The effects of the environmental conditions of some Zener voltage references

We measured variations of output voltages of four Zener standards (Fluke732B) with change of environmental conditions such as humidity, temperature and pressure. Four Zener standards have negligibly small humidity characteristics. The characteristic of the temperature was the order of 0.01 ppm/K. The characteristic of the pressure was approximately 0.02 ppm/10 hPa.