Difference between thermocouple and thermal resistance

                                  Difference between thermocouple and thermal resistance

Thermocouple and thermal resistance belong to contact temperature measurement in temperature measurement. Although their functions are the same, they both measure the temperature of objects, but their principles and characteristics are different
Thermocouple is the most widely used temperature device in temperature measurement. Its main characteristics are wide measurement range, relatively stable performance, simple structure, good dynamic response, and more able to transmit 4-20mA electrical signal remotely, which is convenient for automatic control and centralized control. The temperature measurement principle of thermocouple is based on thermoelectric effect. When two different conductors or semiconductors are connected into a closed loop, when the temperature at the two contacts is different, the loop will generate thermal potential. This phenomenon is called thermoelectric effect, also known as Seebeck effect. The thermoelectric potential generated in the closed loop consists of two kinds of potential: thermoelectric potential and contact potential.
Thermoelectric potential refers to the potential generated by different temperatures at both ends of the same conductor. Different conductors have different electron densities, so they produce different potentials. Contact potential, as the name suggests, refers to the potential formed when two different conductors contact, because their electron densities are different, they produce a certain amount of electron diffusion. When they reach a certain equilibrium, The magnitude of contact potential depends on the material properties of two different conductors and the temperature of their contact point. At present, there is a standard specification for thermocouples used internationally. Internationally, thermocouples are divided into eight different divisions, namely B, R, s, K, N, e, J and t. the lowest measurable temperature is 270 degrees below zero Celsius, and the highest can be 1800 degrees Celsius. Among them, B, R and s belong to platinum series thermocouples. Because platinum is a precious metal, So they are also called precious metal thermocouples, and the remaining few are called cheap metal thermocouples.
There are two types of thermocouple structures, ordinary type and armored type. Ordinary thermocouples are generally composed of thermal electrodes, insulating tubes, protective sleeves and junction boxes, while armored thermocouples are a solid combination of thermocouple wires, insulating materials and metal protective sleeves after stretching.
The electric signal of thermocouple needs a special wire to transmit, which is called compensation wire. Different thermocouples need different compensation wires. Its main function is to connect with the thermocouple to keep the reference end of the thermocouple away from the power supply, so as to stabilize the temperature of the reference end. The compensation wire is divided into compensation type and extension type. The chemical composition of the extension wire is the same as that of the compensated thermocouple, but in practice, the extension wire is not made of the same material as the thermocouple, and is generally replaced by a wire with the same electronic density as the thermocouple. The connection between the compensation wire and the thermocouple is generally clear. The positive pole of the thermocouple is connected to the red wire of the compensation wire, and the negative pole is connected to the remaining color. Most of the general compensation wires are made of copper nickel alloy.
Although thermal resistance is widely used in industry, its application is limited due to its temperature measurement range. The temperature measurement principle of thermal resistance is based on the characteristic that the resistance value of conductor or semiconductor changes with the change of temperature. It also has many advantages. It can also transmit electrical signals remotely. It has high sensitivity, strong stability, good interchangeability and accuracy, but it requires power excitation and cannot measure temperature changes instantaneously.
PT100, pt10, cu50, cu100 are generally used for industrial thermistors. The temperature measurement range of platinum thermistor is generally minus 200-800 ℃, and that of copper thermistor is minus 40-140 ℃. Thermal resistance is the same as thermocouple, but it does not need compensation wire, and it is cheaper than hot spot couple.