Working principle and installation precautions of thermocouple

1, Working principle

When two different conductors or semiconductors A and B form a circuit and their two ends are connected to each other, as long as the temperatures of the two nodes are different, with one end having a temperature of T, called the working end or hot end, and the other end having a temperature of t0, called the free end (also known as the reference end) or cold end, an electromotive force will be generated in the circuit. The direction and magnitude of electromotive force are related to the material of the conductor and the temperature of the two contacts. This phenomenon is called thermoelectric effect. A circuit composed of two conductors is called a thermocouple. These two conductors are called thermoelectric poles. The generated electromotive force is called thermoelectric power.

Thermoelectric power consists of two parts

One part is the contact electromotive force of two conductors, and the other part is the thermoelectric electromotive force of a single conductor. The magnitude of thermoelectric force in a thermocouple circuit is only related to the temperature of the conductor material and two contacts that make up the thermocouple, and is independent of the shape and size of the thermocouple. When the two electrode materials of the thermocouple are fixed, the thermoelectric force is at two contact temperatures T and t0. The difference in functionality means that this relationship has been widely used in actual temperature measurement. Because the cold end t0 is constant, the thermoelectric force generated by the thermocouple only varies with the temperature change of the hot end (measuring end), that is, a certain thermoelectric force corresponds to a certain temperature, and we can only measure the temperature by measuring the thermoelectric force.

The basic principle of thermocouple temperature measurement is that two conductors with different compositions form a closed loop. When there is a temperature gradient at both ends, current will pass through the circuit and electromotive force will exist between the two ends. Thermoelectric power, also known as the Seebeck effect, consists of two uniform conductors with different compositions: the thermoelectric electrode, the working end at high temperature, and the free end at low temperature, which is usually at a constant temperature. A thermocouple calibration table was created based on the functional relationship between thermoelectric potential and temperature. The calibration table is obtained under the condition of a free end temperature of 0 ℃, and different thermocouples have different calibration tables.

When a third metal material is connected to a thermocouple circuit, 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, i.e. not affected by the connection of the third metal to the circuit. Therefore, when using thermocouples to measure temperature, measuring instruments can be connected and the temperature of the measured medium can be known after measuring the thermoelectric force. When measuring temperature, thermocouples require the temperature of their cold end (the measuring end is the hot end, and the end connected to the measuring circuit through a wire is called the cold end) to remain constant, so that the thermoelectric potential is proportional to the measured temperature. If the (ambient) temperature at the cold end changes during the measurement process, the measurement accuracy will be severely affected.

It is normal to take certain measures at the cold end to compensate for the impact caused by temperature changes, which is called thermocouple cold end compensation. The calculation method for cold junction compensation of a dedicated compensating conductor thermocouple connected to a measuring instrument: from millivolts to temperature: measure the cold junction temperature, convert it to the corresponding millivolt value, add it to the millivolt value of the thermocouple, and convert it to temperature; From temperature to millivolts: Measure the actual temperature and cold junction temperature, convert them into millivolts, subtract the millivolts to obtain the temperature.

2, Thermocouple structure

Thermocouples are commonly used temperature measuring components in temperature measuring instruments. They directly measure temperature and convert the temperature signal into a thermoelectric power signal, which is then converted into the temperature of the measured medium through electrical instruments (secondary instruments). Due to the need, various thermocouples usually have very different shapes, but their basic structures are roughly the same. They are usually composed of main components such as hot electrodes, insulation sleeves, protective tubes, and junction boxes, and are typically used in conjunction with display instruments, recording instruments, and electronic regulators.

3, Characteristics and advantages related to thermocouple principle

1. Simple assembly and easy replacement;

2. The compression spring type temperature sensing element has good seismic performance;

3. High measurement accuracy;

4. Large measurement range (-200 ℃~1300 ℃, in special cases -270 ℃~2800 ℃);

5. Fast thermal response time;