Ultrasonic Testing is a method which uses sound waves to measure a piece of material. The operator uses an ultrasonic device connected to a probe by a probe lead.
A Flaw Detector uses short sound waves to check the integrity of the material. This can detect small irregularities and can help you to plot out the shape of the defect.
A Thickness Meter uses short sound waves to test the time the sound takes to travel from the probe to the back wall of the material and back. The time taken will provide an accurate measurement of the thickness of the material.