On the Maintenance of Saturated Standard Cells at 3°C through the Use of Thermoelectric Cooling

Saturated standard cells at temperatures between 20°C and 40°C exhibit a high negative-voltage coefficient with respect to temperature and have required temperature stabilization within ±0.01°C over an extended period to achieve a voltage stability of ±1 μV. Examination of standard cell data in the vicinity of 0°C, on the other hand, indicated higher voltages and a much lower temperature coefficient. In fact, the cell voltage reaches a maximum and consequently the voltage coefficient is zero at approximately 3°C, and is near zero over a range of several degrees. A cell maintenance temperature of 3°C, therefore, appears the most appropriate operating environment for a saturated cell in order to obtain maximum voltage stability. Heretofore, a convenient, high-stability, maintenance-free temperature bath operating near 3°C has not been available for laboratory use. The utilization of thermoelectricity to cool (Peltier effect) is currently available, and this method has been employed to provide a stable low-temperature ambient for saturated cells. This paper describes a practical air chamber-controller combination with thermoelectric heat pumping designed to test the behavior of saturated cells near 3°C. Details of the thermal and electrical characteristics of the unit will be discussed. Results show a ±0.2°C control is adequate to provide a primary voltage reference with ±1 μV stability at 3.1°C. Advantages and precautions in maintaining cells at low temperatures will also be presented.