Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

The preferred type of probe is an absolute reflection probe, preferably spring
loaded, and with V-notches if curved surfaces are to be tested. Reflection
probes are preferred because they show a more linear lift-off curve and they
respond to a greater range of lift-off values. Note that differential probes
should not be used because they eliminate or reduce the lift-off signal. The
coil diameter is important because the size of the magnetic field increases as
the coil diameter increases. The magnetic field from a coil at a distance of
one coil diameter from the coil is only approximately 10% of that adjacent to
the coil, and a very low eddy current intensity is produced at this distance.
The magnetic fields of small diameter coils may not extend through a thick
coating to the substrate. If this is the case, a lift-off signal cannot be obtained
and measurement of coating thickness is not possible. In general, therefore,
the thicker the coating to be measured, the larger the coil diameter should be.
Note that the size of a probe is not necessarily a good guide to coil size.
Some spot probes have a small diameter coil in a large diameter housing.
Keywords:
In general, therefore, the thicker the coating to be measured, the larger the
coil diameter should be.
Charlie Chong/ Fion Zhang