Dakota Ultrasonics VX - Manual (Page 26)

Dakota Ultrasonics
22
APPENDIX B
Application Notes
• 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
200
°
F (100
°
C), 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 (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 VX 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 that 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.