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Magnetic Saturation 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. Charlie Chong/ Fion Zhang http://202.141.40.218/wiki/index.php/Hysteresis
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. Charlie Chong/ Fion Zhang http://202.141.40.218/wiki/index.php/Hysteresis
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Electromagnetic Testing Study Guide
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E&P Applications Charlie Chong/ Fio
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Charlie Chong/ Fion Zhang
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乱七八糟 - 随看随记 C
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Charlie Chong/ Fion Zhang http://ww
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IVONA TTS Capable. Charlie Chong/ F
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EDDY CURRENT an Overview Descriptio
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Tangential Probe Charlie Chong/ Fio
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Electromagnetic Testing Advantages
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Factors Affecting Eddy Current Resp
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from this it can be seen that depth
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http://www.eng.morgan.edu/~hubert/I
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Robotic Charlie Chong/ Fion Zhang
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Measurement Techniques (EN!) a) Abs
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Figure 1.1 Arago’s Experiment Cha
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Oersted discovered the presence of
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Figure 1.3: Induced current electro
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Charlie Chong/ Fion Zhang http://en
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Charlie Chong/ Fion Zhang http://en
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The law of physics describing the p
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Charlie Chong/ Fion Zhang http://hy
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Magnetic Force The magnetic field B
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Generation of Eddy Currents When a
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Factors Affecting Inductance There
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COIL LENGTH: All other factors bein
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Coil Inductance L An approximation
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Note the direction of the primary c
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Generation of Eddy Current With a p
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Charlie Chong/ Fion Zhang Permeabil
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A Dual Variable inductors Figure 1:
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Figure 3: Here is another inductor
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Figure 4: The two inductors on this
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Grundig radio satellit 750 ■ http
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Figure 1.7: Phasor Diagram of Coil
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Current Density The distribution of
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Figure 1.8: Relative eddy current d
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Relative Magnetic Permeability Perm
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It should be observed at this point
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δ = (π x 100 x 10 3 x 4 x π x 0.
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Using 1.35 mm as depth x from surfa
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Standard Depth for Different Conduc
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Current (?) Lagging Voltage lagging
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σº∙πμ■δ∝∞ωΩθ√ρβ
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Figure 1.9 should be used as a rela
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Figure 1.8: Relative eddy current d
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Q.1.1 Generation of eddy currents d
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Q.1.5 Eddy current generated a test
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Q.1.9 Refer to Figure1.9 using exam
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Chapter 2 The Coil Arrangements Cha
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Probe coils and probe coil forms ca
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Encircling Coils Encircling coil, o
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Discussion Subject; Encircling Coil
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Bobbin Coils Bobbin coil, inside di
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Figure 2.5: Test coil configuration
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Differential coils Differential coi
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Figure 2.6: External reference diff
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Figure 2.7: Hybrid coil (through tr
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Additional Coil Characteristics Coi
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Q.2.1 Differential coils are usuall
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Q.2.6 An absolute coil measurement
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The Answers Charlie Chong/ Fion Zha
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As discussed earlier test coil desi
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Demonstration model of a moving iro
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Thus, the resistance of a 10 ft len
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SPECIFIC RESISTANCE OR RESISTIVITY
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Problem: What is the resistance of
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The inductance of a multilayer air
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Example: A coil whose dimensions ar
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Example: Using the 32 μH coil calc
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Impedance To explain the addition o
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Maximum transfer of power is accomp
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Rectangular Notation Because reacta
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Vectors With Polar Notations. Stand
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Rectangular Notation - Prefix “j
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Imaginary Component In “rectangul
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Imaginary Charlie Chong/ Fion Zhang
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To convert from rectangular to pola
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More Reading on Polar & Rectangular
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Eddy current coils with included im
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Permeability and Shielding Effects
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Another technique of shielding uses
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Saturation Approach Another coil de
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Figure 3.5: Magnetic bias probe Cha
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Materials must be chemically compat
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Q.3.1 A coil's resistance is determ
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Q.3.7 The Q or merit of a coil is d
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Jackfruit Tree Charlie Chong/ Fion
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Electrical Conductivity σ In elect
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As previously discussed, conductivi
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Figure 4.1; Conductivity curve Char
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The coil's inductive reactance is r
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Effects of Heat Treatment on Conduc
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Permeability Permeability of any ma
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When increases in the magnetizing f
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Skin Effect Charlie Chong/ Fion Zha
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Discussion Subject: Response to the
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Lift Off Electromagnetic coupling b
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Fill factor will always be a number
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Figure 4.4: Fill factor ratios When
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Example: Calculate the resultant lo
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Figure 1.8 is again useful to illus
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Signal-to-Noise Ratio Signal-to-noi
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Q.4.4 A (?) prime factor affecting
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Q.4.10 Temperature changes, vibrati
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Test Frequencies It is the responsi
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Single Frequency Systems Unfortunat
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Eddy Current Density Since the sens
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Applicable Scenario Eddy Current De
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Standard Depth δ Formula The depth
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Example 1: As an example of how thi
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Frequency Selection - Ferromagnetic
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A frequency can always selected to
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Figure 5.1(a) shows the effect on p
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Figure 5.1: Effects of frequency ch
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Figure 5.2: Normalized impedance di
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Figure 5.3: Impedance variations ca
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Figure 5.4: Impedance variations ca
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Multiparameter Techniques It become
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Multiparameter Techniques - Multipl
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First, a frequency is selected to g
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More Reading on Characteristic Para
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Figure 1: Eddy Current Testing Char
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Figure 2: Types of Probes Charlie C
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Surface (Pancake) Probes Surface pr
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Encircling Probes Encircling probes
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Design of eddy current probes In mo
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In brief, probe design is usually d
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Analytical Approach (Pc & f g ) Ana
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RECENT TRENDS IN EDDY CURRENT PROBE
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Pressurized Heavy Water Reactors (P
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Q.5.4 Calculate the limit frequency
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Q.5.7 Primary resistance is subtrac
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Q5.9 A 25% deep crack open to the n
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Chapter 6 Instrument Systems Charli
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Eddy Current Instrumentation Most o
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Figure 6.1: Internal functions of t
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Chernobyl Nuclear Power Plant Charl
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Figure 6.2: Four types of simple ed
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Figure 6.2(b) shows an impedance br
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Figure 6.2(d) illustrates a balanci
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Eddy current testing - Signal Analy
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Impedance Testing With impedance ma
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Phase Analysis Testing - Vector Poi
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Phase Analysis Testing - Ellipse As
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Figure 6.4: Cathode ray tube displa
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Figure 6.5: Screen image of a linea
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Impedance Plane Testing - Mode of O
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Figure 6.6: Null balance instrument
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Figure 6.7: Typical surface riding
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For deep subsurface crack detection
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Multiple circuits are used througho
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This type of digital instrumentatio
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Read Out Mechanisms - Audio Alarms
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An qualitative meter response could
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Figure 6.13: A qualitative analog m
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Read Out Mechanisms - Cathode Ray T
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Figure 6.14: Numerical readouts/dig
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Read Out Mechanisms - Computer Eddy
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Probe Delivery System Instead of mo
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Chapter 6 Review Questions Charlie
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Q.6.1 Signal preparation is usually
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Q.6.7 Amplitude gates provide a tec
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Chapter 7 Eddy Current Applications
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Discontinuity Detection The theoret
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Heat Exchanger Charlie Chong/ Fion
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Heat Exchanger TSP Charlie Chong/ F
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Heat Exchanger TSP Charlie Chong/ F
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Figure 7.2: Phase-to-depth calibrat
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Page 433 and 434: The Remote Field Technique is an el
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Page 443 and 444: RFT Probes Probes for inspection of
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Page 447 and 448: RFT Reference Standards Reference s
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American Society for Testing and Ma
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Military Standard The United States
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EDDY CURRENT INSPECTION OF NONFERRO
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. Probe positioning and feeding sha
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3. Tube Inspection General (Refer t
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F. INSPECTION RESULTS AND DOCUMENTA
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G. REFERENCES The following documen
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Answers Charlie Chong/ Fion Zhang
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Q.9.4 Military Standards are design
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Q.9.10 The system in QA3 is calibra
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More Reading http://www.allaboutcir
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Discussion Subject: discuss on the
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Inspection from the Tube OD Most AS
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Approximately 25 years ago, the AST
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One additional advantage of OD ECT
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The baseline eddy current test The
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There is no standard accept/reject
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Pulsed Eddy Currents Systems Charli
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Although still an active research f
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LOI (Lift-Off point of Intersection
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Gap Point : Similar to the LOI, the
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Driving Coil Charlie Chong/ Fion Zh
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Hall effect sensors In opposition t
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Crack Detection using Pulsed Eddy C
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Fig. 1 (a) ARMANDA - Automated scan
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Fig. 2. Images of the Eddy current
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Reading 2: 4.9.2.3 Pulsed Eddy Curr
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Good Luck Charlie Chong/ Fion Zhang
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