Pressure coefficients of some Zener diode-based electronic voltage standards

We present results of measurements of the pressure dependence of the 1.018 V and 10 V outputs of twenty electronic voltage standards. Seventeen Fluke 732B and three Fluke 732A instruments were studied, these being the types most often used with Josephson voltage standards and in international comparisons for which the smallest uncertainties are demanded. Fifteen of the 732B instruments have relative pressure coefficients near 1.9×10^-9/hPa for both outputs; the other two have negative coefficients, the absolute values of which are smaller by about a factor of ten. Small, but statistically significant, coefficients of both signs were found for the three Fluke 732A instruments. The pressure effects are highly reproducible and so they permit the use of corrections to eliminate errors which, in extreme cases, can attain relative values of nearly four parts in 10⁷