Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

5.1.6.3 Reference samples
The substrate reference sample for non-conducting coating thickness
measurement should have the same conductivity, surface contour and
thickness as the material to be tested. For reference samples for the coating,
stiff plastic shims of appropriate thicknesses are required. If the measurement
is a go/no-go type, to verify whether the thickness lies within acceptable limits,
only two shim thicknesses are necessary, one with the thickness of the lower
limit and one with the thickness of the higher limit. However, a third shim, with
thickness in the middle of the range, is desirable. If, however, the actual
thickness of the coating is to be measured, at least three shim thicknesses
are required. These should have thicknesses equal to the maximum and
minimum expected thicknesses, and one approximately midway between
these. If the coating required to be measured is thin, bare metal may be used
for calibration. More shim thicknesses may be required if the response is
notably nonlinear.
Charlie Chong/ Fion Zhang