In order to avoid the effects of heat treatment, cold processing and other factors, the instrument should be calibrated using an iron substrate with the same properties as the test piece metal. Base Metal Thickness Each instrument has a critical thickness of the base metal. Measurements greater than this thickness are not affected by the thickness of the substrate. The thickness of the base metal magnetic magnetic method is affected by the magnetic change of the base metal (in practical applications, the change in the magnetic properties of the low carbon steel can be considered to be slight).

The edge effect of the image measuring instrument, the instrument is sensitive to the steep shape of the surface of the test piece. Therefore, it is unreliable to measure near the edge of the test piece or at the inner corner. The curvature of the curvature test piece has an effect on the measurement, and this effect always increases significantly as the radius of curvature decreases. Therefore, it should not be measured on a curved surface where the test piece exceeds the allowable radius of curvature.

Surface roughness The surface roughness of the base metal coating has an effect on the measurement. The roughness increases and the influence increases. Rough surfaces can cause systematic errors and accidental errors. The number of measurements should be increased at different locations for each measurement to overcome this occasional error. If the base metal is rough, it is necessary to check the zero point of the image measuring instrument on the base metal test piece with similar roughness without the butcher's roughness; or remove the base metal cover layer with a non-corrosive solution, and then check the zero point of the instrument. . The strong magnetic field generated by various electrical equipment around the magnetic field can seriously interfere with the work of measuring the thickness by the magnetic method.

Adhesive image measuring instruments are sensitive to adhering substances that impede the probe from coming into close contact with the surface of the cover. Therefore, the attached material must be removed to ensure direct contact between the probe and the surface of the cover. The amount of pressure applied by the probe pressure probe placed on the test piece affects the measured reading. Therefore, the image measuring instrument probe maintains a substantially constant pressure with a spring. Placement of the probe The placement of the probe has an effect on the measurement. In the measurement, the probe should be kept perpendicular to the surface of the specimen.

The deformation probe of the test piece deforms the soft cover test piece. Therefore, less reliable data is measured on these test pieces. The number of readings is usually not exactly the same for each reading of the image meter. It is therefore necessary to take several readings in each measurement area, local differences in the thickness of the overlay, and also to perform multiple measurements in any given area. This should be the case when the surface is rough.