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

6.9.3 Position measurements
Position measurements of solid parts or liquid materials in closed metallic
cavities are usually made with pulse echo type equipment. One technique is
to look for changes in back reflection intensity as the position of the search
unit is changed. In one variation of this technique, the oil level in differential
housings was checked to see if the automated equipment used to put the oil
in the housing on an-assembly line had malfunctioned. The test developed for
this application utilized a dual-gated pulse-echo system that employed a 1.6-
MHz immersion-type search unit with a thin, oil filled rubber gland over its
face. The search unit was automatically placed against the outside surface of
the housing just below the proper oil level, as shown in Fig. 60(a).