Optimum design and efficiency of plated thermocouples

Thermocouples may be made by plating alternate portions of a continuous core wire or, alternatively, by stripping alternate portions of a composite metal. In such couples the Seebeck e.m.f. is not equal to the open-circuit voltage, because of circulating currents occurring within the composite portions; and conventional figure-of-merit considerations are inapplicable. An efficiency calculation is carried out, on the basis of which the optimum geometry and maximum efficiency of plated thermocouples are inferred. For metals obeying the Wiedemann-Franz relation, it is shown that the resistance of the plating must be about 30 per cent of the resistance of the core wire, for optimum performance. It is further shown that the maximum efficiency of plated couples is only 36 per cent that of the corresponding conventional couple. This decrease of efficiency is offset by the increased reliability which can be attained when multi-element thermopiles are required.