Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

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.