What are the factors that affect the measurement of coating thickness gauge?
1. Magnetic properties of the base metal
The magnetic thickness measurement is affected by the magnetic change of the base metal (in practical applications, the magnetic change of low carbon steel can be considered slight). In order to avoid the influence of heat treatment, cold working and other factors, the instrument should be calibrated with an iron substrate having the same properties as the test piece metal.
2. Base metal thickness
Every instrument has a critical thickness of the base metal. Thickness measurements greater than this are not affected by substrate thickness.
3. Edge effect
The instrument is sensitive to abrupt changes in the surface shape of the test piece. Measurements near the edges or inside corners of the test strip are therefore unreliable.
4. Curvature
The curvature of the test piece has an influence on the measurement, and this influence always increases significantly with the decrease of the radius of curvature. Therefore, it should not be measured on the curved surface of the test piece exceeding the allowable radius of curvature.
5. Surface roughness
The surface roughness of the base metal and the overlay has an effect on the measurement. As the roughness increases, the influence increases. Rough surfaces can cause both systematic and accidental errors. For each measurement, the number of measurements should be increased at different positions to overcome this occasional error. If the base metal 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 remove the base metal coating with a non-corrosive solution, and then calibrate the zero point of the instrument.
6. Magnetic field
The strong magnetic field generated by various electrical equipment around will seriously interfere with the work of measuring thickness by magnetic method.
7. Probe pressure and position
The amount of pressure exerted by the probe on the test piece will affect the reading of the measurement. Therefore, the instrument probe uses a spring to maintain a substantially constant pressure. During the measurement, the probe should be kept perpendicular to the surface of the sample.
8. Number of readings
Often the readings from the instrument are not exactly the same for each reading. Several readings must therefore be taken within each measurement area, and local variations in overlay thickness also require multiple measurements within any given area. Rough surfaces are even more so.
