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

DGS is a primarily mathematical technique originally based on the ratio of a
circular probe’s calculated beam profile and measurable material properties
to circular disk reflectors. The technique has since been further applied to
square element and even dual element probes, although for the latter, curve
sets are empirically derived. It is always up to the user to determine how the
resultant DGS calculations relate to actual flaws in real test pieces.
An example of a typical DGS curve set is seen below. The uppermost curve
represents the relative amplitude of the echo from a flat plate reflector in
decibels, plotted at various distances from the transducer, and the curves
below represent the relative amplitude of echoes from progressively smaller
disk reflectors over the same distance scale.