Measurement principle
Eddy current thickness measurement technology is based on the principle of electromagnetic induction. When the coil built into the instrument probe is connected to high-frequency alternating current, an alternating magnetic field is generated. If the probe is close to a conductive metal coating, the magnetic field will induce eddy currents in the coating. This eddy current itself creates an antimagnetic field in the opposite direction of the original magnetic field, which affects the impedance of the probe coil. The change of coating thickness will change the strength of the eddy current effect, which in turn will cause the regular change of the impedance of the coil. The thickness value of the coating is obtained by accurately measuring the change in this electrical parameter, calibrating and calculating. For non-conductive substrates (e.g., plastics, ceramics, composites), the technology enables accurate measurement of the thickness of the surface metal coating because it does not interfere with the electromagnetic processes described above.
Suitable for plating
This method is mainly suitable for various types of non-magnetic metal coatings applied to non-conductive substrates. Common combinations include copper, chromium, zinc, tin, gold, and silver plating on plastic parts, silver electrodes on ceramic surfaces, and heat-reflective films on glass. Measurements on magnetic metal coatings or conductive substrates often require other principles (such as magnetic thickness measurement) and are not covered in this article.
| Typical matrix materials | Typical metal plating |
| All kinds of plastics (ABS, PC, etc.) | Copper, chrome, gold, silver |
| Ceramics | Silver, palladium |
| glass | Aluminum, silver, ITO |
| Composite materials | Zinc, tin |
Measurement procedure
To ensure measurement accuracy, calibration must be performed before formal measurement. The calibration should use a standard sheet similar to the substrate material, curvature and surface roughness of the workpiece to be measured, and its coating material and thickness nominal value should cover the expected measurement range. The basic operation process is as follows: first, turn on the instrument to warm up; secondly, "zero calibration" is carried out in the substrate area without coating of the standard sheet; Subsequent "range calibration" is performed on a standard sheet coating of known thickness; After completing the calibration, place the probe vertically and smoothly on the surface of the workpiece to be measured for measurement; For reliability, it is recommended to average multiple measurements in the same area and multi-point measurements at different locations to evaluate coating uniformity.
Influencing factors
In practice, a variety of factors can affect the reliability of measurement results. The operator needs to fully understand and control it. Excessive surface roughness can cause unstable probe contact, causing readings to fluctuate. A coating radius that is too small can affect the effective coupling area of the probe to the surface and often requires a dedicated microprobe or curved probe to compensate. Temperature changes in the measurement environment can cause slight drift in the instrument's electronics. In addition, the conductivity of the coating itself will be affected by the alloy composition and heat treatment process, and if there is a difference from the calibration standard, it will introduce system errors. Therefore, it is recommended to establish a dedicated calibration curve for a specific material system.
| Influencing factors | Control and compensation methods |
| Matrix roughness | Sand flat or use a special probe |
| Curvature of the workpiece | Choose a matching surface probe |
| Difference in the conductivity of the coating | Calibrated using standard sheets of the same material |
| Edge effect | Measure point away from edge (usually > 3mm) |
| temperature | Operate and calibrate in a stable environment |
Technical advantages
When measuring non-conductive matrix metal coatings, eddy current thickness measurement has the characteristics of non-destructive, fast measurement speed, and easy on-site or online inspection. Its probe design is flexible and can accommodate a wide range of workpiece shapes. However, there are clear application boundaries for this method. It is not suitable for measuring non-metallic coatings or magnetic metal coatings (such as nickel coatings under certain conditions). For very thin coatings, typically below a few microns, the measurement accuracy decreases. At the same time, the performance of the instrument is highly dependent on the accuracy of the calibration and the degree of specification of the operator.
Summary
Eddy current thickness gauges are effective tools for measuring the thickness of non-magnetic metal coatings on non-conductive substrates. Its successful application relies on a deep understanding of electromagnetic principles, rigorous calibration procedures, and comprehensive control over variables such as matrix state, environmental factors, and more. In actual industrial quality control and scientific research analysis, combined with specific material systems and process conditions, standardized measurement procedures can be formulated and followed to obtain reliable and repeatable thickness data, which provides a basis for product performance evaluation and process optimization.
References
ISO 2360: Non-conductive coatings on non-magnetic electrically conductive basis materials - Measurement of coating thickness - Amplitude-sensitive eddy-current method.
GB/T 4957 Non-conductive overlay on non-magnetic matrix metals -- Measurement of overlay thickness -- Eddy current method.
ASTM B244 Standard Test Method for Measurement of Thickness of Anodic Coatings on Aluminum and of Other Nonconductive Coatings on Nonmagnetic Basis Metals with Eddy-Current Instruments.