• Sound energy can be generated over a broad frequency spectrum. Audible sound, for example, is restricted to a low frequency range with a typical upper limit of 20,000 cycles/sec, or 20 kHz. Ultrasound is sound at frequencies above 20 kHz, too high to be detected by normal human hearing. Corrosion thickness guages typicallyoperated at much higher frequencies, ranging from 1 MHz to 10 MHz.

• Ultrasound - because of its short wavelength - has the advantage that it can make very accurate thickness measurements on metals (as well as on plastics, glass, rubber, and other engineering materials). Equally important, measurements are nondestructive and allow an inspector to obtain wall thickness from one side without having to cut the test piece open. Meausrements are repeatable, meaning an inspector has the ability to perform the same inspection at various time intervals and monitor the degree of wall thinning. Ultrasonic thickness guages can play a vital role in the predictive or preventative maintenance of pipes, tanks, or other metal sturctures subject to corrosion, erosion or pitting.

• Recent advances in the design of ultrasonic corrosion thickness guages utilizing dual elements transducers have made it possible to take accurate metal thickness measurements with no need to remove paint or coatings. This feature is often referred to as echo-to-echo thickness measurements. Traditional ultrasonic corrosion guages make thickness measurements by determining pulse transit time to the first backwall echo. This technique generally works very well, except for the specialized case where the surface of the pipe or tank is covered with a layer of paint or other coating. In these cases, traditional corrosionguages will measure the total thickness of both the coating and the metal substrate. Because paint and similar coating normally have a sound velocity that is much slower than the metal substrate.

* used with permission from PANAMETRICS