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mvip@mitech-ndt.com
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.
