Principle overview
A coating thickness gauge is an instrument used to measure the thickness of a dry film coated on the surface of a substrate, which is widely used in quality control in industries such as anti-corrosion, automotive, marine, and steel structure. Its measurement principle is mainly based on non-destructive physical methods such as electromagnetic induction, eddy current and ultrasonic method, which achieve thickness measurement by detecting the difference in physical properties between the coating and the substrate. The choice of these methods often depends on the conductive/magnetic properties of the substrate and the specific properties of the coating.
Electromagnetic induction
The electromagnetic induction method is suitable for measuring the thickness of non-magnetic coatings on magnetic substrates (e.g., steel, iron). The instrument probe has a built-in coil that generates an alternating magnetic field when it is connected to an alternating current. When the probe is close to the magnetic substrate, the magnetic field induces magnetic flux in the substrate, and its strength is affected by the distance between the probe and the substrate (i.e., the thickness of the coating). The thicker the coating, the greater the magnetic resistance, leading to a change in the induced EMF or inductance of the coil. This change in electrical signal is converted into a thickness reading by calibrating the instrument. Its basic relationship can be approximated as:
ΔV ∝ d
where ΔV is the change of induced voltage, and d is the thickness of the coating. In practical applications, factors such as substrate permeability and surface roughness need to be considered, and calibrated through standard sheets to reduce errors.
Eddy current
The eddy current method is used to measure the thickness of non-conductive coatings on non-magnetic metal substrates such as aluminum, copper, stainless steel. The probe coil is driven by high-frequency alternating current to generate an alternating magnetic field that induces eddy currents in the conductive substrate. The secondary magnetic field generated by the eddy current acts in reverse on the probe coil, changing its impedance. Increasing coating thickness can lead to an increase in the distance between the probe and the substrate, which changes the degree of coupling and changes the coil impedance. By measuring the impedance change and referring to the calibration curve, the coating thickness is derived. This method is sensitive to substrate conductivity and usually needs to be calibrated for different substrate materials.
Ultrasound
Ultrasonic methods are suitable for coating measurement on a wide range of substrates, including metals, plastics, and concrete. The instrument probe emits a high-frequency ultrasonic pulse, which is reflected at the interface between the coating and the substrate, and the coating thickness is calculated by measuring the time of ultrasonic propagation in the coating, combined with the known propagation speed of ultrasonic waves in the coating material. The formula is expressed as:
d = (v × t) / 2
where d is the thickness of the coating, v is the sound velocity of the ultrasound in the coating, and t is the echo time. This method requires the difference in acoustic impedance between the coating and the substrate, and requires pre-measurement of sound velocity or calibration using standard blocks.
Measurement method
To ensure measurement accuracy, the appropriate measurement principle should be selected according to the substrate and coating type before operation. Calibration is a critical step and is typically performed on similar substrates using standards of known thickness. When measuring, it is necessary to pay attention to the influence of factors such as substrate shape, surface curvature, roughness, and coating uniformity. Averaging multiple measurements improves reliability. The following is a comparison of applicable scenarios of common measurement principles:
| Measurement principle | Applicable substrate type |
| Electromagnetic induction method | Magnetic metals (e.g. steel) |
| Eddy current method | Non-magnetic metals (e.g., aluminum) |
| Ultrasonic method | Multiple materials (metallic, non-metallic) |
Influencing factors
Measurement accuracy is affected by several factors. Changes in the electromagnetic properties of the substrate (e.g., permeability, conductivity) can lead to biased readings, so they need to be calibrated separately for different substrates. Surface roughness is too large to introduce measurement errors, and it is recommended to measure in smooth areas or take multi-point averages. The composition, density, and multilayer structure of the coating itself can also affect the speed of sound or electromagnetic response. Environmental factors such as temperature and humidity may alter the physical properties of the material and require operation under standard conditions. The probe should be placed vertically and constant pressure should be applied during operation to avoid errors caused by inconsistent angles and pressures.
Summary
Coating thickness gauges enable fast, non-destructive measurement of dry film thickness through the principles of electromagnetic induction, eddy current, or ultrasonic. Correct selection of measurement methods, strict implementation of calibration procedures and control of measurement conditions are the basis for obtaining reliable data. These technologies provide an effective means for quality control of the coating process, helping to ensure the durability and performance of products.
References
ISO 2808:2019, Paints and varnishes — Determination of film thickness.
ASTM D7091-13, Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals.
Industry Technical Manual: Non-Destructive Coating Thickness Measurement Methods.