Appendix B Applications Notes
Measuring
pipe
and
tubing.
When measuring a piece of pipe to determine the thickness of the pipe wall, orientation of the transducers is
important. If the diameter of the pipe is larger than approximately 4 inches, measurements should be made with the
transducer oriented so that the gap in the wearface is perpendicular (at right angle) to the long axis of the pipe. For
smaller pipe diameters, two measurements should be performed, one with the wearface gap perpendicular, another
with the gap parallel to the long axis of the pipe. The smaller of the two displayed values should then be taken as 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.
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