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will cause abnormal readouts even no display (generally, the abnormal
readout is less than actual thickness). In this situation, this kind of
material doesn't apply to be measured with this instrument.
5.9 Reference test piece
When making accurate measuring for different materials under different
conditions, the more the standard test piece is near to the material to be
measured, the more accurate the measurement is. The ideal reference
test pieces should be a group of test pieces with different thickness made
of materials to be measured, the test pieces can provide calibrating
factors for the instrument (such as the microstructure of the material,
heat-treating condition, direction of particles, surface roughness, etc.). To
meet the highest requirement on measuring accuracy, a set of reference
test pieces will be critical.
Under most situations, one can get satisfactory measuring accuracy with
only one reference test piece, which should have same material and
similar thickness with the object. Take an even object, measure it by using
a micron micrometer, then it can be used as a test piece.
For thin material, when its thickness is near to the low limit of the probe’s
measuring range, one can use test piece to determine the accurate low
limit. Never measure a material with a thickness lower than the low limit.
If the thickness range can be estimated, the thickness for the test piece
should select the high limit.
When the object is thick, especially for alloy with complex internal
structure, please select a test piece similar to the object from a group of
test pieces, thus to can have idea of calibration.
For most casting and forging, their internal structures have some direction.
In different direction, the sound velocity will have some change. To solve
the problem, the test piece should have an internal structure with same
direction as that of the object, and the transmitting direction of sound
wave in it should also be same as that for the object.
Under certain circumstances, look up the speed-of-sound table for given
materials can replace reference test pieces. But this is approximately to
substitute some test pieces. Under some situations, the value in the
speed-of-sound table will have some difference from the actual measured
values, this is due to difference in the material’s physical and chemical
characteristics. This way is usually used for measuring low-carbon steel,
and can only be taken as a rough measurement.
Thickness Gauge can measure sound velocity. Measure the sound velocity
