DC Voltage

The voltage unit

At SP the unit of voltage, the volt (V), is realised at dc in the range 140 µV – 10 V with traceability to the unit for frequency and the numerical value of the ratio of Planck's constant to twice the elementary charge known as the Josephson constant K J-90. The realisation is performed with a "Josephson voltage standard" based on the Josephson effects in superconductivity. The measurement uncertainty is on the order of 1 nV/V for the realisation at the 1 volt level.

The realised voltage unit is transferred to a group of 1.018 V primary voltage standards (Weston cells) and groups of 1 V and 10 V secondary standards (electronic zener voltage standards) maintaining the voltage unit. Between realisations, the primary and secondary standards are compared regularly to secure the maintenance of the voltage unit.

In order to further secure traceability, SP also participates regularly in international comparisons.

Calibration service

SP offers calibration of dc voltage standards and different types of instruments such as voltage dividers, voltmeters and calibrators in the range 1 nV - 500 kV.

Most of the calibrations up to 10 V are automated comparison measurements against primary or secondary voltage standards. Calibration times of up to several weeks for the determination of long term stability of voltage standards are used. The best measurement capability is 0.3 μV/V.
Calibration up to 10 V can be offered with calibration directly against the Josephson voltage standard. The best measurement capability is 3 nV/V.

Other types of measurements are also performed, for example, determination of temperature coefficients. See also ac voltage and inductance.

The final uncertainty given at a calibration depends not only on the uncertainty contribution from the calibration equipment but also from the device under test (stability, temperature coefficient, voltage dependence, etc.). A preliminary measurement uncertainty can often be estimated from experience prior to calibration.