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

In another position measurement system, a set of two contact-type 4-MHz
search units was utilized in a through transmission pitch-catch arrangement to
determine the movement of a piston in a hydraulic oil accumulator as both
precharge nitrogen-gas pressure and standby oil pressure varied (Fig. 61).
The two search units were placed 180° apart on the outside surface of the
accumulator wall at a position on the oil side of the piston, as shown in Fig.
61.
When a high energy pulse was sent from the transmitting unit, the beam was
able to travel straight through the oil, and a strong signal was picked up by
the receiving unit. However, as the search units were moved toward the
piston (see locations drawn in phantom in Fig. 61), the sloping sides of the
recess in the piston bottom deflected the beam so that very little signal was
detected by the receiving unit.