The principle of eddy current measurement for coating thickness gauge
High frequency communication signals generate an electromagnetic field in the probe coil, and when the probe approaches the conductor, eddy currents are formed within it. The closer the probe is to the conductive substrate, the greater the eddy current and reflection impedance. This feedback action characterizes the distance between the probe and the conductive substrate, which is the thickness of the non-conductive coating on the conductive substrate. Due to the fact that this type of coating thickness gauge probe is specifically designed to measure the coating thickness on non-ferromagnetic metal substrates, it is commonly referred to as a non-magnetic probe. The non-magnetic probe uses high-frequency materials as the coil core. 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. A coating thickness gauge using the principle of eddy current can measure non-conductive coatings on all conductive substrates, such as paint, plastic coatings, and anodized films on surfaces of aerospace aircraft, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products. The coating material has a certain degree of conductivity, which can also be measured through calibration, but the ratio of conductivity between the two is required to be at least 3-5 times different. Although the steel substrate is also a conductor, it is still more appropriate to measure the coating thickness using magnetic principles for such tasks.
Several factors affecting the measurement of coating thickness gauge. The magnetic method for measuring thickness is affected by changes in the metallic properties of the substrate (in practical applications, the magnetic changes in low-carbon steel can be considered minor). To avoid the influence of heat treatment and cold working factors, the instrument should be calibrated using a standard piece with the same properties as the substrate metal of the specimen; 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. Use a standard piece with the same properties as the base metal of the specimen to calibrate the instrument; Each instrument has a critical thickness, above which the measurement is not affected by the thickness of the base metal; Sensitive to steep changes in the surface shape of the specimen, it is unreliable to measure near the edge or inner corner of the specimen; The curvature of the specimen has an impact on the measurement, and it increases significantly with the decrease of curvature radius. Therefore, measuring on the surface of the bent specimen is also unreliable; The probe will cause deformation of the soft coating specimen, so reliable data cannot be measured on these specimens; The surface roughness of the base metal and coating has an impact on the measurement. As the roughness increases, the impact increases. Rough surfaces can cause systematic and accidental errors. During each measurement, the number of measurements should be increased at different positions to overcome this accidental error. If the substrate metal is rough, it is necessary to take several positions on the uncoated substrate metal specimen with similar roughness to calibrate the zero point of the instrument, or dissolve and remove the coating with a solution that does not corrode the substrate metal, and then calibrate the zero point of the instrument; The strong magnetic field generated by various electrical equipment around can seriously interfere with the magnetic thickness measurement work; Those adhering substances that hinder the close contact between the probe and the coating surface must be removed. During measurement, it is necessary to maintain a constant pressure, keep the probe perpendicular to the surface of the specimen, and the coating thickness gauge can achieve the measurement.
