart. The 

I p = Primary Current 

Ф p =Primary magnetic flux 

Ф E = Secondary Eddy current magnetic flux 

I E = Secondary Eddy current 

Figure 1.4: Induced current relationships 


Generation of Eddy Currents 


http://www.suragus.com/en/company/eddy-current-testing-technology

Factors Affecting Inductance 

There are four basic factors of inductor construction determining the amount 

of inductance created. These factors all dictate inductance by affecting how 

much magnetic field flux will develop for a given amount of magnetic field 

force (current through the inductor's wire coil): 

NUMBER OF WIRE WRAPS, OR "TURNS" IN THE COIL: All other factors 

being equal, a greater number of turns of wire in the coil results in greater 

inductance; fewer turns of wire in the coil results in less inductance. 

Explanation: More turns of wire means that the coil will generate a greater 

amount of magnetic field force (measured in amp-turns!), for a given amount 

of coil current. L ∝ N 2 


http://www.allaboutcircuits.com/vol_1/chpt_15/3.html

COIL AREA: All other factors being equal, greater coil area (as measured 

looking lengthwise through the coil, at the cross-section of the core) results in 

greater inductance; less coil area results in less inductance. 

Explanation: Greater coil area presents less opposition to the formation of 

magnetic field flux, for a given amount of field force (amp-turns). L ∝ A 



COIL LENGTH: All other factors being equal, the longer the coil's length, the 

less inductance; the shorter the coil's length, the greater the inductance. 

Explanation: A longer path for the magnetic field flux to take results in more 

opposition to the formation of that flux for any given amount of field force 

(amp-turns). L ∝ (l) -1 

COIL LENGTH 

COIL LENGTH 

L ∝ (l) -1 L ∝ (l) -1 



CORE MATERIAL: All other factors being equal, the greater the magnetic 

permeability of the core which the coil is wrapped around, the greater the 

inductance; the less the permeability of the core, the less the inductance. 

Explanation: A core material with greater magnetic permeability results in 

greater magnetic field flux for any given amount of field force (amp-turns). 

L ∝ μ 

μ 0 = 4π x 10-7 H.m -1 μ r = 600, μ iron = 600 x μ 0 

μ 0 = 4π x 10-7 H.m -1 



Coil Inductance L 

An approximation of inductance L, for any coil of wire can be found with this formula: The electromagnetic field 

produced about an unloaded test coil can be described as decreasing in intensity with distance from the coil 

and also varying across the coil's cross section. The field is most intense near the coil's surface. The field 

produced about this coil is directly proportiona1 to the magnitude of applied current, rate of change of current or 

frequency and the coil parameters. Coil parameters inc1ude inductance, diameter, length, thickness, number 

of turns of wire and core material. 

L = μ r • (N 2 x A /l) • 1.26 x 10 -6 Henry 

μ 0 = 4π x 10 -7 H.m -1 or 1.26 x 10 -6 H.m -1 

EMF = L di/dt Volt 

Where: 

L= inductance in Henry H 

N = Numbers of turn in coil wire (straight wire N=1) 

μ r = relative permeability 

l = average length of coil in m 

A = area of coil (not wire area?) in m 2 

μ o = relative permeability in air 4π x 10-7 H.m -1 or 1.26 x 10-6 H.m -1 



Coil Inductance L 



Note the direction of the primary current (I p ) and the resultant eddy current (I E ). 

I E extends some distance into the test object. Another important observation 

is that I E is generated in the same plane in which the coil is wound. Figure 1.6 

emphasizes this point with a loop coil surrounding a cylindrical test object (4). 

Important observation is that 

I E is generated in the same 

plane in which the coil is 

wound. 

Figure 1.6 Induction current flow in a cylindrical part. 


Note the direction of the primary current (I p ) and the resultant eddy current (I E ). 

I E extends some distance into the test object. Another important observation 

is that I E is generated in the same plane in which the coil is wound. Figure 1.6 

emphasizes this point with a loop coil surrounding a cylindrical test object (4). 

Important observation is that 

I E is generated in the same 

plane in which the coil is 

wound & in opposite direction 

of I p 

Figure 1.6 Induction current flow in a cylindrical part. 


Generation of Eddy Current 

With a primary current 1p flowing through the coil, a primarr electromagnetic 

field фp is produced about the coil. When this excited test coil is placed on an 

electrically conductive test object, eddy currents IE will be generated in that 

test object Figure 1.5 illustrates this concept. 

Figure 1.5 Generation of eddy current I E in a test object 


It must be understood that this formula yields approximate figures only. One 

reason for this is the fact that permeability changes as the field intensity 

varies (remember the nonlinear "B/H" curves for different materials). 

Obviously, if permeability (µ) in the equation is unstable, then the inductance 

(L) will also be unstable to some degree as the current through the coil 

changes in magnitude. If the hysteresis of the core material is significant, this 

will also have strange effects on the inductance of the coil. Inductor designers 

try to minimize these effects by designing the core in such a way that its flux 

density never approaches saturation levels, and so the inductor operates in a 

more linear portion of the B/H curve. 

If an inductor is designed so that any one of these factors may be varied at 

will, its inductance will correspondingly vary. Variable inductors are usually 

made by providing a way to vary the number of wire turns in use at any given 

time, or by varying the core material (a sliding core that can be moved in and 

out of the coil). An example of the former design is shown in this photograph: 




Permeability changes as the field intensity varies 

(remember the nonlinear "B/H" curves for different 

materials).

Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its 

length. The unit shown happens to be an air-core inductor used in early radio work. 

Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit 

built for radios. The connection terminals can be seen at the bottom, as well as the few turns of 

relatively thick wire: 

Figure 3: Here is another inductor (of greater inductance value), also intended for radio 

applications. Its wire coil is wound around a white ceramic tube for greater rigidity: 

Figure 4: The two inductors on this circuit board are labeled L1 and L2, and they are located to 

the right-center of the board. Two nearby components are R3 (a resistor) and C16 (a capacitor). 

These inductors are called "toroidal" because their wire coils are wound around donut-shaped 

("torus") cores. 

Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" 

as well. The following photograph shows just how small an inductor can be when packaged as 

such: A pair of inductors can be seen on this circuit board, to the right and center, appearing as 

small black chips with the number "100" printed on both. The upper inductor's label can be seen 

printed on the green circuit board as L5. Of course these inductors are very small in inductance 

value, but it demonstrates just how tiny they can be manufactured to meet certain circuit design 

needs. 



A Dual Variable inductors 

Figure 1: This unit uses sliding copper contacts to tap into the coil at different points along its length. The 

unit shown happens to be an air-core inductor used in early radio work. 


Figure 2: A fixed-value inductor is shown in the next photograph, another antique air-core unit built for 

radios. The connection terminals can be seen at the bottom, as well as the few turns of relatively thick wire: 


Figure 3: Here is another inductor (of greater inductance value), also intended for radio applications. Its 

wire coil is wound around a white ceramic tube for greater rigidity: 


Figure 4: The two inductors 

on this circuit board are labeled 

L1 and L2, and they are located 

to the right-center of the board. 

Two nearby components are 

R3 (a resistor) and C16 (a 

capacitor). These inductors are 

called "toroidal" because their 

wire coils are wound around 

donut-shaped ("torus") cores. 



Figure 4: The two inductors 

on this circuit board are labeled 

L1 and L2, and they are located 

to the right-center of the board. 

Two nearby components are 

R3 (a resistor) and C16 (a 

capacitor). These inductors are 

called "toroidal" because their 

wire coils are wound around 

donut-shaped ("torus") cores. 



Figure 5: Like resistors and capacitors, inductors can be packaged as "surface mount devices" as well. The following 

photograph shows just how small an inductor can be when packaged as such: A pair of inductors can be seen on this circuit board, 

to the right and center, appearing as small black chips with the number "100" printed on both. The upper inductor's label can be 

seen printed on the green circuit board as L5. Of course these inductors are very small in inductance value, but it demonstrates 

just how tiny they can be manufactured to meet certain circuit design needs. 



Grundig radio satellit 750 

■ 

https://www.youtube.com/embed/yD7WAcSwz8o 


http://www.universal-radio.com/catalog/portable/0750.html

Phasor Vector Diagram of Coil Voltage 

A more precise method of describing the relationships of magnetic flux, voltage and current is the 

phase vector diagram or phasor diagrams (4). Figure 1.7 compares the electromagnetic events 

associated with an unloaded test coil and what happens when that same coil is placed on a 

nonferromagnetic test object. The components of phasor diagrams are as follows: 

Fig.17(b) 

E p = Primary coil voltage 

I = Exciting current (Primary coil current) 

Ф p = Primary flux 

Ф s = Secondary flux 

Fig.17(b) 

E p = Primary coil voltage 

I = Exciting current (Primary coil current) 

Ф p = Primary flux 

Ф s = Secondary flux 

E s = Secondary voltage 

E T = Total voltage 

Ф T = Total flux 


Figure 1.7: Phasor Diagram of Coil Voltage (?) 

In Figure 1.7(a) the current (I) and primary magnetic 

flux (ф p ) are plotted in phase. The primary voltage (E p ) 

is shown separated by 90 electrical degrees. The 

secondary magnetic flux (ф s ) is plotted at zero 

because without a test object no secondary flux exists. 

Figure 1.7(b) represents the action of placing the coil on a 

nonferromagnetic test object Observing the figure, one can see by 

vectorial addition of E p and E s that a new coil voltage (E T ) is arrived 

at for the loaded condition. The primary magnetic flux ф p and 

secondary magnetic flux ф s are also combined by vectorial addition 

to arrive at a new magnetic flux (ф T ) for the loaded coil. 


In Figure 1.7(a) the current (I) and primary magnetic flux (ф p ) are plotted in 

phase. The primary voltage (E p ) is shown separated by 90 electrical degrees. 

The secondary magnetic flux (фs) is plotted at zero because without a test 

object no secondary flux exists. 

Figure 1.7(b) represents the action of placing the coil on a nonferromagnetic 

test object Observing the figure, one can see by vectorial addition of E p and 

Es that a new coil voltage (E T ) is arrived at for the loaded condition. The 

primary magnetic flux ф p and secondary magnetic flux фs are also combined 

by vectorial addition to arrive at a new magnetic flux (ф T ) for the loaded coil. 

Notice that for the condition of the test object in the test coil, ф T is no longer in 

phase with the excitation current I. Also observe that the included angle 

between the excitation current and the new coil voltage E T is no longer at 90 

electrical degrees. These interactions will be discussed in detaillater in this 

study guide. 


Current Density 

The distribution of eddy currents in a test object varies exponentially. The 

current density in the test object is most dense near the test coil. This 

exponential current density follows the mathematical rules for a natural 

exponential decay curve (1/ e) where ε (epsilon) is 2.718. Usually a natural 

exponential curve is illustrated by a graph with the ordinate (Y axis) 

representing magnitude and the abscissa (X axis) representing time or 

distance. A common point described on such a graph is the knee of the curve. 

The knee occurs at the 37% value on the ordinate axis. 

This 37% point is chosen because changes in X axis values produce 

significant changes in Yaxis values from 100% to 37% and below 37% 

changes in X axis values þroduce less signlficant changes in Y axis values 

(?). 


Applying this logic to eddy current testing, a term is developed to describe the 

relationship of current distribution in the test object. The eddy current 

generated at the surface of the test object nearest the test coil is 100%. The 

point in the test object thickness where this current is diminishedto37% ofits 

previous strength is known as the standard depth of penetration. The term δ 

(delta) is used to represent this point in the material. Figure 1.8 is a relative 

eddy current density curve for a plane wave of infinite extent with magnetic 

field parallel to the conducting test object surface. 


Figure 1.8: Relative eddy current density 


The current density at any depth can be calculated as: 

J x =J 0 e -x√(πfμσ) 

Where: 

J x = Electrical density at depth x in A∙m -2 

J 0 = Electrical density at the surface x=0 

x = distance fro surface in meter m 

f = Frequency of the AC primary current Hz 

μ = Permeability of the test object in H∙m -1 

σ = Conductivity of the test object in Siemen∙m -1 

e = Natural logarithm 


Relative Magnetic Permeability 

Permeability of free space μ 0 = 4π x 10-7 HM -1 

Permeability of material can be expressed as relative to μ 0 

μ material = μ r ∙μ 0 


The Standard Depth of Penetration δ 

The Standard Depth of Penetration can be expressed as: 

δ = (πfμσ) -½ 

Where: 

δ = One standard depth of penetration; 1/e of the surface current 

density (37%) in meter, m 

f = Frequency of the AC primary current in Hz 

μ = Permeability of the test object in Henry per meter, H∙m -1 

σ = Conductivity of the test object in Siemens per meter, S∙m -1 


It should be observed at this point that as frequency, conductivity or 

permeability is increased, the penetration of current into the test object will be 

decreased. The graph in Figure 1.8 is used to demonstrate many eddy 

current characteristics. 

Using an example of a very thick block of stainless steel being interrogated 

with a surface or probe coil operating at a test frequency of 100 kHz, the 

standard depth of penetration can be determined and current densities 

observed at other depths. Stainless steel (300 Series) is nonferromagnetic. 

Magnetic permeability (μ) is 4πX 10 -7 H∙m -1 , the conductivity σ is 0.14 X 10 7 

siemens (mhos) per meter for 300 Series stainless steel. 

δ = (πfμσ) -½ 

δ = (π x 100 x 10 3 x 4 x π x 0.14) -½ 

δ = 0.00135m or 1.35mm # 


δ = (π x 100 x 10 3 x 4 x π x 0.14) -½ 

as 1000*(pi*x*4*pi*0.14*10^3)^(-.5) 


http://graph-plotter.cours-de-math.eu/

δ = (π x 100 x 10 3 x 4 x π x 0.14) -½ 

as 1000*(pi*x*4*pi*0.14*10^3)^(-.5) 


http://fooplot.com/

δ = (π x 100 x 10 3 x 4 x π x 0.14) -½ 

as 1000*(pi*x*4*pi*0.14*10^3)^(-.5) 


http://rechneronline.de/function-graphs/

Using 1.35 mm as depth x from surface, a ratio of depth/depth of penetration 

would be 1 Referring to Figure 1.8, a depth/ depth of penetration of 1 

indicates a relative eddy current density of 0.37 or 37%. What is the relative 

eddy current density at 3 mm? 

The relative standard depth D relative of x = 3mm is: 

D relative = 3/δ = 3/1.53 mm = 2.22δ 

This ratio indicates a relative eddy current densityof about 0.1 or 10% 

[ (1/e) 2.22 = 10.9% ]. With only 10% of the available current flowing at a depth 

of 3 mm, detectability of variables such as conductivity, permeability and 

discontinuities would be very difficult to detect. The obvious solution for 

greater delectability at a depth of 3 mm depth is to lower the test frequency. 

Frequency selection will be covered in detaillater in this text. 


f(x) = (1/e) x where x = depth/δ 

Relative current density 

Relative Standard depth x = depth/δ 


http://rechneronline.de/function-graphs/

Standard Depth for Different Conductive Materials 


https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/PopUps/applet7/applet7.htm

Phase/Amplitude and Current Time Relationships 

Figure 1.9 reveals another facet of eddy current. Eddy currents are not 

generated at the same instant in time throughout the part. Eddy currents 

require time to penetrate the test part. Phase and time are analogous 

meaning - phase is an electrical term used to describe timing relationships of 

electrical waveforms. 

Phase Lag = x/δ radian 

Where: 

x =depth below surface 

δ = Standard depth 


Current (?) Lagging 

Voltage lagging or current lagging? 


Current (?) Lagging 

Voltage lagging or current lagging? 


σº∙πμ■δ∝∞ωΩθ√ρβααδπ 


Phase is u,sually expressed in either degrees or radians. There'are 

2πradians per 360 degrees. Each radian therefore is about 57 degrees 

(360/2π). Using the surface eddy current near the test coil as a reference, the 

deeper the eddy current the greater the phase lag. The amount of phase lag 

is determined by: 

β = x/δ = x∙√(πfμσ) 

β or Φ = Phase lag angle in radian. 

Others as defined earlier 


Figure 1.9 should be used as a relative indicator of phase lag. The exact 

phase relationship for a particular system may be different due to other 

variables, such as coil parameters and excitation methods. 

The amount of phase lag for a given part thickness is an important factor 

when considering resolution. Resolution is the ability to separate variables 

occurring in the test object; for example, distinguishing two discontinuities 

occurring at different depths in the same test object. As an example, using a 

standard depth of penetration at 1 mm in a 5 mm thick test object. Refer to 

Figure 1.9 and observe the phase lag of the current at one standard depth of 

penetration. Where depth of interest (x) is 1 mm and depth of penetration (δ) 

is 1 mm, the x/ δ ratio is 1 and the current at depth x lags the surface current 

by 1 radian or 57 degrees. 


Projecting this examination, observe the phase lag for the entire part 

thickness. The standard depth of penetration is 1 mm, the part thickness is 5 

mm; therefore, the ratio x/δ equals to 5. This produces phase lag of 5 radians 

or about 287 degrees for the part thickness. Having a measurement capability 

of 1 degree increments, the part thickness could be divided into 287 parts 

each part representing 0.017mm. That would be considered excellent 

resolution. 

There is an obvious Iimitation. Refer to Figure 1.8 and observe the resultant 

relative current density with an x/δ ratio of 5. The relative current density is 

near 0. It should become apparent that the frequency can be adjusted to 

achieve optimum results for a particular variable. These and other variables 

will be discussed in Chapter 5 of this Study Guide. 




Chapter 1 

Review Questions 


Q.1.1 Generation of eddy currents depends on the principle of: 

A. wave guide theory. 

B. electromagnetic induction. 

C. Magnetostriction force 

D. All of the above 

Q.1.2 A secondary field is generated by the test object and is; 

A. Equal and opposite to the primary field 

B. Opposite to the primary field but much smaller 

C. In the same plane as the coil is wound. 

D. In phase with the primary field. 

Q.1.3 When a non ferromagnetic part is placed in the test coil, The coil' s 

voltage: 

A. increases 

B. remains constant because this is essential. 

C. decreases. 

D. shifts 90 degrees in phase. 


Q.1.4 Refer to Figure 1.7(b). If ET was produced by the test object being 

stainless steel, what would the effect be if the test object were copper? 

A. ET would decrease and be at a different angle. 

B. ET would increase and be at a different angle. 

C. Because both materials are non-ferromagnetic, no change occurs 

D. None of the above. 


Q.1.5 Eddy current generated a test object flow; 

A. in the same plane as magnetic flux 

B. in the same plane as the coil is wound 

C. 90 degrees to the coil winding plane. 

D. eddy currents have no predictable direction. 

Q.1.6 The discovery of electromagnetic induction is credited to 

A. Arago 

B. Oersted. 

C. Maxwell. 

D. Faraday. 

Q.1.7 A standard depth of penetration is defined as the point in a test object 

where the relative current density is reduced to: 

A. 25%. 

B. 37% 

C. 50%. 

D. 100% 


Q.1.8 Refer to Figure 1.8. If one standard depth of penetration was 

established at 1 mm in an object 3 mm thick, what is the relative current 

density on the far surface? 

A. 3 

B.

Q.1.9 Refer to Figure1.9 using example in question 1.8, what is the phase 

difference between the near and far surfaces? 

A. the far surface current leads the near surface current by 57 degrees. 

B. the far surface current leads the near surface current by 171 degrees. 

C. the far surface current lags the near surface current by 171 degrees. 

D. the far surface current lags the near surface current by 570 degrees. 

Q.1.10 Calculate the standard depth of penetration at 10KHz in Copper with σ 

= 5.7 x 10 7 Siemens per meter. 

A. 0.1 mm (3.9 x10 -3 in.) 

B. 0.02 mm (7.9 x10 -4 in.) 

C. 0.66 mm (0.026 in.) 

D. 66 mm (2.6 in.) 

β = x/ δ x 57.3º 

δ = (πfμσ) -½ = √(10 x 10 3 x π x 4 π x 10 -7 x 5.7 x 10 7 ) x 1000 mm 


The Answers 


Chapter 2 

The Coil Arrangements 


Test Coil Arrangement 

Test coils can be categorized into three main mechanical groups: probe coils, 

bobbin coils and encircling coils. 

(Surface coil, internal bobbing coil, encircling coil) 

Probe Coils 

Surface coil, probe coil, flat coil or pancake coil are all common terms used to 

describe the same test coil type. Probe coils provide a convenient method of 

examining the surface of a test object. Figure 2.1 below illustrates a typical 

set of probe coils used for several surface scanning applications. 


Probe coils and probe coil forms can be shaped to fit particular geometries to 

solve complex inspection problems. As an example, probe coils fabricated in 

a pencil shape (pencil probe) are used to inspect threaded areas of mounting 

studs and nuts or serrated areas of turbine wheels and turbine blade 

assemblies. Probe coils may be used where high resolution is required by 

adding coil shielding (2). When using a high resolution probe coil, the test 

object surface must be carefully scanned to ensure complete inspection 

coverage. This careful scanning is very time consuming. For this reason, 

probe coil inspections of large test objects are usually limited to critical areas. 

Probe coils are used extensively in aircraft inspection for crack detection near 

fasteners and fastener holes. In the case of fastener holes (bolt holes, rivet 

holes), the probe coil may be rotated either manually or mechanically to 

provide a helical scan of the hole using a spinning probe technique (Figure 

2.2). 


Figure 2.2: Bolt hole inspection probes 


Encircling Coils 

Encircling coil, outside diameter coil and feed through coil are terms 

commonly used to describe a coil that surrounds the test object. Figure 2.3 

illustrates a typical encircling coil. Encircling coils are primarily used to inspect 

tubular and bar-shaped products. The tube or bar is fed through the coil (feed 

through) at relatively high speed. The cross section of the test object within 

the test coil is simultaneously interrogated. For this reason, the 

circumferential location of discontinuities cannot be determined with an 

encircling coil. 

The volume of material examined at one time is greater using an encircling 

coil than a probe coil; therefore, the relative sensitivity is lower for an 

encircling coil. The additional advantage that a probe coil would have over the 

encircling coil is that the probe coil could define where within the 

circumferential plane the discontinuity exists. The encircling coil cannot make 

that distinction. If there are multiple signal sources within the coil's field of 

view the encircling coil response will indicate the average of all of those 

events. 


Figure 2.3: Encircling coil 


Discussion 

Subject; Encircling Coil 

If there are multiple signal sources within the coil's field of view the encircling 

coil response will indicate the average of all of those events. 

Question: Why average? why not sum of all signals? 


When using an encircling coil, it is important to keep the test object centered 

in the coil. If the test object is not centered, a uniform discontinuity response 

is difficult to obtain. To ensure proper centering it is corrunon practice to run 

the calibration standard several times, each time indexing the artificial 

discontinuities to a new circumferential location in the coil. As in all 

discontinuity detection schemes, it is essential to select a reasonable 

operating frequency, properly adjust the system display parameters and 

ensure that the tube is centered in the coil at all times to achieve optimum test 

sensitivity. 


Bobbin Coils 

Bobbin coil, inside diameter coil and inside probe are terms that describe 

coils used to inspect from the inside diameter or bore of a tubular test object. 

Bobbin coils are inserted and withdrawn from the tube inside diameter by long, 

semi flexible shafts or simply blown in with air and retrieved with an attached 

pull cable. These mechanisms will be described later in the text. Bobbin coil 

information follows the same basic rules stated for encircling coils. Figure 2.4 

illustrates a typical bobbin coil. 


Coil Arrangements 

Probe coils, encircling coils and bobbin coils can be additionally classified. 

These additional classifications are determined by how the coils are 

electrically connected. The three coil categories are absolute, differential and 

hybrid. Figure 2.5 shows various types of absolute and differential coil 

arrangements. 


Figure 2.5: Test coil configurations for eddy current testing of small-diameter tubing 

• Absolute 

• Differential- Self 

comparisons, external 

reference 

• Thru transmission 

• Reflection (Double) - 

Sending & Receiving 


Absolute Coils 

An absolute coil makes its measurement without direct reference or 

comparison to a standard as the measurement is being made (6). Some 

applications for absolute coil systems would be measurements of conductivity, 

permeability, dimensions and hardness. 


Differential coils 

Differential coils consist of two or more coils electrically connected to oppose 

each other. Differential coils can be categorized into two types: (1) self 

comparison differential and (2) external reference differential. 

a) The self-comparison differential coil compares one area of a test object 

to another area on the same test object. A common use is two coils, 

connected opposing, so that if both coils are affected by identical test object 

conditions, the net output is 0 volts or no signal change. The self-comparison 

arrangement is insensitive to test object variables that occur gradually. 

Variables such as slowly changing wall thickness, diameter or conductivity 

are effectively discriminated against with the selfcomparison differential coil. 

Only when a different condition affects one or the other test coils will an 

output signal be generated. The coils usually being mechanically and 

electrically similar allows the arrangement to be very stable during 

temperature changes. Short discontinuities such as cracks, pits or other 

localized discontinuities with abrupt boundaries can be readily detected using 

the self-comparison differential coli. 


) The external reference differential coil, uses an external reference to 

affect one coil while the other coil is affected by the test object. Figure 2.6 

illustrates this concept. This system is used to detect differences between a 

standard object and test objects. It is particularly useful for comparative 

conductivity, permeability and dimensional measurements. Obviously in 

Figure 2.6 it is imperative to normalize (or balance) the system with one coil 

affected by the standard object and the other coil affected by an acceptable 

test object. The external reference differential coil system is sensitive to all 

measurable differences between the standard object and test object. For this 

reason it is often necessary to provide additional discrimination to separate 

and define variables present in the test object. 


Figure 2.6: External reference differential system 


Hybrid Coils 

Hybrid coils may be defined as driver/pickup, through transmission, reflection 

or primary/secdndary coil assemblies. Hybrid coils may or may not be the 

same size and are not necessarily adjacent to each other. Figure 2.7 shows 

one possible hybrid coil arrangement. In the through transmission coil, the 

excitation coil is on one side of the test object and the sensing coil is on the 

other. The driver coil induces eddy currents and a secondary magnetic field in 

the test specimen. Any variation of these secondary events should be 

detected by the smaller probe coil on the opposite side of the thin plate. 


Figure 2.7: Hybrid coil (through transmission) 


A hybrid coil arrangement consists of an excitation coil and a sensing coil 

(reflection coils). In most cases a single probe housing assembly contains 

both the driver and the pickup coil(s). The primary magnetic flux interacts with 

both coils. The voltage developed in the sensing coil is a function of the 

current magnitude and frequency applied to the excitation coil, coil 

parameters of the exciting and sensing coils and the test object 

characteristics. 

Most hybrid coils are designed to improve test sensitivity for a specific 

application. One example of this is for better detection of subsurface 

discontinuities in multilayer structures. The concept of using a smaller pickup 

coil enhances the ability to detect lower level impedance variations from small 

volume discontinuities deeper in the test sample. It should be pointed out that 

if larger volume discontinuities are encountered that a measurable impedance 

change might be generated by both the exciter and the pick up coil(s). 


Additional Coil Characteristics 

Coil configuration is but one of many factors to consider when setting up test 

conditions. Other coil characteristics of importance are mechanical, thermal 

and electrical stability; sensitivity, resolution and dimensions. The geometry of 

the coil is usually dictated by the geometty of the test object. Selection of 

smaller probe sizes may affect test sensitivity and/or resolution. The relative 

importance of the coil characteristics depends on the nature of the test. A 

blend of theory and experience usually succeeds in selection of proper coil 

parameters. Coil design and interactions with test objects will be discussed 

later in this Study Guide. 


Chapter 2 



Q.2.1 Differential coils are usually used in: 

A. bobbin coils. 

B. probe coils. 

C. outside diameter coils. 

D. any of the above. 

Q.2.2 When using a probe coil to scan a test object: 

A. the object must be dry and polished. 

B. the object must be scanned carefully to ensure inspection coverage. 

C. the object must be scanned in circular motions at constant speeds. 

D. the probe must be moving at all times to get a reading. 

Q.2.3 A spinning probe would most likely be: 

A. a bobbin coil 

B. an inside diameter coil. 

C. an outside diameter coil. 

D. a probe coil. 


Q.2.4 A feed through coil is: 

A. a coil with primary/ secondary windings connected so that the signal is fed 

through the primaq to the secondary. 

B. an encircling coil. 

C. an outside diameter coil. 

D. both Band C. 

Q.2.5 When inspecting a tubular product with an encircling coil, which 

statement is not true? 

A. Outside diameter discontinuities can be found. 

B. Axial discontinuity locations can be noted. 

C. Circumferential discontinuity locations can be noted. 

D. Inside diameter discontinuities can be found. 


Q.2.6 An absolute coil measurement is made: 

A. by comparing one spot on the test object to another. 

B. without reference to or direct comparison with a standard. 

C. only with probe coils. 

D. by comparative measurement to a known standard. 

Q.2.7 When coils in a self-comparison differential arrangement are affected 

simultaneously with the same test object variables, the output signal: 

A is directly proportional to the number of variables. 

B. is 0 or near 0. 

C. is indirectly proportional to the number of variables. 

D. is primarily a function of the exciting current. 


Q.2.8 Which coil type inherently has better thermal stability? 

A bobbin 

B. absolute 

C. outside diameter 

D. self-comparison differential 

Q.2.9 A hybrid coil is composed of two or more coils. The coils: 

A. must be aligned coplanar to the driver axis. 

B. may be of widely different dimensions. 

C. must be impedance matched as closely as possible. 

D. are very temperature sensitive. 

Q.2.10 Proper selection of test coil arrangement is determined by: 

A. shape of test object. 

B. redolution required. 

C. sensitivity required. 

D. stability. 

E. all of the above. 


The Answers 


Chapter 3 

Test Coil Design 


As discussed earlier test coil design and selection is a blend of theory and 

experience. Many factors must be considered. These important factors are 

determined by the inspection requirement for resolution, sensitivity, 

impedance, size, stability and environmental considerations. To better 

understand coil properties and electrical relationships, a short refresher in 

alternating current theory is necessary. First, the electrical units must be 

examined. For example, current and its representative symbol I. Current not 

only suggests electron flow but also the amount. The amount of electrons 

flowing past a point in a circuit in 1 second is expressed in amperes: 

2π x 10 18 electrons passing a point in 1 second is called 1 ampere. 


Ampere 

The ampere (SI unit symbol: A), often shortened to "amp", is the SI unit of electric 

current (dimension symbol: I) and is one of the seven SI base units. It is named after 

André-Marie Ampère (1775–1836), French mathematician and physicist, considered 

the father of electrodynamics. 

The ampere is equivalent to one coulomb (roughly 6.241×10 18 times the elementary 

charge) per second. Amperes are used to express flow rate of electric charge. For any 

point experiencing a current, if the number of charged particles passing or the charge 

on the particles is increased, the amperes of current at that point will proportionately 

increase. 

The ampere should not be confused with the coulomb (also called "ampere-second") 

or the ampere-hour (A·h). The ampere is a unit of current, the amount of charge 

transiting per unit time, and the coulomb is a unit of charge. When SI units are used, 

constant, instantaneous and average current are expressed in amperes (as in "the 

charging current is 1.2 A") and the charge accumulated, or passed through a circuit 

over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). 

The relation of the ampere to the coulomb is the same as that of the watt to the joule, 

and that of metre per second to metre. 


http://en.wikipedia.org/wiki/Amperec

Demonstration model of 

a moving iron ammeter. 

As the current through 

the coil increases, the 

plunger is drawn further 

into the coil and the 

pointer deflects to the 

right. 


http://en.wikipedia.org/wiki/Amperec

Resistance 

Resistance is an opposition to the flow of electrons and is measured in ohms. 

Ohm's Law is stated by the equation: 

E = IR 

Where: 

I = Current in Ampere A 

R = Resistance in Ohm Ω 

E = Electrical potential difference in volt V 

The resistance of a coil is determined primarily by the length of wire used to 

wind the coil; its specific resistance is determined by the type of wire (e.g., 

copper, silver) and the cross-sectional area of the wire. 

Resistance = (Specific resistance X Length) / Area 

Resistance = 


Thus, the resistance of a 10 ft length of 40 gage copper wire with a specific 

resistance of 10.4 circular-mil-foot at 20ºC would be found as follows: 

R = (10.4 X 10) / 9.888 = 10.518 ohm. 

In an alternating current circuit containing only resistance, the current and 

voltage are in phase. In phase means the current and voltage reach their 

minimum and maximum values, respectively, at the same time. The power 

dissipated in a resistive circuit appears :in the form of heat. For example, 

electric toasters are equipped with resistance wires that become hot when 

current flows through them, providing a heat source for toasting bread. 


Circular-Mill-Foot 

A circular-mil-foot (figure 1-3) is a unit of volume. It is a unit conductor 1 foot in length and has a cross-sectional 

area of 1 circular mil. Because it is a unit conductor, the circular-mil-foot is useful in making comparisons 

between wires consisting of different metals. 

For example, a basis of comparison of the RESISTIVITY (to be discussed shortly) of various substances may 

be made by determining the resistance of a circular-mil-foot of each of the substances. 

In working with square or rectangular conductors, such as ammeter shunts and bus bars, you may sometimes 

find it more convenient to use a different unit volume. A bus bar is a heavy copper strap or bar used to connect 

several circuits together. Bus bars are used when a large current capacity is required. 

Unit volume may be measured as the centimeter cube. Specific resistance, therefore, becomes the resistance 

offered by a cube-shaped conductor 1 centimeter in length and 1 square centimeter in cross-sectional area. 

The unit of volume to be used is given in tables of specific resistances. 


http://www.tpub.com/neets/book4/11b.htm

SPECIFIC RESISTANCE OR RESISTIVITY Specific resistance, or resistivity, is the 

resistance in ohms offered by a unit volume (the circular-mil-foot or the centimeter cube) of a substance to the 

flow of electric current. Resistivity is the reciprocal of conductivity. A substance that has a high resistivity will 

have a low conductivity, and vice versa. Thus, the specific resistance of a substance is the resistance of a unit 

volume of that substance. Many tables of specific resistance are based on the resistance in ohms of a volume 

of a substance 1 foot in length and 1 circular mil in cross-sectional area. The temperature at which the 

resistance measurement is made is also specified. If you know the kind of metal a conductor is made of, you 

can obtain the specific resistance of the metal from a table. The specific resistances of some common 

substances are given in table 1-1. 



The resistance of a conductor of a uniform cross section varies directly as the 

product of the length and the specific resistance of the conductor, and 

inversely as the cross-sectional area of the conductor. Therefore, you can 

calculate the resistance of a conductor if you know the length, cross-sectional 

area, and specific resistance of the substance. Expressed as an equation, the 

"R" (resistance in ohms) of a conductor is 



Problem: 

What is the resistance of 1,000 feet of copper wire having a cross-sectional 

area of 10,400 circular mils (No. 10 wire) at a temperature of 20°C? 

Solution: 

The specific resistance of copper (table 1-1) is 10.37 ohms. Substituting the 

known values in the preceding equation, the resistance, R, is determined as 

R = ρ∙ l / A = 10.37 x 1000 / 10400 = 1Ω approximately 

This equipment operates on the principle that the resistance of a line varies 

directly with its length. Thus, the distance between the test point and a fault 

can be computed accurately. 



Inductance 

Heat generation is an undesirable trait for an eddy current coil. If the 10 ft 

length of wire used in the previous example was wound into the shape of a 

coil, it would exhibit characteristics of alternating current other than resistance. 

By forming the wire into the shape of a coil, the coil also would have the 

property of inductance. The role of inductance is analogous to inertia in 

mechanics, because inertia is the property of matter that causes a body to 

oppose any change in its velocity. The unit of inductance is the Henry (H). A 

coil is said to have the property of inductance when a change in current 

through the coil produces a voltage in the coil. More precisely, a circuit in 

which an electromotive force of 1 V is induced when the current is changing 

at a rate of 1 Ampere per second will have an inductance of 1 H. 


The inductance of a multilayer air core coil can be expressed by its physical 

properties or coil parameters. Coil parameters such as length, diameter, 

thickness and number of turns of wire affect the coil's inductance. Figure 3.1 

illustrates typical coil dimensions required to calculate coil :inductance. An 

approximation of small, multilayer, air core coil inductapce is as follows: 

L = 0.8(rN) 2 ∙ (6∙r+9∙l+10∙b) -1 μHenry 

L = self inductance in μH 

N = number of turns 

r = mean radius in inches 

l = length of coil in inches 

b = coil depth 


Multi Layer Induction Coil 


http://info.ee.surrey.ac.uk/Workshop/advice/coils/air/area.xhtml

Example: 

A coil whose dimensions are as follows: 

r = 0.1 in 

I = 0.1 in 

b = 0.1 in 

N = 100 turns 

for L = 0.8(rN) 2 ∙ (6∙r+9∙l+10∙b) -1 μH 

L = 0.8(0.1x100) 2 (6x0.1+9x0.1+10.0.1) -1 

L = 32 μH 

As stated earlier, this inductance is analogous to inertia in mechanical systems in that inductance 

opposes a change in current as inertia opposes a change in velocity of a body. In alternating 

current circuits the current is always changing; therefore inductance is always opposing this 

change. As the current tries to change, the inductance reacts to oppose that change. This 

reaction is called innductive reactance. 


Inductive Reactance 

The unit of inductive reactance (X L ) is in ohms. For a given coil the inductive 

reactance is a function of the rate of change of current or frequency. A 

formula relating frequency, inductance and inductive reactance is: 

XL = ωL = 2πf L 

Where: 

XL = Inductive reactance Ohm 

f = Frequency Hz 

L = Inductance Henry 


Example: 

Using the 32 μH coil calculated earlier operating at 100 KHz, its inductive 

reactance would be found as follows: 

L = 32 μH or 0.000032 H 

f = 100 KHz or 100000 Hz 

X L = ωL = 2πfL = 2π x 100 x 10 3 x 32 x 10 -6 = 20.106Ω 

Therefore, this coil would present an opposition of 20.096 ohms to currents 

with a rate of change of 100 kHz due to its reactive component. Unlike a 

resistive circuit, the current and voltage of an inductive circuit do not reach 

their minimum and maximum values at the same time. In a pure inductive 

circuit the voltage leads the current by 90 electrical degrees. This means that 

when the voltage reaches a maximum value, the current is at 0. 


Power is related to current and voltage as follows: 

Power P = E x I 

P = Power 

E = Potential volt 

I = Current in Ampere 

Notice that in a pure inductive circuit, when the voltage is maximum, the 

current is 0. Therefore, the product P = E x I = 0, Inductive reactances 

consume no alternating power where resistive elements consume power and 

dissipate power in the form of heat. The opposition to current flow because of 

the resistive element of the coil and the reactive element of the coil do not 

occur at the same time; therefore, they cannot be added as scalar quantities. . 

A scalar quantity is one having only magmtude, that is a quantity fully 

described by a number, but which does not involve any concept of direction. 

Gallons in a tank, temperature in a room, miles per hour, for example, are all 

scalars. 


Impedance 

To explain the addition of reactance and resistance witha minimum of 

mathematical calculations, it is ·possible to use vector or phasor diagrams. A 

vector diagram constructed with imaginary units on the ordinate or Y axis and 

real units on the abscissa or X axis is shown in Figure 3.2. Z=√(X L2 +R 2 ) 

Observation of Figure 3.2 reveals X L , R and 

Z appear to form the sides of a right triangle. 

The mathematical solution of right triangles 

states the square of the hypotenuse is equal 

to the sum of the squares of the other two 

sides, or c 2 = a 2 + b 2 

Figure 3.2 Vector Diagram 

Substituting Z, X L and R, the statement 

becomes: Z 2 = X L2 + R 2 , further simplified 

Z = √(X L2 +R 2 ) 


Example: 

Example: What is the impedance of a coil having an inductance. of 100μH 

and a resistance of 5 ohms and being operated at 200 kHz? 

X L = 2π x 200 x 10 3 x 100 x 10 -6 = 125.7Ω 

Z =√(X L2 + R 2 ) = (125.7 2 + 5 2 ) .5 = 125.8 Ω 

First, convert inductance to inductive reactance and then, by vector addition, 

combine inductive reactance and resistance to obtain the impedance. 


Maximum transfer of power is accomplished when the driving impedance and load 

impedance are matched. If, for instance, an eddy current instrument had a driving 

impedance of 50 ohms, the most efficient test coils would also have impedances of 50 

ohms. Other, more common examples of impedance matching are home stereo 

systems rated at 100 W per channel into 8 ohms. Impedance can be discussed in a 

more detailed manner by mathematically noting variables using imaginary numbers). 

The square root of a negative number is known as an imaginary number (√-1). 

The imaginary number √(-16) could be written as √(-1x16) or √-1∙ √(16) or 

√(-1)∙4. The notation √(-1) is used extensively and is mathematically noted by a lower 

case letter "i". Because i is also used in electrical terms for current, the i notation for 

electrical calculations is changed to the letter "j". The term j, often called operator j, is 

equal to the √(-I). Instead of noting √(-16) as √(-1)∙4. note it as j4. Because reactance 

is known as an imaginary component, then impedance In Cartesian form: 

Z = R + jX m 

where the real part of impedance is the resistance R and the imaginary part is the 

reactance X. 


http://en.wikipedia.org/wiki/Electrical_impedance

Rectangular Notation 

Because reactance is known as an imaginary component, then impedance In 

Cartesian form: 

Z = R + jX = |Z|∠ θ= 

The term R + jX is known as a rectangular notation. As an example, a 

resistance of 4 ohms in series with an inductive reactance of 3 ohms could 

be noted as Z = 4 + j3 ohms. The impedance 

j3 ohms 

Z =|5|∠ 48.59º Ω. 

Z = 4 + j3 ohms. 

4 ohms 



Rectangular Notation 

Because reactance is known as an imaginary component, then impedance In 

Cartesian form: 

Z = R + jX = |Z|∠ θ= 

The term R + jX is known as a rectangular notation. As an example, a 

resistance of 4 ohms in series with an inductive reactance of 3 ohms could 

be noted as Z = 4 + j3 ohms. The impedance 

Rectangular 

Notation 

j3 ohms 

Polar Notation 

Z =|5|∠ 48.59º Ω. 

Z = 4 + j3 ohms. 

4 ohms 



Polar and Rectangular Notation 

In order to work with these complex numbers without drawing vectors, we first need 

some kind of standard mathematical notation. There are two basic forms of complex 

number notation: polar and rectangular. 

Polar form is where a complex number is denoted by the length (otherwise known as 

the magnitude, absolute value, or modulus) and the angle of its vector (usually 

denoted by an angle symbol that looks like this: ∠). To use the map analogy, polar 

notation for the vector from New York City to San Diego would be something like 

“2400 miles, southwest.” Here are two examples of vectors and their polar notations: 

(Figure below) 



Vectors With Polar Notations. 

Standard orientation for vector angles in AC circuit calculations defines 0º as 

being to the right (horizontal), making 90º straight up, 180º to the left, and 

270º straight down. Please note that vectors angled “down” can have angles 

represented in polar form as positive numbers in excess of 180º, or negative 

numbers less than 180. For example, a vector angled ∠ 270º (straight down) 

can also be said to have an angle of -90º. (Figure below) The above vector 

on the right (7.81 ∠ 230.19º) can also be denoted as 7.81 ∠ -129.81º. 



The Vector Compass 

Rectangular form, on the other hand, is where a complex number is denoted 

by its respective horizontal and vertical components. In essence, the angled 

vector is taken to be the hypotenuse of a right triangle, described by the 

lengths of the adjacent and opposite sides. Rather than describing a vector's 

length and direction by denoting magnitude and angle, it is described in terms 

of “how far left/right” and “how far up/down.” 

These two dimensional figures (horizontal and vertical) are symbolized by two 

numerical figures. In order to distinguish the horizontal and vertical 

dimensions from each other, the vertical is prefixed with a lower-case “i” (in 

pure mathematics) or “j” (in electronics). These lower-case letters do not 

represent a physical variable (such as instantaneous current, also symbolized 

by a lower-case letter “i”), but rather are mathematical operators used to 

distinguish the vector's vertical component from its horizontal component. As 

a complete complex number, the horizontal and vertical quantities are written 

as a sum: (Figure below) 



Rectangular Notation – Prefix “j” 

These two dimensional figures (horizontal and vertical) are symbolized by two numerical figures. 

In order to distinguish the horizontal and vertical dimensions from each other, the vertical is 

prefixed with a lower-case “i” (in pure mathematics) or “j” (in electronics). These lower-case 

letters do not represent a physical variable (such as instantaneous current, also symbolized by a 

lower-case letter “i”), but rather are mathematical operators used to distinguish the vector's 

vertical component from its horizontal component. As a complete complex number, the horizontal 

and vertical quantities are written as a sum: (Figure below) 



Rectangle & Polar Notation 

• In rectangular notation, the vertical and horizontal components’ ordinate 

and abscissa are shown. Z = R + jX, where j denoting vertical component. 

• In polar notation, vector length hypotenuse and angle are shown |Z|∠ θ, 

where |Z| denoting vector length and ∠ θdenoting the angle. 

Z = R + jX m = |Z|∠ θ= 

j3 ohms 

Z =|5|∠ 48.59º Ω. 

Z = 4 + j3 ohms. 

4 ohms 



Imaginary Component 

In “rectangular” form the vector's 

length and direction are denoted in 

terms of its horizontal and vertical 

span, the first number representing 

the horizontal (“real”) and the 

second number (with the “j” prefix) 

representing the vertical 

(“imaginary”) dimensions. The 

horizontal component is referred to 

as the real component, since that 

dimension is compatible with normal, 

scalar (“real”) numbers. The vertical 

component is referred to as the 

imaginary component, since that 

dimension lies in a different direction, 

totally alien to the scale of the real 

numbers. (Figure below) 



Vector compass showing real and 

imaginary axes. The “real” axis of 

the graph corresponds to the 

familiar number line we saw earlier: 

the one with both positive and 

negative values on it. The 

“imaginary” axis of the graph 

corresponds to another number 

line situated at 90º to the “real” 

one. Vectors being twodimensional 

things, we must have 

a two-dimensional “map” upon 

which to express them, thus the 

two number lines perpendicular to 

each other: (Figure below) 



Imaginary 


Vector compass with real and imaginary “j” number lines. 

Either method of notation is valid for complex numbers. The primary reason for having two 

methods of notation is for ease of longhand calculation, rectangular form lending itself to addition 

and subtraction, and polar form lending itself to multiplication and division. 

Conversion between the two notational forms involves simple trigonometry. To convert from polar 

to rectangular, find the real component by multiplying the polar magnitude by the cosine of the 

angle, and the imaginary component by multiplying the polar magnitude by the sine of the angle. 

This may be understood more readily by drawing the quantities as sides of a right triangle, the 

hypotenuse of the triangle representing the vector itself (its length and angle with respect to the 

horizontal constituting the polar form), the horizontal and vertical sides representing the “real” 

and “imaginary” rectangular components, respectively: (Figure below) 

To convert from polar to rectangular notation 



To convert from rectangular to polar notation, find the polar magnitude through 

the use of the Pythagorean Theorem (the polar magnitude is the hypotenuse of a right triangle, 

and the real and imaginary components are the adjacent and opposite sides, respectively), and 

the angle by taking the arctangent of the imaginary component divided by the real component: 

Z = 4+j3 Magnitude vector in terms of real (4) and imaginary (j3) components. 



REVIEW on Rectangular & Polar Notations 

• Polar notation denotes a complex number in terms of its vector's length and 

angular direction from the starting point. Example: fly 45 miles ∠ 203º (West by 

Southwest). 

• Rectangular notation denotes a complex number in terms of its horizontal and 

vertical dimensions. Example: drive 41 miles West, then turn and drive 18 miles 

South. 

• In rectangular notation, the first quantity is the “real” component (horizontal 

dimension of vector) and the second quantity is the “imaginary” component 

(vertical dimension of vector). The imaginary component is preceded by a lowercase 

“j,” sometimes called the j operator. 

• Both polar and rectangular forms of notation for a complex number can be related 

graphically in the form of a right triangle, with the hypotenuse representing the 

vector itself (polar form: hypotenuse length = magnitude; angle with respect to 

horizontal side = angle), the horizontal side representing the rectangular “real” 

component, and the vertical side representing the rectangular “imaginary” 

component. 



More Reading on Polar & Rectangular Vector Notations 

http://en.m.wikipedia.org/wiki/Vector_notation 

http://www.daycounter.com/Calculators/Polar-To-Rectangular- 

Calculator.phtml 

https://filebox.ece.vt.edu/~LiaB/Lectures/Ch_9/Slides/Mathematics.pdf 

http://www.allaboutcircuits.com/vol_2/chpt_2/5.html 


The term Z = R + jX is known as a rectangular notation. As an example, a 

resistance of R= 4 ohms in series with an inductive reactance of jX = 3 ohms 

could be noted as Z = 4 + j3 ohms. The impedance calculation is then : 

Z = √(4 2 + 3 2 ) = √(25) = 5Ω 

In coil design it is often helpful to know also the included angle between the 

resistive component and impedance. A convenient method of notation is the 

polar form where tan θ =X L + R and θ is the included angle between 

resistance and impedance. In the previous example the impedance 

magnitude is 5 ohms, but at what angle? A proper form of notation is Z∠θ 

where Z is impedance and ∠θ is the included angle. Therefore, the complete 

notation for R=3, X L =4 is: 

Z = √(42 + 32) = √(25) = 5Ω 

tan θ = ¾ = 0.750 = 36.9º 

Z = |5|∠ 36.9º or Z = 3+j4 


Eddy current coils with included impedance angles of 60 degrees to 90 

degrees usually make efficient test coils. As the angle between resistance 

and impedance approaches 0 degrees, the test coil becomes very inefficient 

with most of its energy being dissipated as heat. 


Q or Figure of Merit 

The term used to describe coil efficiency is Q or merit of the coil. The higher 

the Q or merit of a coil, the more efficiently the coil performs as an inductor. 

The merit of a coil is mathematically stated as: 

Q = X L / R 

For example, a coil having an inductive reactance of 100 ohms and a 

resistance of 5 ohms would have a Q of 20. 


Permeability and Shielding Effects 

The addition of permeable core in certain designs dramatically improves “Q” 

factor. For example, are wound on a form that allow the powdered iron rod or 

slug to be placed in the center of coil. It is common to increase the coil 

impedance by a factor of 10 by the addition of coil materials. This increase in 

impedance without addition winding greatly enhances the Q of the coil. Some 

core materials are cylindrical or cup shaped. A common term is cup core 

(Fig3.3). the coil is first wound and then placed inn the cup core. In the case 

of a probe coil in the cup core,not only is the impedance increased, but the 

benefit of shielding is also gained. 

Shielding with a cup core, prevent the electromagnetic field from spreading at 

the sides of the coil. This greatly reduces signal produced by edge effect of 

adjacent member of the test area, such as fasteners on air wings. Shielding, 

while improving resolution, usually sacrifices some amount of penetration into 

fue part. 


Figure 3.3: Effects of cup cores 

(a) Unshielded coil -field spread might be up to twice the coil diameter. 

(b) Shielded coil - magnetic field extension restricted to the core geometry. 


Another technique of shielding uses high conductivity material, such, as 

copper or aluminum, to suppress high frequency interference from other 

sources and also to shape the electromagnetic field of the test coil. A copper 

cup would restrict the electromagnetic field in much the same manner as the 

powdered iron cup core. A disadvantage of high conductivity, low or no 

permeability shielding is that the coil's impedance is reduced when the 

shielding material is placed around the test coil. The net effect is that the 

coil's “Q” factor is less than it was when the coil was surrounded by air. 


Discussion 

Subject 1: Shielding, while improving resolution, usually sacrifices some 

amount of penetration into fue part. 

Subject 2: A disadvantage of high conductivity, low or no permeability 

shielding is that the coil's impedance is reduced when the shielding material 

is placed around the test coil. The net effect is that the coil's “Q” factor is less 

than it was when the coil was surrounded by air. 


Saturation Approach 

Another coil design used for inspection of ferromagnetic materials is the saturation 

approach. A predominant variable that prevents eddy current penetrating in 

ferromagnetic material is called permeability. Permeability effects exhibited by the test 

object can be reduced by means of magnetic saturation(Figure 3.4). Saturation coils 

for steels are usually very large and surround the test object and test coil. A steady 

state (DC) current is applied to the saturation coil. When the steel test object is 

magnetically saturated it may be inspected in the same manner as a nonferromagnetic 

material. In the case of mild steel many thousands of tesla are required to produce 

saturation. 

In some inherently nonferromagnetic tubing materials like high temperature nickel 

chromium alloy there may be low level permeability variations because of 

manufacturing discontinuities. In this case the use of small permanent magnets 

adjacent to the bobbin probe coils may improve the inspection quality by reducing the 

permeability effects. Figure 3.5 shows the use of disk type magnets placed close to 

the coil. It is also possible to use an array of bar magnets arranged around the probe 

housing if higher magnetic potential is required to offset the material permeability 



Figure 3.4: Magnetic saturation inspection process 


Figure 3.5: Magnetic bias probe 


Coil Fixtures 

Coil fixtures or holders may be as varied as the imagination of the designers 

and users. After the size, shape and style have been decided on, the next 

consideration should be the test environment. Characteristics of wear, 

temperature, atmosphere, mechanical stress and stability must be considered 

(4). Normally wear can be reduced by selection of wear resistant compounds 

to protect the coil windings. If severe wear is expected, artificial or genuine 

jewels may be used. Less expensive and very effective wear materials, such 

as aluminum oxide or ceramics, are more commonly used. Temperature 

stability may be accomplished by using coil holder material with poor heat 

transfer characteristics. Metals have high heat transfer characteristics and 

often coils made with metal holders are sensitive to temperature variations 

caused by human touch. For high temperature applications, materials must 

be chosen carefully. Most common commercial copper coil wire may be used 

up to 150°C to 200°C. For temperatures above 200 ac, silver or aluminum 

wire with ceramic or high temperature silicone insulation must be used. 


Materials must be chemically compatible with the test object. As extreme 

examples, a polystyrene coil form would not be used to inspect an acetone 

cooler or a lead or graphite housing allowed to come in contact with a high 

temperature nickel chromium alloy jet engine tail cone. The chemical 

interactions between these material combinations could cause cracking and 

lead to component failure. 

Mechanical and electrical stability of the test coil can be enhanced by an 

application of epoxy resin between each layer of coil winding. This 

accomplishes many objectives: 

1) it seals the coil to exclude moisture; 

2) it provides additional electrical insulation; and 

3) it provides mechanical stability. 

Characteristics listed are not in order of importance. The importance of each 

characteristic is determined by specific test requirements. 


Chapter 3 



Q.3.1 A coil's resistance is determined by: 

A. wire material. 

B. wire length. 

C. wire cross-sectional area. 

D. all of the above. 

Q.3.2 Inductance might be referred to as being analogous to: 

A. force. 

B. volume. 

C. inertia. 

D. velocity. 

Q.3.3 The unit of inductance is the: 

A. henry. 

B. maxwell. 

C. ohm. 

D. farad. 


Q.3.4 The inductance of a multilayer air core coil with the dimensions I = 0.2, 

r = 0.5, b = 0.1 and N = 20, is: 

L = 0.8(rN) 2 ∙ (6∙r+9∙l+10∙b) -1 μHenry 

A. 1.38 H. 

L = self inductance in μH 

B. 13.8 μH. 

N = number of turns 

C. 13.8 ohms. 

r = mean radius in inches 

l = length of coil in inches 

D. 1.38 ohms. 

b = coil depth 

Q.3.5 The inductive reactance of the coil in Q.3.4, operating at 400 kHz, 

would be: 

A. 1380 ohms. 

X L = 2πfL = (2π∙400∙10 3 ∙13.8∙10 -6 ) 

B. 5520 ohms. 

C. 34.66 ohms. 

D. 3466 oluns. 

Q.3.6 The impedance of a 100μH coil with a resistance of 20 ohms operating 

at 100kHz would be: 

A. 62.8 ohms. 

X L = 2πfL, Z =√(X L2 + R 2 ) 

B. 4343.8 ohms. 

C. 628 ohms. 

D. 65.9 ohms. 


Q.3.7 The Q or merit of a coil is denoted by the ratio: 

A. Z/X L 

B. X L /Z 

c. X L /R 

D. R/X L 

Q.3.8 The incorporation of ferromagnetic shielding materials around a coil: 

A. improves resolution. 

B. decreases field extension. 

C. increases impedance. 

D. Does all of the above. 

Q.3.9 The purpose of a steady state winding w near a test coil is to: (? – 

scanned copy missing wording) 

A. reduce material permeability effects. 

B. produce possible magnetic saturation in the test material. 

C. provide a balance source for the sensitive coil. 

D. both A and B. 


Q.3.10 The most important consideration when selecting a test coil is: 

A. sensitivity. 

B. resolution. 

C. stability. 

D. meeting established inspection criteria 


The Answers 


Chapter 4 

Effects of Test Object on Test Coil 


Jackfruit Tree 


Operating or Test Variables 

As previously seen, the eddy current technique depends on the generation of 

induced currents within the test object. Disturbances in these small induced 

currents affect the test coil. The result is a variation of the test coil impedance 

due to test object variables. These variances are called operating or test 

variables. The range of test variables encountered might include electrical 

conductivity, magnetic permeability, skin effect, lift off, fill factor; end effect, 

edge effect and signal-to-noise ratio. 

Coil impedance was discussed at length in Chapter.3. In this chapter coil 

impedance changes will pe represented graphically to more effectively 

explain the interaction of the operating variables. 


Electrical Conductivity σ 

In electron theory the atom consists of a positive nucleus surrounded by 

orbiting negative electrons. Materials that allow these electrons to be easily 

moved out of orbit around the nucleus are classified as conductors. In 

conductors electrons are moved by applying an outside electrical force. The 

ease with which the electrons are made to move through the conductor is 

called conductance. A unit of conductance is the siemens (mho). The 

siemens is the reciprocal of the ohm or conductance G = l / R where G is 

conductance in siemens and R is resistance in ohms. In eddy current testing, 

instead of describing conductance in absolute terms, an arbitrary unit has 

been assigned. Since the relative conductivity of metals and alloys varies 

over a wide range, the need for a conductivity bench-mark is of prime 

importance. 


The International Electrochemical Commission established in 1913 a 

convenient technique of comparing one material to another. 

The commission established that a specific grade of high purity copper, 1 m in 

length, with a uniform cross section of 1 mm 2 , measuring 0.017241 ohms at 

20°C would be arbitrarily considered to be 100% 'conductive. 

The symbol for conductivity is σ (sigma) and the unit is percent IACS or 

percent of the International Annealed Copper Standard. Table 4.1 lists 

materials by their electrical properties: conductivity and resistivity. A 

statement can be made about a conductor in terms of conductance or 

resistance. Note that a good conductor is a poor resistor. Conductance and 

resistance are direct reciprocals as stated earlier. Conductivity and resistivity, 

however, have different origins and units; therefore, the conversion is not so 

direct. 


As previously discussed, conductivity is expressed on an arbitrary scale in 

percent IACS. Resistivity is expressed in absolute terms of micro-ohmcentimeters. 

To convert values on one scale to the other system of units a 

conversion factor of 172.41 is required. Once you know either the conductivity 

or the resistivity value for a material the other electrical property can be 

calculated. 

Shanghai- 上海 

172.41 


%IACS & Resistivity ρ (rho) in micro-ohm-centimeter 

■ 

■ 

%IACS = 172.41 / ρ micro-ohm-centimeter (μΩ∙cm) 

ρ micro-ohm-centimeter = 172.41 / %IACS 

These numerical values will be necessary when additional calculations are 

needed to determine issues of frequency choice, depth of penetration and I or 

phase spread to meet specific inspection criteria. As the test coil is influenced 

by different conductivities, its impedance varies inversely to conductivity. A 

higher conductivity causes the test coil to have a lower impedance value. 

Figure 4.1 illustrates this concept. 


Figure 4.1; Conductivity curve 


Discussion 

Subject: A higher conductivity causes the test coil to have a lower 

impedance value. Figure 4.1 illustrates this concept.- Reason out the 

statement. 


The coil's inductive reactance is represented by theY axis and coil resistance 

appears on the X axis. The 0% conductivity point, or air point, is when the 

coil's empty reactance (X L ) is maximum. Figure 4.1 represents a measured 

conductivity locus . Conductivity is influenced by many factors. Table 4.1 is a 

comparative listing of materials with various chemical compositions. There 

are various manufacturing or in situ factors that must be considered when 

hying to measure the conductivity of various alloys. In metals, .as. the 

temperature is increased, the conductivity wlll decrease. This is a major factor 

to consider when accurate measurement of conductivities is required. 


Figure 4.1; Conductivity curve 

Key Word: 

The 0% conductivity point 

Air point 


Effects of Heat Treatment on Conductivity 

Heat treatment affects electrical conductivity by redistributing elements in the 

material. Dependent on materials and degree of heat treatment, conductivity 

can either increase or decrease as a result of heat treatment. Stresses in a 

material due to cold working produces lattice distortion or dislocation. This 

mechanical process changes the grain structure and harness of the material, 

changing its electrical conductivity. Hardness in age hardened aluminum alloy 

changes the electrical conductivity of the alloy. The electrical conductivity 

decreas as hardness increase. As an example Brinell hardness 60 is 

represented by conductivity 23 while Brinell hardness 100 of the same alloy 

would have a conductivity of 19. 


Table 4.1: Electrical resistivity and conductivity of several metals and alloys 

ρ = 172.41 / %IACS 


Permeability 

Permeability of any material is a measure of the ease with which its magnetic 

domains can be aligned or the ease with which it can be establish lines of 

force. Materials are rated on a comparative basis. Air is assigned a 

permeability of 1. Ferromagnetic metals and alloys including nickel, iron and 

cobalt tend to concentrate magnetic flux lines. As discussed in Chapter 3, 

some ferromagnetic materials or sintered ionic compounds are also useful in 

concentrating magnetic flux. Magnetic permeability is not constant for a given 

material. The permeability in a test sample depends on the magnetic field 

acting on it. As an example, consider a magnetic steel bar placed in an 

encircling coil. As the coil current is increase, the magnetic field of the coil will 

increase. The magnetic flux within the steel will increase rapidly at first and 

then will tend to level off as the steel approaches magnetic saturation. This 

phenomenon is called the saturation effect. 


BH Curve 


When increases in the magnetizing force produce littile or no change on the 

flux within the steel bar, the bar is magnetically saturated. When 

ferromagnetic materials are saturated, permeability becomes constant. With 

magnetic permeability constant, ferromagnetic materials may be inspected 

using tpe eddy current method. Without magnetic saturation ferromagnetic 

materials exhibit such a wide range of permeability variation that signals 

produce by discontinuities or conductivity variations are masked by the 

permeability signal. 


Skin Effect 

Electromagnetic tests in many applications are most sensitive to test object 

variables nearest the test coil because of skin effect. Skin effect is a result of 

mutual interaction of eddy currents, operating frequency, test object 

conductivity and permeability. The skin effect, the concentration of eddy 

currents in the test object nearest the test coil, becomes more evident as test 

frequency, test object conductivity and permeability are increased. For current 

density or eddy current distribution in the test object, refer to Figure 1.8 in 

Chapter 1. 

δ = (πfμσ) -½ 


http://www.geocities.ws/raobpc/EC-Def.html

Skin Effect 


http://www.jmag-international.com/catalog/50_SteelWire_InductionHeating.html

Edge Effect 

The electromagnetic field produced by an excited test coil extends in all 

directions from the coil. The coil's field precedes the coil by some distance 

determined by coil parameters, operating frequency and test object 


As the coil approaches the edge of a test object, eddy current flow in the test 

sample becomes distorted by the edge. This is known as edge effect. 

Edge effect can create a change in the coil's impedance that is similar to a 

discontinuity (Figure4.2). The response would move back along the 

conductivity curve toward the air point. The coil is responding to a slightly less 

conductive situation (air) at the leading edge of the coil's field of view. It is 

therefore essential that edge effect be eliminated as a variable during a 

surface scanning test. Response to the edges of test objects can be reduced 

by: incorporating magnetic shields around the test coil, increasing the test 

frequency, reducing the test coil diameter or by changing the scanning pattern 

used. Edge effect is a term most applicable to the inspection of sheets or 

plates with a probe coil. 


Discussion 

Subject: Response to the edges of test objects can be reduced by: 

incorporating magnetic shields around the test coil, increasing the test 

frequency, reducing the test coil diameter or by changing the scanning pattern 

used. 


Figure 4.2: Edge effect 


Lift Off 

Electromagnetic coupling between test coil and test object is of prime importance 

when conducting an eddy current examination. The coupling between test coil and test 

object varies with spacing between the test coil and test object. This spacing is called 

lift off (4). The effect on the coil impedance is called lift off effect. Figure 4.3 shows the 

relationship between air, conductive materials and lift off. The electromagnetic field, as 

previously discussed, is strongest near the coil and dissipates with distance from the 

coil. This fact causes a pronounced lift off effect for small variations in coil to object 

spacing. 

As an example, a spacing change from contact to 0.0254 mm (0.001 in.) will produce 

a lift off effect many times greater than a spacing change of 0.254mm (0.010 in.) to 

0.2794 mm (0.011 in.) (15), Lift off effect is generally an undesired effect causing 

incre,ased noise and reduced coupling resulting in po6r measming ability (12). In 

some instances, equipment having phase discrimination capability can readily 

separate lift off from conductivity or other variables. Lift off can be used to advantage 

when measuring nonconductive coatings on conductive bases. A nonconductive 

coating such as paint or plastic causes a space between the coil and conducting base, 

allowing lift off to represent the coating thickness. Lift off is also useful in profilometry 

and proximity applications. Lift off is a term most applicable to testing objects with a 

surface or probe coil. 


Fill Factor 

Fill factor is a term used to describe how well a test object will be ectromagnetically 

coupled to a test coil that surrounds or is inserted into the test object. Fill factor then 

pertains to inspections using bobbin or encircling coils. Like lift off, electromagnetic 

coupling between test coil and test object is most efficient when the coil is nearest the 

surface of the part. The area of a circle (A) is determined using the equation: 

A Area = πd 2 /4 

Fill factor can be described as the ratio of test object diameter to coil diameter squared 

(Figure 4.4). The diameters squared is a simplified equation resulting in the.division of 

effective coil and part areas. Because the term π /4 both the numerator and the 

denominator of this fractional equation the term π/4 cancelled out, leaving the ratio of 

the diameters squared; 

η (eta) = d 2 /D 2 , fill factor 


Fill factor will always be a number less than 1 and efficient fill factors 

approaching 1, fill factor of 0.99 is more desirable than a fill factorof 0.75. The 

effect of fill factor on the test system is that poor fill factors do not allow the 

coil to be sufficiently coupled to the test object. This is analogous to the 

effect of drawing a bow only slightly and releasing an arrow. The result is, 

with the bow slightly drawn and released, little effect is produced to propel the 

arrow. 

In electrical terms it is said that the coil is loaded by the test object. How 

much the coil is loaded by the test object due to fill factor can be calculated in 

relative terms. A test system with constant current capabilities being affected 

by a conductive nonmagnetic bar placed into an encircling coil can be used to 

demonstrate this effect. 


For this example, the system parameters are as follows: (a) Unloaded coil 

voltage equals 10 V. (b) Test object effective permeability equals 0.3 (c) Test 

coil inside diameter equals 25.4 mm (1 in.) (d) Test object outside diameter 

equals 22.9 mm (0.9 in.) 

η (eta) = d 2 /D 2 , fill factor = (0.9/1) 2 = 0.81 

An equation demonstrating coil loading is given by: 

E = E o (1- η + η∙μ eff ) 

Where: 

E o = Coil voltage with coil affected by air 

E = Coil voltage with coil affected by the test material 

η = Fill factor 

μ eff = Effective Permeability 


Figure 4.4: Fill factor ratios 

Whena nonfermmagnetic test object is inserted into the test coil, the coil's 

voltage will decrease. 

E = E o (1- η + η∙μ eff ) 

E = 10 (1-0.81 + 0.81 x 0.3) 

E = 4.33 Volts 

This allows 10- 4.3 or 5.7 V available to respond to test object changes 

caused by discontinuities or decreases in effective conductivity of the test 

object. It is suggested that the reader calculate the resultant loaded voltage 

developed by a 12.7 mm (0.5 in.) bar of the same material and observe the 

relative sensitivity difference. 


Figure 4.4: Fill factor ratios 

Whena nonfermmagnetic test object is inserted into the test coil, the coil's 

voltage will decrease. 

∆E = E o (1- η + η∙μ eff ) 

∆E = 10 (1-0.81 + 0.81 x 0.3) 

∆E = 4.33 Volts 



object. It is suggested that the reader calculate the resultant loaded voltage 

developed by a 12.7 mm (0.5 in.) bar of the same material and observe the 

relative sensitivity difference. 


Example: 

Calculate the resultant loaded voltage developed by a 12.7 mm (0.5 in.) bar of 

the same material and observe the relative sensitivity difference. 

η (eta) = d 2 /D 2 , fill factor = (0.5/1) 2 = 0.25 

An equation demonstrating coil loading is given by: 

∆E = E o (1- η + η∙μ eff ) 

∆E = 10 (1-0.251 + 0.25 x 0.3) 

∆E = 8.25 Volts 



object. 


Discontinuities 

Any discontinuity that appreciably changes the normal eddy current flow can 

be detected. Discontinuities, such as cracks, pits, gouges, vibrational damage 

and corrosion, generally cause the effective conductivity of the test object to 

be reduced. Discontinuities open to the surface are more easily detected than 

subsurface discontinuities. Discontinuities open to the surface can be 

detected with a wide range of frequenciesi subsurface investigations require a 

more careful frequency selection. 

Discontinuity detection at depths greater than 12.7 mm (0.5 in.) in stainless 

steel is very difficult. This is in part due to the sparse distribution of magnetic 

flux lines at the low frequency required for such deep penetrations. 


Figure 1.8 is again useful to illustrate discontinuity response because of 

current distribution. As an example, consider testing a nonferromagnetic tube 

at a frequency that establishes a standard depth of penetration at the 

midpoint of the tube wall. This condition would allow a relative current density 

of about 20% on the far surface of the tube. With this condition, identical near 

and far surface discontinuities would have greatly different responses. Due to 

current magnitude alone, the near surface discontinuity response would be 

nearly five times that of the far surface discontinuity. 

Discontinuity orientation has a dramatic effect on response. As seen earlier, 

discontinuity response is maximum when eddy currents and discontinuities 

are at 90 degrees or perpendicular. Discontinuities parallel to the eddy 

current flow produce little or no response. 

The easiest technique to ensure detectability of discontinuities is to use a 

reference standard or model that provides a consistent means of adjusting 

instrumentation. 




Signal-to-Noise Ratio 

Signal-to-noise ratio is the ratio of signals of interest to unwanted signals. 

Common noise sources are test object variations of surface roughness, 

geometry and homogeneity. Other electrical noises can be due to such 

external sources as welding machines, electric motors and generators. 

Mechanical vibrations can increase test system noise by physical movement 

of test coil or test object. In other words, anything that interferes with a test 

system's ability to define a measurement is considered noise. Signal-to-noise 

ratios can be improved by several techniques. If a part is dirty or scaly, signalto-noise 

ratio can be improved by cleaning the part. Electrical interference 

can be shielded or isolated. Phase discrimination and filtering can improve 

signal- to-noise ratio. It is common practice in non destructive testing to 

require a minimum signal-to-noise ratio of 3 to 1. This means a signal of 

interest must have a response at least three times that of the noise at that 

point. 


Chapter 4 



The Answers 


Q.4.1 Materials that hold their electrons loosely are classified as: 

A. resistors. 

B. conductors. 

C. semiconductors. 

D. insulators. 

Q.4.2 100% IACS is based on a specified copper bar having a resistance of: 

A. 0.01 ohms. 

B. 100 ohms. 

C. 0.017241 ohms. 

D. 172.41 ohms. 

Q.4.3 A resistivity of 13 μohm cm is equivalent to a conductivity in percent 

IACS of: 

A. 11.032. 

B. 0.0625. 

c. 1652. 

D. 13.26. 


Q.4.4 A (?) prime factor affecting conductivity is: 

A. temperature. 

B. hardness, 

C. heat treatment. 


Q.4.5 Materials that tend to concentrate magnetic flux lines are: 

A. conductive. 

B. permeable. 

c. resistive. 

D. inductive. 

Q.4.6 Diamagnetic materials have: 

A. a permeability greater than air. 

B. a permeability less than air. 

C. a permeability greater than ferromagnetic materials. 

D. no permeability. 


Q.4.7 Edge effect can be reduced by: 

A. shielding. 

B. selecting a lower frequency. 

C. using a smaller coil. 

D. both A and C. 

Q.4.8 Calculate the effect of fill factor when a conducting bar 12.7 mm (0.5 in.) 

in diameter with an effective permeability of 0.4 is placed into a 25.4 mm (1 in.) 

diameter coil with an unloaded voltage of 10V. The loaded voltage is: 

A. 2V. 

B. 4.6V: 

C. 8.5V: 

D. 3.2V. 

η = (0.5) 2 = 0.25 

∆E = E o ( 1-η+ ηxμ eff ) 

∆E = E o ( 1-0.25 + .25x.4) = 8.5 Volts 

Q.4.9 Laminations are easily detected with the eddy current method. 

A. True 

B. False 


Q.4.10 Temperature changes, vibration and environmental effects are test 

coil inputs that generate: 

A. unwanted signals. 

B. magnetic fields. 

c. eddy currents. 

D. drift. 


Chapter 5 

Selection of Test Frequency 


soursop-prickly-custard-apple-durian-belanda-annona-muricata

Test Frequencies 

It is the responsibility of nondestructive testing engineersand technicians to 

provide and perform non destructive testing that in some way ensures the 

quality, usefulness of industly products. To apply a nondestructive test, it is 

essential that the parameters affecting the test be understood. Usually 

industry establishes a product or component and then seeks a method to 

inspect it. This practice establishes test object geometry, conductivity and 

permeability before the application of the eddy current examination. 

Instrument, test coil and test frequency selection become the tools used to 

solve the problem of inspection. Test coils were discussed previously and 

instrumentation will be discussed later in this text. Test frequencies and their 

selection will be examined in detail in this Chapter. 


Frequency Selection 

In Chapter 1, it was observed that eddy currents are exponentially reduced as 

they penetrate the test object. In addition, a time or phase difference in these 

currents was observed (current lagging with penetration w.r.t surface current). 

The currents near the test coil happen first or lead the current that is deeper 

in the object. 

A high current density allows good detectability and a wide phase difference 

between near and far surfaces allows good resolution. 

Keypoint: 

Current deeper into the test object lag the surface current by β = x/δ radian. 


Single Frequency Systems 

Unfortunately, if a low frequency is selected to provide good penetration and 

detectability, the phase difference between near and far surface is reduced. Selection 

of frequency often becomes a compromise. It is common practice in inservice 

inspection of thin-wall, nonferromagnetic tubing to establish a standard depth of 

penetration just past the midpoint of the tube wall. This permits about 25% of the 

available eddy current to flow at the outside surface of the htbe wall. In addition, this 

establishes a phase difference of about 150 degrees to 170 degrees between the 

inside and outside surface of the tube wall. The combination of 25% outside, or 

surface current and 170 degrees included phase angle provides good detectability and 

resolution for thin-wall tube inspection. (This is accomplished by properly selecting the 

driving frequency of the coil to limit the penetration) 

Calculation: 

δ = (πfσμ) -½ 

β = x/ δ radian = x / δ∙ 57.3º 

Where: 

x = depth below surface 


Standard penetration depth δ 

The depth that eddy currents penetrate into a material is affected by the frequency of 

the alternating current, the electrical conductivity and magnetic permeability of the 

sample. The depth of penetration decreases with increasing frequency and increasing 

conductivity and magnetic permeability. The depth at which eddy current density has 

decreased to 1/e, or about 37% of the surface density, is called the standard depth of 

penetration (δ or 1 δ) and used as criteria of ideal measurement. At three standard 

depth of penetration (3δ), the Eddy Current density is down to only 5% of the surface 

density. So, defects or variation deeper than the three standard depth of penetration 

cannot be recognized because the EC density in this depth is too low to detect. Thus, 

achieving the standard penetration depth is the most important factor at Eddy Current 

testing and this is realized by selecting appropriate frequency suitable for a material 

property. 


http://www.suragus.com/en/company/eddy-current-testing-technologyc

Eddy Current Density 

Since the sensitivity of Eddy Current inspection depends on the Eddy Current density at the 

defect location, it is important to know the strength of the Eddy Currents at this location. When 

detect flaws, a frequency is often selected which places the expected flaw depth within one 

standard depth of penetration. This assures that the strength of the Eddy Currents would be 

sufficient to produce a flaw indication. 


http://www.suragus.com/en/company/eddy-current-testing-technology

Eddy Current Density of a Solid bar D=2δ 

D=2δ 

δ 

1/e = 37% of surface current density 

2δ 

(1/e) 2 = 13.5% of surface current density 


Applicable Scenario 

Eddy Current Density of a Solid bar D≤2c 

The field in air 

on the far 

surface ? 


http://www.suragus.com/en/company/eddy-current-testing-technologyc

Eddy Current Density of a Solid bar D>3δ 

D>3δ 

δ 

2δ 

32δ 

100% surface current density 

1/e = 37% of 

surface current 

density 

(1/e) 2 = 13.5% of 


density 

(1/e) 3 = 5% of 


density at 3δ 


Standard Depth δ Formula 

The depth of penetration formula discussed in Chapter 1, although correct, has rather 

cumbersome units. Conductivity is usually expressed in percent of the International Annealed 

Copper Standard (% IACS). Resistivity is usually expressed in terms of micro-ohm-centimeter 

(μΩcm) (16). Depths of penetration are normally much less than 12.7 mm (0.5 in.). A formula 

using these units may be more appropriate and easier to use. In Chapter 1 a formula for 

calculating depth of penetration in the metric units was presented. Another derivative of this 

formula using resistivity, frequency and permeability with δ expressed in mm or inches can be 

expressed as follows: (standards form δ = (πf∙σμ r μ o ) -½ , σ = 1/ρ) 

δ = K√ [ρ/(fμ r )] , δ= K [ρ/(fμ r )] ½ 

Where: 

δ = Standard penetration in mm or inches 

K = 50 for δ in mm and 1.98 for δ in inches 

ρ = Resistivity in micro-ohm-centimeter (μΩcm) 

f = Frequency 

μ r = Relative permeability (for non-magnetic conductor μ r =1) 

δ = K√ [ρ/(f)] 

for non-magnetic conductor where μ r =1 


Frequency Selection – Non Ferromagnetic Conductor 

The prime variable is frequency. By adjusting frequency technicians can be 

selectively responsive to test object variables. Solving the nonferromagnetic 

depth of penetration formula for frequency requires a simple algebraic 

manipulation as follows: 

δ = K√ [ρ / (f)] for non-magnetic conductor where μ r =1 

ρ /f = (δ/K) 2 , 

f = ρ∙(K/δ) 2 


Example 1: 

As an example of how this may be used, consider inspecting a 7.6 mm (0.3 

in.) thick aluminum plate, fastened to a steel plate at the far surface. Effects of 

the steel part are undesirable and require discrimination or elimination. The 

aluminum plate has a resistivity of 5 μΩ∙cm. By establishing a depth of 

penetration at 2.54 mm (0.1 in., the far surface current will be less than 10% 

(5%) of the available current, thus reducing response to the steel part. The 

frequency required for this can be calculated by applying: δ = 0.1” 

f = ρ∙(K/ δ) 2 , f = 5(1.98/0.1) 2 = 1960Hz (use inches) 

0.3 in. Thick Al. 

Steel plate 


Example 2: 

If detection of the presence of the steel part was required, the depth of 

penetration could be reestablished at 7.6 mm (0.3 in.) in the aluminum plate 

and a new frequency could be calculated. δ = 0.3 in. 

f = ρ∙(K/ δ) 2 , f = 5(1.98/0.3) 2 = 218Hz (use inches) 


Frequency Selection – Ferromagnetic Conductor 

For ferromagnetic conductor δ = K√ [ρ/(fμ r )], the parameter μ r ≠ 1, need to 

be addressed in the above example: 

f = ρ /μ r ∙(K/ δ) 2 instead of f = ρ ∙ (K/ δ) 2 


Bessel function for f g (?) 

Another approach to frequency selection uses argument A of the Bessel 

function where argument A is equal to unity or 1. 

A = f μ r σd 2 / 5066 

Where: 

f = frequency Hz 

μ r = Relative permeability 

σ = Conductivity in meter / Ω.mm 2 

d = Diameter of the coil in cm 


A frequency can always selected to established factor A = 1, the frequency is 

known as limiting frequency and is denoted by f g . By substituting 1 for A and 

f g for f, the equation becomes; 

1 = f g ∙ μ r ∙ σ∙ d 2 / 5066 

f g = 5066/ (μ r ∙ σ∙ d 2 ) 

Limiting frequency f g is then established in term of conductivity, permeability, 

some dimensional properties and a constant 5066. 

Because limiting frequency f g is based on these parameters, a techniques of 

frequency determination using a test frequency to limit frequency ration f/f g 

can be accomplished. 

High f/f g ratios are used for near surface tests and lower f/f g ration is used for 

subsurface tests. 


Often results of such tests are represented graphically by diagrams,These 

diagrams are called impedance diagrams. Impedance illustrated by vector 

diagrams in Chapter 3 shows inductive reactance represented onthe Y axis, 

ordinate and resistance on the X axis, abscissa. 

The vector sum of the reactive and resistive components is impedance. This 

impedance is a quantity with magnitude and direction that is directly 

proportional to frequency. To construct a universal impedance diagram valid 

for all frequencies, the impedance must be normalized (4). Figure 5.1 

illustrated a normalization process. 


Figure 5.1(a) shows the effect on primary impedance Zp with changes in 

frequency (ω= 2πf). Figure 5.1(a) represents primary impedance without a 

secondary circuit or test object. 

Figure 5.1(b) illustrates the effect of frequency on primary impedance with a 

secondary circuit or test object present. The primary resistance R, in 

Figure5.1(a) has been subtracted in Figure 5.1(b) because resistance is not 

affected by frequency. The term ωLsG in Figure 5.1(b) represents a 

reference quantity for the secondary impedance. The units are secondary 

conductance (G) and ωLs (secondary reactance). 


Figure 5.1: Effects of frequency change (a) Primary impedance without 

secondary circuit (b) Primary impedance with secondary circuit. 

X L =ωL = 2πf∙L 

f 


Figure 5.1: Effects of frequency change (a) Primary impedance without 

secondary circuit (b) Primary impedance with secondary circuit. 

G 

B, C, D, E, F, loci for selected values of Z p 

G = secondary conductance 

Z p 

=primary impedance 

ω = angular frequency = 2πf 

ωL s 

= secondary reactance 

The primary resistance R, in 

Figure5.1(a) has been 

subtracted in Figure 5.1(b) 

because resistance is not 

affected by frequency. 


Further normalization is accomplished by dividing the reactive and resistive 

components by the term ωL o or the primary inductive reactance without a 

secondary circuit present. 

Figure 5.2 shows a typical normalized impedance diagram. The terms 

ωL/ωL 0 and R/ωL 0 represent the relative impedance of the test coil as 

affected by the test object. Signals generated by changes in ωL or R caused 

by test object conditions such as surface and subsurface discontinuities may 

be noted by ∆ωL or ∆R. The ∆ωL and ∆R notation indicates a change in the 

impedance. 


Figure 5.2: Normalized impedance diagram for long coil encircling solid 

cylindrical nonferromagnetic bar and for thin-wall tube. Coil fill factor = 1.0 

k = √(ωμσ) = Electromagnetic wave propagation 

constant for conducting material 

r = radius of the conductor in m 

μ = magnetic permeability of bar = 4π∙10 -7 H.m -1 

if bar is non-magnetic (μ = μ o ) 

ω = angular velocity = 2πf 

√(ωL o G) = equivalent of √(ωμσ) for simplified 

electrical circuit, where G=conductance (Siemens) 

and L o = inductance in air (Henry) 


Figure 5.3 shows the impedance variation in a nonferromagnetic cylinder 

caused by surface and subsurface discontinuities. Figure 5.3 also illustrates a 

sensitivity ratio for surface and subsurface discontinuities. Notice with an flfg 

ratio of 50, a relatively high frequency, the respouse to subsurface 

discontinuities is not very prononounced. 

Figure 5.4 shows responses to the same discontinuities with an f/fg ratio of 15. 

This lower frequency allows better detection of subsurface discontinuities as 

shown in Figure 5.4. 


Figure 5.3: Impedance variations caused by surface and subsurface cracks 

Impedance variations caused by surface and 

subsurface cracks in nonferromagnetic cylinders, 

at a frequency ratio f/fg = 50. 


Figure 5.3: Impedance variations caused by surface 

and subsurface cracks 


Figure 5.4: Impedance variations caused by surface and subsurface cracks 

Impedance variations caused by surface and 

subsurface cracks in nonferromagnetic 

cylinders, at a frequency ratio f/fg = 15. 


Figure 5.4: Impedance variations caused by surface 

and subsurface cracks 


Multiparameter Techniques 

It becomes obvious that the technician must have a good working knowledge 

of current density and phase relationships to make intelligent frequency 

choices. The frequency choice discussed to date deals with coil systems 

driven by only one frequency. Test systems driven by more than one 

frequency are called multifrequency or multi parameters systems. It is 

common for a test coil to be driven with three or more frequencies. Although 

several frequencies may be applied simultaneously or sequentially to a test 

coil, each of the individual frequency techniques follows rule established by a 

single frequency techniques. Signals generated at the various frequencies 

are often combined or mixed in electronic circuits that algebraically add or 

subtract signals to obtain the desired result. 


Multiparameter Techniques – Broadband Signals 

One multifrequency approach is to apply a broadband signal, with many 

frequency components to the test coil. The information transmitted by the 

signal is proportional to its bandwidth and the logarithm of 1 plus the signalto-noise 

power ration. 

The relation is stated by the equation: 

C = B∙Log 2 (1+ S/N) 

C = rate of information transmitted in bits per second 

B = bandwidth of the signal 

S/N = signal-to-noise ratio 

This is known as the Shannon-Hartley theorem. 


Multiparameter Techniques - Multiplexing 

Another approach to multiparameter techniques is to use multiplexing 

process. The multiplexing process places one frequency at a time on the test 

coil. This results in zero crosstalk between each frequency and eliminates the 

need for channel specific band pass filters. The major advantages of a 

multiplex system, in addition to the crosstalk reduction issues, are lower cost 

and greater flexibility in frequency selection. If the multiplexing switch rate is 

sufficiently high, both broadband and multiplex systems have essentially the 

same results. 

The characterization of eddy current signals by their phase angle and 

amplitude is a common practice and provides a basis for signal mixing to 

suppress unwanted signals from test data. Two frequencies are required to 

remove each unwanted variable. 


Example – Multiparameter Technique 

Practical multi parameter frequency selection can be demonstrated by the 

following example: 

Problem: Eddy current inspection of installed thin wall nonferromagnetic heat 

exchanger tubing. Tubing is structurally supported by ferromagnetic tube 

supports at several locations. It is desired to remove the tube support 

response signal from tube wall data. 

Solution: Apply a multiparameter technique to suppress the tube support 

signal response. 


First, a frequency is selected to give optimum phase and amplitude 

information about the tube wall. This is called the prime frequency. At the 

prime frequency, the response to the tube support and to a calibration 

through wall hole are about equal in amplitude. They may also have about the 

same phase angle. 

A second frequency called the subtractor frequency is selected on the basis 

of the phase angle of the tube support response. Because the tube support 

surrounds the outside diameter of the tube, a lower frequency is selected. At 

the subtractor frequency the tube support signal response is about 10 times 

greater than the calibration through wall hole. The phase difference between 

the support signal and the through wall hole in this lower frequency will be 

about 90 degrees. Parameter separation limitations are greatest for those 

parameters producing nearly similar signals, such as dents . 


If the prime and subtractor channels have been selected properly then signal 

subtraction algorithms should be able to suppress the tube support signal 

leaving only slightly attenuated prime data (discontinuity) information. 

For suppression of inside or near surface signals, a higher subtractor 

frequency would be chosen. A combination of prime, low and high subtractor 

frequencies is often used to suppress both near and far surface signals, 

leaving only data pertaining to the part thickness and its condition. 

Bandwidth of the coil is of prime importance when operation over a wide 

frequency range is required in multifrequency/multi parameter testing. 

Optimization of a test frequency (or frequencies) will therefore depend on the 

desired measurement or parameter(s) of interest. 


More Reading on 

Characteristic Parameter Pc / Frequency fg 


http://www.geocities.ws/raobpc/Probes.html

EDDY CURRENT PROBES (SENSORS) 

INTRODUCTION 

Eddy current (EC) testing is used to ensure pre-service quality and to assess in-service health of industrial components 

made of electrically conducting materials, by way of detection and characterization of defects or discontinuities. This 

technique works on the principles of electromagnetic induction and test phenomenon can be explained using the 

Maxwell’s equations. In this technique, as shown in Fig. 1, a coil (also called probe) is excited with sinusoidal 

alternating current (frequency, f) to induce eddy currents in the component under test and the change in coil impedance 

is measured. Defects such as cracks, inclusions, notches, microstructure variations etc. cause a discontinuity in 

electrical conductivity, and/or magnetic permeability, hence, distort the eddy current flow and in turn, change the coil 

impedance. The measured impedance change is correlated with defect parameters e.g., length, depth, location, and 

orientation etc. The locus of impedance change during the movement of an EC probe is usually called an EC signal or 

the impedance-plane trajectory. Eddy current test phenomenon is controlled by the skin effect, according to which the 

depth of penetration (also standard depth of penetration [SDP]), depends on frequency and material properties (see 

Fig.1). Due to skin-effect, the detection and characterization of surface defects is more reliable as compared to buried 

or sub-surface defects. Popular industrial applications of eddy current testing include defect detection, material 

property measurement, alloy sorting, and material as well as coating thickness measurements. It is also used for 

proximity sensing, level measurements, metal particles/debris in non-conducting media (cardboards, bakery products, 

currency notes, underground mines, insulators etc. ) 

Eddy current probe is the main link between the eddy current instrument and the component under test. Success of 

eddy current testing for a specific inspection application depends on sensor, instrument and optimization of test 

parameters. The probe plays two important roles: it induces the eddy currents, and it senses the distortion of their flow 

caused by defects. Design of probe / sensor is an important task and a variety of aspects such as component geometry, 

impedance matching, magnetic field focusing, and environment etc. need to be considered for its design and 

development. In this contribution, some important aspects concerning probe design and development are covered. 



Figure 1: Eddy Current Testing 



TYPES OF EDDY CURRENT PROBES / SENSORS 

Design and development of eddy current probes is very important as it is the 

probe that dictates the probability of detection and the reliability of 

characterization. In general, defects that cause maximum perturbation of 

eddy currents are detected with high sensitivity. The shape, cross-section, 

size and configuration of coils are varied to design an eddy current probe for 

a specific application. Depending on the geometry of the component three 

types of eddy current probes viz. surface pancake, encircling and bobbin 

probes shown in Fig.2 are employed. The three types of probes can be 

operated in absolute, differential or send-receive modes. In absolute mode 

only one coil is used for exciting and sensing eddy currents. The differential 

probes with two coils usually wound in opposite direction, and the sendreceive 

probes with separate receiver coils, employ different bridge circuits. 

The absolute and differential modes exhibit different characteristics (Table.1) 

and selection depends, primarily on inspection requirement. 



Figure 2: Types of Probes 





Surface (Pancake) Probes 

Surface probe or Pancake probe, usually a spring mounted flat probe or a 

pointed pencil type probe, allows determining the exact location of a defect. 

The probe may be hand held, may be mounted on automated scanners or 

may even be rotated around to get e.g. a helical scan in tube/rod inspections. 

Surface probes possess directional properties i.e. regions of high and low 

sensitivity (Table.2). Usually ferrite cores (absolute cylindrical as well as split- 

D differential types) and shields are used for enhanced sensitivity and 

resolution. Besides ferrites, copper coils are used for shielding purpose. 

Surface probes are extensively used in aircraft inspection for crack detection 

in fastener holes and for detection of corrosion/exfoliation in hidden layers. 

When the component geometry is complex, it is not uncommon to use probe 

guides, shoes, centering-mechanisms to maintain uniform lift-off and 

detection sensitivity. Surface probes were developed for EC imaging, for 

measurement of liquid sodium level in steel tanks and also for measurement 

of thickness of coatings. 



σº∙πμ■δ∝∞ωΩθ√ρβααδπ∠δ 



Encircling Probes 

Encircling probes are used to inspect rods, tubes and wires. In an encircling 

probe the coil is in the form of a solenoid into which the component is placed. 

In this arrangement, the entire outside circumferential surface of the 

component covered by the coil is scanned at a time, giving high-inspection 

speeds. These probes may not detect circumferential defects (Table.2) as the 

edy currents flow parallel to them without getting distorted. Popular industrial 

application of encircling probes is high-speed inspection of tubes from outside 

during the manufacturing stages. Encircling probes were developed NDE of 

thin-walled cladding tubes and thick-walled steam generator magnetic tubes. 



Bobbin Probes 

These probes are the most widely used ones in eddy current NDE. Bobbin 

probes consist of a coil arrangement in the form of a winding over a bobbin, 

which passes through components such as tubes and scans the entire inside 

surface in one-go. Popular application of bobbin probes is high-speed multifrequency 

inspection of heat exchanger tubes in-situ for detection of cracks, 

wall thinning and corrosion in tubes as well as under support plate regions. 

The directional properties of these probes are identical to encircling probes. 

In some instances, bobbin type probes are employed for inspection of bolt 

holes. For inspection of critical components, phased-array probes are slowly 

replacing the traditional bobbin/encircling probes as regards to detection and 

location of circumferential and short defects. 



Design of eddy current probes 

In most EC instruments excitation current is kept constant (in a few tens of 

mA range) and the inductance may vary by a factor of one thousand. The 

usual input impedance could range between 20 and 200 ohms. The number 

of turns and wire gauge (between SWG 30 and SWG 45) are fixed such that 

the coils fill the available cross sectional space in uniform layers and turns per 

layer so that inter-winding effects are minimal. In some situations, it may be 

necessary to use a number of bridge circuits as well as probes operating 

simultaneously, essentially to cover larger area. For good sensitivity to small 

defects, small diameter probes are used. Similarly, in order to detect subsurface 

and buried defects, large diameter high throughput probes are 

necessary. As a general rule, the probe diameter should be less than or equal 

to the expected defect length and also comparable to the thickness of the 

component. 



The sensing area of a probe is the physical diameter of the coil plus an 

extended area governed by magnetic field spread. Hence, it is common to 

use ferrite cores/shields (high permeability and low conductivity) to contain 

the lateral extent of magnetic fields without affecting the depth of penetration. 

It is essential to operate EC probes below the probe/cable resonance 

frequency, especially while using long probe cables and at very high 

frequencies. The probe bodies are usually made of non-conducting plastics. 

Wear of probes is normally be reduced by giving wear resistant coating to the 

probe heads or tips. It must be noted here that such coatings add to the builtin 

lift-off of probes and tend to reduce signal amplitudes. Temperature stability 

of probes is usually accomplished by using coil holder material with poor heat 

transfer characteristics. Most common commercial copper wires are used up 

to about 150º C. For temperatures above this, silver or aluminum wires with 

ceramic or high temperature silicon insulation or MIC are used. The probe 

material must be chemically compatible with the component. 



In brief, probe design is usually done considering the following: 

• Geometry of the component e.g. rod, tube, plate etc. 

• Type of discontinuity expected e.g. fatigue cracks, conductivity variation etc. 

• Likely location of defect e.g. surface, sub-surface. 

• Coil impedance and its matching with the bridge circuit of the EC instrument. 

• Frequency range of the probe i.e. for simultaneous multi-frequency excitation 

• Inspection requirement e.g. detection, evaluation of length, depth etc. 

• Material characteristics e.g. ferromagnetic or non-ferromagnetic. 

• Coil response to a notch, drilled hole or other reference discontinuity. 

• Field distribution in space and eddy current flow distribution in the material. 

• Shape and dimensions of core, coil /coils and lift-off characteristics. 

• Environmental characteristics such as wear, temperature and chemical attack. 

As many factors need to be considered, three different approaches viz. 

experimental, analytical and numerical are often resorted to for designing 

eddy current probes. 



Experimental Approach 

This approach usually involves trail and error fabrication of probes suiting the 

geometry. In this approach, the coil dimensions and the test frequency are 

usually optimized by comparing the detection sensitivity of artificial reference 

notches as well as natural cracks if available. This approach was used to 

design encircling EC probes for inspection of stainless steel cladding tubes of 

Fast Breeder Test Reactor (FBTR) and also to design probes for Cr-Mo 

steam generator tubes of Prototype Fast Breeder Reactors (PFBR). In 

another instance, in order to minimize low sensitivity zones of phased-array 

eddy current probes for inspection of heat exchanger tubes, tandem probe 

was developed. 



Analytical Approach (Pc & f g ) 

Analytical approaches for probe design involve analyzing the eddy current 

testing phenomenon and calculating the coil impedance and examining the 

operating point on the impedance plane as well as the effect of variations in 

coil radius r, shape, material conductivity, thickness t and test frequency f. 

Two popular impedance plane diagram based methods are 

1) calculation of characteristic parameter, Pc introduced by Deeds and Doods for 

planar geometries and 

2) calculation of characteristic frequency ratio f/f g 

, where fg is the characteristic 

frequency introduced by Förster for tubular geometries. 

(f g 

= 5066/ (μ r 

∙ σ∙ d 2 ) 

Using these two methods, coils are designed such that the operating point is 

in the “knee” region on the normalized impedance plane diagrams. 



Numerical Approach 

Eddy current testing phenomenon can also be analyzed numerically using 

finite difference, finite element (FE), boundary element (BEM) and other 

methods. In this approach, coil and core dimensions are varied systematically 

and signals are predicted for a reference defect and the dimensions that 

result in maximum detection sensitivity are chosen. Not only signal amplitude, 

but phase angle from lift-off is also considered for decision making. A few 

typical applications of axis-symmetric FE model, are discussed elsewhere. In 

this model, the region consisting of EC probe and component, is discritised 

into triangular elements and variational principles are applied to compute the 

vector potential at the vertices of the elements. From the vector potential, the 

probe impedance is calculated and in turn, the impedance plane trajectories. 

This model has been used to optimize eddy current probes for location of 

garter springs in the coolant channels of Pressurized Heavy Water Reactors 

(PHWRs) 



RECENT TRENDS IN EDDY CURRENT PROBE DESIGN 

Detection of cracks emanating from edges and corners of components is very important. Often, strong signal from the edges 

mask the small/weak signal from a potentially harmful crack. Focused surface probes are being explored and likewise appropriate 

signal processing methods are being incorporated to suppress edge contributions. In the case of heat exchanger tubes, rotating 

surface probes or array probes with multiplexing are preferred for detection and characterization of defects along the tube 

circumference (location). For detection of defects at roll joints special array probes are being tried. In order to inspect components 

with complex geometries, flexible probes are being tried. These probes can be mounted/scanned over a region for inspection 

purpose and be easily removed. Similarly, for detection of sub-surface and deep-seated defects in multi-layer and other structures 

eddy current probes are mounted and integrated with Hall probes, SQUID, GMR and AMR sensors. The main objective in these 

strategies is to detect the weak magnetic fields from defects, rather than the traditional impedance changes. When more than one 

sensors is used and data fusion methods are adopted to combine the sensors data to form a comprehensive global picture of 

investigated regions. At times, it may be beneficial to combine information of a single sensor, but operating at different 

frequencies to get enhanced information of defects. Such an approach has been used in an intelligent imaging scheme to obtain 

accurate and quick 3-dimensional pictures of defects. 

Inspection of ferromagnetic tubes is difficult due to high and varying magnetic permeability. For testing such tubes from outside, 

encircling D.C. saturation coils are used, where as remote field eddy current probes and permanent magnet based probes are 

used for testing from tube inside. Optimization of frequency and location of receiver coil (usually about 3 to 4 tube diameters away 

from exciter) in the remote field eddy current testing method is very important. FE model and experimental based approaches 

have been successfully used this purpose. 

When surface EC probes are scanned in a raster and the impedance data is displayed, Eddy current C-scan images of defects 

can be formed. EC images provide valuable information of defects. However, these images are blurred due to distributed point 

spread function of the probe. FE model based approach was used to optimize ferrite-core probes for eddy current imaging. In 

case of heat exchangers and steam generators, probes have to negotiate U-bend regions and detect defects, if any, in those 

regions. Design of flexible probes that are insensitive to bend regions is very challenging. For inspection of bend regions in 

ferromagnetic steam generator tubes, flexible remote field eddy current probe, with WC rings on either sides, was developed and 

wavelet transform based signal processing method was incorporated to suppress disturbing signals from bend regions. 



Pressurized Heavy Water Reactors (PHWRs) 


Pressurized Heavy Water Reactors (PHWRs) 


Chapter 5 



The Answers 


Q.5.1 What frequency is required to establish a standard depth of penetration of 

7.6mm (0.1 in.) in Zirconium? 

A. 19.6kHz 

B. 196Hz 

C. 3.4 kHz 

D. 340Hz 

δ = K√ [ρ/(f)] 

f = ρ(K/ δ) 2 , ρ Zr = 40μΩ∙cm 

f = 40(50/7.6) 2 = 1731 Hz 

f = 40(1.98/.3) 2 = 1742 Hz 

Q.5.2 To reduce effects of far surface indications, the test 

frequency: 

A must be mixed. 

B. must be raised. 

C. mnst be lowered. 

D. has no effect. 

Q.5.3 The frequency required to establish the Bessel function argument A equal to 1 is 

called: 

A an optimum frequency. 

B. a resonant frequency. 

C. a limit frequency. 

n. a penetration frequency. 


Q.5.4 Calculate the limit frequency for a copper bar (σ = 50.6 meter I ohm 

mm 2 ) 1 cm in diameter. The correct limit frequency is: 

A. 50kHz. 

B. 50.6Hz. 

C. 100Hz. 

D. 100kHz. 

fg= 5066/ (μ r x σ x d 2 ) 

fg = 5066/ (1x50.6x1 2 ) = 100Hz 

Q5.5 Using the example in Question 5.4, what is the f/fg ratio if the test 

frequency is 60kHz? 

A. 1.2 

B. 120 

c. 60 

D. 600 


Q.5.6 In Fig.5.1(b) the value of ωL s G equaling 1.4 would be indicative of 

A. a high resistivity metal. 

B. a high conductivity metals. 

C. a low conductivity metals. 

D. a nonconductor. 


Q.5.7 Primary resistance is subtracted from Fig.5.1(b) because 

A. resistance is always constant. 

B. resistance is not frequency dependent. 

C. resistance is not added to impedance. 

D. none of the above. 

Q5.8 The reference quantity is different for solid 

cylinder and thin wall tube in Fig 5.2 because: 

A. the frequency is different. 

B. the conductivity is different. 

C. the skin effect is no longer negligible. 

D. the thin wall tube has not been normalized. 

Note: Both materials having the same conductivity. 

The thin wall tube will form a weaker eddy current thus 

weaker secondary flux that oppose the primary flux 


Discussion: 

Subject: 

Q5.8 The reference quantity is different for solid 

cylinder and thin wall tube in Fig 5.2 because: 

A. the frequency is different. 

B. the conductivity is different. 

C. the skin effect is no longer negligible. 

D. the thin wall tube has not been normalized. 

Further normalization is accomplished by dividing the reactive and 

resistive components by the term ωLo or the primary inductive 

reactance without a secondary circuit present. 

Figure 5.2 shows a typical normalized impedance diagram. The 

terms ωL/ωLo and R/ωLo represent the relative impedance of 

the test coil as affected by the test object. Signals generated by 

changes in ωL or R caused by test object conditions such as 

surface and subsurface discontinuities may be noted by ∆ωL or 

∆R. The ∆ωL and ∆R notation indicates a change in the 

impedance. 

k = √(ωμσ) = Electromagnetic wave propagation constant for conducting material 

r = radius of the conductor in m 

μ = magnetic permeability of bar = 4π∙10-7 H.m-1 if bar is non-magnetic (μ = μ o ) 

ω = angular velocity = 2πf 

√(ωL o G) = equivalent of √(ωμσ) for simplified electrical circuit, where G=conductance 

(Siemens) and Lo = inductance in air (Henry) 


Q5.9 A 25% deep crack open to the near surface give a response ____ times 

greater than the same crack 3.3% of diameter under the surface (refer to Fig 

5.4) 

A. 10 

B. 3 

C. 2 

D. 5 


Q5.10 When using multifrequency system, low subtractor frequencies are 

used to suppress: 

A. conductivity changes 

B. far surface signals 

C. near surface signal 

D. permeability changes 

Note: For suppression of inside or near surface signals, a higher subtractor 

frequency would be chosen. 

A combination of prime, low and high subtractor frequencies is often used to 

suppress both near and far surface signals, leaving only data pertaining to the 

part thickness and its condition. 

The subtractor frequency is actually the “detecting secondary” frequency 

which will be subtracted from the prime frequency signal. 


Chapter 6 

Instrument Systems 



Eddy Current Instrumentation 

Most of the eddy current instrumentation is categorized by its final output or 

display mode. There are basic requirements common to all type of eddy 

current instruments. Five different elements are usually required to produce a 

viable eddy current instrumentThese function are excitation, modulation, 

signal preparation, signal analysis and signal display. An optional sixth 

component would be test object handling equipment. Figure 1 illustrated how 

these components interrelate. 

The generator provide excitation signals to the test coil. The signal 

modulation occurs in the electromagnetic field of the test coil assembly. Next, 

the signal preparation section, usually a balancing network, prepares the 

signal for demodulation and analysis. In the signal preparation stage, balance 

network are used to null out steady value alternating current signals. 

Amplifiers and filters are also part of this section to improve signal-to-noise 

ratio and raise signal levels for the subsequent demodulation and analysis 

stage. 


Figure 6.1: Internal functions of the electromagnetic nondestructive test 




Eddy current testing – Signal Balancing 

The demodulation and analysis section is made up of detectors, analyzers, 

discriminators, filters and sampling circuits. Detectors can be a simple 

amplitude type or a more sophisticated phase/amplitude or coherent type. 

The signal display section is the key link between the test equipment and its 

intended purpose. The signals generated can be displayed many different 

ways. The type of display or readout depends on the test requirements. In 

some tests, a simple GO/NO-GO indicator circuit may be all that is required. 

However some applications may require recording of 100% of all raw data 

generated during a test. This data may be imported into other digital devices 

that allow sophisticated data analysis or engineering statistics to be 

generated. One example of this is the inspection of large inservice nuclear 

components so that discontinuity growth can be monitored for determining 

potential failure rates or replacement cycles. Signal display processes will be 

discussed more in Chapter 7. 


Chernobyl Nuclear 

Power Plant 


Series of simple eddy current instruments is shown in Figure 6.2. In Figure 

6.2(a), the voltage across the inspection coil is monitored by an alternating 

current voltmeter. This type of instrument could be used to measure large lift 

off variations where accuracy was not critical. Figure 6.2(b) shows an 

impedance bridge circuit. This instrument consists of an alternating current 

exciting source, dropping resistors and a balancing impedance. 

Figure 6.2(c) is similar to Figure 6.2(b). In Figure 6.2(c) a balance coil similar 

to the inspection coil is used to provide a balanced ridge. Figure 6.2(d) 

illustrates a balancing coil affected by a reference sample. This is commonly 

used in external reference differential coil tests. In all cases, because only the 

voltage change or magnitude is monitored, these systems can all be grouped 

as impedance magnitude types. 


Figure 6.2: Four types of simple eddy current instruments 


In Figure 6.2(a), the voltage across the inspection coil is monitored by an 

alternating current voltmeter. This type of instrument could be used to 

measure large lift off variations where accuracy was not critical. 


Figure 6.2(b) shows an impedance bridge circuit. This instrument consists of 

an alternating current exciting source, dropping resistors and a balancing 

impedance. 


Figure 6.2(c) is similar to Figure 6.2(b). In Figure 6.2(c) a balance coil similar 

to the inspection coil is used to provide a balanced ridge. 


Figure 6.2(d) illustrates a balancing coil affected by a reference sample. This 

is commonly used in external reference differential coil tests. 




Eddy current testing – Signal Analysis 

can be divided into three broad groups. The groups are impedance testing, 

phase analysis testing and modulation analysis testing. 

1. Impedance testing is based on gross changes in coil impedance when 

the coil is placed near the test object. 

2. Phase analysis testing is based on phase changes occurring in the test 

coil and the test object's effect on those phase changes. 

3. Modulation analysis testing depends on the test object passing through 

the test coil's magnetic field at a constant feed rate or speed. These 

systems act like a tuned circuit. The operating frequency of the tester is 

changed (modulated) as a discontinuity passes through the test coil's field. 

The amount of modulation is a function of the transit time of the 

discontinuity through the coil's field. The faster the transit time the greater 

the modulation. If a system is set up for one speed and then the parts are 

scanned at a much slower speed the discontinuities may not be detected. 


Modulation analysis testing depends on the test object passing 

through the test coil's magnetic field at a constant feed rate or speed. These 

systems act like a tuned circuit. The operating frequency of the tester is 

changed (modulated) as a discontinuity passes through the test coil's field. 

The amount of modulation is a function of the transit time of the discontinuity 

through the coil's field. The faster the transit time the greater the modulation. 

If a system is set up for one speed and then the parts are scanned at a much 

slower speed the discontinuities may not be detected. 


Impedance Testing 

With impedance magnitude instrumentation it is often difficult to separate 

desired responses, such as changes in conductivity or permeability, from 

dimensional changes. 

A variation of the impedance magnitude technique is the reactance 

magnitude instrument. 

In reactance magnitude tests, the test coil is part of the fundamental 

frequency oscillator circuit. This operates like a tuned circuit where the 

oscillator frequency is determined by the test coil's inductive reactance. As 

the test coil is affected by the test object, its inductive reactance changes, 

which in turn changes the oscillator frequency. The relative frequency 

variation ∆f/f is, therefore, an indication of test object condition. Reactance 

magnitude systems have many of the same limitations as impedance 

magnitude systems. 


Phase Analysis Testing – Earlier Test System 

Phase analysis processes can be divided into many subgroup depending on 

the type of display. 

Some of the earlier test system output were called vector point, ellipse and 

linear time base. 


Phase Analysis Testing - Vector Point 

The vector point display would simply be a point of light on an analog cathode 

ray tube (Figure 6.3), The point is the vector sum of Y and X axis voltages 

present in test coil. By proper selection of frequency and phase adjustment a 

response in the vertical plan might represent dimensional changes (magnetic 

permeability’s factor) while voltage shift inthe horizontal plane could represent 

change in conductivity. 


Phase Analysis Testing - Ellipse 

As with the vector point techniques, the test object and reference standard 

are used provide a balanced output. A normal balanced output is a 

straight horizontal line. Fig 6.4 shown typical ellipse responses. 

Figure 6.4: Cathode ray tube displays for dimension and conductivity 


Phase Analysis Testing - Ellipse 

As with the vector point techniques, the test object and reference standard 

are used provide a balanced output. A normal balanced output is a 

straight horizontal line. Fig 6.4 shown typical ellipse responses. 







Phase Analysis Testing - Linear Time Base 

An early test system that was better suited to compensate for harmonic distortions present in the 

fundamental waveform used the linear time base technique. The linear time base unit applies a 

sawtooth shaped voltage to the horizontal deflection plates of a CRT. This provides a linear trace 

of the CRT beam from left to right across the CRT screen. The timing of the linear trace function 

is set to same value as the alternating current energy applied to the coil. This allows one 

complete cycle of the sine wave voltage applied to the coil to appear on the CRT. Figure 6.5 

illustrates a linear time base display. A slit or narrow vertical scale is provided to measure the 

amplitude of signals present in the slit. The base voltage is normally adjusted to cross the slit at 0 

volts, the 180 degree point on the sinewave. 

The slit value M is used to analyze results. The slit value M is described by 

the equation: 

M = A sin θ 

Where: 

M = slit value 

A =amplitude of the measurement in the slit 

θ = angle between base angle and measurement effect. 

In Figure 6.5 the angle difference A to B is about 90 degrees. 


Figure 6.5: Screen image of a linear time base instrument with sinusoidal 

signals 

M = A sin θ 

M = slit value 

A =amplitude of the measurement in the slit 

θ = angle between base angle and measurement 

effect. 

θ= 90º 


Presence Method – The Impedance Plane Testing 

The three tester types that have been defined so far (vector point ellipse and linear time base) 

were early attempts to correlate electromagnetic changes detected by a test system with material 

variables. The circuits that they used were fairly primitive by today's standards. These techniques 

were limited by the level of technology available at the time they were built. They were not very 

sensitive to small changes in materials and could not readily display small variations in the signal 

changesthat they did detect. 

As the field of electronics advanced, more sophisticated components became available. In 

today's marketplace many eddy current test systems have the capability to display data in 

multiple modes. The classic X-Y type display mode is a simple way of showing what is meant by 

an impedance plane test system. In Chapter 4 impedance plane diagrams were defined. These 

graphs and curves allow technicians to look at complex sets of information for a number of test 

variables simultaneously. 

Test systems that provide the ability to view both the direction (phase) and amplitude (voltage) of 

the voltage shift across an inspection coil provide much greater detail than the early model test 

systems that were looked at in this chapter. These modern systems give the ability to sort or 

measure material parameters with a much higher degree of accuracy. Some impedance 

measurement systems may only display part of the information derived (meterbased technology) 

but most use a two-dimensional output device. 


Impedance Plane Testing - Mode of Operation 

Test instruments may also be classified by their mode of operation. The mode 

of operation is determined by two functional areas within the instrument. 

1. The first functional consideration might be the degree of compensation, or 

nulling, and the type of detector used. 

2. The second consideration is the method of test coil excitation. The types of 

excitation include single frequency or multifrequency sinusoidal, single or 

repetitive pulse and swept frequency. 


Impedance Plane Testing - Signal Compensation 

Mode 1. Null balance with amplitude detector 

Mode 2. Null balance with amplitude phase detectors, (Figure 6.6) and 

Mode 3. Selected off null balance with amplitude detector. 

Mode 1 responds to any signal irrespective of phase angle. These would typically be 

meter-based instrumentation capable of showing only the voltage change or amplitude 

of the signal of interest. 

Mode 2, using amplitude and phase detectors, can be used to discriminate against 

signals having a particular phase angle. With this type of system, the total 

demodulated signal can be displayed in an X-Y screen presentation format to show 

both amplitude and phase relationships. A classic example of the advantage of this X- 

Y screen presentation in surface scanning applications is to put lift off responses on 

the horizontal with discontinuities responding up on the screen. 

Mode 3 systems are phase sensitive systems although they have only amplitude 

detector. They achieve phase sensitivity by operating in a manually selected off 

balanced condition. Based on this selection, the off null signal change can be set so 

that it may appear larger than the inherent impedance change due to test object 

variables. 


Figure 6.6: Null balance instrument with amplitude phase detectors 

A classic example of the advantage of this 

X-Y screen presentation in surface 

scanning applications is to put lift off 

responses on the horizontal with 

discontinuities responding up on the 

screen. 


Test Coil Excitation 

The second consideration that was previously mentioned for determining the 

mode of operation of a test unit could be the way the probe is being energized. 

Figure 6.7 a typical surface riding pancake coil responses to an array of EDM 

notches on a calibration standard. Fig 6.8 shows a block diagram of a 

stepped, single frequency, phase amplitude instrument. 

The circuit in Fig 6.8 is capable of operating at any of the four frequency, if 

the four frequencies are spread over a wide range, several different test coils 

may be required to use the instrument over the entire range. Most modern 

single frequency instruments use this principle; however one variable 

frequency generator with a wide operating range usually replaced the four 

individual fixed generators. A typical frequency for such an instrument is in 

the low hertz range (50Hz to 100Hz) to several megahertz (8MHz to 10MHz). 

This large dynamic range gives these units a wide variety of possible 

applications. 


Figure 6.7: Typical surface riding pancake coil response to an array of EDM 

notches on a calibration standard 


Figure 6.8: Single frequency selectable instrument 


For deep subsurface crack detection (more than 5mm) the lower frequency would 

be required. This test might be performed with hybrid (driver/pick-up) coil to 

improved detection of the low amplitude responses from smaller discontinuities 

deeper in a product. 

For detection of very small stress or fatigue crack in a near surface inspection 

process the higher frequency range could improve sensitivity to smaller cracks. 

The compromise at very high frequencies is the issue of skin effect or surface 

noise. Special probe or scanning process may be required for this type of test 

also. 

Figure 6.9 shows a block diagram for a multifrequency instrument operating at 

three frequencies simultaneously. In modern systems this is referred to as 

simultaneous injection. This diagram shows three dedicated frequency modules 

but more recent adaptations use multiple variable frequency circuits. In Figure 6.9, 

excitation currents at each selected frequency are impressed across the coil at 

the same time. You will recall from earlier chapters that the electromagnetic 

envelope around an alternating current driven coil is very dynamic. It is very 

difficult to model what the combined electromagnetic flux pattern would look like 

with more than one frequency affecting the coil at a given moment in time. 


Figure 6.9: Multifrequency instrument operating at three frequencies 

simultaneously 


Multiple circuits are used throughout the instrument. The test coil output 

carrier frequencies are separated by filters. Multiple dual phase amplitude 

detectors are used and their outputs summed to provide separation of several 

test object parameters. A system similar to this is described in Inservice 

Inspection of Steam Generator Tubing Using Multiple Frequency Eddy 

Current Techniques. 

Another approach to the multifrequency technique uses a sequential coil drive 

called multiplexing. The frequencies are changed in a step-by-step sequence 

with such rapidity that the test parameters remain unchanged. The multiplex 

technique has the advantages of lower cost, continuously variable 

frequencies and little or no crosstalk between channels. 

Figure 6.10 illustrates a multifrequency instrument capable of generating up 

to 16 channels of data sequentially. Each channel or time slot may be 

adjusted over a wide range of frequencies. In addition, this digital system 

provides for the creation of mixed channel combinations for suppression of 

unwanted test variables. Results of such suppression are described in 

Multifrequency Eddy Current Method and the Separation of Test Specimen 

Variables . 


Figure 6.10: Commercial multifrequency instrument 


This type of digital instrumentation allows all of the test setup parameters to 

be stored to either internal or external storage media. This allows 

preprogrammed test setups to be recalled and used by semi-skilled personnel. 

Systems can be created with programs having supervisory code interlocks 

that prevent reprogramming by other than authorized personnel. These 

instruments can also interface with robotic or computer-based systems for 

both process control and raw data recording purposes. A test system using 

pulsed excitation is shown in Figure 6.11. 

A pulse is applied to the test coil, compensating networks and analyzers 

simultaneously. Systems having analyzers with one or two sampling points 

perform similar to a single frequency tester using sinusoidal excitation. Pulsed 

eddy current systems having multiple sampling points perform more like the 

multifrequency tester shown in Figure 6.10. 


Figure 6.11: Pulsed waveform excitation 




Read Out Mechanisms 

Eddy current test data may be displayed or indicated in a variety of ways. The 

type of display or readout depends on the test requirements. Some common 

readout mechanisms are indicator lights, audio alarms, meters, digital 

displays, CRTs, recorders and computer interfaces. 

Read Out Mechanisms - Indicating Light 

A simple use of the indicating light is to monitor the eddy current signal 

amplitude with an amplitude gate circuit, When the signal reaches a preset 

amplitude limit, the amplitude gate switches a relay that applied power to an 

indicator light or automatic sorting device. With the amplitude gate circuit, 

high-low limits could be preset to give GO/NO-GO indications. 


Read Out Mechanisms - Audio Alarms 

Audio alarm can be as much same as the indicator light. The alarm gives 

qualitative indication without giving any quantitative information. 


Read Out Mechanisms - Meter Display 

The test information generated by any analog system can be processed 

through an analog-to-digital converter if additional signal processing is 

required. Meter-based technology signal responses fall into one of two 

categories: either quantitative or qualitative. One example of a quantitative 

meter response would be a system used for measuring conductivity (Figure 

6.12). When the needle deflects and reaches a specific point on the scale the 

number indicated on the scale should correlate to a specific percent IACS 

value if the system has been properly set up. Some meter-based devices 

(Figure 6.13) that might be used for simple discontinuity detection do not give 

the operator a numerical value other than a percent of full scale. A given 

crack could generate either a small amplitude voltage at a low gain e setting 

or a larger amplitude response at a higher gain setting. This would be a 

qualitative type response. These systems are not used for discontinuity sizing. 


An qualitative meter response could be used in a test situation where a 

minimum discontinuity amplitude response can be accurately defined. This 

might be an EDM ,, notch of a specified depth in a calibration block. As long 

as the meter stays below the preset voltage level from the selected 

discontinuity then the sample is acceptable. If that voltage level is exceeded 

then the part is deemed unacceptable. In some online inspections, this type of 

voltage threshold or gate is used to rapidly sort or grade materials. The use of 

these types of output displays should be limited to applications where a 

qualitative value or discontinuity threshold can be established and would be 

acceptable to meet test criteria. 


Figure 6.12: A quantitative meter response indicating a specific conductivity 

(in percent of the IACS) 


Figure 6.13: A qualitative analog meter response showing only percent of full 

scale 


Read Out Mechanisms - Digital Displays 

Numerical digital displays can also be used to provide qualitative information. 

These might have several applications but the most common would be for 

measuring conductivity values. 


Read Out Mechanisms - Cathode Ray Tubes 

Cathode ray tubes, or CRT type displays, play an important role in the display of 

eddy current information. In more recent times many eddy cm-rent systems have 

become available with digital representations of CRT type screens. In the original 

analog system there were three main elements: the electron gtm, the deflection 

plates and a fluorescent screen. The electron gun would generate, focus and 

direct the electron beam toward the face or screen of the CRT. The deflection 

plates were sih1ated between the electron gun and he screen, arranged in two 

pairs, usually called horizontal and vertical or X and Y. The plane of one pair 

would be perpendicular to the other pair. 

The screen is the imaging portion of the CRT. The screen consists of a coating or 

coatings that produce photochemical reactions when struck by the electron beam. 

The photochemical action appears in two stages. Fluorescence occurs as the 

electron beam strikes the screen. Phosphorescence is the chemical process that 

allows the screen to continue to give off light after the electron beam has been 

removed or has passed over a section of the screen. All analog CRT screen 

materials possess both fluorescence and phosphorescence. 


The duration of the photochemical effect is called persistence. Persistence 

can be grouped as either low, medium ol' high persistence. To display 

repetitive signals, a low or medium persistence CRT may have been used. To 

display noru:ecurxent or single events, a high persistence CRT would have 

been used. Many modern digital cathode ray tube type systems are available. 

Because analog CRTs are no longer manufactured, those systems are being 

replaced with other options digital system provide the additional flexibility for 

the selection of various color and contrastconditions (Figure 6.14). 

This allows the operator a thoice otcolor options that can be established on 

the swnesystem to compensate for use :in different lighting conditions. 

Because the data are outpuftotlte screen in a digital format varying 

persistence values can be selected by defining the timing factor of a rolling 

data buffer or memory. This selection process allows the operators to choose 

how long the digital images created stay on the screen for viewing. 


Figure 6.14: Numerical readouts/digital conductivity tester 


Read Out Mechanisms - Recorders 

Data recorders might be required to meet the inspection criteria. Recording is sometimes 

accomplished on analog paper strip charts or on magnetic tape formats. With most modern 

equipment providing recording capability some form of digital media would be used. The 

data could be stored internally in some test systems, but more often than not the data are 

exported to an external storage device. Most of these digital recording media can retain the 

files created for offline analysis and long term historical use. Early digital systems were 

write once - read many devices. The more recent recording media can be erased and 

reused. The advantage of digital systems is that all of the raw data created by a 

multifrequency test system can be viewed in multiple display formats at the same time. 

Tubing exam data are often reviewed using both the X-Y and strip chart modes to optimize 

discontinuity detection and sizing. 

The s trip chart format is often used where the discontinuity's location down the length of a 

rod or tube is critical. The strip chart length is indexed to time or distance and signal 

response deviation from the baseline indicates various material conditions. The amplitude 

of the X-Y lissajous response in Figure 6.15 (6.66 V) is an 1nd1cat01' of the volume of the 

discontinuity. The phase angle with respect to the/( axis (114 degrees) represents 

discontinuity depthl(in this case, 41%) and discontinuity origin (tube outside diameter), 

indicating whether the discontinuity originated on the inside or outside surface of the tube 

(13). Many comp uter-based systems have multiple display modes available for the same 

raw data set. 


Read Out Mechanisms - Computer 

Eddy current testing may be display with the used of computer. The electronic 

components and connectors that are linked to a remote computer via a local 

area network (LAN) cable. The computer itself handles data display and 

processing functions as well as adjusting tester operating parameters, such 

as frequency, gain, probe drive voltage and mode of operation, etc. Figure 

6.16 shows a multimode output responses of a rotating pancake coil 

inspection in a bolt hole application. The same crack response can be seen in 

all four display formats. 


Figure 6.16: Multimode output responses: rotating pancake coil inspection in 

a bolthple application. The same crack response can be seen in all four 

display formats. 




Test Object Handling Equipment 

Test object handling equipment is often a necessary component of an online test system. 

Bars and tubes can be fed through encircling coils by means of roller feed assemblies. 

Consistent centering of the material is essential. The stock being fed through the coils is 

usually transported at a constant speed. The transport speed needs to be adjusted to allow 

adequate time for testing and for the reject, cutting or marking systems to perform their 

tasks. Should product centering or speed change during the examination system 

performance could be limited. Automatic sorting devices are very common in online 

inspection systems used in a manufacturing environment. 

When a volumetric test is required for heat treatment or hardness verification the probe 

assembly may interrogate the entire test specimen (or some critical region of the specimen) 

in one view. For small specimens like ball bearings this could take just fractions of a 

second per sample. In larger specimens the volumetric test may take a few seconds per 

sample. When crack detection is required the part is normally rotated with one or more 

coils positioned near the surface of the specimen. This type of inspection ensures 100% 

inspection of critical areas in one test The eddy current technique can often demonstrate 

much higher discontinuity sensitivity and more rapid economical testing for surface 

discontinuities in parts than any of the other nondestructive testing processes. If 

unacceptable material conditions were encountered at any inspection station the part 

would be dropped into rejection bin. A digital counter and or remote sensor can be used to 

track the number of rejection and alert the plant staff on the manufacturing processes. 


Probe Delivery System 

Instead of moving the part through an inspection station there are situation 

where motorized probe delivery system is used. These are normally 

employed outside of the of the manufacturing environment to perform in situ 

inspection on existing materials. The term spinning probes originally comes 

from the pipe manufacturing environment. The coil is typically a fairly small 

specialized coil to improve detection potential for small cracks. A probe was 

rotated around the circumference of a tube or bar. The tested material was 

moved past the inspection point at a controlled rate of speed. The probe 

rotational speeds would have been set to be compatible with instrument 

response and translation speeds to obtain the desired inspection coverage 

and test sensitivity. 


As technology has been improved it has been possible to create other types 

of spinning probe possibilities. There are now many situations where spinning 

probes can be used. High speed probe guns are used to perform bolt hole 

inspection after removal of fastener in aerospace structures. 

For heat exchanger inspection, Tubes to be inspected, are identified and their 

coordinates are loaded into a database. Positive feedback is supplied to 

computerized positioning system by encoder or digital pattern recognition 

routines. Although these systems are quite automated, visual. Verification of 

the inspection is confirmed by an inspector via a remote video system. As the 

probe is inserted and withdrawn from each tube the test results are monitored 

in real time for data quality but the data are also recorded for later analysis. 

Remotely Operated Vehicles (ROV) can also be looked at as part of the array 

of technology to enhance eddy current systems in hostile environments. 

These electromechanical devices can be used to perform a wide array of 

nondestructive testing tasks. This could include applications for underwater 

eddy current array probe inspection of welds in either piping or support 

structures for offshore platforms. 


Chapter 6 



Answers 


Q.6.1 Signal preparation is usually accomplished by: 

A. detectors. 

B. samplers. 

C. balance networks. 

D. discriminators. 

Q.6.2 Most eddy current instruments have ______ coil excitation. 

A. square wave 

B. triangular wave 

C. sine wave 

D. sawtooth wave 

Q.6.3 Eddy current systems can be grouped by: 

A. output characteristics. 

B. excitation mode. 

C. phase analysis extent. 

D. both A and B. 


Q.6.4 A multifrequency instrument that excites the test coil with several 

requencies sequentially uses the ______ concept. 

A. multiplexing 

B. time base 

C. broadband 

D. cartesian 

Q.6.5 Reject limits should always be adjusted to: 

A. one-half the screen height. 

B. 5 volts. 

C. ensure unacceptable components are properly identified. 

D. reduce operator training costs. 

Q.6.6 Display requirements are based on: 

A. test applications. 

B. records requirement. 

C. need for automatic control. 



Q.6.7 Amplitude gates provide a technique of controlling: 

A. reject or acceptance limits. 

B. instrument response. 

C. display amplitude. 

D. All of the above. 

Q.6.8 Alarms and lights offer only: 

A. qualitative information. 

B. quantitative information. 

C. reject information. 

D. accept information. 

Note: this is different from UT equipment 

“gate” where area of interest is high lighted 

to display the necessary information, like 

depth, %FSH etc. 

Q.6.9 The length of a strip chart presentation can indicate: 

A. discontinuity severity. 

B. distance or time. 

C. orthogonality. 



Q.6.10 A top view display of the test results from a specimen can be referred 

to as: 

A. an X-Y display. 

B. a C scan. 

C. a crosshatch presentations 


Chapter 7 

Eddy Current Applications 


Eddy Current Applications 

Electromagnetic induction and the eddy current principle can be affected in many 

different ways. These effects may be grouped by discontinuity detection, 

measurement of material properties, dimensional measurements and other 

special applications. With the discontinuity or the detection group, we are 

concerned with locating cracks, corrosion, erosion and mechanical damage. The 

material properties group includes measurements of . conductivity, permeability, 

hardness, alloy sorting or chemical composition and degree of heat treatment. 

Dimensional measurements commonly made are thickness, profilometry, spacing 

or location and coating or cladding thickness. Special applications include 

measurements of temperature, flow metering of liquid metals, sonic vibrations 

and anisotropic conditions. 

Regardless of the specific application, once the test system has been properly 

calibrated there should not be any fundamental changes made to it during the 

testing process. If it has been determined that the instrument has been set up 

incorrectly or is not working as specified in the operational procedures being used, 

all material should be retested since the last time the correct setup and proper 

system operation was verified. 


Discontinuity Detection 

The theoretical response to discontinuities has been discussed in previous 

chapters of this guide. In this chapter, some actual examples are given to 

enhance the understanding of the applied theory. A problem common to the 

chemical and electric power industries is the corrosion of heat exchanger 

tubing. This tubing is installed in closed vessels in a high density array. It is 

not uncommon for a nuclear steam generator or main condenser to contain 

many thousands of tubes. This high density and limited access to the 

inspection areas often precludes the use of other nondestructive testing 

methods. A bobbin coil inspection provides a volumetric inspection of the tube 

wall in a cost effective process. 

Heat exchanger inspection systems and results are described by Libby, Dodd, 

Sagar and Davis. 


Heat Exchanger 
















Figure 7.1: American Society of Mechanical Engineers (ASME) thin-walled 

tubing standard 



TSP 



TSP 



TSP 


Phase angle and amplitude relationships are usually established by using 

reference standards with artificial discontinuities of known and documented 

values. These discontinuities should reflect expected damage modes as 

close as possible. In most thin-walled tubing cases the severity of the 

discontinuity can be determined by analyzing the eddy current signal phase 

and/ or amplitude. The phase angle of small volume discontinuities (cracks, 

pits) is used to establish a phase-to-depth calibration curve (Figure 7.2) and 

to verify the originating surface (inside diameter or outside diameter) of that 

discontinuity. The signal amplitude is an indicator of discontinuity volume. For 

volumetric tube wall loss conditions such as wear and fretting, a volts-todepth 

calibration curve can be created (Figure 7.3). When used properly, 

these curves will provide a more accurate sizing process for mechanically 

driven discontinuity mechanisms. 


Figure 7.2: Phase-to-depth calibration curve 


Figure 7.3: Volts-to-depth calibration curve 


The geometry of real discontinuities may differ from reference standard 

discontinuities. This difference produces interpretation errors as discussed by 

Sagar. Placement of real discontinuities near tube support members causing a 

complex coil impedance change is also a source of error. This, of course, is 

dependent on the size of the discontinuity and its resultant eddy current signal in 

relation to the tube support signal. This follows the basic principle of signal-tonoise 

ratio. The signal-to-noise ratio can be improved at tube-to-tube support 

intersections by the use of multifrequency techniques. 

In multifrequency applications, an optimum (or prime) frequency is chosen for 

response to discontinuities within the tube wall. A lower than optimum or 

suppression frequency (subtractor frequency?) is chosen for response to the tube 

support. The two signals are processed through comparator circuits called mixers 

where the tube support response is subtracted from the tube wall response signal, 

leaving only the response to the tube wall discontinuity. (See Figures 7.4 and 7.5.) 

Both channels must be able to detect both the discontinuity and the noise source 

that is being suppressed. Another market sector that uses eddy current testing 

extensively is the aerospace industry. Many eddy current examinations are 

conducted on engine and airframe structures. 


Figure 7.4: A multifrequency application without discontinuity 

A: The nominal response to a tube 

support plate at the prime 

frequency. 

B: The nominal response to a tube 

support plate at the subtractor 

frequency. 

A-B: The mixer channel residual 

response after support plate 

suppression. 


Figure 7.5: A multifrequency application with discontinuity 

A: The response to a tube support 

plate with a discontinuity at the 

prime frequency. 

B: The response to a tube support 

plate with a discontinuity at the 

subtractor frequency. 

A-8: The mixer channel response to 

the discontinuity after support plate 

suppression. 


A common problem with turbines is fatigue cracking of the compressor blades 

or disks in the root areas. Given the potential safety risks if these components 

fail, the inspection criteria thresholds are set to detect extremely small 

artifacts. Special probe designs and inspection techniques are required to 

deal with the difficult sample geometries and small discontinuity detection 

limits. Many other aircraft inspections are designed to deal with cracking or 

corrosion processes that may not lead to immediate catastrophic failures but 

that do need to be handled in a timely manner. Portable inspection devices 

are often used to perform these tests. Careful test system calibration using 

appropriate procedures and reference specimens is required to maintain 

aircraft fleet serviceability. 


Aerospace Applications 


Aerospace Applications 


The reference specimen and its associated discontinuities are very critical to 

the success of the test. Often models are constructed with artificial 

discontinuities that are exact duplicates of the item being inspected. Field 

degraded specimens are also used to verify test discontinuity sensitivity. 

D.J. Hagemaier discussed low frequency eddy current inspection of aircraft 

structures for subsurface discontinuity detection in an article published in 

Materials Evaluation in 1982. A low frequency (100Hz to 1000Hz) technique 

can be used to locate cracks in thick or multiple layer, bolted or riveted aircraft 

structures. Again, models are constructed with artificial cracks and their 

responses are compared to responses in the actual test object. Most of these 

examinations are performed using single or multifrequency sinusoidal 

alternating current processes. Pulsed eddy current systems, if available, 

might also be used for crack detection in thick structures. 


Pulsed Eddy Currents Systems 

Learn more on Pulsed Eddy Currents Systems 


Dimensional Measurements 

Dimensional measurements, such as thickness, shape and position, or 

proximity of one item to another, are important uses of the eddy cunent 

technique. Materials are often clad with other materials to present a 

resistance to chemicals or to provide wear resistance. Cladding or plating 

thickness then becomes an important variable to the serviceability of the unit. 

For nonconductive coatings on conductive bases, the probe-to-specimen 

spacing, or lift off technique can be applied. The case of conductive plating or 

cladding on conductive bases requires more refinement. The thickness loci 

respond in a complex manner on the impedance plane. The loci fm 

multilayered objects with each layer consisting of a material with a different 

conductivity follow a spiral pattern. In certain cases, two frequency or 

multifrequency systems are used to stabilize results or minimize lift off 

variations on the thickness measurement. 


Figure 7.6 shows a single frequency hardness tester output presentation. The depth of 

case hardening can be determined by measuring the nitride case thickness in 

stainless steel. The nitride case thickness produces magnetic permeability variations. 

The thicker the nitride layer the greater the permeability. The coil's inductive reactance 

increases with a permeability increase. This variable is carefully monitored and 

correlated to actual metallographic results. Eddy current profilometry is another 

common way to measure dimensions. One example is the measurement of the inside 

diameters of tubes using a lift oft technique. For this measurement, several small 

pancake coils are mounted radially in a coil form. 

The coil form is inserted into the tube and each coil's proximity to the tube wall is 

monitored. The resultant output of each coil can provide detailed information about the 

concentricity of the tube. This is especially useful when the amount of tube wall 

deformation due to either manufacturing or operational conditions may require 

corrective action. An obvious problem encmmtered with this technique is centering of 

the coil holder assembly. The center of the coil holder must be near the center of the 

tube. When inspecting for localized dimensional changes, a long coil holder is 

effective in maintaining proper centering. Another function o£ the long coil form is to 

keep the coils from becoming tilted in the tube. This also requires higher probe fill 

factors than might normally be used during other types of tube inspections. 


Figure 7.6: A single frequency hardness tester output presentation 


Eddy Current Profilometry is another common way to measure dimensions. 

One example is the measurement of the inside diameters of tubes using a lift 

oft technique. For this measurement, several small pancake coils are 

mounted radially in a coil form. The coil form is inserted into the tube and 

each coil's proximity to the tube wall is monitored. 


Conductivity Measurements 

Conductivity is an important measured variable. In the aircraft industry, 

aluminum is used extensively. Aluminum conductivity varies not only with 

aHoy but also with hardness and tensile strength. Eddy current instruments 

scaled in percent IACS are normally used to inspect for conductivity 

variations. Secondary conductivity standards are commonly used to check 

instrument calibration. Common secondary conductivity standards range from 

8% IACS to about 100% IACS. The secondary standards are usually certified 

accurate to within ±0.35% or ±1% of value, whichever is less. 

Temperature is an important variable when making conductivity 

measurements. Most instrument and standards are certified at 20°C. 

Primary conductivity standards are maintained at a constant temperature by 

oil bath systems. Primary standards are measured with precision maxwell 

bridge type instruments. This circuit design increases measurement accuracy 

and minimizes frequency dependence of the measurement. The secondary 

standards used for field tester set up and calibration are often required to 

have their listed values recertified on an annual basis. 


Hardness Measurements (Conductivity variable?) 

Hardness measurements can be performed on both ferritic and nonferritic 

materials. Some hardness measurements are performed with a two coil 

comparative process but this is not a strict requirement. When using a two- oil 

system the reference and test coils are both balanced with sample parts of 

known hardness. As parts of unknown hardness affect the test coil, the 

instrument output (impedance) varies. 

The amount of output variation depends on the degree of imbalance created 

by the unknown test object hardness. The detected signal variations can be 

correlated to test object hardness. If an X-Y type display were to be used to 

display this hardness information, the specimens exhibiting an acceptable 

hardness could be adjusted to one region of the screen while those 

specimens defined as unacceptable, or unhardened, could appear in a 

different region of the screen. Once this calibration process is completed a 

high-speed automated system can be allowed to make the measurements 

using an alarm gate process. 


Alloy Sorting 

Alloy sorting can also be accomplished with a two coil comparator bridge 

process but again it is not a strict requirement. Other types of coil 

arrangements may also provide useful information. The key element to keep 

in mind with alloy sorting is that this is not the same as material identification. 

Two very different materials may provide the same load to the coil. Alloy 

sorting using electromagnetic must be verified with the additional verification 

of the mechanical properties of these materials. In the inspection of 

nonferromagnetic alloys it is easiest to separate one alloy or heat treat type 

from another when there is a unique range of conductivities associated with 

each material. This is not always the case within families of alloys. Different 

alloys and heat treats of the alurninum family may have the same conductivity 

value. This could lead to misidentification of the materials being inspected. 


All comparative tests will be strongly influenced by the selection of correct 

and accurate reference specimens. Because most eddy current instruments 

respond to a wide range of variables, the reference specimen parameters 

must be controlled carefully. Test object and reference specimens must be 

the same or very similar in the following characteristics: 

1. geometry, 

2. heat treatment, 

3. surface finish, 

4. residual stresses, 

5. metallurgical structure. 


In addition, it is advisable to have more than one reference specimen for 

backup in case of loss or damage. In the case of steel parts, they should be 

completely demagnetized information .With the right equipment, probes, 

techniques and to remove the effects of residual magnetism on instrument 

readings. As in most comparative tests, temperature of specimen and test 

object should be the same or compensated. Many other measurements can 

be made using eddy current techniques. The electromagnetic technique 

produces so much information about a material that its application is only 

limited by the ability to decipher this training, the experienced operator should 

be capable of making the required distinctions between relevant and 

nonrelevant indications. 


Chapter 7 



Answers 


Q.7.1 Conductivity, hardness and composition are part of the group. 

A. discontinuity detection 

B. material properties 

C. dimensional 

D. special 

Q.7.2 Using an inside diameter coil on tubing and applying the phase I 

amplitude technique of inspection, a signal appearing at 90 degrees on a 

CRT would be caused by: 

A. inside diameter discontinuity. 

B. outside diameter discontinuity. 

C. a dent. 

D. a bulge. 

Q.7.3 Discont:inuities in heat exchangers at tube support locations are easier 

to detect because the support plate concentrates the electromagnetic field at 

that point. 

A. True 

B. False 


Q.7.4 Using multifrequency techniques on installed heat exchanger tubing, a tube 

support plate signal can be suppressed by subtracting a ____ frequency signal 

from the optimum frequency signal. 

A. low 

B. high 

C. A orB 

D. None of the above. 

Q.7.5 In the aircraft industry, a common problem in gas turbine engines is: 

A. corrosion. 

B. fatigue cracking. 

C. vibration damage. 

D. erosion. 


Q.7.6 Subsurface discontinuities located in thick or multilayered aircraft 

structures could be detected by: 

A. low frequency sinusoidal continuous wave instruments. 

B. high frequency sinusoidal continuous wave instruments. 

C. pulsed systems. 

D. A or C. 

Answer to a Mistake – a mistake make is a good lesson learned. 

D.J. Hagemaier discussed low frequency eddy current inspection of aircraft structures for 

subsurface discontinuity detection in an article published in Materials Evaluation in 1982. A low 

frequency (100Hz to 1000Hz) technique can be used to locate cracks in thick or multiple layer, 

bolted or riveted aircraft structures. Again, models are constructed with artificial cracks and their 

responses are compared to responses in the actual test object. Most of these examinations are 

performed using single or multifrequency sinusoidal alternating current processes. Pulsed eddy 

current systems, if available, might also be used for crack detection in thick structures. 


Q.7.7 Response to multilayer varying conductivity structures follow _____ loci. 

A. orthogonal 

B. spiral 

C. linear 

D. stepped 

Q.7.8 Nitride case thickness variations can be detected in stainless steel 

(μ r ≈ 1) cylinders by measuring: 

A. conductivity. 

B. dimensions. 

C. permeability. 

D. none of the above. 

Q.7.9 Conductivity is not affected by temperature. 

A. True 

B. False 

Answer to mistake: The depth of case hardening can be determined by 

measuring the nitride case thickness in stainless steel. The nitride case 

thickness produces magnetic permeability variations. The thicker the 

nitride layer the greater the permeability. 

Question: Both conductivity & permeability count and the weighted 

significant dictated the prime factor? In this case the permeability effect 

dominates. 


Q.7.10 Residual stresses in the test part produce such a small effect that they 

are usually ignored when selecting reference specimens. 

A. True 

B. False 


Chapter 8 

Other Electromagnetic Techniques 


Electromagnetic Testing 

Eddy current testing is just one of a group of teclmiques that as a whole are 

defined as the electromagnetic testing method. The sub disciplines or 

teclmiques listed within the method continue to expand. Following are the 

techniques that fall under this method at the time of publication: Method: 

Electromagnetic Testing Techniques: 

• alternating current field measurement, ACFM 

• eddy current testing, ECT 

• flux leakage testing, MFLT 

• remote field testing, RFT 

The borders are sometimes a little gray between one process and another. 

These techniques have been grouped in this fashion more on the basis of 

their specific market area or specialized applications in the field testing 

envirorunent rather than on a purely scientific basis. Electromagnetics is a 

very broad term. It covers a wide range of energy levels, sources and 

measurement tools. 


Some other technologies that have been suggested to be included in 

electromagnetic testing are: 

• microwave systems, 

• superconducting quantum interference devices, 

• magneto-optical inspection devices, 

• flux leakage testing*. (*Now accepted as a stand -alone method for tank 

floor, wire rope, and down-hole pipe inspection work.) 


The ASNT Electromagnetics Committee, at the time of this revision, has 

selected the first four teclmiques because they are currently available and 

fairly well established to perform specific nondestructive testing inspections in 

the field. In this chapter the generic differences between these techniques will 

be explained. Eddy current testing is most commonly used for detection of 

surface or near surface discontinuities in nonferromagnetic materials. In 

materials with little or no permeability eddy current testing is effective to about 

5.08 mm (0.2 in.) below the test surface. 

For material thicknesses of greater than 5.08 mm (0.2 in.) special probes and/ 

or electronics packages are needed to improve the performance of eddy 

current testing. Although there are applications for eddy current tests on 

ferritic materials, eddy current has no ability to provide subsurface 

discontinuity detection in ferromagnetic alloys. Surface crack detection in 

ferromagnetic materials, especially for weld inspection, is a very viable eddy 

current process when the right technology is applied. Eddy current is often 

more sensitive and more cost effective than either magnetic particle 

inspection or penetrant inspection in this role. 


Alternating current field measurement, flux leakage testing and remote field 

testing are all special electromagnetics testing teclmiques that, if used 

properly, can provide useful non destructive testing information about 

ferromagnetic components. The deciding factor of one over the other is the 

type of material, part size or geometry and the type and size of discontinuities 

that need to be detected. There is no reason to believe that any of these three 

techniques would show any significant advantage over eddy current in the 

nonferromagnetic world except for material thicknesses over 5.08 mm (0.2in.), 

where remote field testing may be used to provide enhanced sensitivity to 

outside diameter discontinuities. Manufacturers and users will debate the 

various capabilities of one of these techniques over another. The following 

discussion will be made as generic as possible. 

Note: 

ACFM does not need magnetic saturation for ferromagnetic materials, unlike ECT. 

RFT more sensitive to outer surface discontinuities detection than ECT. 


Alternating Current Field Measurement 

Primary application: Inspection of weldments Power source: Alternating 

current Advantages Compared to Magnetic Particle and Dye Penetrant 

Inspection 

• Works through nonconductive coatings [up to 10 mm (0.4 in.) thick] so 

there is no need to remove and then reapply paint or to clean off rust. 

• Provides information on depth as well as length, saving time on removing 

discontinuities of insignificant depth. 

• Relatively insensitive to material property changes, so it is ideal for 

inspecting at welds. (permeability μ, thus no need to saturated the specimen & 

conductivity σ) 

• Relatively insensitive to probe lift off, allowing deployment through 

coatings and on rough surfaces. 

• Allows depth sizing of discontinuities up to about 25 mm (1 in.), depending 

on probe type. 


Figure 8.1 shows the basic principles of the technique. With no discontinuity 

present and a uniform current flowing in the Y direction, the magnetic field is 

uniform in the X direction perpendicular to the current flow, while the other 

components are 0. The presence of a discontinuity diverts current away from 

the deepest parts and concentrates it near the ends of a crack 

The effect of this is to produce strong peaks and troughs in Bz above the 

ends of the crack, while Bx shows a broad dip along the whole discontinuity 

with amplitude related to the depth. Alternating current field measurement has 

been developed from the alternating current potential drop technique. 

Alternating current potential drop uses current injection and contact potential 

drop probes. This technique required extensive surface preparation of the 

weld under examination. It could be used to produce crack depth 

measurements. 


Figure 8.1: Alternating current field measurement qualitative explanation of 

the magnetic forces above a notch. 

Z 

X 

Y 

Bx =magnetic flux component normal to electric field 

and parallel to test surface 

Bz =magnetic flux component normal to test surface 

T = time or scan distance (relative scale) 


http://www.ndt.net/article/wcndt00/papers/idn233/idn233.htm

ACFM Principle 


ACFM 

X 

Z 

Y 






ACFM 

Z 

X 






http://iic-hq.co.jp/english/03sp/01ii/01ni/KH-04.html

ACFM Butterfly Plot 


Alternating current field measurement has been developed from the 

alternating current potential drop ACPD technique. Alternating current 

potential drop uses current injection and contact potential drop probes. This 

technique required extensive surface preparation of the weld under 

examination. It could be used to produce crack depth measurements. 

Alternating current field measurement ACFM has its origins in alternating 

current potential drop but instead of using a contact type probe the current 

is induced in the test specimen. The contact probes previously used in 

alternating current potential drop have been changed to (noncontact) 

magnetic field sensitive coils. The models developed in alternating current 

potential drop for mapping surface magnetic fields and electric cmrents have 

been utilized in alternating current field measurement. 


Offshore Subsea ACFM Applications 


http://www.ndt.net/article/platte/platte.htm

The technique in its simplest form uses a handheld probe containing a 

uniform field induction system and two magnetic field sensors. The 

induced alternating current is generated in a limited region of the test 

specimen where the alternating electric current AC is considered to be lineal: 

In this region a magnetic field is produced which is also linear. Any 

disturbances in this region produced by surface discontinuities will affect the 

components of this linear magnetic field. Two or more air wound coils 

mounted with orthogonal axes within a probe will detect these disturbances. 

This is the foundation of altemating current field measurement which is 

different from eddy current testing. 

Keywords: 

The induced alternating current... 


The alternating current field measurement technique is being used by 

inspection companies and owners of fabricated components for weld 

inspection in petrochemical process plants, pharmaceutical plants, offshore 

well structures, highway bridges and roller coasters. Originally introduced to 

the offshore industry for subsea weld inspection, the use of alternating current 

field measurement has now broadened to include inspection of pressure 

vessels, process piping and drillpipe threads and risers. Recent 

developments have included automated and semiautomated systems to 

reduce the reliance on operators and the use of array technology to increase 

inspection speeds. 


Alternating current field measurement can be used for the inspection of 

nonferromagnetic materials but is less effective in this role. The effective 

depth of penetration in nonferritic materials with alternating current field 

measurement is dramatically reduced. This is in sharp contrast to standard 

eddy current philosophy. It should also be noted that volumetric 

discontinuities, such as corrosion pitting or porosity, give much weaker 

signals than planar discontinuities, so it is not recommended that alternating 

current field measurement be used in this role. 


Keywords: 

The effective depth of penetration in nonferritic materials with alternating 

current field measurement is dramatically reduced. This is in sharp contrast to 

standard eddy current philosophy. 


http://v-e.vn/en/alternating-current-field-measurement-acfm-crack-microgauge.html

Magnetic Flux Leakage Testing 

Primary Application: Ferromagnetic Materials: pipe, plate, wire, oil field 

tubulars and pipelines Power Source. Permanent magnets or direct current 

coils Flux leakage testing has been extensively used in the pipe inspection 

industry. This entails the introduction of a moving direct cmrent magnetic field 

into a ferromagnetic test piece. Any localized (normally surface breaking) 

discontinuities that lie within the inspection zone will cause the field to bend or 

leak and extend above the surface at that point. These flux lines cut across a 

moving coit or other magnetic sensors and are used to detect this direct 

current leakage field. 


In pipe inspection, flux leakage testing is used to look for corrosion pits and 

cracks. The locally thinned area puts a higher magnetic flux distribution in the 

space nearer to the flux detection device. This relative increase in field 

strength can be measured. Any discontinuity with its major axis parallel to the 

direction of the flux flowing in the material has little chance of being detected 

using this method. The pull speed of the flux leakage testing probe must be 

maintained at a fairly constant rate or the accuracy of the test is decreased 

even further. Pipeline inspections are performed with what are called smart 

pigs (Figure 8.2). These devices can simultaneously carry out multiple 

nondestructive testing tests. The most common is flux leakage testing. 


Figure 8.2: Equipment for magnetic flux leakage testing of pipes and tubes: 

(a) pig tool; and (b) data acquisition from pig sensors. 

(a) 

(b) 


Defect Detections 

Any discontinuity with its major axis parallel to the direction of the flux flowing 

in the material has little chance of being detected using this method. 

Low 

Detectability 




http://www.ndt.net/apcndt2001/papers/11921/11921.htm

The most commonly used inservice inspection tools utilize flux leakage 

testing to detect internal or external cmrosion. The flux leakage testing 

inspection pig uses a circumferential array of detectors positioned between 

the poles of strong permanent magnets to magnetize the pipe wall to near 

saturation flux density. Abnormalities in the pipe wall, such as corrosion pits, 

result in flux leakage testing near the pipe's surface. The leakage flux may be 

detected by hall effect probes or passive induction coils. 

MFLT- 

Magnetize the pipe 

wall to near saturation 

flux density 


http://www.ndt.net/apcndt2001/papers/11921/11921.htm

The most commonly used inservice inspection tools utilize flux leakage 

testing to detect internal or external cmrosion. The flux leakage testing 

inspection pig uses a circumferential array of detectors positioned between 

the poles of strong permanent magnets to magnetize the pipe wall to near 

saturation flux density. Abnormalities in the pipe wall, such as corrosion pits, 

result in flux leakage testing near the pipe's surface. The leakage flux may be 

detected by hall effect probes or passive induction coils. 

The demands now being placed on magnetic inspection tools are shifting 

from the mere detection, location and classification of pipeline discontinuities, 

to the accurate measurements of discontinuity size and geometry. Modern, 

high resolution flux leakage testing inspection tools are capable of giving very 

detailed signals. However, converting these signals to accurate estimates of 

size requires considerable expertise, as well as a detailed understanding of 

the effects of inspection conditions and the magnetic behavior of the type of 

steel used. 


Magnetic Saturation 


http://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html



http://202.141.40.218/wiki/index.php/Hysteresis


Hysteresis, in general, is defined as the lag in a variable property of a system with respect to the effect producing it as this effect 

varies. In ferromagnetic materials the magnetic flux density B lags behind the changing external Magnetizing field Intensity H. 

Hysteresis curve is drawn by plotting the graph of B-field vs H (or M-H) by taking the material through a complete cycle of H 

values as follows 

First, consider an unmagnetized sample of ferromagnetic material. The magnetic field intensity H is initially zero at O. It is 

increased monotonically, then magnetic induction B increases nonlinearly along the curve (OACDE) called as the magnetization 

curve. At point E almost all of the magnetic domains are aligned parallel with the magnetic field. An additional increase in H does 

not produce any increase in B. E is called as the point of magnetic saturation of the material. Values of permeability μ derived 

from the formula μ = B / H along the curve are always positive and show a wide range of values. The maximum permeability as 

large as 10 5 μ o occurs at the ``knee (point D) of the curve. 

Next H is decreased till it reduces to zero. B reduces from its saturation value at "E" to that at point "F". Some of the magnetic 

domains lose their alignment but some maintain alignment i.e. some magnetic flux density B is still retained in the material. The 

curve for decreasing values of H (i.e. Demagnetization curve EF) is offset by an amount FO from that for increasing values of H 

(i.e. Magnetization curve OE). The amount of offset “FO” is called the retentivity or the remanence or the level of residual 

magnetism. 

As H is reversed in direction and increased, the curve moves to point "G", where B is reduced to zero. Most of the domains are 

flipped and oriented randomly so that net flux density within the material is zero. Portion corresponding to “GO” denotes 

“coercivity”. 

As H is increased to large values in the negative direction, B reaches saturation but in the opposite direction at point "I ". Almost 

all of the magnetic domains are aligned in opposite direction to that at point E of positive saturation. 

H is varied from its maximum negative value to zero. Then B reaches point "J." This point shows residual magnetism equal to that 

achieved for positive values of H (OF =OJ) 

H is increased back from zero to maximum in the positive direction. Then B reaches zero value at “K” i.e. it does not pass through 

the origin of the graph. OK indicates the amount of field H required to nullify the residual magnetism OJ retained in the opposite 

direction. 

H is increased from point “K” further in the positive direction, then again the saturation of B is reached at point “E” and the loop is 

completed. 



The magnetization curve is not retraceable. The domains forced to coalesce into large domains aligned with the external field 

maintain the alignment and retain magnetism even after the external field is removed. The state of a system depends on the 

history of its state. The state (value and direction) of B depends upon the previous state of H (value=zero/ +ve/ -ve and direction 

increasing/ decreasing). Ferromagnetic materials have "memory" of previous exposure to magnetism or magnetic history. This 

phenomenon is called as Hysteresis. 

This property has been used to advantage in magnetic memory devices e.g. recording of audio tape/ video tape, and the magnetic 

storage of data on computer disks. From the hysteresis loop, important magnetic properties of a material can be determined as 

follows 

1. Retentivity : A measure of the residual flux density corresponding to the saturation of a magnetic material. It is a material's 

ability to retain a certain amount of residual magnetic field when the magnetizing force is removed after achieving saturation (The 

value of B at point E on the hysteresis curve). 

2. Residual Magnetism or Residual Flux : The magnetic flux density B that remains in a material when the magnetizing field 

intensity H is zero. Residual magnetism and retentivity are same only when the material is magnetized to the saturation point. 

However, it may be lower than the retentivity value otherwise. 

3. Coercive Force : The amount of reverse magnetizing field intensity which must be applied to a magnetic material to make the 

magnetic flux density return to zero. (The value of H at point G on the hysteresis curve). 

4. Permeability, μ : A property of a material that measures the ease with which a magnetic flux is established in it. μ is negative in 

the II and IV quadrants and positive in the I and III quadrants of the B-H graph (i.e. the Hysteresis curve). 

5. Reluctance : Is the opposition that a ferromagnetic material shows to the establishment of a magnetic field. Reluctance is 

analogous to the resistance in an electrical circuit. 

The knowledge of these properties of materials is useful for selecting materials appropriate for different applications e.g. materials 

having both a large remanence and a large coercivity are selected for designing a permanent magnet. Materials possessing small 

remanences and small coercivities are selected for making transformer circuits. 









Magnetic Flux Leakage 

The basic principle behind MFL involves magnetizing a ferrous metal object 

to saturation level with a powerful magnetic field. 


http://www.mdpi.com/1424-8220/14/6/10361/htm


Rare earth neodymium iron boron magnets power the magnetizer of the inspection unit, providing the ultimate strength to meet 

most pipeline wall thicknesses for the best feature detection and sizing. Special designs can cover extra heavy wall applications. 

ILI tool drive section is the sealing unit that pulls the pig through the pipeline. All sizes of the ILI tool can accommodate multiple 

wall thickness in the same run. Longitudinal distance measurement to assure accurate location of anomalies. Magnetic sensors 

give 3 digital ticks per foot and analog sinusoid quadrature signals to allow for distance interpolation and forward/backward 

movement discrimination. Closely spaced individually calibrated Hall-effect sensors measure the magnetic flux and record MFL 

leakage caused by anomalies in Gauss units. A typical ¼” nominal sensor spacing provides for a true High Resolution inspection 

result. All tools are articulated for short capsule length to achieve bend radius of 1.5 D. The versatility of adding capsules or 

removing capsules allows the recording life of the tool to be changed with batteries to meet most pipeline lengths. Up-to-date 

computer hardware and components, flash memories and signal conditioning electronics record the signals from the sensors in 

full, without any filtering criteria, to allow for best post-run signal analysis and comparison with future runs. ID /OD Sensors 

discriminate between internal and external anomalies. Each signal captured by the sensor is compared with the signals captured 

with the array of Hall-effect sensors monitoring the total body wall response to the magnetic field. 


http://www.pipeway.com/skins/pipeway/standard.aspx?elid=82


Magnetic Flux Leakage (MFL) testing is a widely used, Non-Destructive Testing (NDT) method for the detection of corrosion and 

pitting in steel structures. MFL is often used for integrity assessment of pipelines and storage tanks, but the principle can be 

applied to assets in any industrial sector. 

The Principle 

The basic principle behind MFL involves magnetizing a ferrous metal object to saturation level with a powerful magnetic field. 

Where the object has no flaws, the magnetic flux will remain undisturbed. High magnetization levels are required to differentiate 

corrosion from other pipeline features such as hard spots, stress and strain variations and to minimize the effects of remnant 

magnetization and velocity. Where there is internal or external metal loss, the magnetic flux leaks from the object. In the MFL 

testing device, a magnetic sensor is placed between the poles of a magnet yoke to record the leakage field by Hall-effect sensors. 

Eddy current sensors integrated in the magnetic flux sensors are used to improve the differentiation between internal and external 

defects. 

Applications 

MFL is used to detect metal loss defects (such as corrosion) in a wide range of settings. 


http://www.rosen-group.com/global/company/explore/we-can/technologies/measurement/mfl.html

Remote Field Testing RFT 

Remote field testing should not be looked at as a typical eddy current test. There 

are papers and other reference materials that include remote field eddy current, 

however, to prevent confusion on the range of applications and material test 

situations, the attempt is being made to phase out that terminology. Both 

American Society for Testing and Materials (ASTM) and American Society of 

Mechanical Engineers (ASME) have remote field testing listed as a specific 

technique within electromagnetic testing. 

For the purpose of generic discussion this book will discuss remote field testing 

as it applies to inspection of ferromagnetic tubing in various heat exchangers. 

Remote field testing is an electromagnetic test that utilizes an alternating current 

excitation source. This alternating current electromagnetic energy travels along 

the htbe wall for some distance in both directions from an exciter coil. The 

distribution of the primary Held is dependent on the magnetic properties of the 

tube, the tube wall thickness and the presence of surrounding support structures. 

The transmitted field may be affected by discontinuities within the tube wall or 

support structures on the tube outside cliameter. The changes in the strength 

(amplitude) and phase shift or phase angle of the received signal are measured a 

few tube diameters away from the exciter coil. 


Special hybrid (driver/pick up) coils are necessary to perform remote field 

testing inspections. Because of the need for a significant spacing between the 

exciter coil(s) and the receiver or pick up coils the probes tend to be longer 

that the typical eddy current probe. Remote field testing probe types are 

shown in Figure 8.3. The high magnetic permeability of ferromagnetic 

materials dramatically impacts standard eddy current testing inspection 

techniques. Some electromagnetic testing techniques attempt to compensate 

for and/ or suppress the permeability effects by the use of strong magnets or 

direct current chiven saturation coils. The remote field testing RFT process 

requires no magnetic saturation. Instead it makes use of the natural 

tendency of ferromagnetic materials to channel magnetic energy. Like the 

keeper of a horseshoe magnet, the magnetic lines of flux from the exciter coil 

take the path of least reluctance. They will flow down the tube wall; which 

acts as a wave guide, for a considerable distance. 

Probes tend to be 

longer that the typical 

eddy current probe 


Figure 8.3: Remote field testing probe types 

From top to bottom: Larger diameter tubing with either single or dual exciters, smaller diameter 

tubing and boiler tubing. 


At distances in excess of two tube diameters from the internal exciter coil, the flux field 

has become homogenous and the passive receiver coils, positioned two to three tube 

diameters away from the exciter, receive practically all of their energy from the flux in 

the tube wall. The direct field from the exciter has been almost completely attenuated, 

or absorbed by the tube wall, and the external field is actually stronger than the field 

inside the tube. 

Through transmission is a term that is often used to describe the remote field testing 

process. This term normally implies that there is a source of energy that transmits 

through a medium. For example through transmission,. in both eddy current and 

ultrasonic testing, implies that the power source is on one side of the test product and 

the receiver element is on the opposite side of the material (through wall). In remote 

field testing some of the alternating current primary magnetic energy does extend to 

the outside diameter of the tube. It travels down the tube wall and eventually 

propagates back through the tube to the tube inside diameter. 

The concept of calling a remote field testing test a through wall technique may be hard 

to visualize, but the energy path is actually twice through the wall; once out at the 

exciter and then in at the detector. It is for this reason that short discontinuities show 

two distinct signals when the exciter and detector pass the discontinuity at different 

moments in time. The short discontinuity has interrupted the through transmission 

path twice. 


In remote field testing inspection of tubing it is probably more accurate to look 

at the tube wall as a conduit or wave guide. Magnetic fields are modeled as 

closed loops. The following graphic shows the magnetic flux lines traveling 

out from the exciter coil (at 0 in Figure 8.4), mixing with incoming exciter 

energy in a transition zone (one to two diameters) and finally becoming 

homogenous in the remote field zone (two to three diameters) where the 

detector should be located. 

The main concern is to determine where along the length of the tube the 

primary magnetic flux lines will reverse their direction and start their return 

path back to the driver coil. It is at that point on the tube inside diameter that 

the remote field testing pick up coils should be placed. The driver or exciter 

coil supplie.s a low frequency alternating current magnetic field which couples 

to the tube wall. Electromagnetic induction occurs twice. In the near field or 

direct coupled zone, eddy currents are created in the tube wall. These 

actually decrease the efficiency of the process. Eddy currents are also 

created through induction as the field flux lines cut across the pickup coils on 

reentering the tube inside diameter. 


Figure 8.4: Remote field testing energy distribution 


By making careful measurements it is possible to map the strength and 

distribution of the driver coil's flux density as it travels down the tube wall. A 

graph can be generated, such as Figure 8.4, using experimental data that 

shows there are three distinct areas of interest. In an attempt to define the 

variations in the alternating current energy distributions that are present in the 

tube wall the following terminology has been developed: 

• Near Field (direct coupled) Zone - (0-1.5 tube diameters from the driver 

coil) 

• Transition Zone - (1.5-2 tube diameters from the driver coil) 

• Remote Field Zone - (2-3 tube diameters from the driver coil) 


Remote-Field Energy Zones - in remote field testing. Profiles of B field just inside and 

outside pipe wall are used to indicate direct field region, transition and remote field zones. 


Remote-Field Zone 


http://www.ndt.net/article/v04n08/krzywosz/krzywosz.htm

Remote-Field Zone 

Transition Zone 


http://www.ndt.net/article/v04n08/krzywosz/krzywosz.htm

Near Field Zone - Within the near field zone the eddy currents generated in 

the tube wall by the alternating current driven exciter coil create a shielding 

effect of the exciter's flux. As eddy currents propagate through the material's 

inner wall, an opposing secondary magnetic flux is developed in the material 

that attenuates the primary field strength and limits its extension. Logically, 

the near zone would be the area where there is the greatest sensitivity to 

discontinuities because of the high concentration of magnetic flux. However 

the field tends to be concentrated near the inner surface of the tube, next to 

the exciter and this strong field tends to mask any signals from the tube 

outside diameter which are much weaker. In remote field testing the pickup 

coils are placed at some distance away from the exciter coil in an effort to get 

outside the high internal field area of the near field zone. 


RFT - Flux distribution in pipe (a) 0 rad (0 deg); 

Legend 

IS = inside surface 

OS = outside surface 

PA = pipe axis 


Transition Zone -The region just outside the near field zone is known as the 

transition zone. It is an area that is currently not considered to contain reliable 

data because the location of the transition zone changes with changes in wall 

thickness, permeability and conductivity. In this zone there is a great deal of 

interaction between the flux of one field that is diffusing outward from the 

exciter and the flux of the returning energy that is diffusing inward from the 

outside surface of the tube. The total or resultant field strength in this 

area · tends to be weaker because of the negative interaction of fields with 

differing directional characteristics. When the two opposing fields meet, the 

result is a cancellation of some of their respective energy. 


Remote Field Zone - The third definable region starts to occur at about two 

tube diameters from the exciter coil. The detector coil's signal amplitude 

bottoms out at the base of the logarithmic curve and starts a linear decay. 

Notice that the curves (Figure 8.4) describing signal amplitudes of the inner 

and outer walls parallel each other and are linear after peaking at maximum 

values. Considering the rate of attenuation of the inner wall field strength, the 

result is that in the area where the remote field zone starts, the outer wall field 

strength can be 10 to 100 times the strength of the inner wall field. 


Phase - The phase change of the signals detected at the pick up coil can be 

used to estimate the loss of wall. A thinner wall allows the flux traversing the 

wall to arrive at the detector sooner (similar to the time of flight of ultrasonic 

testing signals). Discontinuities of differing depths can be evaluated 

accurately based on measured phase shift information. In eddy current testing 

there is a well defined difference in phase angle responses for inside 

diameter and outside diameter events; however, in remote field testing data 

inside diameter and outside diameter discontinuities of the same depth will 

have about the same phase angle. 


Amplitude (voltage) - The remote field testing system senses a decrease in 

wall thickness as a stronger alternating current magnetic field cutting across 

the pick up coil. This induces a stronger voltage in the coil. Discontinuities of 

larger volume increase the amplitude of the signal while smaller volume 

discontinuities produce small amplitude signals, but the signal phase still 

represents the wall loss at the discontinuity. Signal location (at or near a 

support versus in free span tube) goes a long way to assisting in signal 

interpretation. The use of specialized voltage dependent phase analysis 

curves can also improve discontinuity resolution. Because some of the 

primary magnetic field extends out beyond the tube outside diameter tube 

support plates or baffles interfere with the magnetic field distribution. Any 

metallic material on the tube outside diameter will tend to block the energy 

transfer down the length of the tube. Because of the spacing between exciter 

and pick up coils this could lead to decreased sensitivity at these locations. 

Remote field testing is capable of detecting both small and large volume 

discontinuities in most ferromagnetic tubing found in a wide range of tubes 

and pipes such as heat exchangers, boilers, piping and pipelines. Some 

limitations do exist, for example in fin fan tubing found in air fin coolers. 


The base tubing is carbon steel, however to improve heat transfer rates, large 

diameter fins of high conductivity metal (normally aluminum) are installed on 

the tube outside diameter. The induced energies in the fins themselves 

prevent the primary magnetic field distribution along the outside diameter 

surface of the tube which dramatically limits the remote field testing inspection 

process. ASTM E-2096 is a good reference document for anyone considering 

remote field testing applications. It references remote field testing technology 

as well as personnel training criteria. It provides a guide to the types of 

minimum detection capability that should be demonstrated by inspection 

personnel when they apply the proper tools and techniques while performing 

remote field testing examinations. 


RFT 


http://www.mdpi.com/1424-8220/14/12/24098/htm

Remote Field Eddy Current Technique (RFT) 

This process is well adapted to the inspection of small-bore ferromagnetic tubes such as carbon steel. Using 

electromagnetic techniques this is now the industry standard inspection for boilers and heat exchangers due to 

its low frequency (typically 50-1000Hz). The probe consists of two coils in a send-receive configuration which 

are inserted into the tube. The energized exciter coil transmits a signal to the detector coil located some 

distance away. This signal passes through to the outside tube wall returning to arrive at the detector coil. With 

wall thinning there is less shielding hence the return time (greater phase) and attenuation (greater amplitude) is 

shorter. Phase and amplitude traces are generated as the probe is pulled through the tube as recorded data 

identifies the metal loss. Flaw sizing is also possible with RFT enabling depth, length and circumference to be 

accurately calibrated. 


http://www.itcl.org.uk/itcl-services/tube-inspection/

The Remote Field Technique is an electromagnetic examination, which utilizes a through-transmission process. 

The resultant field is affected by either ID or OD tube wall anomalies. RFT signal measurements are made a 

few tube diameters away from the AC excitation coil without any attempt at tube wall magnetization or 

saturation. A pair of pick-up coils located in the remote field zone measures the resultant field to give both a 

differential and an absolute signal. The signal phase and amplitude information is used to determine defect 

depth and volume 






http://www.olympus-ims.com/en/ms-5800-tube-inspection/

RFT- Tube Cleanliness 

Tube Cleanliness is as important for the process reasons (i.e. heat transfer) as it is for the 

Remote Field inspection. Inspections that go the smoothest are ones where the tubes are 

adequately cleaned prior to the inspection. Not only does this save inspection time and money, 

but the data acquired from clean tubes VS dirty tubes make the inspection much more accurate. 

Non-relevant indications can occur from Iron deposits, calcium deposits, etc. These non-relevant 

indications can mask real defects located underneath. 

So how can you tell when the tubes are cleaned enough for a Remote Field inspection? We 

have developed a “Dummy” probe chart that customers can use to build probe heads to check 

for tube cleanliness. These probes can be made to screw on to hydro-blasters lance’s and used 

after the cleaning process is complete to make sure there is proper clearance for the Eddy 

Current probe. 

Final Reports - After the inspections and final data analysis is completed a formal report is 

generated showing a tube sheet diagram with the tubes inspected color coded to a percentage 

wall loss. Additional tube sheet diagrams can be generated showing the worst case scenarios 

for tube plugging or selective re-tubing. In addition to this information our reporting format can 

generate corrosion rates, and a projection based on the established corrosion rates. 


http://www.techcorr.com/services/Inspection-and-Testing/Remote-Field-Testing.cfm

More Reading: RFT 

Remote Field Testing or "RFT" is one of several electromagnetic testing methods commonly employed 

in the field of nondestructive testing. Other electromagnetic inspection methods include magnetic flux 

leakage, conventional eddy current and alternating current field measurement testing. Remote field 

testing is associated with eddy current testing and the term "Remote Field Eddy Current Testing" is 

often used when describing remote field testing. However, there are several major differences between 

eddy current testing and remote field testing which will be noted in this section. 

RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have 

difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic 

materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm 

thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to 

achieve the adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency 

results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be 

increased by the use of partial saturation eddy current probes, magnetically biased probes, and pulsed 

saturation probes. However, because of the large volume of metal present as well as potential 

permeability variations within the product, these specialized eddy current probes are still limited in their 

inspection capabilities. 

The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use 

of the remote field testing method. The RFT method has the advantage of allowing nearly equal 

sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is 

highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes 

between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is 

generally considered to be less sensitive than conventional eddy current techniques when inspecting 

nonferromagnetic materials. 


https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/RFT/RFT_Intro.htmc

RFT Theory of Operation 

A probe consisting of an exciter coil and one or more detectors is pulled through the tube. The exciter coil and 

the detector coil(s) are rigidly fixed at an axial distance of two tube diameters or more between them. The 

exciter coil is driven with a relatively low frequency sinusoidal current to produce a magnetic field. 

axial magnetic flux induced 

perpendicularly circumferential eddy currents 

This changing magnetic field induces strong circumferential eddy currents which extend axially, as well as 

radially in the tube wall. 



These eddy currents, in turn, produce their own magnetic field, which opposes the 

magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect 

inductive coupling, the magnetic field from the eddy currents does not fully 

counterbalance the magnetic exciting field. However, since the eddy current field is 

more spread out than the exciter field, the magnetic field from the eddy currents 

extends farther along the tube axis. The interaction between the two fields is fairly 

complex but the simple fact is that the exciter field is dominant near the exciter coil 

and the eddy current field becomes dominant at some distance away from the exciter 

coil. 

Eddy current field 

Exciter field 



The receiving coils are positioned at a distance where the magnetic field from the 

eddy currents is dominant. In other words, they are placed at a distance where they 

are unaffected by the magnetic field from the exciter coil but can still adequately 

measure the field strength from the secondary magnetic field. Electromagnetic 

induction occurs as the changing magnetic field cuts across the pick-up coil array. By 

monitoring the consistency of the voltage induced in the pick-up coils one can monitor 

changes in the test specimen. The strength of the magnetic field at this distance from 

the excitation coil is fairly weak but it is sensitive to changes in the pipe wall from the 

I.D. to the O.D. 



The Zones 

Direct Couple Zone 

The region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy 

currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT 

inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil. 

Transition Zone 

The region just outside the direct couple zone is known as the transition zone. In this zone there is a great deal of interaction 

between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the 

interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" 

field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly on the ID due to the 

interaction of the fields with differing directional characteristics of the two fields. The receiver coil's signal phase, with respect to 

the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled 

and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it 

can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes 

dominate and the start of the remote field. 



Remote Field Zone 

The remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is 

negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic 

field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The 

amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 

1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as 

well as the inside diameter of the tube. 

1.65 tube diameters 



RFT Probes 

Probes for inspection of pipe and tubing are typically of the bobbin (ID) variety. These 

probes use either a single or dual excitation coil to develop an electromagnetic field 

through the pipe or tube. The excitation coils are driven by alternating current. The 

sensing coil or coils are located a few tube diameters away in the remote field zone. 

Probes can be used in differential or absolute modes for detection of general 

discontinuities, pitting, and variations from the I.D. in ferromagnetic tubing. To insure 

maximum sensitivity, each probe is specifically designed for the inside diameter, 

composition, and the wall thickness of a particular tube. 



RFT Instrumentation 

Instruments used for RFT inspection are often dual use eddy current / RFT instruments employing multifrequency 

technology. The excitation current from these instruments is passed on to the probe that contains an 

exciter coil, sometimes referred to as the driver coil. The receiving coil voltage is typically in the microvolt range, 

so an amplifier is required to boost the signal strength. 

Certain systems will incorporate a probe excitation method known as multiplexing. This utilizes an extreme high 

speed switching method that excites the probe at more than one frequency in sequence. Another method of coil 

excitation that may be used is simultaneous injection. In this coil stimulation technique, the exciter coil is 

excited with multiple frequencies at the same time while incorporating filter schemes that subtract aspects of 

the acquired data. The instrument monitors the pickup coils and passes the data to the display section of the 

instrument. Some systems are capable of recording the data to some type of storage device for later review. 



RFT Signal Interpretation 

The signals obtained with RFT are very similar to those obtained with conventional eddy current testing. When all the proper 

conditions are met, changes in the phase of the receiver signal with respect to the phase of the exciter voltage are directly 

proportional to the sum of the wall thickness within the inspection area. Localized changes in wall thickness result in phase and 

amplitude changes. These changes can be indicative of defects such as cracks, corrosion pitting or corrosion/erosion thinning. 



RFT Signal Interpretation 



RFT Reference Standards 

Reference standards for the RFT inspection of tubular products come in many variations. In order to produce 

reliable and consistent test results, the material used for manufacturing calibration standards must closely 

match the physical and chemical properties of the inspection specimen. Some of the important properties that 

must be considered include conductivity, permeability and alloy content. In addition, tube dimensions including 

I.D., O.D. and wall thickness must also be controlled. 

The type of damage mechanisms that are expected to be encountered must also be carefully considered when 

developing or selecting a reference standard. In order to get accurate quantitative data, artificial discontinuity 

conditions are typically machined into the standards that will closely match those conditions that may be found 

in the tubing bundle. 



Chapter 8 



Answers 


Q.8.1 Which of the following electromagnetic testing techniques does not use an 

alternating current coil excitation process? 

A. alternating current field measurement 

B. eddy current testing 

C. flux leakage testing 

D. remote field testing 

Q.8.2 Which of the following electromagnetic testing techniques should provide the 

best discontinuity depth and length sizing capability for cracks in ferromagnetic 

weldments? 





Q.8.3 Which of the following techniques should perform best in nonferromagnetic 

materials? 






Q.8.4 A generally accepted definition of remote field testing is: 

A. electromagnetic testing done at remote locations. 

B. the electromagnetic field which has been transmitted through the test 

object and is observable beyond the direct coupling of the exciter. 

C. through transmission eddy currents, detected on the far side of a material 

or object under test by a remote receiver coil. 

D. the opposite of direct field. 

Q.8.5 When a nonferromagnetic tube is inspected with a self-comparison 

differential encircling coil arrangement a non-detection could occur when a 

discontinuity is: 

A. filled with water. 

B. deep but very narrow. 

C. long with slowly varying depth. 

D. short and wide. 


Q.8.6 The most common electromagnetic testing technique used to locate 

corrosion thinning in large diameter cross country piping systems would be: 

A. alternating current field measurement. 

B. eddy current testing. 

C. flux leakage testing. 

D. remote field testing. 

Q.8.7 Considering the full range of typical probe designs currently in use, in 

which of the following electromagnetic testing techniques could the term 

passive receivers be used? 





E. All of the above. 


Q.8.8 The region of intense electromagnetic interaction at the interface 

between an alternating current coil's outside diameter surface and a tube 

wall's inside diameter surface is called the: 

A. direct couple zone. 

B. fresnel zone. 

C. near field zone. 

D. Both A and C. 

E. None of the above. 

Q.8.9 The operating frequencies that are selected to perform remote field 

testing inspections are: 

A. usually higher than those used in conventional eddy current tests. 

B. usually lower than those used in conventional eddy current tests. 

C. identical to those used in conventional eddy current tests. 

D. about one half of those used in conventional eddy current tests. 

• Near Field (direct coupled) Zone - (0-1.5 tube diameters from the 

driver coil) 

• Transition Zone - (1.5-2 tube diameters from the driver coil) 

• Remote Field Zone - (2-3 tube diameters from the driver coil) 


Q.8.10 The amplitude or voltage of the detected response from a discontinuity 

is most often related to: 

A. the width of the discontinuity. 

B. the location of the discontinuity. 

C. the depth of the discontinuity. 

D. the volume of the discontinuity. 


Chapter 9 

Eddy Current Procedures, Standards 

and Specifications 


Procedures, specifications and standards are produced to provide a means of 

controlling product or service quality. Written instructions that guide a company or 

individual to a desired end result and are acceptable to industry, are the basis of 

procedures, specifications and standards. Many publications are available to 

guide or instruct us. Some of the most frequently used references are the 

American Society for Testing and Materials (ASTM), American Society of 

Mechanical Engineers (ASME), American National Standards Institute (ANSI) 

and Military Standards (MIL-STD-XXXX). 

These publications are laboriously produced by committees made up of scientific 

and technical people. Usually after a committee produces a draft document, it is 

submitted to industry and the scientific community for comment and subsequent 

revision. In certain cases, standards combine to assist each other. 

As an example, ASME Section V Article 8- Appendix IV uses ASTM E1316 to 

provide Standard Terminology for Nondestructive Testing. The military standard, 

MIL-STD-1537C Electrical Conductivity Test for Verification of Heat Treatment of 

Aluminum Alloys, Eddy Current Method, references ASTM B193 Resistivity of 

Electrical Conductor Materials and ASTM E18 Rockwell Hardness and Rockwell 

Superficial Hardness of Metallic Materials. 


American Society for Testing and Materials 

American Society for Testing and Materials (ASTM) standards (practices or guides) usually 

include in the written instructions headings such as scope, referenced documents, terminology, 

significance and use, basis of application, apparatus, reference standards, standardization, 

procedure and keywords. Scope makes a general statement about the document's applicability 

and intent. Referenced Documents refers to other publications used as references within the 

standard. The terminology section usually may contain definitions of unique terms specific to the 

equipment or examination covered by the standard. Significance and Use is a more detailed 

discussion of test results and probable causes of indications expected during the examination. 

The Basis of Application section identifies items which are subject to contractual agreement 

between the parties using or referencing the standard such as personnel qualification, 

qualification of nondestructive testing agencies, procedures and techniques, surface preparation, 

timing of examination, extent of examination, reporting criteria/ acceptance criteria, 

reexamination of repaired/reworked items. Apparatus describes the general requirements for the 

inspection system including instrumentation, coils, positioning and driving mechanisms. The 

fabrication requirements for artificial discontinuity standards used for standardization are 

discussed under reference standards. A discussion of the reference specimen and the 

geometrical requirements of the artificial discontinuities in it is usually included. Standardization 

provides instructions for adjustment of the apparatus used for the examination. The response to 

known discontinuities in the reference standard is usually described in this section. Detailed 

instructions to process the inspection appears under procedure. These instructions may include 

acceptance limits and the handling of components that are not acceptable. 


ASTM publishes several standards pertaining to the eddy current method. 

These standards are numbered; for example: 

• E 571- 98. "E 571" refers to the standard and "98" refers to the year of 

revision. 

Some ASTM standards that pertain to the eddy current method are: 

• E 215 Standard Practice for Standardizing Equipment for Electromagnetic 

Examination of Seamless Aluminum-Alloy Tube. 

• E 243 Standard Practice for Electromagnetic (Eddy Current) Examination 

of Copper and Copper-Alloy Tubes. 

• E 426 Electromagnetic (Eddy-Current) Testing of Seamless and Welded 

Tubular Products, Austenitic Stainless Steel and Similar Alloys. 

• E 571 Standard Practice for Electromagnetic (Eddy Current) Examination 

of Nickel and Nickel Alloy; Tubular Products. 

• E 690 Standard Practice for In Situ Electromagnetic (Eddy-Current) 

Examination of Nonmagnetic Heat Exchanger Tubes. 

• E 1316 Standard Terminology for Nondestructive Testing. 


Military Standard 

The United States Military uses the Military Standard document to control testing and 

materials. Standard procedures are provided by a series of MIL-STD-XXXXX 

documents. Special requirements are specified by the Military Specification system. 

For example, MIL-STD- 537C refers to Electrical Conductivity Test for Verification of 

Heat Treatment of Aluminum Alloys, Eddy Current Method. The Calibration System 

Requirements for MIL-STD-1537C are contained in Military Specification MIL- -45662. 

The MIL-STD usually contains several parts and is very descriptive. These parts 

normally include Scope, Applicable Documents, Definitions, General Requirements, 

Detail Requirements and Notes. The Scope contains a general statement of 

applicability and intent of the Standard. 

Applicable Documents pertains to other reference or controlling documents such as 

other MIL-STD, Military Specification or ASTM publications. Definition contains precise 

definitions of key words and phrases used in the Standard. Under General 

Requirements, equipment, reference specimen and personnel requirements are 

described in sufficient detail to implement the Standard. Included in this part is 

instrument sensitivity and response, test object variables, reference specimen 

requirements and personnel qualification requirements. Detail Requirements 

describes the specific procedure to implement the Standard. Notes contains pertinent 

statements about the process and guidelines for reporting results. 


American Society of Mechanical Engineers 

In 1911 the American Society of Mechanical Engineers (ASME) set up a committee to establish 

rules of safety for design, fabrication and inspection of boilers and pressure vessels. These rules 

have become known throughout industry as the ASME code. The ASME Boiler and Pressure 

Vessel Committee is a large group from industry and the scientific community. The Committee 

has many subcommittees, subgroups and working groups. Each subcommittee, subgroup and 

working group combines as a unit for a specific area of interest. For example, the 

Subcommittee on Pressure Vessels (SC VIII) has two working groups and five subgroups 

reporting to it. The purpose of these groups is to interface with industry to keep pace with 

changing requirements and needs of industry and public safety. 

The ASME Boiler and Pressure Vessel Code is divided into 11 sections. ASME Section V, 

Nondestructive Examination/ is divided into two subsections, A and B. Subsection A deals with 

Nondestructive Methods of Examination. Article 8 is Eddy Current Examination of Tubular 

Products. Subsection B is Documents Adopted by Section V. Eddy current standards are 

described in Article 26. In this case, the ASTM E215 document has been adopted by ASME and 

reassigned the designation SE215. ASME Section V, Article 8, Appendix I gives detailed 

procedure requirements for Eddy Current Examination Method for Installed Nonferromagnetic 

Heat Exchanger Tubing. A procedure designed to meet this requirement can be illustrated by the 

following example, Document QA 3. 


EDDY CURRENT INSPECTION OF NONFERROUS TUBING BY SINGLE FREQUENCY 

TECHNIQUES 

Procedure No. QA 311-1 

A. PURPOSE 

This procedure describes the equipment and methods as well as the personnel 

qualifications to be utilized for the performance of the eddy current examination of steam 

generator tubes. It meets the requirements of the NRC Regulatory Guide 1.83, ASME 

Section XI, Appendix IV and ASME Section V, Article 8 of the ASME Boiler and Pressure 

Vessel Code. 

B. SCOPE 

The scope of the examination to be performed is contained in the eddy current inspection 

program document applicable to the specific plant to be inspected. 

C. PREREQUISITES 

1 . Plant Condition 

The plant must be shut down with the primary system drained. The steam generators shall 

be open on the primary side for access to the channel head and the shell cool down 

sequence shall be complete. Air movers shall be attached to circulate air through the 

generator to dry the tube sheet. 


2. Equipment 

The examinations shall be performed utilizing an XXXX/XX multifrequency eddy current instrument with bobbin 

coil probes designed for testing from the inside of the tubes. The inspection performance shall be monitored by 

the use of a phase sensitive vector display and recorded for later evaluation. 

a. Equipment utilized shall be: 

i. XXXX/XX eddy current instrument. 

ii. Bobbin coil probes capable of operation in the differential and absolute modes. 

iii. Digital recording device(s). 

iv. Communications system. 

v. Reference standard The reference standard shall be manufactured from a length of tubing of the same 

size and type of material that is to be examined in the vessel. The standard shall contain 6 intentional 

discontinuity areas as follows: 

aa. 100% through the wall drill hole (0.052 in. for 0.750 in. outside diameter tubing and smaller, and 0.067 in. 

for larger tubing). 

bb. Flat bottomed drill hole 5/64 in. diameter X 80% through from the outer tube wall surface. 

cc. Flat bottomed drill hole 7/64 in. diameter X 60% through from the outer tube wall surface. 

dd. Flat bottomed drill hole 3/16 in. X 40% diameter through from the outer tube wall surface. 

ee. Four flat bottom holes, 3/16 in. diameter, spaced 90 degrees apart around the tube circumference, 20% 

through the tube wall. 

ff. Circumferential groove 20% deep by 1/16 in. long by 360 degrees on the inside tube wall surface. 

gg. Circumferential groove 10% deep by 1/8 in. long !jy 13,60 degrees on the outer tube wall surface. 

hh. Each standard shall be identified by a serial number etched on one end and be traceable to the master 

standard stored at the facility. 


. Probe positioning and feeding shall be accomplished remotely for in-service inspection. Baseline 

inspection may be done manually. 

c. Personnel communications devices shall be provided. 

3. Personnel Qualifications 

Personnel collecting data in accordance with this procedure shall be qualified to Level! or higher in 

accordance with Document QA 101. Personnel interpreting data collected in accordance with 

procedure shall be qualified to Level IIA or higher in accordance with Document QA 101. Prior to 

receiving a certification, the applicants shall have completed the program recommended by SNT-TC-1A 

(1984 edition}, Supplement E. 

D. PRECAUTIONS 

1. All personnel to be engaged in eddy current inspection programs at operating plants shall have 

received instructions in and understand the radiation protection rules and guidelines in effect on the 

plant site. 

2. All personnel to be engaged in the test program shall wear protective clothing to the extent of the 

type defined by the exclusion area work permit. 

3. All personnel entering a radiation work area will have proven their ability to work in a face mask by 

successfully passing the pulmonary function test during their annual physical. 

4. No entries shall be made into the steam generator channel head without the presence of a qualified 

health physics technician. 

5. Ensure that nozzle covers (when applicable) are securely in place inside the vessel before 

commencement of the eddy current inspection program. 


E. PERFORMANCE 

1 . Preparation 

a. Establish location of data acquisition control center. 

b. Arrange power distribution at data acquisition control center. 

c. Install communications system control box at the data acquisition control center. 

d. Establish communication with one or more headsets at the steam generator. 

e. Install XXXX!XX eddy current test instrument, pusher puller and fixture control boxes as the steam generator. 

f. Install remote digital data acquisition computers and recording devices at the data acquisition control center. 

2. Equipment Calibration 

a. Prior to the commencement of the eddy current examination, of the steam generator tubes and after the 

replacement of any component, the equipment shall be calibrated in accordance with the following steps: 

Insert the reference bobbin coil probe into a reference standard. 

i. Insert the test bobbin coil probe into a section of the reference standard, which is tree of discontinuities. 

ii. Select the desired frequencies as per the Site Specific Data Acquisition Procedure. 

iii. Select the probe drive voltage and channel gain as per the Site Specific Data Acquisition Procedure. 

iv. Perform a hardware null. 

v. Remotely pull the test probe through the reference standard at the speed selected for actual testing in the heat 

exchanger. Data from the heat exchanger will also be acquired on the pull unless noted. 

vi. Set the display sensitivity setting for each channel per the site specific calibration procedures. 

vii. Set the rotation (phase) value so that the probe motion signals in the discontinuity sensitive differential channels 

are horizontal (as per the specific calibration procedure) with the first lobe of the 100% through the wall drill hole 

going down first as the probe is withdrawn from the standard. 

viii. Set the rotation (phase) value so that the probe motion signals in the discontinuity sensitive absolute channels are 

horizontal (as per the specific calibration procedure) with the response of the 100% through the wall drill hole 

going up as the probe is withdrawn from the reference standard. 

ix. Complete the digital calibration summary form, update it with all pertinent information and store this information to 

the selected digital storage device. 


3. Tube Inspection General 

(Refer to Site Specific Calibration Procedure QA 2) 

a. Eddy current inspection activities shall be performed with equipment sensitivities and speeds set 

per the Site Specific Data Acquisition Procedure. 

b. Visual verification of the identity of the specific tube being inspected shall be performed before and 

after each fixture change and at the beginning and end of each row or column. Verification of the 

positive identification of tube location shall be noted by a digitally recorded message. 

c. Should the performance of the tube identity verification reveal an error has occurred in the 

recording of probe location, all tubes examined because the previous verification of location shall 

be reexamined. 

d. The equipment calibration shall be verified and recorded at the beginning and end of each 

calibration cycle. At a minimum, the calibration will be verified at 4 hours intervals and after any 

equipment change. 

e. Should the equipment be found to be out of calibration, the equipment will be recalibrated as per 

Section E-2 of this procedure. The data interpreter will determine if it is necessary to re-inspect any 

of the tubes. 

4. Tube Inspection Manual 

a. The data recording shall be made during probe withdrawal. Withdrawal speed is 14 in. per second 

maximum. No minimum speed specification is required, but a good uniform pull of 12 in. per second 

is preferred. 

b. Because no inspection is performed during probe insertion, the speed may be as rapid as possible. 

c. Due to radiation exposure probe pusher/pullers should be used to facilitate the inspection. 


5. Tube Inspection Automatic Remote 

NOTE: Ensure that all probe positioner, probe feeder and probe and communication connecting cables 

are clear of access walkways and secured to available supports. 

a. Install remotely operated probe feeder local to steam generator. 

b. Check the operation of the remotely operated eddy current positioner and connect the flexible 

probe conduits to the probe guide tube and the probe pusher. 

c. Install remotely operated probe positioner on the manway or the tube sheet of the steam generator 

to provide coverage of the area to be examined. 

d. Connect power and air supply lines to remote hardware as required. 

e. Verify the correct operation and control of the remotely operated platform hardware. 

f. Operate the position er to locate the probe beneath the tube to be examined. 

g. If probe insertion is to be done manually, utilize the probe pusher controls to feed the probe into 

and up the tube to the desired height. Monitor the extent of insertion by reference to impedance 

signals from known tube reference locations (tube end, top of tube sheet, supports) on the display 

screen. 

h. If operating in the Auto Acquire mode, verify that the proper landmark tables have been installed, 

axial encoders are functioning properly and that the correct voltage thresholds have been 

established for auto locate of supports and tube ends. 

i. If performing manually or automatically ensure that the tube alphanumeric identifier has been 

properly updated. Monitor the withdrawal of the probe from the tube until the impedance signal on 

the screen indicates that the probe is clear of the tube sheet. Concurrent with the probe withdrawal, 

visually monitor the signals on the display screen while recording all data in real time. 

j. Reposition the probe beneath the next tube selected for examination. 

k. Repeat the procedures described in the preceding steps until all the tubes selected for inspection 

have been examined. 


F. INSPECTION RESULTS AND DOCUMENTATION 

1 . Requirements 

a. The data interpreter shall be certified to Level IIA or IIIA as per Procedure QA 101. 

b. Data shall be collected with an eddy current test system with a current certification 

of calibration as per CSP procedure. 

c. The data collection system shall be calibrated with an approved reference 

standard that is serialized and traceable to a master reference standard. 

d. The identify of the plant site, the steam generator, the operators name and 

certification, the date, the test frequencies, the reference standard serial numbers, 

equipment serial numbers and certification dates, software revisions and probes 

design and serial number shall be recorded at the start of each calibration cycle. 

e. data collection station shall be set up and calibrated as per Procedure OA 3. 


2. . Performance 

a. The data interpreter shall: 

i. Determine that all tubes selected for inspection have been tested. 

ii. Report tubes whose data are incomplete or un-interpretable. 

iii. 

iv. 

Require a retest of any tubes exhibiting excessive noise or unusual responses. 

ln-service inspections 

a. Report all discontinuities > 19%. 

b. Report all other indications that appear to be relevant. 

c. Identify the axial position of all indications with respect to a known structural member. 

v. Pre-service inspections 

a. Report all indications observed. Include the axial position of the indication with respect 

to a known structural member. 

b. Interpretation 

i. All data shall be reported on a digital Final Report form. 

ii. The conversion from signal phase angles (or amplitudes) to discontinuity depths shall be 

accomplished per calibration curves established on the appropriate channels using the 

calibration standards and techniques defined in the site specific data analysis specifications. 

iii. All data shall be reviewed in its entirety. 

iv. Any abnormal signals observed shall be reported. 


G. REFERENCES 

The following documents or files are required for the performance of eddy 

current inspection programs utilizing the methods described in this procedure. 

1. Required Documentation 

a. Eddy current inspection specific calibration procedure documents applicable to the plant to 

be inspected. 

b. Inspection plans showing tube sheet maps marked to designate the extent of examination to 

be performed and extent of completion. 

c. Final Reports including all indications resolved by the Data Resolution Analyst. 


Chapter 9 



Answers 


Q.9.1 A precise statement of a set of requirements to be satisfied by a 

material, product, system or service is a: 

A. standard. 

B. specification. 

C. procedure. 

D. practice. 

Q.9.2 A statement that comprises one or more terms with explanation is a: 

A. practice. 

B. classification. 

C. definition. 

D. proposal. 

Q.9.3 A general statement of applicability and intent is usually presented in 

the of a _____ standard? 

A. summary 

B. scope 

C. significance 

D. procedure 


Q.9.4 Military Standards are designated by MIL-C-(number). 

A. True 

MIL-STD-XXXXX 

B. False 

Q.9.5 In the structure of American Society of Mechanical Engineers (ASME) 

the subcommittee reports to the subgroup. 

A. True 

B. False 

Q.9.6 In example QA 3, personnel interpreting results must be: 

A. Level I or higher. 

B. Level II or higher. 

C. Level IIA or higher. 

D. Level Ill. 


Q.9.7 The prime artificial discontinuity used to calibrate the system described 

in QA 3 is: 

A. 20% inside diameter. 

B. 50% outside diameter. 

C. 100%. 

D. 50% inside diameter. 

Q.9.8 In QA 3, equipment calibration must be verified at least: 

A every hour. 

B. each day. 

C. every 4h. 

D. every 8 h. 

Q.9.9 QA 3 specifies a maximum probe traverse rate of: 

A. 305 mm/ s (12 in./ s). 

B. 355.6mm/s (14in./s). 

C. 152.4 mm/s (6 in./s). 

D. not specified. 


Q.9.10 The system in QA3 is calibrated with an approved standard that is 

traceable to: 

A. NBS. 

B. American Society of Mechanical Engineers (ASME). 

C. a master standard. 

D. American Society for Testing and Materials (ASTM). 

Q.9.11 In accordance with QA 3, a tube whose data are incomplete must be: 

A. reinspected. 

B. reported. 

C. reevaluated. 

D. removed from service. 


■ωσμ∙Ωπ∆º≠δ≤>η 


More Reading 

http://www.allaboutcircuits.com/vol_1/index.html 


Further Reading 


Discussion 

Subject: discuss on the standard requirements on the differences in frequency Hz 

used on specific applications for thickness checks and weld examination. 

BS EN 1711:2000 

6.4.2 Surface probes 

6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block 

To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument 

when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the 

structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the 

probes shall be clearly marked with their operating frequency range. (See Figure 1). 

6.4.2.2 Probes for weld examination 

For examination of ferritic welds, probes specially designed for this purpose shall be used. The probe assembly shall be 

differential, orthogonal, tangential or equivalent which is characterized by having a minimal dependency on variations in 

conductivity, permeability and lift off in the welded and heat-affected zones. The diameter of the probe shall be selected relative to 

the geometry of the component under test. Such probes shall be able to operate when covered by a thin layer on non-metallic 

wear-resistant material over the active face. If the probe is used with a cover, then the cover shall always be in place during 

calibration. The probe shall operate at a selected frequency in the range from 100 kHz to 1 MHz. 

Key 

• 1,2,3,4 Deflections representing variations of thickness of 

simulated coatings on calibration block 

• 5 Deflection representing material of calibration block 

• 6,7 Deflection representing range of material to be examined 

using calibration block 0 Balance 


BS EN 1711:2000

Discussion 

Subject: discuss on the standard requirements on the frequency used on the 

specific applications for thickness testing and defect detections. 

BS EN 1711:2000 

6.4.2 Surface probes 

6.4.2.1 Probes for measuring thickness of coating and material evaluation relative to calibration block 

To be acceptable for this purpose, the probe shall be capable of providing a full screen deflection lift off signal on the instrument 

when moved from an uncoated spot on a calibration block to a spot covered with the maximum coating thickness expected on the 

structure to be tested. The probe shall operate in absolute mode at a selected frequency in the range from 1 kHz to 1 MHz. All the 

probes shall be clearly marked with their operating frequency range. (See Figure 1). 

6.5.2 Procedure for examination of welds in ferritic materials 

6.5.2.1 Frequency 

The frequency shall be optimized with respect to the sensitivity, the lift off and other unwanted signals. Underusual conditions a 

frequency of about 100 kHz is recommended. 

Key 

• 1,2,3,4 Deflections representing variations of thickness of 

simulated coatings on calibration block 

• 5 Deflection representing material of calibration block 

• 6,7 Deflection representing range of material to be examined 

using calibration block 0 Balance 


BS EN 1711:2000


Comparison of OD and ID Eddy Current 

Inspection of Tubing 

Scope 

The Eddy Current test (ECT) test is the primary nondestructive test (NDT) used in 

tube mill certification testing for condenser, feedwater heater, and balance of plant 

(BOP) power generation tubing. 

When tested at the tube mill, the procedure is performed using encircling differential 

outside diameter (OD) coils. Such OD testing techniques are well accepted by industry 

and consumers alike. This technique has, in fact, been the standard tubing NDT 

practice for several decades and is incorporated into the ASME Boiler & Pressure 

Vessel Code. 

Details of this type of test and its advantages and limitations are defined in the HEI 

Tech Sheet #129. Although ultrasonic testing (UT), remote field testing (RFT) and flux 

leakage testing may be acceptable alternatives, they are only used when specified by 

the customer, or by a small number of product specifications such as ASTM B338. As 

a result, this document will only discuss the EC test. Once the tubing is manufactured, 

the owner or end user may specify an additional EC test using an ID probe. The 

purpose of this document is to describe the major differences between the two tests 

and what each is intended to accomplish. 


http://www.heatexchange.org/pdf/techsheets/TechSheet132.pdf

Inspection from the Tube OD 

Most ASTM and ASME tubular product specifications require a 

nondestructive electric test NDE. The NDE tests may include eddy current 

testing, ultrasonic testing, or flux leakage testing. The product specifications 

do not necessarily designate which of these three must be used, and unless 

agreed upon in the purchase order, the test choice is at the option of the tube 

producer. The test that is the quickest, with the highest reliability and provides 

good sensitivity for finding sharp, abrupt defects is the OD eddy current test. It 

is the overwhelming choice of both tube manufacturers and end users. 



The ASTM has developed recommended practices on how those tests may 

be performed. These include as follows: 

1. ASTM E309 / SE309 –Standard Practice for Eddy-Current Examination of 

Steel Tubular Products Using Magnetic Saturation 

2. ASTM E426 / SE426 –Standard Practice for Electromagnetic (Eddy- 

Current) Examination of Seamless and Welded Tubular Products, 

Austenitic Stainless Steel and Similar Alloys 

3. ASTM E571 / SE571 –Standard Practice for Electromagnetic (Eddy- 

Current) Examination of Nickel and Nickel Alloy Tubular Products 

4. ASTM E2096 - Standard Practice for In Situ Examination of Ferromagnetic 

Heat-Exchanger Tubes Using Remote Field Testing 

5. ASTM E-690 / ASTM E690 - Standard Practice for In Situ Electromagnetic 

(Eddy-Current) Examination of Nonmagnetic Heat Exchanger Tubes 



Approximately 25 years ago, the ASTM A01.09/A01.10 NDE task group 

recognized that the “E” practices identified above did not have sufficient detail 

to ensure that tube mills were incorporating all of the necessary ASTM 

requirements. In addition, there was no validation that the procedures used at 

one manufacturing plant would provide similar test results as those at another 

mill. As a result, the ASTM developed a number of additional requirements 

which were then added into the general tubular product specifications. These 

additional requirements specified items such as calibration size & location 

(artificial defect size & type, i.e., drilled hole or notch), and calibration 

procedures to ensure consistent & repeatable results. These requirements 

also included training and certification of operators, signal to noise ratio 

recommendations, and required equipment calibration standards. The 

general specifications that include the additional requirements are as follows: 

■ 

ASTM A450 / A450M – Standard Specification for General Requirements for 

Carbon and Low Alloy Steel Tubes 

■ 

ASTM A1016/A1016M - Standard Specification for General Requirements for 

Ferritic Alloy Steel, Austenitic Alloy Steel, and Stainless Steel Tubes 



The ASTM/ASME OD EC testing of tubes is usually accomplished using only one 

frequency, typically within a range of 25 KHz to 100 KHz. As the magnetic field 

penetrates the metal, the ability to receive a signal lessens or attenuates. This 

phenomenon is called standard depth of penetration or alternatively, the “skin effect”. 

The depth of penetration decreases with increasing frequency, conductivity and 

magnetic permeability. As a result, the signal returning from an imperfection near the 

OD will be stronger than an identically sized imperfection away from the OD surface. 

The specifications do not address the imperfection’s location. Rejection is typically 

decided on a go/no-go signal amplitude criteria from an artificial defect described in 

the general specification or in the supplementary requirements of the product 

specifications. 

Magnetic properties or anomalies can be created in non-magnetic materials through 

minor parent metal alloy excursions, manufacturing, welding, strain-induced cold work 

and other processes and may not be detected using conventional OD saturation. 

These signals are defined as anomalies or discontinuities and are not considered a 

manufacturing defect. This magnetic coupling is achieved by using encircling coils to 

create a saturating magnetic field. This magnetic saturation does not necessarily 

improve the testing sensitivity or repeatability but does allow penetration of eddy 

currents in magnetic materials. 



One additional advantage of OD ECT inspection is that the tubing can be fully 

magnetically saturated during testing to ensure the maximum level of 

sensitivity and repeatability, vastly reducing the occurrence of false 

indications on those materials which have ferromagnetic domains. As noted 

earlier, carbon and alloy steels, stainless steels and some nickel alloys may 

contain small magnetic domains that must be magnetically coupled during 

testing to minimize “noise” providing for “quiet” or higher signal to noise 

inspection with eddy currents. 

Even austenitic stainless steels which are considered to be non-magnetic, 

may have small magnetic regions from residual delta ferrite formed during the 

welding process or strain induced martensite from cold working. 

Special tube configurations such as integral ID and/or OD fins will require 

unique technologies that are not covered in this document but should be 

reviewed with the manufacturer prior to the onset of testing. 



Inspection from the tube ID 

ID eddy current testing employing internal probe coils was developed as an in-service 

or baseline inspection tool to identify tube damage, discontinuities or operational wear. 

This damage may include pitting, cracking, wear from vibration or abrasion and other 

environmentally induced mechanisms. The indications are identified using ID probes 

that are passed down the length of the tube on a tethered cable that is connected to 

specially designed equipment containing an alternating current power source and 

electronics for recording and analyzing the output. The probes can be designed with 

differential encircling coils highly sensitive in identifying sharp, abrupt or axial damage 

(similar to OD testing), or can use pancake coils to identify longitudinally oriented 

damage. Internal probe coils can be operated in both the differential and absolute 

modes simultaneously for identifying both abrupt and gradually occurring 

discontinuities. 

The ID test is not only sensitive to tube damage and wear but may also identify other 

discontinuities including scratches and dents caused by transport and handling, 

installation, and OD and ID debris that can come from a variety of sources. Therefore, 

if a one-time test is performed on an existing heat exchanger, it may be difficult to 

determine which indications are the results of service vs. those that are a result of the 

manufacturing and installation process. 



The baseline eddy current test 

The baseline eddy current test was developed to separate service related 

damage vs. manufacturing/installation process defects. The baseline test is most 

effective when performed immediately after the installation of the tubing and is 

typically done in the fabricator’s shop. It should be noted however that even with 

specialized electronics, the use of a bobbin coil will have considerable difficulty 

determining the precise discontinuity shape. Depth can be determined with a 

single frequency but multiple frequencies will improve the analysis. Signal length 

and a comparison of the absolute and differential signals from the same 

discontinuity can also help. The ability & knowledge of the signal analyst to 

correctly & accurately interpret potential failure mechanisms for the tubes 

serviced and the signals’ location in the heat exchanger becomes of paramount 

importance. A baseline test can be performed for the following reasons: 

■ 

■ 

To determine if the tube was damaged during installation in the heat 

exchanger 

To develop a database of discontinuities and anomalies, including their 

locations in the heat exchanger for comparison with future examinations 



An initial ID baseline map facilitates in-service evaluations by comparing the 

initial readings to a second test after a given service exposure. Tracking 

indication changes becomes a useful tool to understand the effect of pits, 

cracks and other wall loss damage in the tubing over the service timeline. 

This information can then facilitate predictive maintenance programs. 

During ID testing, indications are normally identified as a percentage of wall 

loss which is determined by a combination of phase angle shift and different 

responses to multiple frequencies. 

Because natural damage may not provide identical size data to the artificial 

defects used to calibrate the equipment, accurate sizing of the damage needs 

to be verified by removing samples with indications and comparing them to 

the calculation made during the analysis. However, removal of actual 

samples may not always be practical; in that case, the analysis must rely on 

past experience. 


There is no standard accept/reject criteria for ID electric testing of feedwater 

heater, condenser or balance of plant heat exchangers. Steam generator 

tubing has several criteria for rejection and many are associated to the 

artificial defects machined into the reference standard per ASME, Section V, 

Article 8, Appendix II. This reference standard does not simulate the 

feedwater heater, condenser and BOP heat exchanger indications and may 

result in excessive reject rates from non-injurious indications. 

ID testing to develop a baseline condition map is a mature technology and 

can be a very useful tool to help track tube damage and wear and future heat 

exchanger tube problems. Testing should be performed after the tubing is 

installed, rolled, seal welded and other surrounding manufacturing processes 

are completed. Detailed test information such as frequencies, probe speeds, 

phase angles and other parameters need to be carefully documented as well 

as probe descriptions and model numbers. When the test is duplicated, it can 

be compared easily to the baseline map to identify any changes to the tubes. 


Conclusion 

Investigative efforts in producing this HEI Tech Sheet have not identified any known research or studies that 

have been performed comparing the results of OD EC testing vs. ID EC testing of new commercial grade tubes. 

As such, this Tech Sheet is compelled to identify concerns relative to this issue as follows: 

1. The impact of attenuation needs to be better understood and addressed. Testing performed from the OD 

will accentuate OD imperfections while ID testing will accentuate those on the ID. 

2. The use of different frequencies will also have a significant impact on the signal vs. depth of the 

discontinuity. 

3. With any eddy current testing, fill factor, or the distance between the coil and the tube, is critical for 

determining discontinuity sizing. A high fill factor and precise coil centering improves sensitivity while a low 

fill factor results in a less precise response. When OD testing is performed the tubing is rigidly held and 

centering within the coil is ensured through the use of stationary rolls in both in-line and offline testing. 

Depending on the calibration process, OD-tested tubes can either be held stationary or rotated during 

testing. ID probes rarely have effective centering devices and no requirement or specification currently 

exists to prove centering. In the case of ID probe coils, a high fill factor results in better centering. Poor 

centering results in less sensitivity in the hemisphere of the tube that has a larger gap between the probe 

coil and tube wall. In a baseline test, a good fill factor is usually achievable because the tubes are clean. 

Testing tubes that have been in service may result in lower fill factors because of ID fouling. 

4. Most ID eddy current probes do not have a method for saturation to ensure that small magnetic domains 

do not produce false indications. Those probes with saturation only have sufficient energy to saturate thin 

walls and the testing is significantly slower. 

5. If the ID testing is performed before installation in the bundle, imperfections developed during the 

installation process are typically ignored. 

The OD ECT is the current industry norm for NDE certification of new tubing. Considering all of the issues above and in the 

absence of detailed comparative studies, the use of ID testing as an acceptance criterion for new tubing is not only controversial 

but highly subjective. In light of these concerns, it is therefore recommended that users discuss these issues in detail with the 

proposed tube manufacturers before specifying an ID test. 




Reading 1: 

Pulsed Eddy Currents - PEC Technology 

Conventional eddy current testing uses a single frequency sinusoidal to excite a coil 

and, among many applications, measure flaw responses as voltage and phase 

changes on an impedance plane. Since different frequencies present different 

sensitivity behaviors, multi-frequency testing is sometimes performed. In that case, 

multi-frequency eddy current measurements are either performed by simultaneous 

injection or multiplexing of multiple frequency components. 

In pulsed eddy current (PEC) testing, multi-frequency inspections are performed by 

driving a coil with a broadband pulse instead of a monochromatic excitation. This 

results in broader frequency contents than standard eddy current signals, as well as 

offering a better penetration into the depth of a material. The measured response of a 

PEC inspection is a waveform, similar to an ultrasonic A-Scan, from which features 

can be extracted to characterize flaws or for example perform thickness 

measurements. A temporal analysis of the transient response of the coil that results 

from this excitation can provide useful information about the depth of a defect. Pulsed 

eddy current is an ongoing research field as novel probes as well as new ways of 

interpreting and quantifying results are still required to fully exploit their potential. 


http://www.pecscan.ca/BasicsPECtech.html

Although still an active research field, PEC technology already has its place 

among the NDT techniques. The first application that benefits from the use of 

PEC is the detection of corrosion under insulation (CUI), where PEC has 

been used for many years to measure the remaining wall thickness of 

material buried below up to 6” of insulation material. PEC has also been used 

to detect deeply embedded corrosion or cracks in the multi-layered aluminum 

structures used in the aerospace industry. 



PEC Definitions 

Transient response: 

A transient event or response is a short-lived oscillation caused by a sudden change 

of voltage, current, or load. In pulsed eddy current, it expresses the time-dependant 

behavior of the coil-inspected material response to the input pulse. 

Balanced signal: 

Result of the subtraction of a voltage response by a reference signal, generally taken 

on an unflawed area of a test sample. The balanced signal is null for unflawed regions 

and displays amplitude variations when a defect or thickness change is encountered. 

It is similar in nature to performing a null with a standard eddy current apparatus. 



LOI (Lift-Off point of Intersection): 

The LOI is the location of the crossing point between a transient voltage response 

acquired on a sample and a response taken with a certain probe lift-off: the LOI is a 

position where the signal does not vary with probe lift-off. Monitoring the voltage 

response in the vicinity of the LOI point location therefore provides a mean of 

performing Pulsed Eddy Current inspections that are free of lift-off. 



Monitoring the actual displacement of the LOI point also provides useful information as 

the position of the LOI is dependant of the sample properties (material, thickness, etc.). 

An important application of shift is the possibility to perform thickness measurements 

from the variations of the LOI point coordinates. 



Gap Point : 

Similar to the LOI, the gap point defines a coordinates of the voltage response that is 

independent of gap variations between two layers of a multi-layered sample. This 

point is located further in time than the LOI. 

Spectral analysis: 

Spectral analysis consists of performing a conventional eddy current analysis of the 

frequencies contained in a PEC signal. The spectral analysis approach is a variation 

of the multi-frequency eddy current field but benefits a complete spectrum instead of 

finite frequencies. 



Probes 

Probes play an important part in Pulsed Eddy Current inspections. Their selection basically 

depends on the dimensions and shape of the flaws that need to be detected in relation with the 

properties of the part (material, number of layers, etc.). The probe that is most commonly used 

for Pulsed eddy current inspections is a reflection type, which means that the device used to 

induce the pulsed eddy currents is different than the device that receives their effects. Different 

combinations of driving/receiving sensors can be used to perform pulsed eddy current 

measurements. 

Driving Coil: Induction of Pulsed eddy currents 

The generation or induction of the pulsed eddy currents is typically done using a coil. The 

purpose of the coil is to convert an electrical pulse (driving pulse) into a magnetic field which 

induces eddy currents into the tested material following Faraday's laws of induction. The physical 

and electromagnetic characteristics of the driving coil partly define the bandwidth and footprint of 

the induced pulsed eddy currents. 



Driving Coil 



Coil receiver: Conventional eddy current probes 

Conventional eddy current probes use both a coil as the driving and receiving sensor. 

The reception of the eddy currents is again based on Faraday's induction laws. When 

a voltage I applied to the driving coil, it creates a magnetic field that induces eddy 

currents in the tested material. In return, these eddy currents generate an additional 

magnetic field that interacts with the initial one. A receiving coil picks up the variations 

of that resulting magnetic field and converts it into a measurable electrical signal. 

Driving Coil 

Receiving Coil 



Hall effect sensors 

In opposition to coils which measure variations of a magnetic field, Hall effect sensors 

allow for the direct measurement of a magnetic field. This difference allows for a better 

measurement of magnetic fields that do not vary rapidly. 

GMR sensors 

Giant Magneto Resistance sensors (GMR sensors) make use of a phenomenon 

discovered in 1988 and observed in thin film structures composed of alternating 

ferromagnetic and nonmagnetic layers, where the electrical resistance of the GMR 

varies in the presence of a magnetic field. While it does not rely on the same 

principles, this sensor is equivalent to a Hall sensor in the sense that it also provides a 

voltage output that is proportional to the magnetic field. 

GMR sensors 



PEC Results 

PEC is considered a new technology rather than an improvement of conventional 

eddy current. By changing the pulse excitation to a square wave, we input and receive 

signals that are quite different form conventional eddy currents. For this reason, PEC 

requires particular signal processing techniques which differ from the usual amplitude 

and phase analysis techniques. There is no denying that considerable information is 

available in the temporal and spectral analysis of these pulses. Because of the 

considerable amount of information available and inherent to the technology, the 

physical phenomenon must be well understood to discriminate between flaws and 

other artifacts 



Crack Detection using Pulsed Eddy Currents - PEC 

Crack detection on multilayered aircraft structures is achieved with two different PEC analysis methods. The 

PEC analysis method is selected based on the layer thickness and the rivet head physical properties. The 

designated PEC system requires proper calibration to obtain the desired detection. 

Introduction 

The detection of cracks is of great importance in aerospace structures as they can rapidly grow to cause 

catastrophic failures. Eddy currents, ultrasounds and radiography are the most common ways of inspecting this 

type of defect. While radiography has a limited use in tight spaces and because of security reasons, eddy 

current and ultrasonic inspections fail to detect cracks in all situations. Ultrasonic inspections require a 

mechanical bonding in order to propagate through multiple layers, which is not always the case for riveted 

structures. On the other hand, eddy currents can penetrate through unbounded layers, but at limited depths 

(typically 2 layers). Like eddy currents, Pulsed Eddy Currents have the particular advantage of being able to 

monitor multiple layers without the need for mechanical bonding. In the case of multilayered aerospace 

structures, a magnetic field that is strong enough to penetrate all layers of interest must be generated. When 

this is achieved, pulsed eddy currents are produced on both surfaces of each layer and, from the principles of 

mutual-inductance, generate an additional magnetic field that interact with the one coming from the driving coil. 

The presence of cracks affects the pulsed eddy currents and can be monitored in the resulting field. Multiple 

features can be used to detect cracks from either the transient waveform or its spectral representation. 


http://www.pecscan.ca/PDFs/Application-note-Cracks-Revised.pdf

Experiments 

C-Scan inspections of multilayered aircraft structures can be done using the 

ARMANDA scanner (figure 1a), which is a portable scanner that can be fixed 

on the structure. A PEC inspection was performed using this scanner on a 

riveted eddy current standard (2 aluminum layers of 0.04” with the bottom 

layer containing EDM notches of lengths of 0.250”, 0.200”, 0.150” and 0.100” 

on the rivet holes edge and identified from {1} to {4} on figure 2a). For sample 

inspection, we selected a conventional reflection eddy current probe (700 Hz 

- 15 kHz). The PecScan driver/receiver unit (figure 1b) is used to drive the 

probe, generate and receive the PEC signals. 



Fig. 1 (a) ARMANDA - Automated scanner Pulsed Eddy Current generation 

and reception. utomated for PEC testing (b) PecScan Driver/Receiver unit 

for Pulsed Eddy Current generation and reception. 



A picture of the sample is presented in figure 2a. Figure 2b shows the result 

obtained by analyzing the PEC waveforms using a temporal method (feature: 

total energy in a time gate) in the form of a C-Scan image. On the other hand, 

figure 2c shows the C obtained through spectral analysis (feature: single 

frequency component of 10 kHz extracted from the PEC waveforms). This 

spectral analysis allows displaying the content of the selected frequency on 

an impedance plane the same way it is performed in conventional eddy 

current inspections. Based on the impedance response measured on a good 

rivet, a rotation is applied on the 10 kHz component to minimize the effects of 

the rivet edge, leading to the result presented in figure 2 (c). 



Fig. 2. Images of the Eddy current standard samples {1}, 0.200” {2}, 0.150” {3} and 0.100” waveforms 

(energy within a time gate); all scales in analysis of the PEC waveforms: imaginary part of the 10 kHz 

component after rotation of the rivet edge signals. (d) Color palette used to display the C samples. (a) Picture 

showing the EDM notches of 0.250” {4}. (b) C-Scan obtained from the temporal analysis of the PEC mm. (c) C- 

Scan obtained from the spectral C-Scans. 




Pulsed Eddy Currents offer great potential for corrosion detection and location in thick structures. The wide 

band frequency spectrum of Pulsed Eddy Currents allows the determination of a large number of parameters, 

such as defect size and location. In fact, Pulsed Eddy Current techniques have the potential to become the 

primary method of corrosion detection in multi-layered structures. 

Our research concerning Pulsed Eddy Current technologies concentrates on the detection of corrosion and 

measurement of wall thickness of insulated pipelines. In order to optimize inspection productivity and costs, it is 

imperative to improve the quality of inspection and corrosion data interpretation. Our research efforts therefore 

revolve around the interpretation of corrosion data and the integration of Pulsed Eddy Current techniques to 

commercial inspection systems. 


http://www.tecscan.ca/solutions/advanced/pulsed-eddy-current/

Reading 2: 

4.9.2.3 Pulsed Eddy Current Testing. 

Conventional multifrequency systems usually utilize two or three frequencies. Additional frequencies require very 

complex multiplex mixing systems to analyze the information from the test. A variety of experimental techniques have utilized 

the multifrequency characteristics of a short electrical pulse to achieve the same type of results as the multifrequency test 

technique. In principle, this technique is advantageous in that it requires simpler electronics to process the data. It can 

potentially generate higher frequencies than fixed frequency systems. This would allow testing of thinner materials, and materials 

with very low electrical conductivity (high resistivity). The eddy current pulse can also be a very short, high voltage pulse that can 

be used to momentarily produce magnetic saturation in a ferromagnetic part. This will allow detection of subsurface flaws in 

ferromagnetic materials. 

4.9.2.4 Low Frequency Eddy Current Inspection. 

In the past most eddy current testing utilized test frequencies of 10 kHz to 1 MHz .Improved equipment and data 

processing techniques now allow the use of test frequencies as low as 55 Hz. Along with impedance plane equipment to measure 

signal phase, this has provided a means for testing multilayer materials and thick materials. Detection of deep subsurface 

cracks, cracking in intermediate layers of material, and corrosion on the backside of a material are possible. 

4.9.2.5 Barkhausen Noise Testing Of Ferromagnetic Materials. 

Abnormal stresses induced by shot peening, other cold working processes, and grinding burns affect the structural 

properties of a material and can lead to flaw growth and part failure. In ferromagnetic materials, these processes affect the ease 

with which the magnetic domains in the surface of the material can be moved. In un-magnetized ferromagnetic material, the 

magnetic domains are randomly oriented. If the material is subjected to a magnetic field, the magnetic domains tend to align 

themselves in the direction of the magnetic field. When the domains move to align themselves, electrical pulses are generated 

during the domain movement. This is called Barkhausen noise. This electrical noise can be detected and measured 

by Hall effect sensors. If the material is free of abnormal stresses, the domains are relatively free to move and little 

Barkhausen noise is generated. Areas of tensile stress parallel to the applied magnetic field cause an increase in Barkhausen 

noise. Examples of applications of this test method are ferromagnetic engine components and landing gear. 

Barkhausen noise measurements are also used to detect the quality of drilling and reaming of holes in ferromagnetic 

material. 


http://chemical-biological.tpub.com/TM-1-1500-335-23/css/TM-1-1500-335-23_419.htm

Good Luck 


Good Luck 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Electromagnetic Testing

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