The difference between thermocouples and thermal resistors in the field of industrial control
Thermocouples and thermal resistors both belong to contact temperature measurement in temperature measurement. Although they have the same function of measuring the temperature of objects, their principles and characteristics are not the same.
Firstly, let's introduce thermocouples. Thermocouples are widely used temperature devices in temperature measurement, characterized by a wide temperature measurement range, stable performance, simple structure, good dynamic response, and the ability to transmit 4-20mA electrical signals remotely, making it easy for automatic and centralized control. The temperature measurement principle of thermocouples is based on the thermoelectric effect. Connecting two different conductors or semiconductors into a closed circuit, when the temperatures at the two junctions are different, a thermoelectric potential will be generated in the circuit. This phenomenon is called thermoelectric effect, also known as Seebeck effect. The thermoelectric potential generated in a closed circuit consists of two types of potentials: thermoelectric potential and contact potential.

Thermoelectric potential refers to the electric potential generated at the two ends of the same conductor due to different temperatures. Different conductors have different electron densities, so the electric potential they generate is also different; And contact potential, as the name suggests, refers to the potential formed when two different conductors come into contact, due to their different electron densities, resulting in a certain amount of electron diffusion, and when they reach a certain equilibrium. The magnitude of the contact potential depends on the material properties of two different conductors and the temperature at their contact points. At present, the thermocouples used in applications have a standard specification, which stipulates that thermocouples are divided into eight different divisions, namely B, R, S, K, N, E, J, and T. The lowest temperature they can measure is minus 270 degrees Celsius, and the highest can reach 1800 degrees Celsius. Among them, B, R, and S belong to the platinum series thermocouples, and because platinum is a precious metal, they are also known as precious metal thermocouples. And the remaining ones are called cheap metal thermocouples.

There are two types of thermocouple structures, ordinary type and armored type. Ordinary thermocouples are generally composed of hot electrodes, insulating tubes, protective sleeves, and junction boxes, while armored thermocouples are a solid combination of thermocouple wires, insulating materials, and metal protective sleeves assembled and stretched. However, the electrical signal of a thermocouple requires a special type of wire for transmission, which we call a compensating wire. Different thermocouples require different compensation wires, which are mainly used to connect with the thermocouple and keep the reference end of the thermocouple away from the power supply, 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 with a wire with the same electron density as the thermocouple. The connection between the compensating wire and the thermocouple is generally clear, with the positive pole of the thermocouple connected to the red wire of the compensating wire and the negative pole connected to the remaining color. The material of most compensation wires is copper nickel alloy.

Next, let's introduce the thermistor. Although thermal resistors are widely used in industry, their application is limited due to their temperature measurement range. The temperature measurement principle of thermal resistors is based on the characteristic that the resistance value of conductors or semiconductors changes with temperature. It also has many advantages, such as remote transmission of electrical signals, 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 -200-800 degrees Celsius, while that of copper thermistors is -40 to 140 degrees Celsius. Thermistors and thermocouples have the same type of differentiation, but they do not require compensating wires and are cheaper than thermocouples.