Dakota Ultrasonics
24
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 (refer to page 11) on a sample piece of known
thickness, which is at or near the temperature of the material to be
measured. This will allow the
PR-8
2
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 itself, 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.
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