Measurements of Nonconducting Coatings on Conductive Materials
The thickness of nonmetallic coatings on metal substrates can
be determined simply from the effect of liftoff on impedance.
This method has widespread use for measuring thickness of paint
and plastic coatings. The coating serves as a spacer between the
probe and the conductive surface. As the distance between the
probe and the conductive base metal increases, the eddy current
field strength decreases because less of the probe's magnetic
field can interact with the base metal. Thicknesses between 0.5 and
25 µm can be measured to an accuracy between 10% for lower
values and 4% for higher values. Contributions to impedance changes
due to conductivity variations should be phased out, unless it
is known that conductivity variations are negligible, as normally
found at higher frequencies.
Fairly precise measurements can be made with a standard eddy
current flaw detector and a calibration specimen. The probe is
nulled in air and the direction of the lift-off signal is established.
The location of the signal is marked on the screen as the probe
is placed on the calibration specimen in areas of decreasing coating
thickness. When the probe is placed on the test surface, the position
of the signal will move from the air null position to a point
that can be correlated to the calibration markings.
Specialized eddy current coating thickness detectors are also
available and are often pocket-sized with the probe resembling
a small pencil. They are usually operated by a small battery and
provide a digital read-out in the appropriate units. Calibration
adjustments, some of which are laid down by standards such as BS
EN 2360 (1995) and ASTM B 244 and E 376, may be assisted by the
use of an inbuilt microprocessor.