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 ??V. 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 ??V stability at 3.1??C. Advantages and precautions in maintaining cells at low temperatures will also be presented.