The electrical conductivity of non-ferrous metals with conductivities in the range 2.2 MS/m to 35.8 MS/m, provided by NPL in the form of bars (600 mm by 80 mm by 10 mm) and square blocks (80 mm side by 10 mm), was measured using an AC method at NPL and a DC method at PTB and VSL. The purpose of the comparison was to establish if there is a difference in the electrical conductivity measured for AC and DC conditions. Conductivity standards are used by industries involved with non-destructive testing (NDT) and evaluation (NDE) to calibrate the instruments used. These instruments operate at frequencies of at least 60 kHz and it was therefore necessary to establish agreement between AC and DC methods.

For all cases the uncertainty for AC conductivity measurements is considerably larger than that for DC measurements due to the increased complexity of the unique NPL bridge method. For DC measurements on blocks the PTB and VSL values agreed within the measurement uncertainties provided. A small difference between the values for a bar made from Nordic gold was removed when the CMC uncertainty for PTB was used. A better agreement between the DC measurement for blocks is believed to be due to a reduced sensitivity to material homogeneity as well as fewer dimensional measurements for the Van der Pauw technique used. A significant difference between the measured AC and DC conductivities for the titanium bar was found. This is believed to be due to a variation in the properties of the material through the thickness. The AC method is surface sensitive and so such a change in properties will influence the value compared to DC methods that are a volume average.

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