In order to shift some of our human resources to other essential activities, such as establishing the calculable capacitor and collaborating on the BIPM watt balance project, while at the same time continuing our support for the CCEM and our programme of comparisons and calibrations, the CCEM (2007) agreed the following guidelines for the calibration of electrical standards at the BIPM:
- NMIs are asked 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.
- Users of the BIPM electrical calibration services should please limit the number of standards of a given type to no more than three standards of the same nominal value.
- The calibrations will be carried out at the BIPM in two or three batches during the year.
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 the CIPM MRA database.
Further information on the calibrations available for electrical standards of national laboratories is provided in the "Voltage", "Resistance" and "Capacitance" pages above.
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 (excluding 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 (excluding uncertainty components associated with transportation). These uncertainties have been confirmed by several international comparisons.