The BIPM and NIST have successfully conducted a new pilot study, marking a significant advancement in the accuracy and reliability of ac voltage comparisons. This achievement represents a major step forward for on-site comparisons of ac voltages.

Background

Since 2016, the International Bureau of Weights and Measures (BIPM) has conducted multiple pilot studies to expand its on-site comparison programme from dc (direct current) Josephson voltage standards to ac (alternating current) voltages. These studies aim to decrease the uncertainties associated with the differential sampling technique applied to Programmable Josephson Voltage Standards (PJVSs) within the range of 10 Hz to 1 kHz, for root-mean-square (rms) voltages of 0.7 V and 7 V. This effort has been supported by several National Metrology Institutes (NMIs), including including NMIJ, CENAM, MIKES, KRISS, NIST, VNIIM, NMIA, and PTB.

Key Results

In August 2024, Dr Stéphane Solve and Mr Régis Chayramy from the BIPM spent three weeks at the NIST Boulder laboratories, alongside a KRISS scientist, to carry out a new pilot study. The BIPM’s transportable PJVS was directly compared to NIST’s Josephson Arbitrary Waveforms Synthesizer (JAWS). As a primary standard, JAWS produces sinusoidal voltage signals in the hertz to megahertz range with very high spectral purity up to 2 V. Importantly, unlike other ac voltage sources, the JAWS does not exhibit any voltage drift, ensuring reliable results.

Key achievements of this study include:

• Stability and reproducibility: The NIST JAWS system allowed the BIPM PJVS to be tested under a wide variety of configurations, pushing the limits of reliability of the differential sampling setup.
• Breakthrough in uncertainty: For the first time, the study achieved a Type A uncertainty (a measure of statistical uncertainty) of a few parts in 10⁹ for a 10 Hz sine wave at 2 V rms. At 1 kHz, a relative level of 2 × 10⁻⁷ was reached.

Future Implications

The technical protocol for this new BIPM comparison was validated by experts from the Consultative Committee for Electricity and Magnetism (CCEM) and released in June 2023. A follow-up questionnaire was sent to the NMIs of the BIPM’s Member States to gauge interest in participating in the comparison. Out of 21 NMIs that expressed interest in participating, 16 requested the option for ac measurements.

These successful pilot comparison results confirm the high reliability of the BIPM’s transportable system and pave the way for on-site comparison exercises for ac voltages in the coming years. This will help NMIs to demonstrate the performance of their ac voltage measurement systems and helps to underpin equivalence of ac voltage measurements worldwide.

Conclusion

The successful results of the pilot study at NIST confirm the high reliability of the BIPM transportable system. It represents the beginning of on-site comparison exercises for ac voltages.

From left to right: A. Rüfenacht (NIST), R. Chayramy (BIPM),
R. Johnson (NIST), S. Solve (BIPM), M.-S. Kim (KRISS)

About our Experts

Dr Solve has been working at the BIPM for 23 years. He was previously involved in the fields of thermometry, photometry and radiometry before joining the voltage metrology laboratory in 2005. He is responsible for the BIPM voltage standards comparison programmes that have been in progress since the early 1990s.

Régis Chayramy joined the BIPM in 1998. He is mainly involved in dc voltage standards and ac measurements in providing technical support for calibrations of secondary voltage standards for NMIs and for the comparison programme.