An Explanation of the Echo-to-Echo Measurement Technique

Introduction:

The ultrasonic echo-to-echo technique utilizes gates set around two consecutive backwall echoes and measures the difference between them. This difference negates the coating thickness and returns just the part thickness.

The echo-to-echo technique is useful because the coating can add a substantial error to the overall thickness reading. This is especially true on painted piping and structural steel.

The error incurred is actually about 2 to 3 times the thickness of the coating, because the velocity of sound through paint is that much slower than through steel.

Explanation

How exactly does the echo-to-echo technique manage to subtract the paint thickness? The simple math in the following figure will demonstrate.