GB/T 7991.5-2014 "Test methods for vitreous and porcelain enamels - Part 5: Measurement of thickness by electromagnetic method" specifies the electromagnetic measurement method for the thickness of vitreous enamel layers on magnetic substrates, applicable to vitreous enamel equipment with magnetic substrates. Based on the principle of electromagnetic induction, measurements are taken by placing the probe of an electromagnetic thickness gauge perpendicular to the lining surface. For flat samples, at least three points should be selected per 100 cm², with edge areas being at least 20 mm from the edge. The inspection requires the use of an electromagnetic thickness gauge, calibration foils, and standard test blocks, enabling non-destructive testing and providing data for evaluating the uniformity and quality of the vitreous enamel layer.