..... Click the link for more information. ; thermoelectricity thermoelectricity, direct conversion of heat into electric energy, or vice versa. The term is generally restricted to the irreversible conversion of electricity into heat described by the English physicist James P.
..... Click the link for more information. .
thermocouple
or thermal junction or thermoelectric thermometerTemperature-measuring instrument consisting of two wires of different metals joined at each end. One junction is placed where the temperature is to be measured, and the other is kept at a constant lower (reference) temperature. A measuring instrument is connected in the electrical circuit. The temperature difference causes the development of an electromotive force that is approximately proportional to the difference between the temperatures of the two junctions. Temperature can be read from standard tables, or the instrument can be calibrated to display temperature directly.
thermocouple
thermocouple [′thər·mə‚kəp·əl]
Thermocouple
A device in which the temperature difference between the ends of a pair of dissimilar metal wires is deduced from a measurement of the difference in the thermoelectric potentials developed along the wires. The presence of a temperature gradient in a metal or alloy leads to an electric potential gradient being set up along the temperature gradient. This thermoelectric potential gradient is proportional to the temperature gradient and varies from metal to metal. It is the fact that the thermoelectric emf is different in different metals and alloys for the same temperature gradient that allows the effect to be used for the measurement of temperature.
The basic circuit of a thermocouple is shown in the illustration. The thermocouple wires, made of different metals or alloys A and B, are joined together at one end H, called the hot (or measuring) junction, at a temperature T1. The other ends, CA and CB (the cold or reference junctions), are maintained at a constant reference temperature T0, usually but not necessarily 32°F (0°C). From the cold junctions, wires, usually of copper, lead to a voltmeter V at room temperature Tr. Due to the thermoelectric potential gradients being different along the wires A and B, there exists a potential difference between CA and CB. This can be measured by the voltmeter, provided that CA and CB are at the same temperature and that the lead wires between CA and V and CB and V are identical (or that V is at the temperature T0, which is unusual). Such a thermocouple will produce a thermoelectric emf between CA and CB which depends only upon the temperature difference T1 - T0. See Temperature measurement, Thermoelectricity
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| Type designation | Materials |
|---|---|
| B | Platinum-30% rhodium/platinum-6% rhodium |
| E | Nickel-chromium alloy/a copper-nickel alloy |
| J | Iron/another slightly different copper-nickel alloy |
| K | Nickel-chromium alloy/nickel-aluminum alloy |
| R | Platinum-13% rhodium/platinum |
| S | Platinum-10% rhodium/platinum |
| T | Copper/a copper-nickel alloy |
| *After T. J. Quinn, Temperature, Academic Press, 1983. | |
A large number of pure metal and alloy combinations have been studied as thermocouples, and the seven most widely used are listed in the table. The thermocouples in the table together cover the temperature range from about -420°F (-250°C or 20 K) to about 3300°F (1800°C). The most accurate and reproducible are the platinum/rhodium thermocouples, types R and S, while the most widely used industrial thermocouples are probably types K, T, and E.
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