Working principle and structure of thermocouple

What is a thermocouple

Thermocouple is one of the temperature detection elements commonly used in industry. The operation principle of thermocouple is based on seeback effect, that is, two conductors of different components are connected into a loop at both ends. If the temperature of the two connecting ends is different, the physical phenomenon of thermal current will occur in the loop.

Two, the basic principle of thermocouple temperature measurement

Thermocouple is a temperature sensing element, is a surface, it directly measures the temperature, and the temperature signal into thermoelectric electromotive force signal, and then through the electrical surface (secondary surface) into the temperature of the medium to be measured.

The basic principle of thermocouple temperature measurement is that two different components of the material conductor formed a closed loop, when there is a temperature gradient at both ends, there will be a current through the loop, at the moment there is electromotive force between the two ends - thermoelectric emf, this is the so-called Seebeck effect.

The homogeneous conductor of two different components is the thermal electrode, the higher temperature end is the operating end, the lower temperature end is the free end, the free end is generally at a stable temperature. According to the thermoelectric electromotive force and the function of temperature, made thermocouple indexing meter; The indexing meter is obtained under the condition of the free end temperature at 0℃. Different thermocouples have different indexing meters.

When a third metal data 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 data is the same, that is, it will not be affected by the third metal access circuit. Therefore, in the thermocouple temperature measurement, can be connected to the measurement surface, measured thermoelectric electromotive force, you can know the temperature of the measured medium.

A thermocouple welds two different data conductors or semiconductors A and B together to form A closed loop, as shown in the figure.

When there is A temperature difference between the two junctions 1 and 2 of conductor A and B, an electromotive force will occur between the two, thus forming A large and small current in the circuit. This phenomenon is called thermoelectric effect. Thermocouples operate using this effect.

Two conductors of different components (called thermocouple wire or thermal electrode) are connected to a synthetic circuit at both ends. When the temperature of the junction is different, electromotive force will occur in the circuit. This phenomenon is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouple is the use of this principle for temperature measurement, which is directly used to measure the temperature of the medium is called the operating end (also known as the measurement end), the other end is called the cold end (also known as the compensation end); The cold end is connected to an appearance or matching appearance, which indicates the thermoelectric potential generated by the thermocouple.

Thermocouple is actually a kind of energy converter, which converts heat energy into electricity and measures temperature with the generated thermoelectric potential. For thermoelectric potential of thermocouple, we should pay attention to the following problems:

1: the thermoelectric potential of the thermocouple is the difference of the temperature function at both ends of the thermocouple, rather than the function of the temperature difference at both ends of the thermocouple;

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

3: when the data composition of two thermocouple wires is determined, the size of the thermocouple thermoelectric potential is only related to the temperature difference of the thermocouple; If the temperature of the cold end of the thermocouple is constant, the thermoelectric potential of the input thermocouple is only a univalued function of the temperature of the operating end.