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Dec 04, 2018

# Structure and working principle of thermocouple (ii)

## Basic principle of thermocouple temperature measurement

Thermocouple is a kind of temperature sensing element, is a primary instrument, it directly measures the temperature, and the temperature signal into thermoelectric electromotive force signal, and then through the electrical instrument (secondary instrument) into the temperature of the medium under test.

The basic principle of thermocouple temperature measurement is that two material conductors with different components form a closed loop. When there is a temperature gradient at both ends, there will be a current passing through the loop. At this time, there will be an electromotive force -- thermoelectric electromotive force between the two ends, which is the so-called seebeck effect. Two homogeneous conductors of different components are hot electrodes, the higher temperature end is the working end, the lower temperature end is the free end, the free end is usually at a constant temperature.According to the function of thermoelectric electromotive force and temperature, thermocouple indexing meter is made.Indexing table is free end temperature at 0 ℃, under the condition of different thermocouples with different indexing table.

When the third metal material is connected to the thermocouple circuit, the thermoelectric potential generated by the thermocouple will remain unchanged as long as the temperature of the two contacts of the material is the same, that is, it will not be affected by the access circuit of the third metal.Therefore, when the thermocouple measures the temperature, it can be connected to the measuring instrument. After measuring the thermoelectric emf, the temperature of the medium under test can be known.

A thermocouple welds conductors or semiconductors A and B of two different materials together to form A closed loop, as shown in the figure. →TO the 4-20mA PT100 Temperature Transmitter Gauge

When there is A temperature difference between the two persistent points 1 and 2 of conductor A and conductor B, an electromotive force is generated between them, and A large current is formed in the circuit, which is called thermoelectric effect.Thermocouples use this effect to work.

When two conductors of different components (called thermocouple wires or hot electrodes) are connected to the synthesis circuit at both ends, when the temperature of the junction point is different, the electromotive force will be generated in the circuit, this phenomenon is called thermoelectric effect, and this electromotive force is called thermoelectric potential.Thermocouple is to use this principle for temperature measurement, in which, directly used as a measurement of the temperature of the medium at one end is called the working end (also known as the measurement end), the other end is called the cold end (also known as compensation end);The cold end is connected to the display instrument or supporting instrument, which will indicate the thermoelectric potential generated by the thermocouple.

A thermocouple is actually an energy converter, which converts heat energy into electrical energy and measures temperature with the generated thermoelectric potential. For the thermoelectric potential of a thermocouple, the following problems should be noted:

1: the thermoelectric potential of a thermocouple is the difference between the temperature functions of the two ends of the thermocouple, rather than the function of the temperature difference between the two ends of the thermocouple;

2: the thermoelectric potential generated by the thermocouple, when the material of the thermocouple is uniform, has nothing to do with the length and diameter of the thermocouple, but only with the composition of the thermocouple material and the temperature difference between the two ends;

3: when the composition of two thermocouple wire materials is determined, the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple;If the temperature at the cold end of the thermocouple remains constant, the thermoelectric potential of the incoming thermocouple is only a single value function of the temperature at the working end.

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