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Appendix D. Ultrasonic Thickness Gauge Theory of Operation
Transport® Model PT878 Portable Liquid Flowmeter User’s Manual 249
Appendix D. Ultrasonic Thickness Gauge Theory of
Operation
All ultrasonic thickness gauging involves timing the round trip of a sound
pulse in a test material. Because solid metal has an acoustic impedance that
differs from that of gasses, liquids, or corrosion products such as scale or
rust, the sound pulse will reflect from the far surface of the remaining metal.
The test instrument is programmed with the velocity of sound in the test
material, and computes the wall thickness from the simple formula
Distance = Velocity × Time
Single element transducers use one element as both transmitter and receiver.
Dual element transducers incorporate separate transmitting and receiving
elements. These elements are mounted on delay lines that are usually cut at
an angle to the horizontal plane (the roof angle), so that the transmitting and
receiving beam paths cross beneath the surface of the test piece. This
crossed-beam design of duals provides a pseudo-focussing effect that
optimizes measurement of minimum wall thickness in corrosion
applications. Duals will be more sensitive than single element transducers
to echoes from the base of pits that represent minimum remaining wall
thickness. Also, duals may often be used more effectively on rough outside
surfaces. Couplant trapped in pockets on rough sound entry surfaces can
produce long, ringing interface echoes that interfere with the near surface
resolution of single element transducers. With a dual, the receiver element
is unlikely to pick up this false echo. Finally, duals may be designed for
high temperature measurements that would damage single element contact
transducers.