Ultrasonic thickness gauges can be as accurate as most measuring techniques, but the transducer surface, and frequency need to be tailored to the specific piece being measured. In most applications, the sending element and receiving element of the transducer are housed in the same package. It is critical that the transducer be positioned exactly perpendicular to the surface being tested. any angle that deviates will give exaggerated, higher readings. Picture a circle, within a circle. A point at 6:00 on each circle will be the actual thickness, but if the transducer is not facing straight on, you might be at 6:00 on the outside, and the beam will strike the 9:00position on the inside. This can be cured with a transducer having an exact radius machined on it's shoe, not practical for any but the diameter it was machined to fit.
Mechanical wall thickness gauges are POINT-TO-POINT devices, like a large mouthed caliper, which measure from an exact point on the OD, to an EXACT point on the ID, Not the inside surface in general. The barrel can only be touching these two points.
Ultrasonics, both in flaw detection, and thickness measurement, have their place, and are the best way to measure when the opposite side of the wall is difficult to reach, such as the hull of a ship, wall of a tank, or inside cavity of a complex shape.
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