Coating thickness gauges are a critical tool used in industrial inspection to measure the thickness of non-conductive coatings on conductive substrates, or non-magnetic coatings on magnetic substrates. Among them, magnetic method and eddy current method are the two most widely used non-destructive measurement principles. Although both are based on electromagnetic induction, there are essential differences in their physical mechanisms, applicable scenarios, and signal processing methods. Understanding these differences is crucial for proper instrument selection and ensuring measurement accuracy.
Magnetic method
The magnetic method is mainly suitable for measuring the thickness of non-magnetic coatings (such as paint, plastic, zinc, chromium, etc.) on ferromagnetic substrates (such as steel, iron). Its core principle is based on the change in magnetoresistance of the magnetic flux path between a permanent magnet or electromagnetic probe and a ferromagnetic matrix.
When the probe is close to the object being tested, the probe forms a closed magnetic circuit with the ferromagnetic matrix. The presence of a non-magnetic coating increases the air gap between the probe and the matrix. According to Ohm's law of the magnetic circuit, magnetic resistance (Rm) is directly proportional to the length of the magnetic circuit (i.e., the thickness of the air gap, which can be approximated as the thickness of the coating d), and is inversely proportional to the permeability (μ) and cross-sectional area (A), and the relationship can be simplified as:
Rm ≈ d / (μ · A)
The thicker the coating, the greater the magnetic resistance, resulting in a corresponding change in the magnetic attraction or magnetic flux felt by the probe. The instrument calibrates to convert this change in magnetism (such as the magnitude of magnetic attraction, the induced EMF of the induced coil, or the output voltage of the Hall element) into the thickness value of the coating. This method is sensitive to the magnetic properties of the substrate, such as permeability.
Eddy current method
The eddy current law is mainly used to measure the thickness of non-conductive coatings (e.g., anodized films, paints, ceramics) on non-ferromagnetic metal substrates (e.g., aluminum, copper, brass, stainless steel). Its working principle is based on the eddy current effect induced by high-frequency alternating electromagnetic fields in a conductive matrix.
The coil in the thickness gauge probe is transmitted with high-frequency alternating current (usually a few MHz) to generate an alternating magnetic field. When the probe is close to the conductive substrate, the magnetic field induces eddy currents within the substrate. The eddy current itself will generate an antimagnetic field opposite to the direction of the original magnetic field, which will affect the impedance of the probe coil. The presence of a coating changes the distance between the probe coil and the conductive matrix (i.e., the lifting effect), which in turn changes the intensity and distribution of the eddy current, ultimately resulting in a change in the impedance (Z) of the probe coil. Impedance is complex and contains resistance (R) and inductive (ωL) components:
Z = R + jωL
where ω is the angular frequency. The instrument detects changes in coil impedance and derives coating thickness through calibration and signal processing. The method is sensitive to the conductivity of the substrate, probe frequency and temperature.
Comparison of differences
| Contrast dimensions | Magnetic method |
| Physics foundations | Magnetic resistance change in magnetic circuit (magnetic attraction/magnetic flux) |
| Mainly suitable for substrates | Ferromagnetic materials (e.g., steel, iron) |
| Mainly suitable for coatings | Non-magnetic coatings (lacquer, plastic, zinc, etc.) |
| Key impact parameters | Matrix permeability, geometry, remandence |
| Typical probe composition | Permanent magnets with Hall elements, or electromagnetic coils |
| Contrast dimensions | Eddy current method |
| Physics foundations | Changes in coil impedance due to high-frequency eddy currents |
| Mainly suitable for substrates | Non-ferromagnetic conductive metals (e.g., aluminum, copper, stainless steel) |
| Mainly suitable for coatings | Non-conductive coatings (lacquers, oxide films, ceramics, etc.) |
| Key impact parameters | Substrate conductivity, measurement frequency, temperature |
| Typical probe composition | High-frequency excitation and detection coils |
Selection considerations
In practical applications, the choice of principle depends first of all on the properties of the matrix material. For non-magnetic coatings on ferromagnetic substrates, the magnetic method is a straightforward and often more stable option. For insulating coatings on non-ferromagnetic metal substrates, the eddy current method must be used. Some modern instruments integrate dual-function probes (F-type or N-type) that automatically identify substrate materials and switch principles to deal with mixed materials or unknown substrates.
Both methods require calibration on a similar matrix with a standard of known thickness to establish a signal-to-thickness curve. When measuring, it is necessary to pay attention to the characteristics of the matrix itself (such as the change of the permeability of the matrix of the magnetic method, the change of the conductivity of the matrix of the eddy current method), edge effects, curvature, surface roughness, and the conductivity/conductivity of the coating itself. For the measurement of thin or ultra-thick coatings, it is necessary to pay attention to the linear measurement range and resolution limits of each principle.
Summary
The difference between magnetic method and eddy current method are the two pillars of coating thickness measurement technology, and their differences are rooted in different electromagnetic physical mechanisms. The magnetic method relies on the magnetic resistance of the magnetic circuit and is oriented towards the ferromagnetic matrix. The eddy current law relies on high-frequency eddy current impedance and is oriented towards non-ferromagnetic conductive substrates. A correct understanding of the differences in principles, applicable boundaries and influencing factors is the basis for ensuring the reliability and accuracy of coating thickness measurement results. In actual work, the selection and calibration of the material properties, shape and size and process requirements of the specific test object are the key to the best performance of the instrument.
References
1. International Organization for Standardization. ISO 2178:2016, Magnetic measurement of the thickness of non-magnetic coatings on magnetic substrates.
2. International Organization for Standardization. ISO 2360:2017, Eddy current measurement of the thickness of non-conductive coatings on non-magnetic conductive substrates.
3. National Standardization Administration of China. GB/T 4956-2003, Measurement of the thickness of non-magnetic overlays on magnetic substrates -- Magnetic method.
4. National Standardization Administration of China. GB/T 4957-2003, Measurement of the thickness of non-conductive overlays on non-magnetic metal substrates -- Eddy current method.
5. The chapter on the principle of coating thickness measurement in the non-destructive testing manual (electromagnetic volume).