Thermocouple (redirected from Thermo electric thermometer)

thermocouple: see thermometer; thermoelectricity.

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thermocouple

 or thermal junction or thermoelectric thermometer

Temperature-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.

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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, C_A and C_B (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 T_r. Due to the thermoelectric potential gradients being different along the wires A and B, there exists a potential difference between C_A and C_B. This can be measured by the voltmeter, provided that C_A and C_B are at the same temperature and that the lead wires between C_A and V and C_B and V are identical (or that V is at the temperature T₀, which is unusual). Such a thermocouple will produce a thermoelectric emf between C_A and C_B which depends only upon the temperature difference T₁ - T₀. See Temperature measurement, Thermoelectricity

Basic circuit of a thermocouple

Letter designations and compositions for standardized thermocouples*

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.