12
Measuring hot surfaces
The velocity of sound through a substance is dependent upon its temperature. As materials heat up, the
velocity of sound through them decreases. In most applications with surface temperatures less than
about 100
℃
, no special procedures must be observed. At temperatures above this point, the change in
sound velocity of the material being measured starts to have a noticeable effect upon ultrasonic
measurement. At such elevated temperatures, it is recommended that the user perform a calibration
procedure on a sample piece of known thickness, which is at or near the temperature of the material to
be measured. This will allow the gauge to correctly calculate the velocity of sound through the hot
material.
When performing measurements on hot surfaces, it may also be necessary to use a specially
constructed high-temperature transducer. These transducers are built using materials which can
withstand high temperatures. Even so, it is recommended that the probe be left in contact with the
surface for as short a time as needed to acquire a stable measurement. While the transducer is in
contact with a hot surface, it will begin to heat up, and through thermal expansion and other effects, may
begin to adversely affect the accuracy of measurements.
Measuring laminated materials.
Laminated materials are unique in that their density (and therefore sound-velocity) may vary
considerably from one piece to another. Some laminated materials may even exhibit noticeable changes
in sound-velocity across a single surface. The only way to reliably measure such materials is by
performing a calibration procedure on a sample piece of known thickness. Ideally, this sample material
should be a part of the same piece being measured, or at least from the same lamination batch. By
calibrating to each test piece individually, the effects of variation of sound-velocity will be minimized.
An additional important consideration when measuring laminates, is that any included air gaps or
pockets will cause an early reflection of the ultrasound beam. This effect will be noticed as a sudden
decrease in thickness in an otherwise regular surface. While this may impede accurate measurement of
total material thickness, it does provide the user with positive indication of air gaps in the laminate.
Measuring through paint & coatings.
Measuring through paints and coatings are also unique, in that the velocity of the paint/ coating will be
significantly different form the actual material being measured. A perfect example of this would be a mild
steel pipe with approximately 0.6mm of coating on the surface. Where the velocity of the pipe is 5920m/s,
and the velocity of the paint is 2300m/s. If the user is calibrated for mild steel pipe and measures through
both materials, the actual coating thickness will appear to be 2.5 times thicker than it actually is, as a
result of the differences in velocity. This error can be eliminated by using a special echo-echo mode to
perform measurements for applications such as these. In echo-echo mode, the paint/ coating thickness
will be eliminated entirely and the steel will be the only material measured.
Suitability of materials
Ultrasonic thickness measurements rely on passing a sound wave through the material being measured.
Not all materials are good at transmitting sound. Ultrasonic thickness measurement is practical in a wide
variety of materials including metals, plastics, and glass. Materials that are difficult include some cast
materials, concrete, wood, fiberglass, and some rubber.
Couplants
All ultrasonic applications require some medium to couple the sound from the transducer to the test
piece. Typically a high viscosity liquid is used as the medium. The sound used in ultrasonic thickness
measurement does not travel through air efficiently.
