The magnetic thickness measurement method is a method of measuring the thickness of the magnetic coating on the magnetic substrate by using the principle of electricity. This method is suitable for the thickness measurement of non-productive coatings or chemical protective layers on magnetic substrates.
In this method, the instrument should be adjusted systematically on the standard sample before measurement, so as to ensure its measurement accuracy. It can also be adjusted on the uncoated substrate. At this time, the standard sample and the uncoated substrate should be compared at zero. The difference in thickness is called the correction value. Both the supplier and the buyer use the same sample and method to determine the correction value. When the thickness of the layer is determined by the magnetic method, the following conventions are followed.
When the reference plane is 1cm², the measurement of the minimum local thickness is shown in the table below:
Note: 1. The contact between the curved surface probe and the coating surface is one point, which is indicated by "×" in the figure;
2. The contact between the flat probe and the coating surface is one side, indicated by "○" in the figure, and the area of the flat probe should be smaller than the circle area of φ 3mm.
When the datum is 1dm², no matter what type of instrument and what kind of probe is used for measurement, the 10-point method shown in the figure should be used to make 10 measurements on the datum, and the average value should be taken.
Factors affecting the measured value:
1. Some factors of the coating itself, such as thickness, electrical conductivity and surface roughness;
2. Some factors of the base metal itself, such as thickness, magnetism, curvature, surface roughness, machining direction and residual magnetism, etc.;
3. The influence of external factors, such as the surrounding magnetic field and foreign attachments, etc.;
4. The impact of measurement techniques, such as probe pressure, probe orientation and edge effects, etc.
The influence of the above factors should be excluded and corrected according to the provisions of GB 4956.
