thermocouple(redirected from Thermocouples)
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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
|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|
|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.
a temperature sensor consisting of two unlike electrically conductive elements—usually metal conductors but occasionally semiconductors—that are joined to one another. A thermocouple makes use of the Seebeck effect. If the junctions of the conducting elements (often called thermoelectrodes) are at different temperatures, a thermal electromotive force (emf) is generated in the circuit. The magnitude of the emf is unambiguously determined by the hot and cold terminal temperatures and the composition of the electrodes.
Thermocouples are used in an extremely wide temperature range (see Table 1). The emf of a thermocouple using metal conductors is usually 5–60 millivolts. The accuracy of temperature indication is usually several degrees K; some thermocouples attain an accuracy of ~0.01°K. The emf’s of semiconductor thermocouples may be an order of magnitude higher, but they are quite unstable.
|Table 1. Operating temperature ranges of some types of thermocouple|
|Positive element||Negative element||Temperatures (°K)|
|Gold-iron alloy||Copper or Chromel||4–270|
Thermocouples are used in temperature measurement equipment and in various automatic control and monitoring systems. A thermoelectric thermometer is produced by combining a thermocouple with an electrical measurement instrument, such as a millivoltmeter or potentiometer. The measurement instrument is connected to the ends of the thermoelectrodes or to a break in one of the electrodes. When making a temperature measurement, one of the junctions must be maintained at a reference temperature, usually 273°K.
Thermocouples can be divided into a number of types, depending on their design and purpose. They may be of the submersible or surface type, and they may be unjacketed or with an ordinary, explosion-proof, moisture-proof, or other jacket, which may be sealed or unsealed. In addition, they may be shock-resistant or vibration-resistant and stationary or portable.
REFERENCESosnovskii, A. G., and N. I. Stoliarova. Izmerenie temperatur. Moscow, 1970.
D. N. ASTROV