Coating thickness gauge eddy current measurement principle
High frequency AC signals generate electromagnetic fields 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 the higher the reflection impedance. This feedback action quantity characterizes the distance between the probe and the conductive substrate, that 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 thickness of coatings 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. The coating thickness gauge based on the principle of eddy current can measure non-conductive coatings on all conductive substrates, such as surfaces of spacecraft, vehicles, household appliances, aluminum alloy doors and windows, and other aluminum products, including paint, plastic coatings, and anodized films. The coating material has a certain degree of conductivity, which can also be measured through calibration, but it is required that the ratio of conductivity between the two be at least 3-5 times different. Although the steel substrate is also a conductor, it is still more appropriate to use magnetic principles to measure coating thickness for such tasks.
Several factors affecting the measurement of coating thickness gauge. The magnetic method for measuring thickness is affected by changes in the properties of the base metal (in practical applications, changes in the magnetic properties of low carbon steel can be considered slight). In order to avoid the influence of heat treatment and cold processing factors, standard plates with the same properties as the base metal of the specimen should be 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. Use standard plates with the same properties as the base metal of the specimen to calibrate the instrument; Every instrument has a critical thickness, beyond which the measurement is not affected by the thickness of the base metal; It is sensitive to the steep changes in the surface shape of the specimen, so measuring near the edge or inner corner of the specimen is unreliable; 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 bent specimens is also unreliable; The probe will cause deformation of the soft coating specimens, so reliable data cannot be measured on these specimens; The surface roughness of the base metal and coating has an impact on the measurement. The increase in roughness leads to an increase in impact, and rough surfaces can cause systematic and accidental errors. Therefore, the number of measurements should be increased at different positions during each measurement to overcome this accidental error. If the base metal on the substrate is rough, it is necessary to take several positions on the uncoated base 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 base metal, and then calibrate the zero point of the instrument; The strong magnetic field generated by various electrical equipment around can seriously interfere with magnetic thickness measurement work; The attached substances that hinder the close contact between the probe and the coating surface must be removed. In the measurement, the pressure must be kept constant, and the probe must be kept perpendicular to the surface of the specimen in order to achieve accurate measurement.
