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 m ost often used with Josephson voltage standards and in international compar isons for which the smallest uncertainties are demanded. Fifteen of the 732 B instruments have relative pressure coefficients near 1.9 x 10(-9)/hPa for both outputs; the other two have negative coefficients, the absolute value s of which are smaller by about a factor of ten. Small, but statistically s ignificant, coefficients of both signs were found for the three Fluke 732A instruments. The pressure effects are highly reproducible and so they permi t the use of corrections to eliminate errors which, in extreme cases, can a ttain relative values of nearly four parts in 10(7).