Electrical conductivity is an important material property that attests to how well a metal allows electricity to pass through it while also allowing inferences to be drawn about the composition, microstructure or mechanical properties of that metal.
The phase-sensitive eddy current test method is used in accordance with the DIN EN 50994 standard to determine this important quantity.
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 φ).
The phase φ depends directly on the electrical conductivity of the material.
In the phase-sensitive eddy current method, the amplitude of the complex impedance exerts no influence on the measurement signal. For this reason, the probe does not need to be placed directly on the object to be tested. Lift-off compensation enables measurements at a distance of around 200–700µm. This effect can be used to inspect parts that are wrapped in a foil or are painted.
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.
Factors that can strongly influence conductivity measurements using the phase-sensitive eddy current method are mainly the temperature and the thickness of the sample. In addition, for all measurements the operator should always ensure that the probe is correctly positioned.
The temperature has a strong influence on the conductivity of a metal. Therefore, by convention, the conductivity is given at the reference temperature of 20°C. If the ambient temperature during the measurement is different, the measured conductivity can be converted into the conventional specification. For this purpose, some of Fischer's conductivity probes are equipped with a temperature sensor.
If the sample is not thick enough, e. g. as with some coins or thin sheets, the propagation of the eddy currents is limited. This can have a major impact on the measurement result. Therefore, the penetration depth of the eddy currents must be chosen appropriately via the probe frequency.
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.
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