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Appendix D. Ultrasonic Thickness Gauge Theory of Operation
252 Transport® Model PT878 Portable Liquid Flowmeter User’s Manual
D.1.2 Calibration
The accuracy of measurements are only as good as the accuracy and care
with which the gauge has been calibrated. It is essential that the thickness
gauge be calibrated (as shown on page 159) whenever the transducer is
changed or you have a reason to doubt the accuracy of the readings.
Periodic checks with samples of known thicknesses are recommended to
verify that the gauge is operating properly.
D.1.3 Taper or Eccentricity
If the contact surface and the back surface are tapered or eccentric with
respect to each other, the return echo again becomes distorted and the
accuracy of measurement is diminished.
D.1.4 Acoustic Properties of the Material
There are several conditions found in engineering materials that can
severely limit the accuracy and thickness range that can be measured.
D.1.4a Sound Scattering
In some materials, notably certain types of cast stainless steel, cast irons,
and composites, the sound energy is scattered from individual crystallites in
the casting or from dissimilar materials within the composite. This effect
reduces the ability to discriminate a valid return echo from the back side of
the material and limits the ability to gauge the material ultrasonically.
D.1.4b Velocity Variations
A number of materials exhibit significant variations in sound velocity from
point-to-point within the material. Certain types of cast stainless steels and
brass exhibit this effect due to a relatively large grain size and the
anisotropy of sound velocity with respect to grain orientation. Other
materials show a rapid change in sound velocity with temperature. This is
characteristic of plastic materials where temperature must be controlled in
order to obtain maximum precision in the measurement.
D.1.4c Sound Attenuation or Absorption
In many organic materials, such as low density plastics and rubber, sound is
attenuated very rapidly at the frequencies used in normal ultrasonic
thickness gaging. Therefore, the maximum thickness that can be measured
in these materials is often limited by sound attenuation.