Problems in high temperature thermocouple thermometry

Thermocouples are the most common thermometers used in science and industry for high temperature measurements. Contrary to common belief, thermocouples are not a panacea for high temperature thermometry, for they too have significant problems associated with their use. Accurate temperature measurements with these instruments can be achieved only when their applications are planned carefully. With the instruments now available, accurate thermocouple thermometry above 1200 C is a herculean task. The error sources in high temperature thermocouple thermometry can be divided into seven categories: thermal shunting errors, electrical shunting errors, calibration errors, decalibration errors, extension wire errors, reference junction errors, and data acquisition errors. Each error source is described in this paper, specifically: the temperature intervals in which each source is operative; the magnitudes of the sources; difference in sources for various types of thermocouples and environments; and where possible, methods for minimizing, calculating, or measuring the magnitude of the errors, to aid in a more fundamental understanding of error sources, a brief review of the theory of thermocouple thermometry precedes the discussion of the error sources.