- Presence of the big internal resistance that allows carrying out remote measurements;
- In small overall dimensions and persistence, enabling their uses in remote places;
- High mechanical durability,
- Big service life,
- Rather low cost.
Essential lacks semi-conductor thermistor are:
- Nonlinearity of dependence of value of resistance from temperature,
- Significant disorder of values of resistance from the sample to the sample and a number of others.
Resistance SCT for any temperature t can be defined from the following ratio:
(2.8)
where B= t2* t1 /( t2 – t1)*ln(R1/R2); R1 and R2 – resistance at temperatures t1 and t2.
In test chambers and at direct measurements of temperature thermoelectric converters find application. Thermoelectric converters are based on the phenomenon thermoelectricity, opened in 1823 by Seebeck. This phenomenon consists that if to make a circuit of two various conductors (or semiconductors) A and B, having connected them among themselves the ends provided that the temperature t1 one junction is distinct from temperature t2 other junction, in a circuit will appear current under action of emf, named thermal electromotive force.
The further transformation of a measuring circuit, disconnection of a contour, inclusion of conductors and millivoltmeter allows to define value of thermal electromotive force:
. (2.9)
Received total thermal electromotive force for the given pair conductors depends only from value of absolute temperatures Т1, and Т2; also does not depend on length and diameter of thermoelectrodes, and also from their specific electric resistance. Junction of the thermocouples, placed on Wednesday with the measured temperature, refers to as the working end (hot junction), and junction which temperature is maintained by constant, free (cold junction).
Thermal electromotive force, developed by the thermocouple, makes 0,01...0,06 mV·оС, and for its measurement use millivoltmeter or potentiometer included between the free ends of the thermocouple or in break between parts of one of thermoelectrode. In the first case the thermocouple will have three junction: one hot with temperature t1 and two cold which should have constant temperature t2. In the second case there will be four junction: one hot with temperature t1, one cold with temperature t2 and two neutral, having one temperature t3 (figure 2.4).
For measurement of environment temperature it is necessary, that the free ends (cold junction) were at constant temperature that demands use rather long and flexible thermoelectrode the thermocouples providing their accommodation at a distance from the working (hot) junction. With a view of economy of precious metals used for manufacturing thermoelectrodeconnecting wires А1, and В1 make of other metal. These wires going from clips of a head of the thermocouple to vessel V in which the constant temperature t0 is maintained, refer to lengthening thermoelectrode.
Figure 2.4. The scheme of action principle of thermoelectric converters.
However lengthening thermoelectrodes should thermoelectrically be identical to the basic thermocouple. So, for example, to the thermocouple platinum apply lengthening thermoelectrodes from copper and alloy ТП forming identical thermocouple. Further they connect a usual wire to the measuring device.
It is obvious that to use what or device for maintenance of a constancy of temperature - is inconvenient, therefore thermocouples graduate at the temperature of the free ends equal to zero. In practice the temperature of the free ends differs from 0оС on size + t0 and measured thermal electromotive force will be less real, that leads to necessity of corrective action for indications of the thermometer. Using calibration curve it is possible to define the valid value of temperature.
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