Magnetic Induction Measurement Principle of Coating Thickness Gauges
When using the principle of magnetic induction, the thickness of the coating is determined by the magnitude of the magnetic flux flowing into the ferromagnetic substrate from the probe through the non ferromagnetic coating. The corresponding magnetic resistance can also be measured to indicate its coating thickness. The thicker the coating, the greater the magnetic resistance and the smaller the magnetic flux. A thickness gauge based on the principle of magnetic induction can, in principle, measure the thickness of non-magnetic coatings on a magnetic substrate. Generally, it is required that the magnetic permeability of the substrate be above 500. If the coating material also has magnetism, it is required that the difference in magnetic permeability between it and the substrate is large enough (such as nickel plating on steel). When the probe with a coil wound around the soft core is placed on the tested sample, the instrument automatically outputs a test current or test signal. Early products used pointer type gauges to measure the magnitude of induced electromotive force, and the instrument amplified the signal to indicate the thickness of the coating. In recent years, new technologies such as frequency stabilization, phase locking, and temperature compensation have been introduced in circuit design, utilizing magnetoresistance to modulate measurement signals. We also adopted integrated circuits designed with and introduced microcomputers, which greatly improved the measurement accuracy and reproducibility (almost by an order of magnitude). Modern magnetic induction thickness gauges have a resolution of 0.1um, an allowable error of 1%, and a measurement range of 10mm.
The magnetic principle thickness gauge can be applied to measure the paint layer, porcelain and enamel protective layer, plastic and rubber coating, various non-ferrous metal electroplating layers including nickel chromium, and various anti-corrosion coatings of chemical and petroleum industries on the surface of steel.
