Comparison of two principles
In the world of industrial inspection, accurate measurement of coating thickness is crucial for quality control. Magnetic induction and eddy current are two widely used physical principles, and thickness gauges based on these principles have their own emphasis on applicable scenarios, technical characteristics and operational considerations. This article aims to systematically compare these two principles to assist users in selecting according to specific application needs.
How it works:
The principle of magnetic induction is mainly suitable for measuring the thickness of non-magnetic coatings on magnetic substrates (such as steel and iron). At its core, the electromagnets inside the probe generate a low-frequency magnetic field. As the probe approaches the object under test, the magnetic field forms a loop in the magnetic matrix. Changes in coating thickness can change the magnetic resistance of the magnetic circuit, which in turn causes measurable changes in the induced voltage or inductance value of the probe coil. This amount of change is a function of coating thickness and can usually be determined by calibration curves. Its basic relationship can be approximated as:
ΔV ∝ 1 / (d + k)
Among them, ΔV is the change of the induced signal, d is the coating thickness, and k is the constant related to the instrument and matrix.
The eddy current principle is mainly used to measure the thickness of non-conductive coatings on non-ferromagnetic metal substrates (such as aluminum, copper, stainless steel). The high-frequency AC coil inside the probe generates a high-frequency electromagnetic field. When the probe is close to the conductive substrate, eddy currents are induced on the surface of the substrate. The eddy current effect acts against the probe coil, changing its impedance. The presence of a coating changes the distance between the probe and the substrate (lift-off effect), causing a change in impedance. By measuring this impedance change, the coating thickness can be deduced. Its relationship is more complex and is usually expressed as:
Z = f(σ, μ, f, d)
Among them, Z is the coil impedance, σ is the conductivity of the matrix, μ is the magnetic permeability, f is the excitation frequency, and d is the thickness of the coating.
Application characteristics
The following table provides a general comparison of the two principles from multiple dimensions, and the content is condensed for easy access.
| Contrast dimensions | Magnetic induction principle | Eddy current principle |
| Mainly suitable for substrates | Ferromagnetic metals (steel, iron) | Non-ferromagnetic conductive metals (aluminum, copper, brass, austenitic stainless steel) |
| Mainly measure coatings | Non-magnetic overlays (paint, plastic, zinc, chrome) | Non-conductive overlays (paints, anodized films, ceramics, plastics) |
| Matrix impact | It is significantly affected by the magnetism of the matrix (such as steel grade, heat treatment). | It is significantly affected by the conductivity and magnetic permeability of the matrix |
| Measurement range typical | Wider, often a few microns to several millimeters | They are usually thinner, often ranging from a few microns to hundreds of microns |
| Surface and shape influence | Small radius of curvature surfaces may require special probes or calibrations | Sensitive to small radius of curvature surfaces and requires targeted calibration |
| Calibration requirements | It is necessary to calibrate zero on an uncoated matrix of the same material and the same curvature | Calibration is required on a matrix of the same material, the same curvature, and known conductivity |
| Common application industries | Steel structure anti-corrosion, automobile body, heavy machinery | Aerospace aluminum parts, electronic shells, hardware surface treatment |
Selection and Operation
In the actual selection and operation, the following factors need to be comprehensively considered. The primary factor is the material of the substrate, which is the fundamental basis for the selection principle. For ferromagnetic substrates, magnetic induction is the standard choice; For non-ferromagnetic metal substrates, the eddy current method is used. Some instruments integrate dual-principle probes that automatically identify substrate types, improving their applicability on mixed-material production lines.
Secondly, the influence of matrix characteristics and state cannot be ignored. The magnetic induction method is affected by the magnetism of the substrate, and different alloy compositions, heat treatment states, or cold working processes will change the permeability, which may introduce errors. The eddy current law is sensitive to the conductivity of the matrix, and temperature changes and uneven alloy composition will also affect the measurement stability. Therefore, when measuring on a potentially changing matrix, it is recommended to calibrate with a sample block with the same material and condition as the DUT.
In addition, the measurement environment and the geometry of the workpiece are important considerations. A strong electromagnetic field environment can interfere with measurements of both principles. The radius of curvature of the workpiece, its dimensions, and the proximity of the measurement area (e.g., edges, internal corners) can affect the measurement accuracy. Typically, the probe size needs to match the curvature, and measurements should be performed in a flat area as much as possible, with calibration done on a sample block of similar curvature.
Conclusion
Magnetic induction and eddy current coating thickness gauges are mature technologies based on different physical principles, each with a clear application field. The magnetic induction principle is the mainstream solution for the measurement of non-magnetic coatings on magnetic substrates, while the eddy current principle provides a solution for the measurement of non-conductive coatings on non-ferromagnetic conductive metal substrates. There is no one principle that can be applied to all scenarios, and the key to the choice lies in a comprehensive analysis of the substrate material, coating type, workpiece geometry and production environment. Proper calibration procedures and awareness of the effects of matrix properties are common keys to ensuring the reliability of measurement results from both principles. In practical applications, it is important to clarify the measurement requirements and follow the instrument's operating specifications to effectively monitor coating thickness.
References
1. International Organization for Standardization. ISO 2178:2016 Non-magnetic overlays on magnetic substrates — Measurement of overlay thickness — Magnetic method.
2. International Organization for Standardization. ISO 2360:2017 Non-conductive overlays on non-magnetic conductive substrates - Measurement of overlay thickness - Amplitude-sensitive eddy current method.
3. National Standardization Administration of China. GB/T 4956-2003 Non-magnetic overlay on magnetic substrates - Measurement of overlay thickness - Magnetic method.
4. National Standardization Administration of China. GB/T 4957-2003 Non-conductive overlay on non-magnetic substrates - Measurement of overlay thickness - Eddy current method.
5. NDT Manual (Comprehensive Guide to Materials Testing and Evaluation).