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D A C

Developing a Distance Amplitude Correction (DAC) Curve involves using a reference standard with reflectors like side drilled holes or notches located at varying depths to establish a graphic reference level sensitivity curve as a function of sweep distance on an A-scan display. This allows signals from similar discontinuities to be evaluated accounting for signal attenuation with depth. The document then describes constructing a DAC curve for a 1 3/4” thick material using an ASME calibration block with side drilled holes at 1/4T and 3/4T, peak each signal at 80% full screen height and connect the dots to complete the curve.

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136 views2 pages

D A C

Developing a Distance Amplitude Correction (DAC) Curve involves using a reference standard with reflectors like side drilled holes or notches located at varying depths to establish a graphic reference level sensitivity curve as a function of sweep distance on an A-scan display. This allows signals from similar discontinuities to be evaluated accounting for signal attenuation with depth. The document then describes constructing a DAC curve for a 1 3/4” thick material using an ASME calibration block with side drilled holes at 1/4T and 3/4T, peak each signal at 80% full screen height and connect the dots to complete the curve.

Uploaded by

naiyar alam
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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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, ie.. using a 1” thick standard, calibrate the sweep for 2” of material travel.

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.

ASME 1 ¾” Basic Calibration Block


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.

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