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

What is DGS
TCG is a time-corrected DAC so that equal dimension reflectors give equal amplitude
responses for all sound path distances. Used for PAUT Sectorial scans where it would
be otherwise impossible to set every angle and sound path to the same sensitivity
level using DAC's.
ASTM E-1316: DGS (distance gain size-German AVG) distance amplitude curves
permitting prediction of reflector size compared to the response from a back surface
reflection.
The probe manufacturer supplies data sheet diagams for each probe which shows the
amplitude response curves from the backwall and a range of diameters of flat-bottom
holes along the length of the soundfield.
Have a look at EN 583-2:2001 Sensitivity and range setting for excellent authoritative
descriptions of DAC/TCG and DGS. You'll have to look at AWS D1.1. for instance
for knowledge of their sensitivity setting requirements.
Knowledge of these techniques is desirable but will such knowledge really improve
your inspection method? You use DAC because the Codes and standards you work to
require you to assess indications to those DAC's. A report that a reflector was 3,5 mm
equivalent FBH size to DGS would most probably be rejected.