Working principle of thermocouple and thermal resistor temperature sensor!

Thermocouples and thermal resistors are temperature sensing elements and primary instruments. They directly measure temperature and convert temperature signals into thermoelectric potential signals, which are then converted into the temperature of the measured medium through electrical instruments (secondary instruments). The basic principle of thermocouple temperature measurement is that two conductors of different compositions form a closed loop. When there is a temperature gradient at both ends, current will flow through the loop. At this time, there is an electromotive force between the two ends - thermoelectric potential, which is the so-called Seebeck effect. Two homogeneous conductors of different compositions are thermocouples. The end with a higher temperature is the working end, and the end with a lower temperature is the free end. The free end is usually at a constant temperature. According to the functional relationship between thermoelectric potential and temperature, a thermocouple scale is made; the scale is obtained when the free end temperature is 0°C, and different thermocouples have different scales.
When a third metal material is connected to the thermocouple loop, as long as the temperature of the two contacts of the material is the same, the thermoelectric potential generated by the thermocouple will remain unchanged, that is, it will not be affected by the third metal being connected to the loop. Therefore, when measuring the temperature with a thermocouple, a measuring instrument can be connected. After measuring the thermoelectric potential, the temperature of the measured medium can be known.
Working principle of thermocouple temperature sensor
Two conductors of different components (called thermocouple wire or hot electrode) are connected at both ends to form a loop. When the temperature of the junction is different, an electromotive force will be generated in the loop. This phenomenon is called the thermoelectric effect, and this electromotive force is called the thermoelectric potential. Thermocouples use this principle to measure temperature. Among them, the end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end); the cold end is connected to the display instrument or the matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.
Thermocouple is actually an energy converter, which converts heat energy into electrical energy and uses the generated thermoelectric potential to measure temperature. For the thermoelectric potential of thermocouple, the following issues should be noted:
1: The thermoelectric potential of thermocouple is the difference of the temperature function of the two ends of the thermocouple working end, not the function of the temperature difference between the cold end and the working end of the thermocouple;
2: The size of 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 material composition of the two thermocouple wires of the thermocouple is determined, the size of the thermoelectric potential of the thermocouple is only related to the temperature difference of the thermocouple; if the temperature of the cold end of the thermocouple is kept constant, the thermoelectric potential of the thermocouple is only a single-valued function of the working end temperature. Weld two conductors or semiconductors A and B of different materials together to form a closed loop, as shown in the figure. When there is a temperature difference between the two attachment points 1 and 2 of conductors A and B, an electromotive force is generated between the two, thus forming a current of a certain magnitude in the circuit. This phenomenon is called the thermoelectric effect. Thermocouples work by utilizing this effect.