Capacitance is an important physical quantity in electromagnetic measurement which is closely associated with the value of impedance in the field of electric power, electrical appliance manufacturing industry, and scientific research. Therefore, capacitance has always been the key standard value of the Consultative Committee for Electricity and Magnetism (CCEM). NIM has achieved excellent results in the 2017 international key comparison of capacitance standards (CCEM.K4-2017) organized by CCEM. The comparison results were recently published in the BIPM key comparison database (BIPM KCDB).
The comparison plan is that the pilot laboratory BIPM conducted bilateral comparisons with the other seven participating laboratories (METAS, NIM, NIST, NMIA, NPL, PTB and VNIIM). The 10 pF and 100 pF capacitance standards used were provided by participating laboratories, while BIPM provided standard capacitors as a standard.
The capacitance standard NIM used for comparison can be traced to a new vertical calculable cross-capacitor with a movable guard electrode applying electrical compensation approach. The device is based on a National Science and Technology Support Program named “Modern Measurement Standards Based on Quantum Physics” in 2007, and a subsequent innovative team project of the NIM, as well as the "Study of Precision Measurement and Traceability Technology of Basic Physical Constants for the Redifinition of SI" under the “Research and Application of Common Technology of National Quality Infrastructure”, which was a key R&D programme of the Ministry of Science and Technology, China.
Figure 1 and Figure 2 show the final results of the key comparison of 10 pF and 100 pF capacitance. In the figures, the capacitance value of 10 pF provided by China (NIM) is 1.8×10-8 (k=1), showing the lowest uncertainty. The results of comparison of 10pF and 100pF capacitance are very close to the key comparison reference value (KCRV), where 100pF shows the smallest deviation from the standard value. This marks that China's new vertical calculable cross-capacitor and capacitance bridges have received international recognition and reached the world's advanced level.
Figure 1. CCEM.K4-2017 result of key comparison of 10 pF capacitance
Figure 2. CCEM.K4-2017 result of key comparison of 100 pF capacitance
The new vertical calculable cross-capacitor has significantly improved the ability of realizing the SI unit of capacitance, providing a solid technical support for improving the product quality and precision measurement of impedance in the electric power and electrical appliance manufacturing industries; obtaining mutual recognition via international key comparisons; and consolidating and improving the national electromagnetic traceability chain.
At the annual meeting of the Asia Pacific Metrology Program (APMP) in 2018, NIM plans to pilot the next APMP key comparison of capacitance .
(Division of Electricity and Magnetism)