GE Ultrasonic USM Go Operator's Manual page 16

Preface
Test Object Material Effects
If the material of the test object is not homogeneous, the sound waves may propagate at different velocities in different parts of the test
object. An average sound velocity should then be used for the range calibration. This is achieved by using a reference block with a sound
velocity equal to the average sound velocity of the test object.
If substantial sound velocity variations are expected, then the instrument calibration should be adjusted to the actual sound velocity values at
shorter time intervals. Failure to do so may lead to false readings.
Test Object Temperature Effects
The sound velocity also varies as a function of the temperature of the test object. This can cause appreciable errors in measurements if the
instrument has been calibrated with a reference block at one temperature, and the instrument is then used on a test object at a different
temperature. Such measurement errors can be avoided either by ensuring that the reference block and the test object are at the same
temperature, or by using a correction factor obtained from published tables.
xiv USM Go Operator's Manual