Discrepancies Between the DC and AC Measurement of Low Frequency Electrical Conductivity

The measurement of electrical conductivity of non-ferrous metals and alloys is of particular interest to the coin production and handling industries and to the aerospace industry as it provides a measure of the quality of parts. These industries use commercial conductivity meters which measure the parameter using an AC technique. Traceability to national standards in many countries is achieved using DC measurement techniques, although the UK's National Physical Laboratory (NPL) has an established method for achieving this traceability by an AC technique. Due to the effects of frequency and the behaviour of the materials used as reference standards, discrepancies can exist between the AC and DC techniques. This paper describes work undertaken during a project, called Conductivity, aimed at addressing these discrepancies recently completed by a consortium of partners and funded by the European Commission's Framework 5 Programme on Competitive and Sustainable Growth. The partners are 3 national metrology institutes (NPL (UK),' PTB (Germany), NMi (Netherlands)), 2 European instrument manufacturers in this area (Hocking NDT Ltd (UK), Institut Dr Foerster (Germany)) and a representative of the aerospace industry (EADS launch vehicles)