Ultrasonic thickness gauging

What is NDT? 

Because in ultrasonic pulse-echo testing the time of travel of the pulse to a reflector is measured and displayed, it is a very simple adaptation to use this measurement as a thickness gauge. Low-cost small hand-held instruments are available, and the usual read-out is a digital display rather than an oscilloscope screen. Such thickness gauges require either calibration or a knowledge of the ultrasonic velocity in the material under test.