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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.

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Physical principles of coating thickness measurement

The phase-sensitive eddy current method is a further development of the conventional amplitude-sensitive eddy current method for coating thickness measurement. According to ISO 21968, the phase-sensitive eddy current method can be used to test electrically conductive coatings on many substrates: for example, copper on printed circuit boards or nickel on steel or insulating material. 

The phase-sensitive eddy current method is not very susceptible to many kinds of external influences. For example, the curvature of a sample or the roughness of a surface will hardly impact the measurement – a big advantage over magnetic induction or the amplitude-sensitive method. For this reason, phase-sensitive probes are ideally suited for checking the zinc thickness on small parts in the electroplating process – without requiring additional calibration. 

How does the phase-sensitive eddy current method work?

Phase-sensitive eddy current probes consist of a ferrite core around which two coils are wound. First, a current in the exciter coil generates a high-frequency magnetic field (in the kHz-MHz range). This creates eddy currents in the sample. 

The probe’s second coil, the measuring coil, measures the alternating current resistance (impedance). The probe’s impedance is modified by the eddy currents in the sample and – as compared to the excitation current (probe without sample) – subsequently phase-shifted (phase angle Ο†). 

The phase Ο† depends on the layer thickness and the electrical conductivity of the material. If the conductivity is known, the device compares the phase with a stored characteristic curve and converts it into a coating thickness value.

Lift-off effect

The phase-sensitive eddy current method offers great advantages for measuring coating thickness. As described above, the actual measurement signal is generated directly in the coating. This distinguishes the method substantially from the magnetic induction and amplitude-sensitive methods, which measure the attenuation of the signal from the substrate material. 

This is why the probe doesn’t have to directly touch the metallic layer; it can even measure metal layers underneath a coating, e.g. in duplex measurements. 

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!

Electrical conductivity

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 measurement 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).

Curved surfaces

In practice, most 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 coating thickness 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.

Roughness

For rough surfaces, the result can be distorted depending on whether the probe pole is placed in a β€˜valley’ or on a β€˜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, coating thickness measurements on rough surfaces only make sense if the coating is at least twice as thick as the roughness peaks are high. 

See available handheld coating thickness and automated measurement systems here!
 

Thickness of the sample

With metallic base materials, eddy currents are generated not only in the coating material but also in the substrate. If the substrate is very thin (e.g. flat sheet metal), a minimum thickness – which depends on the measuring frequency and the material – is required. 

User influence

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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