Introduction of Eddy Current Measuring Principle of Coating Thickness Gauge
The high-frequency AC signal generates an electromagnetic field in the probe coil, and when the probe is close to the conductor, eddy currents are formed in it. The closer the probe is to the conductive substrate, the greater the eddy current and the greater the reflection impedance. This feedback action characterizes the size of the distance between the probe and the conductive substrate, that is, the thickness of the non-conductive coating on the conductive substrate. Because this type of coating thickness gauge probe is designed to measure the thickness of coatings on non-ferromagnetic metal substrates, it is often called a non-magnetic probe. Non-magnetic probes use high-frequency materials as coil cores. Compared with the principle of magnetic induction, the main difference is that the probe of the coating thickness gauge is different, the signal frequency is different, and the signal size and scale relationship are different. The coating thickness gauge using the principle of eddy current can measure non-conductive coatings on all conductive substrates in principle, such as paint and plastic coatings on the surface of aerospace vehicles, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products. and anodized film. The coating material has a certain conductivity, which can also be measured through calibration, but the ratio of the two conductivity is required to be at least 3-5 times different. Although steel substrates are also conductors, the magnetic principle of measuring coating thickness is more suitable for this type of task.
Several factors affect coating thickness gauge measurements. The thickness measured by the magnetic method is affected by the change of the metal properties of the base (in practical applications, the change of the magnetic properties of low carbon steel can be considered slight). The standard sheet is used to calibrate the instrument; the conductivity of the base metal has an impact on the measurement, and the conductivity of the base metal is related to its material composition and heat treatment method. Calibrate the instrument by using a standard sheet with the same properties as the base metal of the test piece; each instrument has a critical thickness, greater than this thickness, the measurement will not be affected by the thickness of the base metal; it is sensitive to the abrupt change of the surface shape of the test piece, Therefore, it is unreliable to measure near the edge or inner corner of the test piece; the curvature of the test piece has an influence on the measurement, which increases significantly with the decrease of the radius of curvature, therefore, the measurement on the surface of the curved test piece is also unreliable The probe will deform soft coating specimens, so no reliable data can be measured on these specimens; the surface roughness of the base metal and coating have an impact on the measurement. As the roughness increases, the impact increases, and the rough surface will cause systematic errors and accidental errors. When measuring each time, the number of measurements should be increased at different positions to overcome this accidental error. If the base metal on the substrate is rough, it is also necessary to take several positions on the uncoated base metal test piece with similar roughness to calibrate the zero point of the instrument, or use a solution that does not corrode the base metal to dissolve and remove the coating, and then calibrate the instrument The zero point; the strong magnetic field generated by various electrical equipment around will seriously interfere with the magnetic thickness measurement work; those attached substances that prevent the probe from being in close contact with the coating surface must be disposed of. During the measurement, the pressure must be kept constant. The maximum measurement can be achieved only if the surface of the part is kept vertical.
