GE MFE DMS 2 Operating Manual page 7

Important Notice
thickness greater than twice the minimum of the specified
range. This thickness should be determined by calibrating the
instrument/transducer combination on reference blocks that
represent the complete range of possible thicknesses that may
be encountered in testing. This is particularly important when
the test piece is being ultrasonically measured for the first time
or in any case where the history of thickness of the test
specimen is unknown.
Page vi DMS 2 Operating Manual