The difference between temperature sensor thermal resistor and thermocouple

The difference between thermocouples and thermal resistors
1. Different signal properties:
Thermal resistors are resistors themselves. Thermal resistor thermometers work based on the principle that the resistance of metal wires changes with temperature; while thermocouples: weld two wires or semiconductors of different materials together to form a closed loop. When there is a temperature difference between the two contacts of the wire, an electromotive force is generated between the two, thus forming a certain amount of current in the loop. This phenomenon is called the thermoelectric effect. Thermocouples work by using this effect.
2. The temperature ranges detected by the two sensors are different:
Thermal resistors generally detect a temperature range of 0-150 degrees, with a maximum measurement range of about 600 degrees, while thermocouples can detect a temperature range of 0-1000 degrees (or even higher).
3. Different materials:
Thermal resistors are a metal material with temperature-sensitive changes. Thermal resistors are resistance signals, and their output is resistance value (Ω), without positive and negative poles. Commonly used ones are pt100 and cu50. . Thermocouples are bimetallic materials, that is, two different metal materials. Due to the change in temperature, an electric potential difference is generated at the two ends of the two different metal wires. It can be divided into K, S, E, R, T, N and other graduation numbers, which can be used in different measurement media and temperature measurement ranges. Thermocouples are voltage signals, and the output is millivolts (mv), with positive and negative poles. Some are more expensive than resistors, and some are cheaper than resistors, but the comprehensive cost of thermocouples is high when the compensation wire is included.
4. Different relationship with PLC:
The input modules of thermal resistors and thermocouples corresponding to PLC are also different. This sentence is fine, but generally PLCs are directly connected to 4~20ma signals, while thermal resistors and thermocouples are generally connected to PLCs with transmitters. If you connect to DCS, you don't need to use a transmitter. Thermistor is RTD signal, thermocouple is TC signal! (PLC also has thermal resistor module and thermocouple module, which can directly input resistance and thermocouple signal.
The temperature measurement principle of thermal resistor is different from that of thermocouple. Thermal resistor measures temperature based on the thermal effect of resistor, that is, the resistance of resistor changes with the change of temperature. Therefore, as long as the resistance change of temperature-sensitive thermal resistor is measured, the temperature can be measured. At present, there are mainly two types: metal thermal resistor and semiconductor thermistor.
The resistance value and temperature of metal thermal resistor can generally be expressed by the following approximate relationship, that is, Rt=Rt0[1+α(t-t0)]
Where Rt is the resistance value at temperature t; Rt0 is the corresponding resistance value at temperature t0 (usually t0=0℃); α is the temperature coefficient. The relationship between the resistance value and temperature of semiconductor thermistor is Rt=AeB/tWhere Rt is the resistance value at temperature t; A and B depend on the structure of semiconductor material. Constant.
In comparison, thermistors have a larger temperature coefficient and a higher resistance value at room temperature (usually above several thousand ohms), but their interchangeability is poor, nonlinearity is serious, and the temperature measurement range is only about -50~300℃. They are widely used in temperature detection and control of household appliances and automobiles. Metal thermal resistors are generally suitable for temperature measurement in the range of -200~500℃. They are characterized by accurate measurement, good stability, and reliable performance. They are widely used in process control.
From the perspective of the change of resistance with temperature, most metal conductors have this property, but not all of them can be used as temperature measuring thermal resistors. Metal materials used as thermal resistors generally require: as large and stable temperature coefficient as possible, large resistivity (reducing the size of the sensor at the same sensitivity), stable chemical and physical properties within the temperature range of use, good material reproducibility, and resistance value changes with temperature. There should be an intermediate value function relationship (preferably a linear relationship).