Measurement principle
Magnetic thickness gauge is a non-destructive testing instrument based on the principle of magnetic induction or magnetic attraction, which is widely used to measure the thickness of non-magnetic coatings on the surface of ferromagnetic substrates such as steel. Its core principle is that when the instrument probe is close to the measured surface, a closed magnetic circuit is formed between the probe and the ferromagnetic matrix. If there is a non-magnetic coating on the surface (such as paint, plastic, zinc, chromium, etc.), the magnetic resistance in the magnetic circuit will increase with the increase of coating thickness. The instrument directly displays the coating thickness value by detecting the change in magnetic resistance or the magnetism required to maintain a constant magnetic flux through an internal circuit.
The measurement principle can be described by the following relationship: For the magnetic attraction principle, the suction force between the permanent magnet of the probe and the ferromagnetic matrix is a function of the coating thickness d F = f(d). The instrument calculates the thickness by measuring the critical value or balance force at the time of magnetic detachment. For the principle of magnetic induction, the amount of change in induced electromotive force E or permeability is related to the coating thickness d, and the relationship can be approximated as follows:
d ∝ (1/μ) · ΔΦ
Among them, μ is the effective permeability of the magnetic circuit, and ΔΦ is the change of magnetic flux. The instrument is calibrated by a standard thickness sheet before leaving the factory, and the corresponding curve of signal strength and thickness is established and stored in the chip to achieve fast measurement.
Measurement standards
In order to ensure the accuracy and comparability of measurement results, the calibration and use of magnetic thickness gauges must comply with relevant technical standards at home and abroad. These standards specify the accuracy, repeatability, calibration methods and measurement procedures of the instrument. The main reference standards are shown in the table below.
| Standard system | Standard number and name |
| International standards | ISO 2178: Measurement of the thickness of non-magnetic coatings on magnetic substrates |
| Chinese national standards | GB/T 4956: Measurement of the thickness of non-magnetic overlays on magnetic substrates |
| American Materials Institute Standards | ASTM B499: Magnetic Measurement of Non-Magnetic Coating Thickness on Magnetic Substrates |
| Industry application specifications | It is commonly used in ships, bridges, automobiles, steel structure anti-corrosion and other industry technical specifications |
Standards usually require that the instrument must perform "zero point calibration" on an uncoated substrate similar to the material, curvature and surface roughness of the workpiece to be measured before measurement, and complete "multi-point calibration" on a standard piece of known thickness. When measuring, ensure that the probe axis is perpendicular to the surface to be measured and apply constant, appropriate pressure.
Influencing factors
In practice, various factors can affect the measurement results of magnetic thickness gauges. Operators need to identify and control these variables to ensure data reliability.
| Categories of influencing factors | Specific explanations and responses |
| Substrate properties | The magnetism, hardness, and alloy composition of steel affect the permeability. Different steels need to be calibrated separately. |
| geometric shapes | The curvature of the workpiece (convex/concave) changes the magnetic path clearance. Small radius of curvature workpieces should use special probes or curvature compensation. |
| Surface condition | The rough surface causes unstable probe contact and large fluctuations in measured values. It should be measured after taking a multi-point average or polishing and leveling. |
| Coating properties | Errors may be introduced in coating material, thickness uniformity, multi-layer structure, and drying curing degree. |
| Outside interference | Strong magnetic fields, vibration, probe wear, and drastic temperature changes can affect sensor performance. |
| Operation method | The probe placement angle, pressure, stabilization time and measurement point selection should be standardized. |
To reduce errors, it is recommended to take multiple points (usually no less than 3 points) in each measurement area to calculate the average value and troubleshoot outliers.
Operation process
A standardized measurement process is the basis for obtaining trusted data. The basic operation steps include: first, check the power of the instrument and the cleanliness of the probe; secondly, the zeroing operation is carried out on the bare steel matrix of the workpiece to be tested or the provided zero plate; Then, calibration verification is performed using a standard sheet that is close to the expected coating thickness. Then, a representative measurement point is selected on the surface of the coating to be measured and recorded. Finally, statistical analysis was carried out on the data. After the measurement, the probe should be cleaned and the instrument properly stored.
Regular maintenance and verification of instruments is crucial. Violent impact or scratching of the probe should be avoided. It is recommended to check the instrument's readings and repeatability weekly or before important measurement tasks using standard thickness sheets. If it has not been used for a long time, the battery should be removed. When the instrument readings are systematically deviated or cannot be calibrated, it is necessary to return to a qualified institution for verification or repair.
Applications:
Magnetic thickness gauges are widely used in the industrial field due to their portable, fast, and non-destructive characteristics. Typical applications include: evaluating the thickness of anti-corrosion coatings on steel structures (e.g., bridges, storage tanks, transmission towers); control the coating quality of automobile bodies and parts; monitoring of protective coatings on ships and offshore platforms; and check the thickness of the surface treatment layer of home appliances and hardware products.
However, the method also has a clear limitation: it is only suitable for measurements of non-magnetic coatings on ferromagnetic metal substrates. For coatings on non-ferromagnetic substrates (e.g., aluminum, copper, stainless steel austenitic), or magnetic coatings on ferrous substrates (e.g., some nickel plating), instruments with other principles such as eddy current thickness gauges are required. In addition, for extremely thin (<3 μm) or very thick coatings, the measurement accuracy may be reduced, and it is necessary to confirm that the range and resolution of the instrument meet the requirements.
Epilogue
Magnetic thickness gauges are a key tool for quality control of steel surface coating thickness. A deep understanding of its working principle, strict adherence to standard specifications for operation and calibration, and full consideration of various influencing factors are the guarantee of accurate and reliable measurement results. Correct use and maintenance of this instrument can effectively serve multiple industrial links such as anti-corrosion assessment, process control and product inspection, and provide data support for ensuring engineering and product quality.
References
ISO 2178:2016, Measurement of coating thickness — Magnetic method.
GB/T 4956-2003, Non-magnetic overlay on magnetic substrates - Measurement of overlay thickness - Magnetic method.
ASTM B499-14, Standard Test Method for Measurement of Coating Thickness by the Magnetic Method: Nonmagnetic Coatings on Magnetic Basis Metals.