Eddy current conductivity
Conductivity standards are required for the calibration of eddy-current conductivity meters widely used in the aerospace industry to determine the state of heat treatment and effects of work hardening of aluminium alloys. This is particularly important for aircraft manufacturing, when parts are regularly produced by different companies in different countries.
Conductivity is one of the main parameters used in coin vending and coin handling, and a sustainable infrastructure for the Euro currency requires conductivity reference standards with the required value. One of the NPL standards has been produced to match this requirement.
It is necessary to carry out the calibration of reference standards at the working frequency of the instruments which typically starts at 60 kHz. At these frequencies the Eddy current depths of penetration are typically less than 1 mm. Calibrating conductivity meters using material calibrated by a DC method, as is often the procedure in the USA, yields an average for the total thickness of the material. It is difficult to prove that the conductivity of the surface layers, as measured by a conductivity meter operating at a frequency of 60 kHz, is the same as this average or bulk DC value.
Boxed sets of AC conductivity standards can be supplied calibrated at 60 kHz depending upon the values required for specific applications. Depending upon availability, they comprise up to 7 calibrated standards, each approximately 80 mm x 80 mm x 10 mm, having nominal conductivities of 59.5, 36, 22.5, 18.5, 14, 9.6 and 2.2 MS/m (where 58.0 MS/m = 100.0 %IACS on the International Annealed Copper Standard).
Each set is supplied complete with a certificate of calibration accredited by UKAS quoting the conductivities in MS/m and %IACS measured at a frequency of 60 kHz.
DC (bulk) conductivity
The DC conductivity or resistivity can be measured for a range of materials and geometries using various techniques. A number of techniques are available from ‘current clamps/knife edges’ for larger bar samples to four-point needle probes for smaller samples. The DC conductivity measurements are not covered by UKAS accreditation.