Amplitude-sensitive measurements and the most critical influencing factors
With the amplitude-sensitive eddy current method, conductivity measurement, the thickness of coatings can be measured non-destructively according to ISO 2360. The prerequisite for this is that the base material is electrically conductive but not magnetizable: Metals such as copper or aluminum are therefore suitable. The coating itself must be electrically insulating, e. g. made of lacquer or plastic. One of the main applications of the eddy current measurement method is the testing of anodic coatings on aluminum.
The probes used for measuring according to the amplitude-sensitive eddy current method have a ferrite core. A coil is wound around this core and a high-frequency alternating current flows through it. This creates a high-frequency alternating magnetic field around the coil.
When the probe pole comes close to a metal, an alternating current – or ‘eddy current’ – is induced in this metal. This, in turn, generates another alternating magnetic field. Since this second magnetic field is the opposite of the first, the original magnetic field is attenuated (weakened). The extent of the attenuation depends on the distance between the pole and the metal. For coated parts, this distance corresponds exactly to the layer thickness.
Here’s what you need to pay attention to during the measurement
All electro-magnetic test methods are comparative. This means that the measured signal is compared with a characteristic curve that’s stored in the device. In order for the result to be correct, the characteristic curve must be adapted to the current conditions. This is achieved through calibration.
Correct calibration makes all the difference!
Factors that can strongly influence the measurement when using the eddy current method are: the electrical conductivity, the shape and size of the sample and the roughness of the surface. Of course, correct operation of the device is also crucial!
A material’s electrical conductivity influences how well an eddy current can be induced within it. The conductivity can vary greatly depending on the specific alloy and how the metal was processed, and different temperatures can also cause variations. In order to minimize the calibration effort, Fischer eddy current probes have a conductivity compensation. They provide correct results over a wide range of conductivities and only need to be standardized on the respective material (i.e. calibration of the zero point).
In practice, most magnetic coating measurement errors occur due to the shape of the sample. With curved surfaces, the proportion of the magnetic field that passes through the air is different. For example, if a measuring device was calibrated on a flat sheet, measuring on a concave surface would lead to a lower result, whereas measuring on a convex one would lead to a higher result. The errors that occur in this way can be many times the actual value!
Careful calibration is the remedy for this problem. But even here, Fischer has found a way to save time and work: a curvature-compensating probe. With this special probe you can measure without errors on tubes of 2 mm in radius or larger, even if the device was calibrated on a flat sheet.
Small, flat parts
A similar effect can occur if the sample is small or very thin. Also in this case, the magnetic field extends beyond the sample and into the air, which systematically distorts the measurement results. To avoid these errors, you should always calibrate on an uncoated part that corresponds to the end product.
For rough surfaces, the result can be distorted depending on whether the probe pole is placed in a ‘valley’ or on a ‘peak’ of the roughness profile. With such measurements, the results vary widely and it is advisable to repeat the measurements several times in order to accumulate a stable mean. In general, magnetic coating thickness measurements on rough surfaces only make sense if the coating is at least twice as thick as the roughness peaks are high.
For better accuracy, Fischer offers probes with particularly large poles, as well as 2-pole probes. These probes integrate the roughness profile and thus reduce the scatter in the measured values.
Last but not least, the way the measuring device is operated also plays a major role. Always make sure that the probe is set vertically on the surface and without pressure. For better accuracy, a stand can be used to automatically lower the probe onto the sample. Helmut Fischer offers both Handheld Coating Thickness devices and Automated Measurement Systems.
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