17/2023.07
How to solve temperature jump problem in the process of thermocouple temperature measurement
Thermocouple is affected by the measurement environment, atmosphere, operating temperature, and pollution of insulating materials and protective tube materials during temperature measurement. After a period of use,its thermoelectric properties will change, especially in high temperature and corrosive atmospheres.There are several common problems: Indicated value of the thermometer is zero; Indicated value of the thermometer is unstable; Indicated value of the thermometer is larger or smaller than the actual value; Change of thermoelectric potential is not sensitive. 1. Problems & Treatments (1) .The possible reasons for the extreme instability of thermocouple thermal electromotive force are as follows: A. Poor contact between thermocouple terminal and thermode; B. Instrument connections are not secure. There are intermittent short circuit, grounding, disconnection and poor welding in the measurement circuit; C. The thermocouple swings due to loose installation or external vibration makes the measured value unstable; D. There are conductive liquids, moist dust, and metallic liquids inside the junction box of the thermocouple. The junction box is used for connecting thermocouple and compensation wire, and its outlet hole and cover are sealed with gaskets to prevent dirt from falling in and affecting the reliability of the wiring. When there are pollutants, the inside of the thermocouple junction box must be carefully cleaned and dried in a drying oven before use; E. The thermal electrode is oxidized during use, and the crystal grains will become larger at high temperature. At the same time, deformation stress will be generated by external force. The pollution and corrosion of the thermal electrode by the environment will affect the stability of the thermocouple. (2) . Treatments: A. Check whether the contact between the terminal and the thermode is firm and reliable, whether the fixing screw at the terminal is oxidized, deformed or loose, and re-fix the screw and the terminal; B. Use a multimeter to test the loop resistance, and it should meet the requirements of 15Ω or specified. At the same time, check the thermocouple, compensation wire, eliminate the grounding point, weld or replace a new compensation wire, thermal electrode, and tighten the fixing screws; C. Take vibration reduction measures, install the thermocouple firmly, and the insertion depth of the thermocouple should not be less than 10 times the diameter of the protective sleeve itself. For metal protection tube thermocouple, the insertion depth should be 15-20 times the diameter. For non-metal protection tube thermocouple, the insertion depth should be 10-15 times the diameter; D. Check junction box, clean and dry carefully. (3) .After the inspection, the thermocouple is installed, and the temperature jump still occurs when measuring the temperature.It may be due to the negligence of the staff, a...
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