Overview
Coating thickness gauges are commonly used to measure the thickness of overlays, and their working principle is mainly based on the magnetic method and the eddy current method. The magnetic method is suitable for measuring the thickness of non-magnetic overlays on magnetic substrates, and its principle is to estimate the thickness of the overlay by measuring the change in magnetic flux or magnetic resistance. When the probe touches the sample, the magnetic path changes due to the presence of a non-magnetic coating, which is related to the coating thickness and can be expressed as: ΔΦ ∝ d, where ΔΦ is the change in magnetic flux and d is the coating thickness.
The eddy current method is suitable for measuring the thickness of non-conductive overlays on non-ferromagnetic metal substrates. The method uses the high-frequency electromagnetic field generated by the coil in the probe to induce eddy currents in the conductive substrate, and the strength of the eddy current is affected by the thickness of the coating, which in turn changes the impedance of the probe coil. The relationship between impedance change and coating thickness can be roughly described as: Z = Z0 + k·e-d/δ, where Z is the measurement impedance, Z0is the reference impedance, k is the coefficient, and δ is the skin depth.
Method fit
The electromagnetic properties of the substrate are the key basis for choosing a measurement method. Magnetic substrates, such as various types of steel and cast iron, are suitable for magnetic measurement due to their high permeability. In this case, the overlay needs to be made of non-magnetic materials, such as paint, plastic, zinc, chromium, etc.
Non-ferromagnetic metal substrates, such as aluminum, copper, brass, stainless steel (austenitic), etc., are conductive but non-ferromagnetic materials, and should be measured by eddy current method. The thickness of non-conductive coatings on its surface, such as anodized layers, paints, powder coatings, etc., can be accurately measured by the eddy current effect.
For composite substrates or special coatings, it may be necessary to consider a dual-function instrument that integrates both magnetic and eddy current probes to switch measurement modes by automatically identifying the substrate type.
Selection considerations
In the actual selection and application, it is necessary to comprehensively evaluate multiple technical parameters and site conditions. The curvature of the substrate, size, surface roughness, and uniformity of the coating all have an impact on measurement accuracy. Often, a small radius of curvature or rough surfaces can lead to increased fluctuations in measurements.
Calibration of instruments is critical. Calibration should be done using a standard sheet consistent with the substrate and coating properties of the sample to be tested, and a zero-point calibration should be performed on the uncoated area of the same substrate prior to measurement. Environmental factors, such as strong electromagnetic fields or drastic temperature changes, may also interfere with measurements and need to be avoided.
For measurements of thin or ultra-thick coatings, it is necessary to confirm that the range and resolution of the instrument meet the requirements. Some instruments have a lower range of up to 1 micron and an upper limit of several millimeters, but there are differences in the applicable range of different principles and models.
| Considerations | Instructions and suggestions |
| Substrate type | magnetic method for ferromagnetic materials; Eddy current method for non-ferromagnetic conductive metals. |
| Coating properties | Non-magnetic coating magnetometric substrate; Non-conductive coatings measure conductive substrates. |
| Substrate shape | Planes, curved surfaces, and small parts need to consider probe contact and calibration. |
| Surface condition | Rough, uneven surfaces can reduce measurement repeatability. |
| Measurement range | Choose the appropriate range and resolution of the instrument based on the expected coating thickness. |
| Calibration method | Multi-point calibration was performed using a standard sheet consistent with the measured substrate. |
Standard compliance
To ensure the reliability and comparability of measurement results, the operation must comply with relevant domestic and foreign standards. These standards set clear requirements for instrument performance, calibration procedures, measurement steps, and result reporting. Compliance with standards is the basis for ensuring data accuracy and quality control.
Regular validation of instruments with certified standard thickness sheets is a common practice to maintain measurement accuracy. When the measurement results are systematic, factors such as probe wear, calibration settings, and environmental interference should be checked, and the calibration procedure should be re-performed.
References
1. Principle overview: refers to the general physical principles of electromagnetic induction measurement technology and the basic literature of related instrument design.
2. Suitability: Guidelines for classifying electromagnetic properties of materials and selecting measurement methods from various materials science manuals.
3. Selection considerations: Summary of operation suggestions in the technical application notes provided by common industrial testing procedures and multiple instrument manufacturers.
4. Standard Compliance Part: The relevant requirements mentioned are based on the standards for coating thickness measurement methods issued by the International Organization for Standardization and national standardization bodies.