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

5.11: Pulse Repetition Rate and Penetration
The energy of the generated sound depend on the pulse repetition rate, the
higher the repetition rate the higher the energy and the sound able to
penetrate thicker material. However if the PRR is excessive, ghost signal may
formed, this is due to the fact that the next sequence of pulse is generated
before the expected returning signal reaching the receiver.
1. The pulse repetition frequency or pulse repetition rate PRR:
is the number of pulse of ultrasonic energy that leave the probe in a given
time (per second). Each pulse of energy that leave the probe must return
before the next pulse leave, otherwise they will collide causing ghost
echoes.
2. Transit time: The time taken for the pulse to travel from the probe and
return
3. Clock interval: The time between pulse leaving the probe.
The transit time must be shorter than the Clock interval else, ghost signal may
formed. Typically the Clock interval should be 5 time the transit time.