Thermocouples and thermal resistances both belong to the contact type temperature measurement in temperature measurement. Although their functions are the same, they measure the temperature of objects, but their principles and characteristics are not the same.

Firstly, let me introduce thermocouples. Thermocouples are widely used temperature devices in temperature measurement. Their main characteristics are wide measurement range, stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, making it easy for automatic control and centralized control. The temperature measurement principle of thermocouples is based on the thermoelectric effect. When two different conductors or semiconductors are connected into a closed circuit, when the temperatures at the two junctions are different, a thermoelectric effect or Seebeck effect will be generated in the circuit. The thermoelectric potential generated in a closed circuit consists of two types of potentials; Temperature difference potential and contact potential. Thermoelectric potential refers to the potential generated at the two ends of the same conductor due to different temperatures. Different conductors have different electron densities, so the potential they generate is also different. Contact potential, as the name suggests, refers to the potential formed when two different conductors come into contact, and because their electron densities are different, they produce a certain amount of electron diffusion. When they reach a certain equilibrium, the size of the contact potential depends on the material properties of the two different conductors and the temperature at their contact point. At present, there is a standard specification for thermocouples used internationally, which stipulates that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J, and T. The sensitive words for measuring temperature can reach minus 270 degrees Celsius, while the sensitive words can reach up to 1800 degrees Celsius. Among them, B, R, and S belong to the platinum series of thermocouples, which are also known as precious metal thermocouples due to platinum being a precious metal. 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, insulation tubes, protective sleeves, and junction boxes, while armored thermocouples are a solid combination of thermocouple wires, insulation materials, and metal protective sleeves that are assembled and stretched. However, the electrical signal of a thermocouple requires a special type of wire for transmission, which we refer to as a compensating wire. Different thermocouples require different compensation wires, and their main function is to connect with the thermocouple, keeping the reference end of the thermocouple away from the power source, thereby stabilizing the temperature of the reference end. Compensation wires are divided into two types: compensation type and extension type. The chemical composition of the extension wire is the same as that of the compensated thermocouple. However, 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 electron density as the thermocouple. The connection between the compensating wire and the thermocouple is generally very clear. The positive pole of the thermocouple is connected to the red wire of the compensating wire, while the negative pole is connected to the remaining color. Most compensation wires are made of copper nickel alloy.

Next, let's introduce the thermal resistance. Although it is widely used in industry, its application is limited due to its temperature measurement range. The temperature measurement principle of the thermal resistance is based on the characteristic that the resistance value of a conductor or semiconductor changes with temperature. It has many advantages, such as the ability to transmit electrical signals remotely, high sensitivity, strong stability, good interchangeability, and accuracy. However, it requires power excitation and cannot measure temperature changes instantaneously. Industrial thermistors generally use Pt100, Pt10，Cu50，Cu100， The temperature measurement range of platinum thermistors is generally -200 to 800 degrees Celsius, while copper thermistors are -40 to 140 degrees Celsius. Thermistors have the same distinguishing type as thermocouples, but they do not require compensating wires and are cheaper than thermocouples.