Factors Affecting Measurement Values of Coating Thickness Gauges and Solutions
Using a thickness gauge, like using other instruments, requires both understanding of the instrument's performance and knowledge of the testing conditions. The coating thickness gauge using magnetic and eddy current principles is based on the electrical and magnetic properties of the measured substrate and the distance from the probe to measure the coating thickness. Therefore, the electromagnetic physical properties and physical dimensions of the measured substrate affect the magnitude of magnetic flux and eddy current. This affects the reliability of the measured values. Below is an introduction to this issue.
1. Boundary spacing
If the distance between the probe and the boundary, hole, cavity, or other cross-sectional changes of the measured object is less than the specified boundary distance, measurement errors will occur due to insufficient cross-section of the magnetic flux or eddy current carrier. If it is necessary to measure the thickness of the coating at this point, it can only be measured by pre calibrating on an uncoated surface under the same conditions. (Note: The latest product has a unique function of achieving an accuracy of 3-10% through coating calibration.)
2. Surface curvature of the substrate
Calibrate an initial value on a flat comparison sample, and then subtract this initial value after measuring the coating thickness. Or refer to the following article.
3. Minimum thickness of base metal
The base metal must have a given minimum thickness, so that the electromagnetic field of the probe can be completely contained within the base metal. The minimum thickness is related to the performance of the measuring instrument and the properties of the metal substrate. Measurements can be made just above this thickness without correcting the measurement values. The impact caused by insufficient thickness of the substrate can be eliminated by placing a piece of the same material tightly under the substrate. If it is difficult to make a decision or unable to add a substrate, the difference from the rated value can be determined by comparing it with a sample with a known coating thickness. And consider this point in the measurement and make corresponding corrections to the measured values or refer to Article 2 for correction. And those instruments that can be calibrated can obtain accurate direct reading thickness values by adjusting knobs or buttons.
On the contrary, by utilizing the influence caused by the thickness being too small, a thickness gauge that can directly measure the thickness of copper foil can be developed, as mentioned earlier.
4. Surface roughness and surface cleanliness
To obtain a representative average measurement value on a rough surface, multiple measurements must be taken. It is obvious that the rougher the substrate or coating, the less reliable the measurement value. To obtain reliable data, the average roughness Ra of the substrate should be less than 5% of the coating thickness. For surface impurities, they should be removed. Some instruments have upper and lower limits to eliminate those 'flying points'.
