Thermocouples and RTDs are the most commonly used temperature sensors. This paper compares and analyzes the differences between thermocouples and RTDs in terms of their working principle, structure, classification, and application fields, and briefly introduces the applications of thermocouples and RTDs. .
  Temperature is an important thermal parameter in industrial production such as metallurgy and chemical industry. It plays an important role in production control. To measure the temperature efficiently and accurately, it is necessary to select the temperature sensor according to the characteristics and requirements of the temperature measurement point. Although thermocouples and thermal resistances have a delay in temperature measurement, they are simple, reliable, and have high measurement accuracy. They are widely used in industrial production. The correct choice requires detailed understanding of them.
  1. The difference between working principle and structure
  The difference between 1 working principle
  The thermocouple is made of two different conductors or semi-conductor materials welded or hinged into hot and free ends. The hot end is inserted into the device that requires temperature measurement, and the cold end is placed outside the device if both ends are located. Different temperatures will generate thermoelectric potential in the thermocouple circuit. Because the thermoelectric potential is a function of the measured temperature, the value of the electromotive force can be converted into a temperature value.
  Thermal resistance is based on the nature that the resistance of a conductor changes with temperature and converts the change in resistance into an electrical signal for temperature measurement.
  2 Differences in Structure
  Ordinary thermocouples are usually made up of a hot electrode, an insulating material, a galvanic protection sleeve, a junction box, and the like. Thermocouples generally use perforated high-temperature ceramic tubes as the insulating material, and the hot electrodes are drawn from the holes of high-temperature-resistant ceramic tubes. The protection sleeve material needs to have corrosion resistance, high temperature resistance, high mechanical strength, good airtightness, high thermal conductivity, such as metal, nonmetal, and cermet. The most commonly used protective sleeve is 1Cr18Ni9Ti stainless steel. Operating conditions below 900°C.
  The main part of the thermal resistance is the resistor body plus insulating sleeves, protective sleeves, junction boxes and other components. The resistance wire is wound on the insulating framework of quartz, ceramics or plastics, and the protective sleeve is put on the thermal resistance. The wire and the sleeve are filled with a thermally conductive material.
  2. Classification and characteristics of thermocouples
  The standard thermocouple refers to the relationship between the thermoelectric thermoelectric potential and the temperature specified in the national standard. There is a unified standard index table, allowing thermocouples with certain errors.
  Non-standard thermocouples generally do not have a uniform indexing table. They are mainly used to measure some special occasions. The range of use and magnitude are smaller than standard thermocouples. The thermocouples that make up the thermocouples must be firmly welded together. There should be better insulation between them to prevent short circuits; the connection between the compensating wire and the free end of the thermocouple should be firm and reliable, and the protection sleeve must ensure that the thermal electrode is fully isolated from the external medium to ensure reliable and stable operation of the thermocouple.
  3, thermal resistance classification and characteristics
  1 According to thermal resistance compositional structure classification
  Ordinary thermal resistance: According to the principle of temperature measurement of thermal resistance, the measured temperature change is directly reflected by the change of the resistance value. Therefore, the change in the resistance of the various leads caused by the thermal resistance will adversely affect the temperature measurement. . Need to eliminate the impact of lead resistance, usually thermal resistance using three-wire or four-wire compensation.
  Armored thermal resistance: similar to armored thermocouples, it is also composed of temperature sensing elements, leads, insulation materials and stainless steel bushings. The outside diameter is generally between φ2-φ8mm, which is smaller than the common type. Easy to install, impact resistant, bendable and longer life.
  End-face thermal resistance: The temperature-sensing element of the end-face thermal resistance is wound with a specially treated resistance wire, and it is closely attached to the end face of the thermometer. Compared with general thermal resistance, it can reflect the actual temperature of the tested end face more quickly and accurately, and is suitable for the temperature measurement of the end face such as the bearing bush.
  Explosion-proof thermal resistor: The junction box of explosion-proof thermal resistor has special structure. It can control the explosion and flash fire caused by sparks or arcs in the junction box to ensure the open flame at the production site. The explosion-proof thermal resistor is suitable for containing flammable, Explosive and other chemical gases and vapors.
  2 According to the thermal resistance material classification
  Platinum resistance: platinum has a large resistivity, and the relationship between resistance and temperature is non-linear. Platinum resistance has a wide range of temperature measurement, high measurement accuracy, easy purification of materials, and good reproducibility; its physical and Chemical properties are very stable. Industrial platinum resistance grades are Pt100 and Pt10. Pt10 is made of coarse platinum wire and is suitable for measuring temperatures above 600°C. At 0°C, the resistance value of Ptl00 is 100Ω, and the resistance value of Ptl0 at 0°C is 10Ω, so Pt100 is also more commonly used; the platinum resistance temperature measurement range is generally -200°C to 850°C. Between temperatures above 550°C are only suitable for use in oxidizing atmospheres. Vacuum and reducing atmosphere will cause the resistance value to drift rapidly.
  Copper resistance: The resistance value of the copper resistance is almost linear with the temperature. The copper resistance material is easy to purify, the price is low, the resistivity is only 1/2 of platinum and the volume is large, and the thermal response is slow. Copper is easily oxidized at temperatures in excess of 250°C, so industrial copper thermal resistors generally operate within a temperature range of -40°C to 120°C.
  Semiconductor thermistors: Suitable for occasions with low temperature requirements, generally used in the low temperature range of -50 ~ 350 °C, in industrialized countries, the use of semiconductor temperature sensors made of a large number of temperature sensors used in various temperature measurements and Temperature compensation and demanding temperature control such as home appliances and automobiles.
  4, the actual application of the difference
  A thermocouple is a component in which the thermo-electromotive force (voltage) changes with temperature. A thermal resistor is a component whose resistance changes with temperature. Thermocouples are generally used in a high-temperature environment, and thermal resistances are generally used in a low-temperature environment. If the measured temperature difference exceeds 500°C, the resistance of the RTD will be very large, which will directly affect the results of the temperature measurement, even if the measurement results cannot be displayed. The thermocouple is measured by the change of the thermoelectromotive force (voltage) as the temperature changes. The higher the measured temperature, the more intense the movement of the electrons in the atom and the more sensitive the potential response. Second, the thermocouple needs to compensate for the wire. The thermal resistance does not require the compensation of the wire. The price of the thermal resistance is also lower than that of the thermocouple.
  China's thermocouples and thermal resistors are all produced according to IEC international standards, and designated S, B, E, K, R, J, T seven kinds of standardized thermocouple as a unified design thermocouple.
  Temperature is a very important basic parameter control index in industrial production. The main task of temperature measurement is to perform temperature detection in various heat production. Reasonable selection of the essential difference between thermocouple and thermal resistance enables production process personnel to grasp the equipment in time. Operational status to ensure that the equipment is safe, stable, and economical.