GE Ultrasonic USM Go Operator's Manual page 111

3.13 DGS Evaluation Mode (cont.)
Using the DGS mode (Distance Gain Size), you can compare the reflecting power of a natural flaw in the test object with that of a
theoretical flaw (e.g. a circular, disk-shaped equivalent reflector) at the same depth.
CAUTION! You are comparing the reflecting power of a natural flaw with that of a theoretical flaw. No definitive conclusions
may be drawn about the natural flaw (roughness, inclined position, etc.).
The DGS diagram forms the basis for this comparison of the reflecting power. This diagram consists of a set of curves showing the
connection of three influencing variables:
•
Distance (D) between the probe and circular, disk-shaped equivalent reflector
•
Difference in gain (G) between various large circular, disk-shaped equivalent reflectors and an infinitely large backwall
Size (S) of the circular, disk-shaped equivalent reflector. The influencing variable S remains constant for one curve in a set
•
The advantage of the DGS method lies in the fact that you can carry out reproducible evaluations of small discontinuities. This
reproducibility is most important whenever you carry out an acceptance test.
USM Go Operator's Manual
Chapter 3. Making Measurements
93