Measurement uncertainties arising from unpowered shipment of de voltage references

We have measured the hysteresis in output voltage arising from loss of powe r for three Fluke 732B de voltage references. Continuously powering the int ernal temperature-controlled oven provides the best accuracy with such Zene r-diode-based de voltage references. Batteries are used to provide power co ntinuity when shipping between laboratories, We have shown that for these p articular instruments, an apparent hysteresis induced by humidity is likely to dominate any hysteresis arising from loss of battery power when transit between laboratories takes three days or more. Our conclusion is that as m uch, if not more, care needs to be taken when dealing with humidity than wi th loss of battery power when shipping between laboratories. When the highe st accuracy of de voltage comparison is required, it becomes necessary to a pply voltage corrections using the measured values of humidity coefficients and the relative humidity occurring during transit.