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

Locating reflectors with an angle-beam probe
Fig. 53 Scanning a reflector using an angle beam probe
The echo of a discontinuity on the instrument display does not now give us
any direct information about its position in the material. The only available
information for determination of the reflector position is the scale position and
therefore the sound path s, this means the distance of the discontinuity from
the index point (sound exit point) of the probe, Fig. 53.
The mathematics of the right-angled triangle helps us to evaluate the Surface
Distance and the Depth of a reflector which are both important for the
ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly
mark a detected flaw's position on the surface of the test object by
measurement of the surface distance from the sound exit point and to give
the depth. For practical reasons, the reduced surface distance is used
because this is measured from the front edge of the probe. The difference
between the surface distance and the reduced surface distance corresponds
to the x-value of the probe, this is the distance of the sound exit point to the
front edge of the probe, Fig. 54b.