UT Testing-Section 4 Calibration Methods

Section 4: Calibration Methods

Content: Section 4: Calibration Methods 

4.1: Calibration Methods 

4.2: The Calibrations 

4.2.1: Distance Amplitude Correction (DAC) 

4.2.2: Finding the probe index 

4.2.3: Checking the probe angle 

4.2.4: Calibration of shear waves for range V1 Block 

4.2.5: Dead Zone 

4.2.7: Transfer Correction 

4.2.8: Linearity Checks (Time Base/ Equipment Gain/ Vertical Gain) 

4.2.9: TCG-Time Correction Gain 

4.3: Curvature Correction 

4.4: Calibration References & Standards 

4.5: Questions & Answers 

4.6: Video Time

4.1: Calibration Methods 

Calibration refers to the act of evaluating and adjusting the precision and 

accuracy of measurement equipment. In ultrasonic testing, several forms of 

calibration must occur. First, the electronics of the equipment must be 

calibrated to ensure that they are performing as designed. This operation is 

usually performed by the equipment manufacturer and will not be discussed 

further in this material. It is also usually necessary for the operator to perform 

a "user calibration" of the equipment. This user calibration is necessary 

because most ultrasonic equipment can be reconfigured for use in a large 

variety of applications. The user must "calibrate" the system, which includes 

the equipment settings, the transducer, and the test setup, to validate that the 

desired level of (1) precision and (2) accuracy are achieved. The term 

calibration standard is usually only used when an absolute value is measured 

and in many cases, the standards are traceable back to standards at the 

National Institute for Standards and Technology.

Calibrations

In ultrasonic testing, there is also a need for reference standards. Reference 

standards are used to establish a general level of consistency in 

measurements and to help interpret and quantify the information contained in 

the received signal. Reference standards are used to validate that the 

equipment and the setup provide similar results from one day to the next and 

that similar results are produced by different systems. Reference standards 

also help the inspector to estimate the size of flaws. In a pulse-echo type 

setup, signal strength depends on both the size of the flaw and the distance 

between the flaw and the transducer. The inspector can use a reference 

standard with an artificially induced flaw of known size and at approximately 

the same distance away for the transducer to produce a signal. By comparing 

the signal from the reference standard to that received from the actual flaw, 

the inspector can estimate the flaw size.

This section will discuss some of the more common calibration and reference 

specimen that are used in ultrasonic inspection. Some of these specimens 

are shown in the figure above. Be aware that there are other standards 

available and that specially designed standards may be required for many 

applications. The information provided here is intended to serve a general 

introduction to the standards and not to be instruction on the proper use of the 

standards.

Introduction to the Common Standards 

Calibration and reference standards for ultrasonic testing come in many 

shapes and sizes. The type of standard used is dependent on the NDE 

application and the form and shape of the object being evaluated. The 

material of the reference standard should be the same as the material being 

inspected and the artificially induced flaw should closely resemble that of the 

actual flaw. This second requirement is a major limitation of most standard 

reference samples. Most use drilled holes and notches that do not closely 

represent real flaws. In most cases the artificially induced defects in reference 

standards are better reflectors of sound energy (due to their flatter and 

smoother surfaces) and produce indications that are larger than those that a 

similar sized flaw would produce. Producing more "realistic" defects is cost 

prohibitive in most cases and, therefore, the inspector can only make an 

estimate of the flaw size. Computer programs that allow the inspector to 

create computer simulated models of the part and flaw may one day lessen 

this limitation.

The IIW Type Calibration Block

The IIW Type Calibration Block

The IIW Type 2 Calibration Block

The IIW Type I Calibration Block

EN12223:1999 Calibration Block

The IIW Phase Array Calibration Block

The IIW Calibration Block 

1 st Check Index / Check Range


2 nd Check Angle


2 nd Check Angle

Find probe angle 

Find Index/Range/Resolution

The IIW Phase Array Calibration Block 

3 rd Check Resolution

V2 Calibration Block

The IIW 2 Calibration Block 

Check focal point 

Check probe angle 

Check range 

Can not Check resolution

Calibration Blocks

Calibration Blocks- Area Amplitude Block

The standard shown in the above figure is commonly known in the US as an 

IIW type reference block. IIW is an acronym for the International Institute of 

Welding. It is referred to as an IIW "type" reference block because it was 

patterned after the "true" IIW block but does not conform to IIW requirements 

in IIS/IIW-23-59. "True" IIW blocks are only made out of steel (to be precise, 

killed, open hearth or electric furnace, low-carbon steel in the normalized 

condition with a grain size of McQuaid-Ehn #8) where IIW "type" blocks can 

be commercially obtained in a selection of materials. The dimensions of "true" 

IIW blocks are in metric units while IIW "type" blocks usually have English 

units. IIW "type" blocks may also include additional calibration and references 

features such as notches, circular groves, and scales that are not specified by 

IIW. There are two full-sized and a mini versions of the IIW type blocks. The 

Mini version is about one-half the size of the full-sized block and weighs only 

about one-fourth as much. The IIW type US-1 block was derived the basic 

"true" IIW block and is shown below in the figure on the left. The IIW type US- 

2 block was developed for US Air Force application and is shown below in the 

center. The Mini version is shown on the right.

IIW Blocks- US-1 

IIW Type US-1

IIW Blocks- IIW Type US-2

IIW Blocks- IIW Type Mini

V1/5, A2 Block

IIW type blocks are used to calibrate instruments for both angle beam and 

normal incident inspections. Some of their uses include setting metal-distance 

and sensitivity settings, determining the sound exit point and refracted angle 

of angle beam transducers, and evaluating depth resolution of normal beam 

inspection setups. Instructions on using the IIW type blocks can be found in 

the annex of American Society for Testing and Materials Standard E164, 

Standard Practice for Ultrasonic Contact Examination of Weldments. 

The Miniature Angle-Beam or ROMPAS Calibration Block

DSC Block, Mini block, Rompas Block are all mini blocks. 

ROMPAS Calibration Block 

AWS Shear Wave 

Distance/Sensitivity 

Calibration (DSC) Block

A block that closely resembles the miniature angle-beam block and is used in 

a similar way is the DSC AWS Block. This block is used to determine the 

beam exit point and refracted angle of angle-beam transducers and to 

calibrate distance and set the sensitivity for both normal and angle beam 

inspection setups. Instructions on using the DSC block can be found in the 

annex of American Society for Testing and Materials Standard E164, 

Standard Practice for Ultrasonic Contact Examination of Weldments.

A block that closely resembles the miniature angle-beam block and is used in 

a similar way is the DSC AWS Block. This block is used to determine the 

beam exit point and refracted angle of angle-beam transducers and to 

calibrate distance and set the sensitivity for both normal and angle beam 

inspection setups. Instructions on using the DSC block can be found in the 



DSC AWS Block

Calibration Range Using DSC AWS Block 

www.youtube.com/embed/TEQ8Qrz4D-A

AWS Shear Wave Distance Calibration (DC) Block

AWS Shear Wave Distance Calibration (DC) Block

The DC AWS Block is a metal path distance and beam exit point calibration 

standard that conforms to the requirements of the American Welding Society 

(AWS) and the American Association of State Highway and Transportation 

Officials (AASHTO). Instructions on using the DC block can be found in the 



AWS Resolution Calibration (RC) Block 

The RC Block is used to determine the resolution of angle beam transducers 

per the requirements of AWS and AASHTO. Engraved Index markers are 

provided for 45, 60, and 70 degree refracted angle beams.

The RC Block is used to determine the resolution of angle beam transducers 

per the requirements of AWS and AASHTO. Engraved Index markers are 

provided for 45, 60, and 70 degree refracted angle beams.

30 FBH Resolution Reference Block 

The 30 FBH resolution reference block is used to evaluate the near-surface 

resolution and flaw size/depth sensitivity of a normal-beam setup. The block 

contains number 3 (3/64"), 5 (5/64"), and 8 (8/64") ASTM flat bottom holes at 

ten metal-distances ranging from 0.050 inch (1.27 mm) to 1.250 inch (31.75 

mm).

Miniature Resolution Block 

The miniature resolution block is used to evaluate the near-surface resolution 

and sensitivity of a normal-beam setup It can be used to calibrate highresolution 

thickness gages over the range of 0.015 inches (0.381 mm) to 

0.125 inches (3.175 mm).

Step and Tapered Calibration Wedges 

Step and tapered calibration wedges come in a large variety of sizes and 

configurations. Step wedges are typically manufactured with four or five steps 

but custom wedge can be obtained with any number of steps. Tapered 

wedges have a constant taper over the desired thickness range.

Distance/Sensitivity (DS) Block 

The DS test block is a calibration standard used to check the horizontal 

linearity and the dB accuracy per requirements of AWS and AASHTO.

Area Amplitude Blocks provide standards for discontinuities of different size 

at the same depth 

Distance Amplitude Blocks provide standards for discontinuities of same size 

at the different depth

The ASTM basic set of Area/Distance Amplitude Blocks consists of ten, two 

inches diameter blocks

The ASTM basic set of Area/Distance Amplitude Blocks consisits of ten, two 

inches diameter blocks

Distance/Area-Amplitude Blocks 

Distance/area amplitude correction blocks typically are purchased as a tenblock 

set, as shown above. Aluminum sets are manufactured per the 

requirements of ASTM E127 and steel sets per ASTM E428. Sets can also be 

purchased in titanium. Each block contains a single flat-bottomed, plugged 

hole. The hole sizes and metal path distances are as follows: 

• 3/64" at 3" 

• 5/64" at 1/8", 1/4", 1/2", 3/4", 11/2", 3", and 6" 

• 8/64" at 3" and 6" 

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and 

Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the 

requirements of ASTM E127, Standard Practice for Fabricating and Checking 

Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are 

fabricated per the requirements of ASTM E428, Standard Practice for 

Fabrication and Control of Steel Reference Blocks Used in Ultrasonic 

Inspection.

ASTM E 127

Area-Amplitude Blocks 

Area-amplitude blocks are also usually purchased in an eight-block set and 

look very similar to Distance/Area-Amplitude Blocks. However, areaamplitude 

blocks have a constant 3-inch metal path distance and the hole 

sizes are varied from 1/64" to 8/64" in 1/64" steps. The blocks are used to 

determine the relationship between flaw size and signal amplitude by 

comparing signal responses for the different sized holes. Sets are commonly 

sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion 

Resistant Steel. Aluminum blocks are fabricated per the requirements of 

ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy 

Ultrasonic Standard Reference Blocks. Steel blocks are fabricated per the 

requirements of ASTM E428, Standard Practice for Fabrication and Control of 

Steel Reference Blocks Used in Ultrasonic Inspection.

Distance-Amplitude #3, #5, #8 FBH Blocks 

Distance-amplitude blocks also very similar to the distance/area-amplitude 

blocks pictured above. Nineteen block sets with flat-bottom holes of a single 

size and varying metal path distances are also commercially available. Sets 

have either a #3 (3/64") FBH, a #5 (5/64") FBH, or a #8 (8/64") FBH. The 

metal path distances are 1/16", 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 

1-3/4", 2-1/4", 2-3/4", 3-14", 3-3/4", 4-1/4", 4-3/4", 5-1/4", and 5-3/4". The 

relationship between the metal path distance and the signal amplitude is 

determined by comparing signals from same size flaws at different depth. 

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and 

Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the 

requirements of ASTM E127, Standard Practice for Fabricating and Checking 

Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are 

fabricated per the requirements of ASTM E428, Standard Practice for 

Fabrication and Control of Steel Reference Blocks Used in Ultrasonic 

Inspection.

Key Words: 

Distance Amplitude Blocks 

DSC 

DC 

SC 

AWS RC 

Distance sensitivity calibration 

Distance calibration 

Sensitivity calibration 

AWS Resolution Calibration.

Q56: On the area-amplitude ultrasonic standard test blocks, the flat-bottomed 

holes in the blocks are: 

A. All of the same diameter 

B. Different in diameter, increasing by 1/64 inch increments from the 

No. 1 block to the No. 8 block 

C. Largest in the No. 1 block and smallest in the No. 8 block 

D. Drilled to different depths from the front surface of the test block

Q: A primary purpose of a reference standard is: 

A. To provide a guide for adjusting instrument controls to reveal 

discontinuities that are considered harmful to the end use of the 

product. 

B. To give the technician a tool for determining exact discontinuity size 

C. To provide assurance that all discontinuities smaller than a certain 

specified reference reflector are capable of being directed by the test. 

D. To provide a standard reflector which exactly simulates natural 

discontinuities of a critical size.

4.2: The Calibrations 

4.2.1: Distance Amplitude Correction (DAC) 

Distance Amplitude Correction (DAC): Acoustic signals from the same 

reflecting surface will have different amplitudes at different distances from the 

transducer. Distance amplitude correction (DAC) provides a means of 

establishing a graphic ‘reference level sensitivity’ as a function of sweep 

distance on the A-scan display. The use of DAC allows signals reflected from 

similar discontinuities to be evaluated where signal attenuation as a function 

of depth has been correlated. Most often DAC will allow for loss in amplitude 

over material depth (time), graphically on the A-scan display but can also be 

done electronically by certain instruments. Because near field length and 

beam spread vary according to transducer size and frequency, and materials 

vary in attenuation and velocity, a DAC curve must be established for each 

different situation. DAC may be employed in both longitudinal and shear 

modes of operation as well as either contact or immersion inspection 

techniques.

DAC Curve

http://www.huatecgroup.com/china-digital_portable_dac_avg_curves_ultrasonic_flaw_detector_ut_flaw_detector_fd350-632512.html

DAC- Distance Amplitude Correction

DAC- Distance Amplitude Correction 

DGS- Distance Gain Size

A distance amplitude correction curve is constructed from the peak amplitude 

responses from reflectors of equal area at different distances in the same 

material. A-scan echoes are displayed at their non-electronically 

compensated height and the peak amplitude of each signal is marked on the 

flaw detector screen or, preferably, on a transparent plastic sheet attached to 

the screen. Reference standards which incorporate side drilled holes (SDH), 

flat bottom holes (FBH), or notches whereby the reflectors are located at 

varying depths are commonly used. It is important to recognize that 

regardless of the type of reflector used, the size and shape of the reflector 

must be constant. Commercially available reference standards for 

constructing DAC include ASTM Distance/Area Amplitude and ASTM E1158 

Distance Amplitude blocks, NAVSHIPS Test block, and ASME Basic 

Calibration Blocks.

The following applet shows a test block with a side drilled hole. The 

transducer was chosen so that the signal in the shortest pulse-echo path is in 

the far-field. The transducer may be moved finding signals at depth ratios of 1, 

3, 5, and 7. Red points are "drawn" at the peaks of the signals and are used 

to form the distance amplitude correction curve drawn in blue. Start by 

pressing the green "Test now!" button. After determining the amplitudes for 

various path lengths (4), press "Draw DAC" and then press the green "Test 

now!" button.

DAC Java 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/applet2/applet2.htm

Developing a Distance Amplitude Correction (DAC) Curve 

Distance Amplitude Correction (DAC) provides a means of establishing a 

graphic ‘reference level sensitivity’ as a function of sweep distance on the A- 

scan display. The use of DAC allows signals reflected from similar 

discontinuities to be evaluated where signal attenuation as a function of depth 

may be correlated. In establishing the DAC curve, all A-scan echoes are 

displayed at their non-electronically compensated height. 

Construction of a DAC involves the use of reference standards which 

incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches 

whereby the reflectors are located at varying depths. It is important to 

recognize regardless of the type of reflector that is used in constructing the 

DAC, the size and shape of the reflector must be constant over the sound 

path distance. Commercially available reference standards for constructing 

DAC include ASTM Distance/Area Amplitude and ASTM E1158 Distance 

Amplitude blocks, NAVSHIPS Test block, and ASME Basic Calibration 

Blocks.

Sequence for constructing a DAC curve when performing a straight 

beam contact inspection on 1 ¾” thick material. 

1.) Using a suitable reference standard, calibrate the sweep for a distance 

appropriate for the material to be inspected, i.e.. using a 1” thick standard, 

calibrate the sweep for 2” of material travel.

Back Wall Echo

Back Wall Echo 

Sweep 2” / Distance 1”

2.) This example represents the use a 1 3/4” thick reference standard with 

1/8” side drilled holes located at 1/4 T and 3/4 T respectively. ‘T’ being equal 

to the block thickness.

3.) Position the transducer over the 1/4T hole and peak the signal to 

approximately 80% FSH (Full screen height), mark the peak of the echo on 

the display using a suitable marker, and record the gain setting.

4.) With no further adjustments to the gain control, position the transducer 

over the 3/4T hole and peak the signal, mark the peak of the echo on the 

display.

5.) To complete the DAC curve connect the dots with a smooth line. The 

completed curve represents the ‘reference level sensitivity’ for this application.

Plotting DAC Curve

DAC Curve

DAC Curve

Gain Control for FSH: It should be remember that the dB is a means of 

comparing signals. All UT sets are different and a FSH with a gain controls of 

36dB in one UT set and be at FSH at another UT set with a gain control 

reading of 26dB. 

The gain controls allow us to set sensitivity and form the basis of Ultrasonic 

Sizing Techniques.

Birring NDT Series, Ultrasonic Distance Amplitude Correction - DAC 

www.youtube.com/embed/qUqaF0PnLGA?list=UUZncq6JFram3pfQDlzGggwA

Alta Vista UT Calibration DAC Curve 

www.youtube.com/embed/VNgMKlp43I8

4.2.2: Finding the probe index

Exit Point 

A2 Block

Exit Point- A5 Block

Q16: Notches are frequently used as a reference reflector for: 

A. Distance amplitude calibration for shear wave 

B. Area amplitude calibration 

C. Thickness calibration for plate 

D. Determining the near-surface resolution

5.2.3: Checking the probe angle

Probe Angles- A2 Block

Probe Angles- A5 Block

4.2.4: Calibration of shear waves for range V1 Block

Calibration of shear waves for range V1 Block

1 st Echo from circular Section

Echo from 100mm circular Section

Calibration of shear waves for range V1 Block 

Test block 1 for calibrating the 

time base (depth scale) of a flaw 

detector for vertical probes 

(longitudinal waves) for angle 

probes (transverse waves), for 

determining the probe index and 

beam angle of angle probes, and 

for checking the short term 

consistency of the sensitivity of 

vertical probes


25 mm radius from V2 Block

50 mm radius from V2 Block

100 mm radius from K2 Block


Shear Wave Distance Calibration IIW Block & DSC Blocks 

www.youtube.com/embed/RmtHmtOozic

Exit Point /Range/Probe Angle calibration using IIW Block (Repeat-Code1) 

www.youtube.com/embed/Qr0dGNuq9yY

4.2.5: Dead Zone 

Determine the dead zone by finding the hole echo which is easily 

identifiable from the probe noise at the shortest range

Dead Zone 

Determine the dead zone by 

finding the hole echo which is 

easily identifiable from the probe 

noise at the shortest range

4.2.6: 20 dB Profile- A5 Block

20 dB Profile 

Probe Beam Line of Symmetry

20 dB Profile 

Probe Beam Sound Pressure

4.2.7: Transfer Correction 

Methods of compensating for transfer and attenuation loss differences for 

0attenuation 000compression probes and for shear wave compression 

probes. These are based on obtaining similar echo responses on both the 

calibration block and on the component. 

For 0degree probes backwall echoes are used to probes establish transfer 

and attenuation correction. 

For shear wave probes two identical probes are used in “pitch-catch” in 

order to obtain what are effectively backwall echoes. 

either method cannot be used if the either component does not have a 

convenient parallel section.

Example: 

0 degree Probe Calibration 

40mm thick block – Gain to achieve FSH

Example: 

0 degree Probe Calibration 

30mm thick block – Gain to achieve FSH

TRANSFER & ATTENUATION CORRECTION: 

0 degree Probes 

If the results are plotted on 

log -linear paper they will 

form straight parallel lines 

provided that there is no 

attenuation difference if an 

attenuation difference 

occurs then the resultant 

lines will no longer be 

parallel.

Transfer and Attenuation Correction: Shear Probe 

The principle for obtaining transfer correction for shear wave probes is the 

same as it was for compression probes except that backwall echoes are 

replaced by pitch --catch responses.

4.2.8: Linearity Checks (Time Base / Equipment Gain / Vertical Gain)

4.2.8.1: Linearity of time base 

General 

This check may be carried out using a standard calibration block eg A2, 

and a compressional wave probe. The linearity should be checked over a 

range at least equal to that which is to be used in subsequent testing. 

Method 

a) Place the probe on the 25mm thickness of the A2 block and adjust the 

controls to display ten BWEs. 

b) Adjust the controls so that the first and last BWEs coincide with the scale 

marks at 1 and 10. 

c) Increase the gain to bring successive backwall echoes to 80% FSH. The 

leading edge of each echo should line up with the appropriate reticules 

line. 

d) Record any deviations at approximately half screen height. Deviations 

should be expressed as a percentage of the range between the first and 

last echoes displayed (ie 225mm).

Tolerance 

Unless otherwise specified by the testing standard, a tolerance of ±2% is 

considered acceptable. 

Frequency of checking 

This check shall be carried out at least once per week.

Ultrasonic Testing - Horizontal Linearity (Calibration) 

www.youtube.com/embed/NuS6j0SmjKQ

4.2.8.2: Linearity of Equipment Gains 

General 

This is a check on both the linearity of the amplifier within the set and the 

calibrated gain control. It can be carried out on any calibration block 

containing a side-drilled hole and should be the probe to be used in 

subsequent testing. Reject/suppression controls shall be switched off. 

Method 

• Position the probe on a calibration block to obtain a reflected signal from a 

small reflector eg 1.5mm hole in the A2 block. 

• Adjust the gain to set this signal to 80% FSH and note the gain setting (dB). 

- Increase the gain by 2dB and record the amplitude of the signal. 

- Remove the 2dB and return the signal to 80% FSH. 

- Reduce the gain by 6dB and record signal amplitude. 

- Reduce the gain by a further 12dB (18 intotal) and record signal amplitude. 

- Reduce the gain by a further 6dB (24 in total) and record signal amplitude.

Tolerance 

Frequency of checking 

The check shall be carried out at least once per week.

5.2.8.3-1: Linearity of vertical display to EN12668-1 

Procedure: Test the ultrasonic instrument screen linearity by altering the 

amplitude of a reference input using an external calibrated attenuator and 

observing the change in the signal height on the ultrasonic instrument 

screen. Report the gain setting at the beginning of the test. Check the 

linearity at prescribed intervals from 0 dB to - 26 dB of full screen height. 

Repeat the test for centre frequencies for of each filter as measured in 9.5.2. 

Using the same set-up shown in Figure 6 set the external calibrated attenuator 

to 2 dB and adjust the input signal and the gain of the ultrasonic instrument 

so the signal is 80 % of full screen height. Without changing the gain of the 

ultrasonic instrument switch the external calibrated attenuator to the values 

given in the Table 4. For each setting measure the amplitude of the signal on 

the ultrasonic instrument screen. 

Extract from: BS EN 12668-1:2010 Non-destructive testing- Characterization and verification of ultrasonic examination equipment 

Part 1: Instruments

Figure 6 — General purpose set-up for equipment

4.2.8.3-2: Linearity of vertical display to ASTM E317-01 

Vertical Limit and Linearity: 

Significance—Vertical limit and linearity have significance when echo signal 

amplitudes are to be determined from the display screen or corresponding 

output signals, and are to be used for evaluation of discontinuities or 

acceptance criteria. A specified minimum trace deflection and linearity limit 

may 

be required to achieve the desired amplitude accuracy. For other situations they 

may not be important, for example, go/no-go examinations with flaw alarms 

or evaluation by comparison with a reference level using calibrated gain 

controls. 

This practice describes both the two-signal ratio technique (Method A) and the 

input/output attenuator technique (Method B). 

Extract from: ASTM E317-01 Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Examination 

Instruments and Systems without the Use of Electronic Measurement Instruments 

Note: Method A: two-signal ratio technique collecting 2 signal from the 

reflectors of same size at different depth.

Method A: 

6.3.2.1 Apparatus—A test block is required that produces two non interfering 

signals having an amplitude ratio of 2 to 1. These are compared over the 

usable screen height as the instrument gain is changed. The two amplitudes 

will be referred to as HA and HB (HA > HB). The two signals may occur in 

either screen order and do not have to be successive if part of a multipleecho 

pattern. Unless otherwise specified in the requesting document, any 

test block that will produce such signals at the nominal test settings specified 

can be used. For many commonly used search units and test conditions, the 

test block shown in Fig. 1 will usually be satisfactory when the beam is 

directed along the H dimension toward the two holes. The method is 

applicable to either contact or immersion tests; however, if a choice exists, 

the latter may be preferable for ease of set-up and coupling 

stability……(more…)

4.2.9: Time Correction Gain (TCG) 

Please read: 

http://aqualified.com/tcg-dac-ndt-ultrasound/

Q61: The vertical linear range of a test instrument may be determined by 

obtaining ultrasonic responses from: 

A. a set of distance amplitude blocks 

B. steel ball located at several different water path distances 

C. a set of area amplitude blocks 

D. all of the above

Q29: Test sensitivity correction for a metal distance and discontinuity area 

responses are accomplished by using: 

A. An area amplitude set of blocks 

B. An area amplitude and a distance amplitude set of blocks 

C. A distance amplitude set of blocks 

D. Steel balls of varying diameters.

4.3: Curvature Correction 

Curvature in the surface of a component will 

have an effect on the shape of the ultrasonic 

beam. The image to the right shows the beam 

from a focused immersion probe being 

projected on to the surface of a 

component. Lighter colors represent areas of 

greater beam intensity. It can be seen that 

concave surfaces work to focus the beam and 

convex surfaces work to defocus the 

beam. Similar effects are also seen with 

contact transducers. When using the 

amplitude of the ultrasonic signal to size flaws 

or for another purpose, it is necessary to 

correct for surface curvature when it is 

encountered. The "correction" value is the 

change in amplitude needed to bring signals 

from a curved surface measurement to the flat 

surface or DAC value.

Convex surfaces work to defocus the beam 

Diverge if the surface is convex. 

Concave surface contour- 

Focusing effects

Concave surfaces work to focus the beam 

Diverge if the surface is convex. 

Concave surface contour- 

Focusing effects

convex surfaces work to defocus the beam 

Convex surfaces work to defocus the beam

Convex surfaces work to defocus the beam- When sound travels from a 

liquid through a metal, it will converge if the surface is concave or diverge if 

the surface is convex.

Q: In an immersion method, the incident sound path enter the specimen 

interface with convex geometry, the sound path on entry into the specimen, 

the convex surface works to 

a) De-focus the sound 

b) Focus the sound 

c) Has no effect on the focusing or de-focusing the sound 

d) Reflected totally all the incident sound.

Q: In transmitting sound energy into a part shown below in a immersion 

testing, the sound beam will be: 

a) Diverge 

b) Converge 

c) Straight into 

d) Will not enter

A curvature correction curve can be generated experimentally in a manner 

similar to that used to generate a DAC curve, This simply requires a 

component with a representative reflector at various distances below the 

curved surface. Since any change in the radius will change the focus of the 

sound beam, it may be necessary to develop reference standards with a 

range of surface curvatures. 

However, computer modeling can also be used to generate a close 

approximation of the curvature correction value. Work by Ying and Baudry 

(ASME 62-WA175, 1962) and by Birchak and Serabian (Mat. Eval. 36(1), 

1978) derived methods for determining "correction factors" to account for 

change in signal amplitude as a function of the radius of curvature of convex, 

cylindrical components. 

An alternative model for contact and immersion probe inspection was more 

recently by researchers at the Center for NDE at Iowa State University. This 

mathematical model further predicts transducer radiation patterns using the 

Gauss-Hermite model, which has been used extensively for simulation of 

immersion mode inspections.

The resulting model allows computationally efficient prediction of the full 

ultrasonic fields in the component for 

1. any frequency, including broadband measurements. 

2. both circular and rectangular crystal shapes. 

3. general component surface curvature 

4. both normal and oblique incidence (e.g., angle beam wedges) transducers. 

When coupled with analytical models for defect scattering amplitudes, the 

model can be used to predict actual flaw waveforms. The image shown 

above was generated with this model.

The plot to the right shows an example curvature correction curve and DAC 

curve. This curvature correction curve was generated for the application of 

detecting a #4 flat bottom hole under a curved surface as shown in the 

sketch and photograph. An immersion techniques was used generate a 

shear wave since the reflective surface of the target flaw was not parallel with 

the surface. The DAC curve drops monotonically since the water path 

ensures that the near field of the sound beam is always outside the part. The 

correction factor starts out negative because of the focusing effect of the 

curved surface. At greater depths, the correction factor is positive due to the 

increased beam spread beyond the focal zone caused by the surface 

curvature.

Curvature Corrections

A table of correction values and the DAC and curvature correction curves for 

different size radiuses can be found at the following link. 

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/table/table.htm

Curvature Correction

Curvature Correction

4.4: Calibration References & Standards 

What are standards? 

Standards are documented agreements containing technical specifications or 

other precise criteria to be used consistently as rules, guidelines, or 

definitions of characteristics, in order to ensure that materials, products, 

processes, and services are fit for their purpose. 

For example, the format of the credit cards, phone cards, and "smart" cards 

that have become commonplace is derived from an ISO International 

Standard. Adhering to the standard, which defines such features as an 

optimal thickness (0.76 mm), means that the cards can be used worldwide.

An important source of practice codes, standards, and recommendations for 

NDT is given in the 

Annual Book of the American Society of Testing and Materials, 

ASTM. Volume 03.03, Nondestructive Testing 

is revised annually, covering acoustic emission, eddy current, leak testing, 

liquid penetrant, magnetic particle, radiography, thermography, and 

ultrasonics. 

There are many efforts on the part of the National Institute of Standards and 

Technology (NIST) and other standards organizations, both national and 

international, to work through technical issues and harmonize national and 

international standards.

Reference Reflectors: 

are used as a basis for establishing system performance and sensitivity.

Spherical reflectors are often used in immersion techniques for assessing 

sound fields. 

1. Omni direction 

2. Sphere directivity patterns reduce reflectance as compare with plane 

reflector 

3. Sphere of any materials could be used, however steel balls are often 

preferred.

Reference Reflectors are used as a basis for establishing system 

performance and sensitivity.

4.5: Questions & Answers 

Exercises

Q80: The 50 mm diameter hole in an IIW block is used to: 

(a) Determine the beam index point 

(b) Check resolution 

(c) Calibrate angle beam distance 

(d) Check beam angle 

Q81: The 100 mm radius in an IIW block is used to: 

(a) Calibrate sensitivity level 

(b) Check resolution 

(c) Calibrate angle beam distance 

(d) Check beam angle

Q6: The Notches are frequently used for reference reflectors for: 

A. Distance amplitude calibration for shear wave 

B. Area amplitude calibration 

C. Thickness calibration of plate 

D. Determine of near surface resolution 

Q17: Notches provide good reference discontinuities when UT examination is 

conducted to primarily detect defects such as: 

A. Porosity in rolled plate 

B. Inadequate penetration at the root of weld 

C. Weld porosity 

D. Internal inclusion

4.6: Video Time 

http://v.pps.tv/play_315ARS.html

Birring NDT Series, UT of Welds Part 1 of 2 - CALIBRATION 

https://www.youtube.com/embed/SRJktrHUlM4

Birring NDT Series, Ultrasonic Testing # 4, Angle Beam Shear Wave UT as 

per AWS D1.1 

www.youtube.com/embed/vXcAI-Zci30

UT Testing-Section 4 Calibration Methods

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?