Analysis of structure and principle of temperature sensor

(1) Thermocouple thermometer

1. The principle and structure of thermocouple thermometer

Thermocouple thermometer is a temperature measuring instrument made by using the principle of two different metal conductors to generate thermoelectric potential after being heated. It is mainly composed of three parts: thermocouple, compensation wire and electrical measuring instrument (galvanometer). Two different conductors or semiconductors (hot electrodes) are welded to each other to form the working end (hot end) of the thermocouple, which is inserted into the measured medium to measure the temperature. The free end (cold end) of the thermocouple is connected to the measuring instrument through wires respectively. When there is a temperature difference between the working end and the free end of the thermocouple, a thermoelectric potential is generated, so that a current flows on the compensation wire, and the greater the temperature difference, the greater the generated thermoelectric potential and the current on the wire. By observing the deflection angle of the pointer on the measuring instrument, the temperature value of the measured medium can be directly read out. Commonly used ordinary platinum rhodium-rhodium thermocouple (WRLL type) maximum measurement temperature is 1600 ℃. Ordinary platinum rhodium-platinum thermocouple (WRLB) The maximum measuring temperature is 1400 ℃. Ordinary nickel-chromium-nickel silicon thermocouple (WREU) The maximum measurement temperature is 1100 ℃.

The thermocouple thermometer has the advantages of high sensitivity, large measurement range, no external power supply, and is convenient for long-distance measurement and recording. The disadvantage is that it requires compensation wires, and the installation cost is relatively expensive. In industrial boilers, it is commonly used to measure the steam temperature, furnace flame temperature and flue gas temperature in the flue.

2. Requirements for the installation and use of thermocouple thermometers

(1) The installation location of the thermocouple should be easy to work and not affected by collision and vibration.

(2) The thermocouple must be placed in the middle of the measured medium, and should be inclined at a 45 ° angle to the flow direction of the measured medium, and the depth should not be less than 150mm. When measuring the temperature of the furnace, it should generally be inserted vertically. If it is difficult to insert vertically, it can also be installed horizontally, but the length inserted into the furnace should not be greater than 50Omm, otherwise it must be supported.

(3) After the thermocouple is installed, its insertion hole should be tightly plugged with plaster to prevent the external cold air from invading and affecting the measurement accuracy. Thermocouples protected with ceramics should be slowly inserted into the measured medium to prevent the protection tube from breaking due to sudden temperature changes.

(4) The change of the temperature at the free end of the thermocouple has a great influence on the measurement result, and the temperature at the free end must be constantly corrected or kept constant.

(2) Thermal resistance thermometer

(1) The thermal resistance thermometer is made by using the property that the resistance of a conductor or a semiconductor changes with temperature, and the value of the measured temperature is obtained by measuring the resistance value of the metal. Common materials for thermal resistance are platinum and copper.

(2) A protective sleeve is generally added to the outside of the thermal resistance, and the material of the protective sleeve must be resistant to temperature, corrosion, rapid temperature changes, good sealing, and sufficient mechanical strength.

(3) Measuring the temperature below 500 ℃ with the thermal resistance has higher measurement accuracy than the thermocouple. Pin thermal resistance can measure -200 ~ 650 ℃, copper thermal resistance can measure -50 ~ 150 ℃. Thermal resistance can also be measured and displayed at a distance. The installation method is similar to the thermocouple. The thermal resistance and the temperature display instrument are connected by wires to become a temperature measurement display device, which is used to measure the temperature of gas, liquid and steam.

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