3
Calibration of Sound Velocity
The principle of operation of an ultrasonic thickness gauge is that the instrument measures the time of flight of an
ultrasonic pulse through the test piece and multiplies this time by the velocity of sound in the material. Thickness
measuring error is minimized by ensuring that the sound velocity to which the instrument is calibrated is the sound
velocity of the material being tested. Actual sound velocity in materials often vary significantly from the values found in
published tables. In all cases, best results are obtained if the instrument is calibrated on a velocity reference block
made from the same material as the test piece; this block should be flat and smooth and as thick as the maximum
thickness of the test piece. Operator should also be aware that sound velocity may not be constant in the material
being tested; heat treating, for example, can cause significant changes in sound velocity. This must be considered
when evaluating the accuracy of the thickness provided by this instrument. Instruments should always be calibrated
before testing, and the calibration should be checked after testing, to minimize testing errors.
Probe Zero Procedure
The probe zero procedure must be performed as described in this manual. The zero reference block should be clean
and in good condition, without noticeable wear. Failure to properly perform the probe zero procedure will result in
inaccurate thickness readings.
Effects of Temperature on Calibration
Variations in temperature change the sound velocity of materials and transducer delay lines and therefore, zero calibration.
All calibrations should be performed on-site and with test blocks at the same temperature as the test piece, in order to
minimize errors due to temperature variations.
Transducer Selection
The transducer used in testing must be in good condition without noticeable wear of the front surface. Badly worn
transducers will have a reduced effective measuring range. The specified range of the transducer must include the
complete range of thickness to be tested. The temperature of the material to be tested must be within then transducer’s
temperature range.
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