Magnetic induction and eddy current instruments provide measurements that are relatively insensitive to the type of nonferrous and nonconductive coating or material thickness being gaged. However, these gages are sensitive to the properties of the substrate material to which the coatings have been applied, and they can t measure the thickness of coatings on nonmetallic substrates. In addition, magnetic induction and eddy current instruments can t discriminate between individual layers in a multi-layer coating process. Special parts with separated regions are required to obtain an indirect measurement using magnetic induction and eddy current technology. The traditional tool for determining coating thickness on plastic and other nonmetallic substrates is the measuring microscope.
Optical microscopes provide a direct visual thickness measurement.
Unlike magnetic induction and eddy current gages, the measuring microscope distinguishes individual layers in a multilayer process. Microscope measurements require a sample cross section of the part to be gaged. Obtaining the cross section usually requires gouging or cutting the part, and thus the measurement is inherently destructive. In addition, correct preparation of the cross sections is time and labor intensive. Incorrectly prepared cross sections produce inaccurate measurements.