Thermocouple

A thermocouple is a device for the measurement of temperature. Its operation is based upon the findings of Seebeck (1821). His work showed that a small electric current will flow in a closed circuit composed of two dissimilar metallic conductors when their junctions are kept at different temperatures. The electromotive force, emf, produced under these con­ditions is known as the Seebeck emf. Thermo­couple elements or pair of conductors, which constitutes the thermoelectric circuit, is called a thermocouple. Simply stated, a thermocouple is a device which converts thermal energy to electric energy. The amount of electric energy produced can be used to measure temperature.

Certain pairs of thermocouple elements give emfs which vary in a regular way with the temperature differences between their junctions. Thermo-metric use may be made of this behavior by maintaining one of the junc­tions at a known, fixed, reproducible temperature. This temperature is called the reference temperature and, for practical measurements, usually is taken at the melting point of ice (0 C). The junction maintained at such a temperature is known as the reference junction; the other junction is called the measuring junction. When a reference temperature is used, the temperature difference, and consequently the emf of a thermocouple, becomes a function of the temperature of the measuring junction. The functional dependence permits the establishment of tables, curves, or mathematical relationships, which, in turn, provide the emfs generated by various pairs of thermocouple elements in terms of the temperature of the measuring junction. This is the general approach followed in thermo­electric thermometry.

In practical measurements the reference junction need not be held at the reference temperature, provided that the actual temperature of the reference junction is known. Consider a case in which the reference junction is at 25 C instead of 0 C and the measuring junction is at 100 C. The emf of the thermocouple will correspond to a temperature difference of 75 C instead of one of 100 C; its emf will be too small owing to the 25 C difference at the reference junction. Compensation can be made by adding the emf equivalent to a 25 C difference to the output of the thermocouple. Ad­justment* of this kind can be made either manually or automatically, depending upon the electrical measuring device. It should be noted, however, that given differences in temperature generally do not give the same emf differences when measured over different temperature ranges. This is equivalent to saying that the change in emf per degree change in temperature, or the thermoelectric power, is not a constant or, in still other words, that most temperature-emf relationships are not linear. The thermoelectric power provides a useful measure for the description of the behavior of a given thermocouple over ranges of temper­atures. It also is a convenient means for comparing the properties of different types of thermocouples.

Questionnaire:

• What is thermocouple? Explain.