DAC Distance Amplitude Curve

DISTANCE AMPLITUDE CORRECTION 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 the side drilled hole(SDH) as shown below. Maximize the echo , then  bring the echo to 80%FSH(Full 80%FSH(Full Screen Height) by turning 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÷ BPL=D÷ cos Ø to estimate where where the echo should fall fall on the screen. 1ST position

2.4mmØ SDH 80% FSH

D

T

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

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.

80% FSH

40% FSH

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

20% FSH

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

1st pos.

d 2nd pos.

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5.Position the probe in 3 rd position.Do the same procedure as above. Result will be another 3 point mark  for the 3 rd probe position. To estimate the echo location on the screen, BPL= T÷cosØ + D ÷ cos Ø 3rd pos. 80% FSH

D

40% FSH 20% FSH

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

80% FSH

40% FSH 20% FSH

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°.

3.2mm diam. hole

Types of DAC Block  30mm

, Use for T=25mm and above

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