GE 37 400, Technical Reference And Operating Manual

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Evaluation according to the DGS method 5 Operation
USM 36 Issue 2 (12/2013) 5-121
5.21 Evaluation according to the
DGS method
The USM 36 offers the optional evaluation method DGS
(distance - gain - size).
Using the DGS for measurements
You can use the DGS mode to compare the reflecting
power of a natural flaw in the test object with that of a
theoretical flaw (circular disk-shaped equivalent reflec-
tor) at the same depth.
ATTENTION
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 posi-
tion, etc.).
The so-called DGS diagram forms the basis for this
comparison of the reflecting power. This diagram con-
sists of a set of curves showing the connection of three
influencing variables:
● Distance D between the probe coupling face and the
circular disk-shaped equivalent reflector
● Difference in gain G between a circular disk-shaped
equivalent reflector and a reference reflector, e.g. an
infinitely large backwall
● Size S of the circular disk-shaped equivalent reflector
The influencing variable S remains constant for one
curve in each set of curves.
The advantage of the DGS method lies in the fact that
you can carry out reproducible evaluations of small dis-
continuities. This reproducibility is especially important,
e.g. whenever you want to carry out an acceptance test.
In addition to the influencing variables already men-
tioned, there are other factors affecting the curve shape:
● sound attenuation,
● transfer losses,
● amplitude correction value,
● probe.