Truly Transportable Standard-Cell Air Bath

Dissemination of the volt is complicated by the complex temperature response of standard cells. Discribed are the design and performance characteristics of an experimental standard-cell airbath specifically optimized for transportable voltage-dissemination service. Salient features of this bath are 1) ±0.001°C control of the cell temperature in the laboratory, ±0.02°C maximum variation around 28°C in shipment; 2) exposure capacity from over 8 hours at 0°C or 45°C to over 72 hours in moderate temperatures; 3) shipping package weight 14.5 kg, size 20.3 × 22.9 × 30.5 cm; and 4) electrical power drain 0.8-0.2 watt. Cell temperature is determined primarily by an ambient-temperature-biased on-off heater system. Immunity to external environment is achieved by a unique secondary control, which employs the energy storage capacity of two heat-of-fusion alloys, which are reset simply by exposure to laboratory temperature. Experience with this bath has shown that the average value of cell voltage is reproducible to 0.1 μv within a few hours following shipment. This time delay is significantly shorter than the days-to-weeks for most air baths and the weeks-to-months for cells shipped without temperature control.