Design and construction of precision heat fluxmeters

Heat flux is traditionally measured, in a steady flow situation, with the heat passing through a medium of known thermal conductivity which has temperature sensors built in at determined distances. An alternative method consists of making the heat flow between the ends of a thermopile and measuring the resulting emf. The first method has the disadvantage that it is necessary to know the thermal conductivity of the medium and its temperature dependence. Further difficulties include those specific to local temperature measurement; the sensors create errors due to their finite size and due to heat leaks through their temrinals, In the second method, the difficulty lies in the fact that each thermocouple junction must be electrically isolated, which tends to produce a thermal isolation, and there is no guarantee of the homogeneity of the thermal conditions of each thermocouple. Delicate mechanical structure and lack of functionality are other typical defects in thermopiles to date. In order to achieve mechanical strength, substances of generally poor thermal conductivity are used as supports, even in more compact thermopiles made of semiconductors. I We shall now discuss our technique for the design and construction of thermopiles for heat flux measurement in a stationary regime for a one-dimensional temperature gradient. Our method intends to achieve the following conditions: adequate sensitivity and linear electrical responses, absence of heat leaks, easy and accurate calibration, compact structure and mechanical strength, absence of poor thermal conductors, temperature homogeneity of the surfaces at the ends of the thermopiles, where the thermo junctions are located, and good thermal contact with the medium between which they are sandwiched. Our thermopiles (see Fig. 1) consist of N thermocouples arranged electrically in series and thermally in paranel between a heat reservoir H (isothermal metallic block) and a metal plate B which has a built-in electrical resistance R. Each thermocouple is made up of a pair of metals or alloys ab in wires of equal length, /, and section s. The electrical terminals come out of the end which is thermally anchored to the heat reservoir. The thermo junctions form two flat parallel surfaces and are adhesive bonded to two thin metal . plates FI and F2 which act as a thermal bridge with H and B. Vacuum isolation is normally used. When a heat power is dissipated in the setup BR, or in an attached sample (see Fig. 1), a heat flux q passes through the thermopile and a temperature difference Ll T is established between its ends, giving rise to an emf E in the thermopile. If.d Tis small enough to admit linear approximation, we can write