DISTANCE AMPLITUDE CORRECTION CURVE(DAC)

Initially cut out a transparent plastic with the same size as of the CRT screen and put or stick it on the CRT screen .

1.Position the probe targeting the side drilled hole(SDH) as shown below. Maximize the echo , then bring the echo to 80%FSH(Full Screen Height) by turning the dB control. Mark the peak of the echo on the screen. Note your dB gain ,ex. 66dB.Reminder: Use the formula BPL=D÷ cos Ø to estimate where the echo should fall on the screen.

2.Bring the echo further down using the dB control to 40% FSH.You can do this by subtracting a 6db from your previous dB gain,ie 66db-6db=60 dB.Mark the peak of the echo on the screen.

3.Again using the dB control bring down the echo to 20%FSH.Subtract another 6dB from your last dB setting.ex60-6=54 db. You have now 3 point marks for the first probe position.

4.Position the probe as in fig below. Do the same procedure as above. Result will be another 3 point mark for the 2nd probe position. To estimate the echo location on the screen,BPL= T÷cosØ + d ÷ cos Ø

2.4mmØ SDH

2nd pos.

1st pos.

20% FSH

80% FSH

40% FSH

80% FSH

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Ex. of ASME DAC Block use for material Thk. 25mm and below

D

1ST position

d

T

5.Position the probe in 3rd position.Do the same procedure as above. Result will be another 3 point mark for the 3rd probe position. To estimate the echo location on the screen, BPL= T÷cosØ + D ÷ cos Ø

6. To complete the DAC Curve join all the point mark as below.

7.Now DAC curve is ready.

Reminder also that the DAC block has a more smooth and shiny surface than a material that you are going to test, so to compensate for this ,consider also the Transfer Correction Factor.( do this separately) Whatever dB result ex. 4dB,( depends on the roughness of the test material) you are going to get, add this to the dB gain, ex. 66+4=70 dB. This is now your Primary Reference Level(PRE) or the Sensitivity gain setting. During scanning add another 6dB.Scanning dB now is “66+transfer correction+ 6dB”.When you get any indication , remove the 6dB and evaluate the result according to acceptance criteria.(Ex ASME VIII Appendix 12 Ultrasonic Examination of Welds).

7.Provide DAC curve for each probe that you are going to use,70°,60°,45°.

Types of DAC Block

,

3rd pos.

80% FSH

40% FSH

20% FSH

80% FSH

40% FSH

20% FSH

Edmundo T. Botor J r. Senior CSWIP 3.2 No.3147-2 PCN Level 2 No.207763

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Use for T=25mm and above

30mm

3.2mm diam. hole

D