Measurement of electrical conductivity
Electrical conductivity is one of several measured values used to determine material properties. Traceability to a physical basic variable is required for measuring instruments. This calibration must be carried out by a standardization institute such as the Physikalisch-Technische Bundesanstalt (PTB) [National metrology institute of the Federal Republic of Germany] in Braunschweig. The measured value of electrical conductivity is usually stated in MS/m. Alternatively in %IACS, which creates the reference to a pure copper material.
Absolute electrical conductivity is temperature-dependent and, for metals, relative to an ambient temperature of 20°C. If materials have a different temperature, this has to be measured and compensated. The mechanical condition likewise impacts the measurable electrical conductivity. Even curved surfaces (convex/concave) must also be corrected, for example.
To be able to achieve the greatest possible degree of accuracy within a broad conductivity range of 0 to 65 MS/m using eddy current technology, various homogenous materials in this range are used as a reference. They are recorded and saved as part of a factory calibration at various distances to the material, and also at different measuring frequencies using an absolute probe.
For a temperature compensation, values are recorded and correction parameters for the calibration points of the factory calibration are determined at the time of testing by means of reference bodies with known conductivities under the same ambient conditions. This also allows a highly accurate conductivity measured value to be achieved even at different temperatures.