Acta Physica Hungarica 61. (1987)

1987 / 3-4. szám - General Physics - A. Haas: Pressure dependent e.m.f. of thermocouples

Acta Phys. Hung., 61 (3—4), pp. 297—308 (1987) PRESSURE DEPENDENT E.M.F. OF THERMOCOUPLES A. Haas Central Research Institute for Measurement Techniques Budapest, Hungary (Received 2 April 1985) The paper deals with the influence of the stationary pressure on the e.m.f. of a thermocouple, by using Onsager’s irreversible thermodynamic equations. The conductivities, the thermodynamic forces and fluxes are expressed by measurable physical constants, and the pressure dependent e.m.f. of a thermocouple is deduced by the phenomenological equations. 1. Introduction Mechano-electrical phenomena constitute the most neglected part of theoretical physics. Although the very first observed electric phenomenon was triboelectricity, the theory of mechano-electrical phenomena has made very little progress even since the birth of irreversible thermodynamics. To this neglected group of phenomena belongs the pressure dependence of the e.m.f. of thermocouples. Nowadays the measurement of temperatures by thermocouples is sometimes performed under extreme conditions. Such is the temperature measurement under high static pressures for which purpose the thermocouple is quite suitable in view of its small size and pressure resistance. Since the measured thermo e.m.f. is pressure dependent, it is difficult to establish the temperature because of this second variable even if the pressure is precisely known. This is a good example of the often emphasized view against the usual reversible direct methods [1], [2], [3] that an axiomatic irreversible measurement theory based on thermodynamics is missing. If such a theory were available it might well furnish an overall comprehensive picture on the influencing quantities, and the information transport [10] and it would enable rigorous significance tests, in contrast to practices when the significance of variables is decided a priori on the ground of plausibility. Instead of the quantitative evaluation of the conductivity matrix of Onsager [4], [5] which characterizes the direct and indirect interactions in all their respects, only one a priori chosen element or part of this matrix is considered by some reversible theory for a direct interaction, irrespective of the overlooked irreversibility and its neglected consequences. But measurement information or negentropy transport is typically an irreversible feature, which cannot be described by reversible methods. Acta Physica Hungarica 61, 1987 Akadémiai Kiadó, Budapest

Next