DeFelsko PosiTector UTG v.3.1 Full Manual - Manual (Page 40)

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Measuring on hot surfaces
Measurements taken at higher temperatures (above 100° C / 212°
F) require special consideration. Both material sound velocity and
probe zero will change with temperature. For maximum accuracy
at high temperatures, adjustment should be performed using a
material of known thickness heated to the temperature where
measurements are to be performed. The probe should remain on
the surface only as long as it takes to get a measurement.
The surface temperature of the test piece should not exceed (150°
C / 300° F).
Measuring laminated materials
The density (and thus sound velocity) of laminated materials often
varies considerably within a single piece or from piece to piece.
For best accuracy, perform a calibration adjustment to a sample of
known thickness. This sample should be as close as possible to
the density characteristics of the part to be measured. Be aware
that air gaps between laminates will likely cause strong reflections,
resulting in the Gage measuring a lower thickness value than the
actual total thickness.
Calibration
Measurement accuracy is directly related to the accuracy and care
taken when performing a gage calibration adjustment. Adjustments
will likely be required whenever the test material or temperature
changes. Periodic checks with samples of known thickness are
recommended to verify that the Gage is operating properly.
Taper or Eccentricity
If the measured surface and the back wall are not parallel to one
another, measurement accuracy may be diminished due to a
distorted echo.
Sound Scattering
Some materials have conditions which can limit the accuracy and
thickness range of the Gage. For example, individual crystallites in
cast metals and dissimilar materials in composites can scatter the
ultrasonic pulse. The return echo may be too weak for the Gage to
effect a measurement.