According to the gradient theory, the emergence of the thermovoltage takes place only in sections of the thermocouple with temperature gradient. It is shown that, in the case of inhomogeneous thermocouples, the thermovoltage measured does not depend only on the temperatures of hot and cold junctions, but also on the temperature profile alongside the thermocouple. As in the case of an external calibration the temperature profile is never the same as in the real application, any correction of inhomogeneous thermocouples based on it will be imperfect. However, thermocouples with integrated miniature fixed-point cells allow also in case of inhomogenity a correct and automatic in-situ calibration. The paper describes the structure and mode of operation of such thermocouples and reports on the experience of their application in the hot-steam area of power stations.
1. Reed, R. P. Possibilities and limitations of self-validation of thermoelectric thermometry. In Temperature, Its Measurement and Control in Science and Industry. American Institute of Physics, 2003, vol. 7, 507–512.
2. Bernhard, F. (ed.). Technische Temperaturmessung. Springer-Verlag, Berlin, 2004.
3. Henning, F. and Moser, H. (eds.). Temperaturmessung, 3rd ed. Johann Ambrosius Barth, Leipzig, 1977.
4. White, W. P. What is the most important portion of a thermocouple? Phys. Rev., 1908, 32, 535–536.
5. Brixy, H. Thermocouple thermometry. ISOTECH J. Thermometry, 1995, 6, H. 1, 1–17.
6. Quinn, T. J. Temperature, 2nd ed. Academic Press, London, 1990.
7. Pollock, D.D. Thermocouples, Theory and Properties. CRC Press, Boca Raton, 1991.
8. Bentley, R. E. The distributed nature of EMF in thermocouples and its consequences. Australian J. Instrumentation Control, 1982, Dec., 128–132.
9. Körtvélyessy, L. Thermoelement-Praxis – Neue theoretische Grundlagen und deren Umsetzung, 3rd ed. Vulkan-Verlag, Essen, 1998.
10. Brixy, H., Hecker, R., Rittinghaus, K. F. and Höwener, H. Applications of noise thermometry in industry under plant conditions. In Temperature, Its Measurement and Control in Science and Industry. American Institute of Physics, 1982, vol. 5, part 2, 1225–1237.
11. Tischler, M. and Korembit, M. J. Miniature thermometric fixed points for thermocouple calibrations. In Temperature, Its Measurement and Control in Science and Industry. American Institute of Physics, 1982, vol. 5, part 1, 383–390.
12. Lehmann, H. and Bernhard, F. Self-calibrating thermocouples. In Proc. 6th International Symposium on Temperature and Thermal Measurements in Industry and Science. Torino, 1996, 195–206.
13. Boguhn, D., Augustin, S., Bernhard, F., Mammen, H. and Tischler, M. Application of binary alloys in miniature fixed-point cells as secondary fixed points in the temperature range from 500 °C to 660 °C. In Temperature, Its Measurement and Control in Science and Industry, 8th Symposium. Chicago, 2002.
14. Augustin, S., Bernhard, F., Boguhn, D., Donin, A. and Mammen, H. Industrial applicable miniature fixed-point thermocouples. In Proc. 8th International Symposium on Temperature and Thermal Measurements in Industry. Berlin, 2001, vol. 1, 3–8.
15. Bernhard, F., Boguhn, D., Augustin, S., Mammen, H. and Donin, A. Application of self-calibrating thermocouples with miniature fixed-point cells in a temperature range from 500 °C to 650 °C in steam generators. In Proc. XVII IMEKO World Congress on Metrology in the 3rd Millenium. Dubrovnik, 2003, 1604–1608.
