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(1) Thermocouple thermometer

1, the principle and structure of thermocouple thermometer

Thermocouple thermometer is a temperature measuring instrument made of the principle that the contact of two different metal conductors is heated to produce a thermoelectric potential. It is mainly composed of three parts: thermocouple, compensation wire and electrical measuring instrument (galvanometer). One end of two different conductors or semiconductors (thermal electrodes) is welded to each other to form the working end (thermal end) of the thermocouple, which is inserted into the measured medium to measure temperature. The free end (cold end) of the thermocouple is connected to the measuring instrument through a wire. When there is a temperature difference between the working end and the free end of the thermocouple, the thermoelectric potential is generated, so there is a current through the compensation wire, and the greater the temperature difference, 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 read directly. Commonly used common platinum rhodium - rhodium thermocouple (WRLL type)* high measurement temperature of 1600℃. Common platinum-rhodium-platinum thermocouple (WRLB) * has a high measured temperature of 1400 ° C. Common nickel-chromium-nickel-silicon thermocouples (WREU) * have a maximum measured temperature of 1100 ° C.

The advantages of thermocouple thermometers are high sensitivity, large measurement range, no need for external power supply, and easy to measure and record at a distance. The disadvantage is that compensation wire is required, and the installation cost is more expensive. In industrial boilers, it is commonly used to measure steam temperature, furnace flame temperature and flue gas temperature.

2, the installation and use of thermocouple thermometer requirements

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

(2) The thermocouple must be placed in the middle of the measured medium, and should be as far as possible to the flow direction of the measured medium at a 45° Angle, the depth is not 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 plugged with plaster to avoid the intrusion of external cold air affecting the measurement accuracy. The thermocouple protected by ceramic should be slowly inserted into the measured medium to avoid rupture of the protective tube due to sudden temperature change.

(4) The change in the temperature of the free end of the thermocouple has a great impact on the measurement results, and the free end temperature must be corrected or maintained constantly.

(2) Thermal resistance thermometer

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

(2) The protective casing is generally added to the outside of the thermal resistance, and the protective casing material should be temperature resistant, corrosion resistant, withstand temperature changes, good sealing and sufficient mechanical strength.

(3) Using thermal resistance to measure temperatures below 500 ° C has higher measurement accuracy than thermocouples. The pin resistance can measure -200~650℃, and the copper thermal resistance can measure the temperature range of -50~150℃. Thermal resistance can also be measured and displayed remotely. Installation is similar to that of a thermocouple. The thermal resistance is connected with the temperature display instrument through a wire to become a temperature display device to measure the temperature of gases, liquids and vapors.