General-No presently available thickness gauge is applicable to all combinations of coating-substrate thicknesses and materials. The limitations of a particular instrument are generally delineated by its manufacturer.

Magnetic-Magnetic-type gauges measure either magnetic attraction between a magnet and a coating or its substrate, or reluctance of a magnetic flux path passing through the coating and substrate. These gauges are designed to measure thickness of a nonmagnetic coating on a magnetic substrate. Some of them will also measure thickness of nickel coatings on a magnetic or nonmagnetic substrate.

Eddy Current-Eddy-current-type thickness gauges are electronic instruments that measure variations in impedance of an eddy-current inducing coil caused by coating thickness variations. They can only be used if the electrical conductivity of the coating differs significantly from that of the substrate.

Accuracy-The accuracy of a measurement depends on the instrument, its calibration and standardization, and its operating conditions. The accuracy is also affected by the interferences listed in Section 5, such as part geometry (curvature), magnetic permeability, and surface roughness.

Note 2-This practice allows the coating thickness to be determined within ±10 % of its true thickness or to within ±2.5 μm (or ±0.0001-in.), whichever is the greater. (See exceptions in Appendix X2.)