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 Effect of Seebeck
Principle, actions of thermocouple
Dependences voltage on the difference of temperature for thermocouples
General principles of transducers (sensors) 
    In the strict sense of the word transducer is a device that converts a change in one quantity changes in the other. In terms of electronic transmitter ... >>>
The principles of the transformation of the measured physical parameters 
    In each converter, which converts the element is based on certain physical principle, which is associated with the electrical characteristics of the device so that a change in the quantity being measured   >>>


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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.17).
   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. 18). 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 17. Seebeck effect, at which the current emerges in the layout that consists of junctions from two heterogeneous metals, being at different temperature.

   Picture 18. Principle of thermocouple operation

   Picture 19. 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 19 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:
Thermoelectricity of metals and semiconductors
Thermoelectric cooling devices
Thermoelectric converters - generators of EMF

Thermoelectric transducers - thermocouples:
Making of thermocouple. Video
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 ...
Alloys for thermocouples
For making of thermocouples mostly use next alloys: kopel, alumel, chromel ...>>>
Calibrating of thermocouple
Calculation of the temperature on the value of the thermopower thermocouple

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)
platinum-30% rhodium/platinum-6% rhodium
(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:
Датчики, преобразователи. Sensors, transducers
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Датчики, преобразователи, sensors, gauge