1. Introduction to Thermocouples
Thermoelectric temperature recorders often use thermocouples as temperature measuring elements. It is widely used to measure the temperature in the range of -200℃~1300℃. In special cases, it can measure the high temperature of 2800℃ or the low temperature of 4K. It has simple structure, Low price, high accuracy, wide temperature measurement range, etc. Since the thermocouple converts temperature into electricity for detection, it is very convenient to measure, control, and amplify the temperature signal. It is suitable for long-distance measurement and automatic control. In contact thermometry, thermoelectric thermometers are widely used.​
Second, the principle of thermocouple temperature measurement:
1. Definition: A sensor that is composed of two conductors and converts temperature into thermoelectromotive force is called a thermocouple. 2. Temperature measurement principle: The temperature measurement principle of thermocouple is based on the thermoelectric effect. Connect conductors A and B of two different materials in series to form a closed loop. When the temperatures of the two contacts 1 and 2 are different, if T>T0 (as shown in Figure 12-1 thermoelectric effect), heat will be generated in the loop. The electromotive force generates a certain amount of current in the loop, which is called the thermoelectric effect. The thermoelectromotive force is recorded as EAB, and conductors A and B are called thermoelectrodes. Contact 1 is usually welded together, and it is placed in the temperature measurement place to feel the measured temperature during measurement, so it is called the measurement end (or working end, hot end). Contact 2 requires a constant temperature and is called the reference end (or cold end). 3. Thermoelectric effect: The thermocouple closed circuit composed of conductors A and B has two contact potentials eAB (T) and eAB (T0) at the two junctions, and because T>T0, there are also two contact potentials in conductors A and B. There is a thermoelectric potential. So the closed loop total thermoelectromotive force EAB (T, T0) should be the algebraic sum of the contact electromotive force and the thermoelectric potential, namely: 4. The closed loop total thermoelectromotive force For the selected thermocouple, when the reference temperature is constant, the total thermoelectromotive force is It becomes a single-valued function of the temperature T at the measurement end, that is, EAB(T, T0)=f(T). This is the basic principle of thermocouples to measure temperature. In actual temperature measurement, connecting wires and instruments must be introduced into the thermocouple closed loop.​
3. Basic principles of thermocouple temperature measurement
A closed loop composed of a homogeneous conductor, regardless of the conductor's cross-sectional area, length, and temperature distribution, does not generate thermal electromotive force. If the two hot electrodes of the thermocouple are composed of two homogeneous conductors, then the thermoelectromotive force of the thermocouple is only related to the temperature of the two junctions, and has nothing to do with the temperature distribution of the thermocouple; In the temperature field of the temperature gradient, an additional potential will be generated. If the temperature is judged only from the thermoelectromotive force of the thermocouple, it will cause errors. 1. Homogeneous conductor rule: 2. Intermediate conductor rule: When a conductor of the third material is connected to the thermocouple circuit, as long as the temperatures at both ends are equal, the access of the conductor will not affect the total thermoelectromotive force of the thermocouple circuit. . According to this rule, one contact of the thermocouple can be disconnected and connected to the third conductor, or one conductor of the thermocouple can be disconnected and connected to the third conductor, as long as the temperature at both ends of each conductor is the same, Neither affects the total thermal EMF of the circuit. In the actual temperature measurement circuit, there must be a connecting wire and a display instrument. If the connecting wire and the display instrument are regarded as the third conductor, as long as the temperatures at both ends of them are the same, the total thermal electromotive force will not be affected. 3. Reference electrode rule: The two conductors A and B respectively form a thermocouple with the reference electrode C (or standard electrode). If the thermoelectromotive force generated by them is known, the thermoelectromotive force after the pairing of the A and B poles can be used. It can be seen that as long as the thermoelectromotive force when the two conductors form a thermocouple with the reference electrode are known, the thermoelectromotive force when the two conductors form a thermocouple can be calculated according to the reference electrode rule. This simplifies the selection of thermocouples. Due to the stable physical and chemical properties of platinum, high melting point and easy purification, people mostly use high-purity platinum as the reference electrode.​
4. Commonly used thermocouples
There are more than 300 kinds of materials suitable for making thermocouples, of which 40 to 50 are widely used. The International Electrotechnical Commission recommends 8 kinds of thermocouples as standardized thermocouples to countries around the world. There are also 8 kinds of standardized thermocouples in my country. They are: platinum-rhodium 10-platinum (graduation number is S), platinum-rhodium 13-platinum (R), platinum-rhodium 30-platinum-rhodium 6 (B), nickel-chromium-nickel-silicon (K), nickel-chromium-constantan (E), Fe-Constantan (J), Cu-Constantan (T), and NiCrSi-NiSi (N). A few of them are briefly described below:
5. Several commonly used thermocouples
1. Platinum-rhodium 10-platinum thermocouple 1. Composition: It is made of φ0.5mm pure platinum wire and platinum-rhodium wire with the same diameter. The graduation number is S. The platinum-rhodium wire is the positive electrode and the pure platinum wire is the negative electrode. 2. Features: good thermoelectric performance, strong oxidation resistance, suitable for continuous use in oxidative and inert atmosphere. The long-term applicable temperature is 1400°C. When the temperature exceeds this temperature, the pure platinum wire will recrystallize even in the air to increase the grain size. The short-term use temperature is 1600 ℃. Among all thermocouples, it has a high level of accuracy and is usually used as a standard or thermocouple for measuring high temperature.