Measuring with the conductivity method.
Electrical conductivity is an important material property. Conductivity measurement provides information about the specific electrical conductivity of a metal and allows conclusions to be drawn about its composition, microstructure and mechanical properties. Thus, in many applications, conductivity is an important part of process control or product monitoring.
This is how electrical conductivity measurement works.
The 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 (usually in the kHz range). This creates eddy currents in the sample.
The probe contains a second coil – the measuring coil, which measures the AC resistance (impedance). This probe impedance is modified by the eddy currents in the sample and is out of phase (phase angle φ) compared to the excitation current (probe without sample).
The phase angle φ depends on the film thickness and the electrical conductivity of the material. If the coating or bulk material is saturation thick, the phase angle is compared in the instrument with a stored characteristic curve and converted into the conductivity.
What happens during the lift-off effect?
For conductivity, the phase-sensitive eddy current method has a useful advantage. As we are measuring the phase angle, there is no need to for the probe to be directly in contact with the material. Instead, the conductivity of gold coins, for example, can be measured inside a plastic protective sheath.
Where is this process used?
- Conductivity of aluminum alloys for anodizing processes
- Conductivity measurement for decorative anodized coatings
What factors can influence the measurement?
All electromagnetic measuring methods are comparative. This means that the measured signal is compared with a characteristic curve stored in the device. To ensure that the result is correct, the characteristic curve must be adapted to the current conditions. This is done by calibrating the measuring device for conductivity measurements.
The right calibration makes the difference
Factors that can strongly influence conductivity measurement using the phase-sensitive eddy current method are the temperature and the thickness of the test part. In addition, the operator should always ensure the correct probe position for all measurements.
Influence of the temperature on the measurement results
Temperature strongly influences the conductivity of a metal. Therefore, by convention, conductivity is specified at the reference temperature of 20 °C. If the ambient temperature differs during conductivity measurement, the measured conductivity can be converted to the conventional specification. Some of our probes are equipped with a temperature sensor for this purpose. This greatly simplifies the measuring of conductivity, but still ensures high measuring accuracy.
Thickness of the test piece
In order to maintain the effect of eddy currents in the material under investigation, care must be taken to ensure that the measurement frequency and material-dependent minimum thickness is guaranteed, especially in the case of a thin sample (such as flat sheet metal).
Operating the conductivity meter
Last but not least, how the instrument is operated also plays a major role in conductivity. Always make sure that the probe is positoned level above the sample being tested. For better accuracy, a tripod can also be used to automatically lower the probe onto the test piece. In addition, we offer set-up aids for various probes, such as prisms for curved surfaces.
Which standard is applied here?
Measuring the electrical conductivity according to DIN EN 50994