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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.