GE DM5E Operating Manual page 12

Preface
Ultrasonic Thickness Measurement Critical Operating Procedures
The following operating procedures must be observed by all users of ultrasonic thickness gauges in order to minimize errors in test results.
1. 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 velocities 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.
Operators should also be aware that the 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.
2. Probe Zero Procedure
Before performing calibration with a contact probe, the probe zero procedure must be performed first. If the zero procedure is not performed
or if it is performed improperly, inaccurate thickness readings will result.
x DM5E Operating Manual