Working principle and precautions of platinum rhodium S-type thermocouple temperature sensor

From a professional perspective, two conductors with different compositions (called thermocouple wires or thermoelectric electrodes) are connected at both ends to form a circuit. When the temperature of the junction is different, an electromotive force is generated in the circuit, which is called thermoelectric effect, and this electromotive force is called thermoelectric potential. Thermocouples use this principle for temperature measurement, where the end directly used to measure the temperature of the medium is called the working end (also known as the measuring end), and the other end is called the cold end (also known as the compensating end); Connect the cold end to the display instrument or matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.

In fact, in simple terms, the platinum rhodium S-type thermocouple is actually an energy converter that converts thermal energy into electrical energy and measures temperature using the generated thermoelectric potential. Regarding the thermoelectric potential of the thermocouple, the following three issues should be noted:

After the composition of the two thermocouple wires is determined, the magnitude 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 remains constant, the thermoelectric potential entering the thermocouple is only a single value function of the working end temperature.

Secondly, the thermoelectric potential of a thermocouple is the difference in temperature function between the two ends of the working end of the thermocouple, rather than the temperature difference between the cold end and the working end of the thermocouple;

Thirdly, the magnitude of the thermoelectric potential generated by the thermocouple is not related to the length and diameter of the thermocouple when the material is uniform, but only to the composition of the thermocouple material and the temperature difference at both ends;