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mvip@mitech-ndt.com
the thickness at that point.
Measuring hot surfaces
The velocity of sound through a substance is dependant 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.
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.
