Eddy current lift off signal can be used for both coating thickness assessment and also wall thickness assessment.
Coating Thickness Assessment
In the simple case of a non-conductive coating on a conductive material, such as paint, the eddy current lift off signal can be used. The probe type to be used should have an absolute response with reflection spot face probes offering some advantage in temperature stability and frequency range. Higher frequencies are preferred (100 kHz and higher) and for non-ferrous materials, it should be checked that the frequency is sufficiently high so as not to be influenced by material thickness, say 10 times that to make the wall thickness equal the effective depth of penetration.
To obtain quantitative readings a calibration piece with several different coating thicknesses, in the range of interest is essential, and a calibration curve created. Some instruments have an intrinsic function as part of conductivity measurement for obtaining direct readings.
For the more complex case where the coating is conductive, then the following needs to be taken into account. The two materials must have different conductivities and/or relative permeabilities and the top coating must be non-magnetic. Choose a frequency that will make the effective depth of penetration equal the nominal wall thickness. If the surface coating has a higher resistivity than the lower coating, then by using a frequency that is sufficiently low to penetrate the surface coating, results will be similar to that for non-conductive coatings.
Wall Thickness Assessment
This is possible in the same way as it is possible to determine non-ferrous conductive coating thickness and the same rules apply to choice of frequency.
If you have a coating thickness application that you think you maybe able to solve through using Eddy Current NDT, please email us at firstname.lastname@example.org