Although the calibration of electrical standards continues to be an important activity of the BIPM Electricity Section, at its meeting in March 2007 the CCEM reviewed our workplan and resources. In order to shift some of our human resources to other essential activities in the Section, such as establishing the calculable capacitor and collaborating on the watt balance, while at the same time continuing our support for the CCEM and our programme of comparisons and calibrations, we are instituting new guidelines for the calibration of electrical standards:
- We are asking NMIs to limit the frequency of their BIPM calibrations of resistors, capacitors and electronic voltage standards (Zeners) to once every two years for the same type of standard.
- At the same time, we are asking users of the BIPM electrical calibration services to limit the number of standards of a given type to no more than three standards of the same nominal value.
- Furthermore, we are studying the organization of calibrations into batches such that the calibration work would be carried out during two or three periods of the year. Details of the batching must still be worked out.
Further information on the calibrations available for electrical standards of national laboratories is given below:
| Voltage | Resistance | Capacitance |
Electronic voltage standards referenced to Zener diodes are now widely used, creating the need to carry out calibrations at 10 V as well as 1.018 V. The BIPM uses 10 V arrays of Josephson junctions as the reference voltage standard and all votages are defined with respect to KJ-90. Calibrations at 10 V are carried out either by comparison with a Zener working standard that is calibrated with a 10 V array or by direct comparison with an array. Calibrations at 1.018 V are carried out either by comparison with a calibrated standard cell or by direct comparison with an array. The combined standard uncertainty (relative to the nominal output voltage) assigned to the calibration of a 1.018 V or 10 V Zener voltage standard is typically 1.4 parts in 108. This excludes uncertainty components associated with transportation.
In the area of resistance calibrations, the BIPM was the first laboratory to use a cryogenic current comparator for routine resistance calibrations. This provides a significant increase in the precision of the comparisons and makes it possible to obtain, for example, precise estimates for the pressure coefficients of travelling standards received for calibration by measuring variations in resistance caused by natural changes in atmospheric pressure. Routine calibrations are carried out at 1 , 100 and 10 k. With respect to RK-90, the combined standard uncertainty assigned to the calibration of a 1 resistance standard is typically 1.7 parts in 108 relative to the nominal value; that assigned to the calibration of a 100 or 10 k resistance standard is typically 1.8 parts in 108. This excludes uncertainty components associated with transportation.
The link between 1 pF, 10 pF and 100 pF capacitance standards and the quantized Hall resistance has been established at the BIPM. With respect to RK-90, the combined relative standard uncertainty assigned to the calibration of a 1 pF, 10 pF or 100 pF capacitance standard is typically 5 parts in 108 at 1000 Hz and 4 parts in 108 at 1592 Hz. These values exclude uncertainty components associated with transportation. These uncertainties have been confirmed by several international comparisons and a calibration service is now offered to national metrology institutes.
In addition to calibrations, a bilateral comparison scheme using conventional standards is offered to those laboratories that desire well-documented traceability to the BIPM voltage, resistance, and capacitance standards. This uses methods and procedures similar to those used in the former large-scale international comparisons and the results are proposed to the CCEM for inclusion in Appendix B of the CIPM MRA.