Description of factors affecting measuring accuracy of coating thickness gauge
A) Magnetic properties of the base metal
Thickness measurement by magnetic method is affected by the change of the magnetic properties of the base metal (in practical application, the change of the magnetic properties of the low carbon steel can be considered to be slight). In order to avoid the influence of heat treatment and cold working factors, the standard sheet with the same properties as the sample base metal should be used to calibrate the instrument.It can also be calibrated with the specimen to be coated.
B) Electrical properties of matrix metal
The electrical conductivity of the base metal has an influence on the measurement, and the electrical conductivity of the base metal is related to the composition of the material and the heat treatment method.The instrument is calibrated using a standard sheet that has the same properties as the sample base metal.
C) Thickness of matrix metal
Each instrument has a critical thickness of the base metal.Above this thickness the measurement is not affected by the thickness of the base metal.The critical thickness of the instrument is shown in Table 1.
D) Edge effect
This instrument is sensitive to abrupt changes in specimen surface shape.Therefore, it is not reliable to measure near the edge or inner corner of the specimen.
The curvature of the specimen has an effect on the measurement.This effect always increases appreciably as the radius of curvature decreases.Therefore, the measurement on the surface of the curved specimen is not reliable.
F) Deformation of specimen
The probe can deform soft overburden specimens, so reliable data can be measured on these specimens.
G) Surface roughness
The surface roughness of the substrate metal and the coating has an effect on the measurement.The effect increases with the increase of roughness.Rough surface can cause systematic error and accidental error. In each measurement, the number of measurement should be increased at different positions to overcome such accidental error.If the base metal is rough, several positions must be taken to check the zero point of the instrument on the base metal specimen with uncoated roughness similar to that of the base metal specimen;Or use a solution that does not corrode the base metal to remove the covering layer, and then calibrate the zero point of the instrument.
G) magnetic field
The strong magnetic field produced by all kinds of electrical equipment around will seriously interfere with the magnetic thickness measurement.
H) Adherent substances
The instrument is sensitive to attachments that prevent the probe from making close contact with the surface of the covering layer. Therefore, the attachment must be removed to ensure direct contact between the probe and the surface of the part to be tested.
I) Probe pressure
The amount of pressure applied by the probe on the specimen will affect the measured reading, so the pressure should be kept constant.
J) Probe orientation
The way the probe is placed has an effect on the measurement.During measurement, the probe should be kept perpendicular to the sample surface.