The working principle of thermocouple / thermal resistance and the difference between them

1 . Basic principle of thermocouple temperature measurement
Solder the conductors or semiconductors A and B of two different materials to form a closed loop, as shown in Figure 2-1-1. When there is a temperature difference between the two attachment points 1 and 2 of the conductors A and B, an electromotive force is generated between the two, and thus a current of a magnitude is formed in the loop. This phenomenon is called a thermoelectric effect. Thermocouples use this effect to work.
2 . Thermocouple type and structure formation
( 1 ) Type of thermocouple
Commonly used thermocouples can be divided into two categories: standard thermocouples and non-standard thermocouples. The standard thermocouple that is called refers to the thermocouple that the national standard specifies the relationship between the thermoelectric potential and the temperature, the allowable error, and the uniform standard index table. It has a display instrument that is compatible with it. Non-standardized thermocouples are not as good as standardized thermocouples in terms of use or order of magnitude. Generally, there is no uniform indexing table, which is mainly used for measurement in some special occasions.
Standardized Thermocouples Since January 1, 1988, thermocouples and thermal resistors have all been produced in accordance with IEC international standards, and seven standardized thermocouples, S, B, E, K, R, J, and T, have been designated as China's unified design. Thermocouple.
(2) Structure of thermocouple In order to ensure reliable and stable operation of the thermocouple, its structural requirements are as follows :
1 The welding of the two hot electrodes constituting the thermocouple must be firm;
2 The two hot electrodes should be well insulated from each other to prevent short circuits;
3 The connection between the compensation wire and the free end of the thermocouple should be convenient and reliable;
4 The protective sleeve should ensure that the hot electrode is sufficiently isolated from harmful media.
3 . Temperature compensation of the cold junction of the thermocouple
Because thermocouple materials are generally more expensive (especially when using precious metals), and the temperature measurement point is far away from the meter, in order to save thermocouple materials and reduce costs, the compensation wire is usually used to cool the cold end of the thermocouple (free The end extends to a relatively stable temperature control room and is connected to the meter terminals. It must be pointed out that the function of the thermocouple compensation wire only extends the hot electrode, so that the cold end of the thermocouple moves to the instrument terminal of the control room, which itself does not eliminate the influence of the temperature change of the cold end on the temperature measurement, and does not compensate. Therefore, other correction methods are needed to compensate for the influence of the cold junction temperature t0 ≠ 0 ° C on the temperature measurement.
When using the thermocouple compensation wire, it must be noted that the model is matched, the polarity cannot be connected incorrectly, and the temperature between the compensation wire and the thermocouple connection terminal cannot exceed 100 °C.
Thermal resistance
Thermal resistance is the most commonly used temperature detector in the mid-low temperature zone. Its main features are high measurement accuracy and stable performance. Among them, platinum heat is the highest measurement accuracy, it is not only widely used in industrial temperature measurement, but also made into a standard reference instrument.
1 , the principle and material of thermal resistance temperature measurement
Thermistor temperature measurement is based on the fact that the resistance value of a metal conductor increases as the temperature increases. Most of the thermal resistances are made of pure metal materials. Currently, platinum and copper are the most widely used. In addition, thermal resistances such as nickel, manganese and tantalum have been used.
2 , the type of thermal resistance
1 ) Ordinary type thermal resistance
From the temperature measurement principle of the thermal resistance, it is known that the change in the measured temperature is directly measured by the change in the resistance of the thermal resistance. Therefore, changes in the resistance of various wires such as the lead wires of the thermal resistor body may affect the temperature measurement.
2 ) armored thermal resistance
The armored thermal resistor is a solid body composed of a temperature sensing element (resistor), a lead wire, an insulating material, and a stainless steel sleeve. Its outer diameter is generally φ2--φ8 mm and the minimum is φ mm. Compared with ordinary type thermal resistance, it has the following advantages: 1 small volume, no air gap inside, thermal inertia, small measurement lag; 2 good mechanical properties, vibration resistance, impact resistance; 3 can bend, easy to install 4 long service life .
3 ) end face thermal resistance  
The end face thermal resistance temperature sensing element is wound by a specially treated resistance wire and is placed close to the end face of the thermometer. Compared with the general axial thermal resistance, it can reflect the actual temperature of the tested end face more correctly and quickly, and is suitable for measuring the end face temperature of the bearing bush and other parts.
4 ) Flameproof thermal resistance
Explosion-proof thermal resistance is limited to the explosion caused by sparks or arcs in the explosion-mixed gas inside the casing through a special-structure junction box, and the production site will not lead to an explosion. Explosion-proof thermal resistors can be used for temperature measurement in explosion-proof locations in the Bla--B3c class.

Thermocouple temperature measurement principle and several basic laws

Two different conductors or semiconductors are connected in a closed loop at both ends. When the two contacts are placed at different temperatures T and T0, a thermoelectric potential is generated in the loop to form a loop current. This is called the Seebeck effect, which is also called the thermoelectric effect. The generated thermoelectric potential consists of two parts: the contact potential and the temperature difference potential. Thermocouples work based on thermoelectric effects.

1. Law homogeneous conductor

A closed loop is formed by welding the same homogeneous material (conductor or semiconductor) at both ends. No matter the conductor cross section and the temperature distribution, no contact potential will be generated, the temperature difference potential will cancel, and the total potential in the loop will be zero.
It can be seen that the thermocouple must be composed of two different homogeneous conductors or semiconductors. If the hot electrode material is not uniform, an additional thermoelectric potential will be generated due to the presence of a temperature ladder.

2. Intermediate temperature law

The thermoelectric potential between the two junctions of the thermocouple loop (temperature T, T0) is equal to the thermoelectric potential of the thermocouple at temperatures T and Tn and the algebraic sum of the thermoelectric potentials at temperatures Tn and T0. Tn called intermediate temperature

Application : Due to the non-linear relationship between thermocouples ET, when the cold junction temperature is not 0 degrees Celsius, the actual thermoelectric potential E(t, t0) of the known loop cannot be used to directly look up the hot end temperature value; Use the actual thermoelectric potential E(t, t0) of the known loop to directly look up the obtained temperature value, and add the cold junction temperature to determine the measured temperature value of the hot end, which needs to be corrected according to the intermediate temperature law. Beginners often do not correct according to the intermediate temperature law!

3. Intermediate conductor law

The intermediate conductor (third conductor) is connected to the thermocouple loop. As long as the temperature at both ends of the intermediate conductor is the same, the introduction of the intermediate conductor has no effect on the total potential of the thermocouple loop. This is the intermediate conductor law.
Application : According to the intermediate conductor law, in the actual temperature measurement application of thermocouple, hot-end welding and cold-end open circuit are often used, and the cold end is connected with the display instrument to form a temperature measurement system.
Some people worry that the copper wire is used to connect the cold junction of the thermocouple to the meter to read the mV value. The contact potential generated at the junction of the wire and the thermocouple will cause additional errors in the measurement. According to this law, there is no such error!