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Home >> Temperature, thermoelectricity >> Thermoelectric transducers. Thermocouple
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Thermal electric transducer
Thermoelectric transducer elements transform change of the measured value (temperature) to change the current, arising from differences in temperature at junction of two dissimilar materials in which there is the Seebeck effect (Fig.1).
Thermal electric transducer is more known as a thermocouple, where a probe consists of two junctions, one of which is located in the point, where the temperature is measured, and the second one - in the point of reference temperature (Pic. 2). The difference of potentials V1-V2, which is formed in two junctions (known as contact potentials) depends on temperature of junctions and is measured by voltmeter. Thus, the indications of voltmeter show the difference of temperatures between junctions.
Picture 1. Seebeck effect, at which the current emerges in the layout that consists of junctions from two heterogeneous metals, being at different temperature.
Picture 2. Principle of thermocouple operation
Picture 3. Typical dependencies of voltage on difference of temperature for thermocouple, which junctions are made of different metals:
1 - nickel-chromium/constantan (E); 2 - iron/constantan (J type); 3 - copper/constantan (T type); 4 - nickel-chromium/nickel-manganese-aluminum-silicon (K type); 5 - platinum-rhodium/platinum (R type); 6 - platinum/rhodium/platinum (S type).
Picture 3 shows the curves that characterize the dependence of voltage on difference of temperatures for some typical materials, from which junctions of thermocouples are made. Although these dependencies seem to be linear, the detailed researches demonstrate that it is not the case.
In practice it is not so easy to achieve the necessary precision for temperature measurements using thermocouples as it may seem from abovementioned considerations as the connection of voltmeter with thermal electrical circuit means a new junction in the layout itself. Besides, the object, which temperature is changed, may be located in some distance from voltmeter that complicates the process for formation of stable reference temperature.
Literature:
1. Measuring is in industry. т. 1-3. Reference book edited D.I.Ageykin. 1990.
2. Tenzomeasuring of building constructions and materials. A.B.Rensky, D.S.Baranov, A.Makarov. М. - 1977.
3. Measuring transducers. K.Bridli. Energoatomizdat.1991.
See also:
Thermoelectric transducers - thermocouples:
For measuring and researches in the laboratory terms of thermocouple it is possible to make independently. For this purpose the produced use by industry of wire from materials suitable for making ...
For making of thermocouples mostly use next alloys: kopel, alumel, chromel ...>>>
Calibrating of thermocouples (calibration charts)
For a condition, when a free end(shut) of thermocouple is at the zero of degrees celsius. A temperature is driven to °C, E.M.F. (tension) in mV.
chromel - alumel (-270 °с to 1370 °с)
(0 °с to 1820 °с)
chromel - constantan (-260 °с to 1000 °с)
copper - constantan (-270 °с to 400 °с)
chromel - kopel (-20 to 200 °с)
Making thermocouple:
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CONVERTERS, GAUGES, SENSORS
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