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Improved characterization of the BIPM secondary voltage standards used for bilateral comparisons

The BIPM offers a programme of bilateral comparisons of calibrations of Zener voltage standards (BIPM.EM-K11) to those NMIs which do not possess primary Josephson voltage standards (JVSs) or as a first step before an on-site comparison of JVSs.

These comparisons are carried out by using BIPM secondary Zener voltage standards, which are calibrated at 1 V and at 10 V by the BIPM and by the participating NMI.

A change of temperature at the heart of the standard, the Zener diode and the associated resistance voltage divider, induces a change of the voltage output of the standard up to a relative level of a few parts in 109. Since the temperature of the Zeners at the BIPM and at the NMI might be different, it is necessary to correct the voltages to a common reference temperature.

The temperature coefficients of the BIPM Zeners were last measured fourteen years ago. In order to guarantee the smallest possible uncertainties of the BIPM bilateral Zener comparisons, the temperature coefficients have been measured again in the temperature range from 18 °C to 28 °C. The internal temperature of a Zener standard is monitored by a thermistor, the intrinsic parameters of which are also known to possibly change over time.

A complete data analysis is still under way to confirm the provisional conclusions.

For about half of the Zeners, the 2016 temperature coefficients agree with those obtained in 2002. The other coefficients appear to have changed by a significant amount with the observed changes being consistent between the 10 V and the 1.018 V outputs.

In addition, the thermistor resistances at the reference temperature (23 °C) appear to have changed, but this has no influence on the result of a comparison. There does not appear to be any correlation between the change in reference temperature and the change in temperature coefficient.

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