The influence of the ambient humidity on the output voltage of the nominal 10 V terminal of a prototype Zener dc voltage standard was investigated. In this measurement, the main module of the Zener standard driven with an external power supply unit was placed in a thermohygrostat chamber, and the output voltage was measured using a Josephson voltage standard system. When the humidity in the chamber was changed in the range 30%rh - 80%rh, no corresponding effects in voltage outputs were observed.
