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...

The thermocouple measurment errors 's reasons (1) .Insertion Depth When the thermocouple is inserted into the measured object, the heat flow will be generated along the sensor body. When the ambient temperature is low, there will be heat loss, which will cause the temperature of the thermocouple and the measured object to be inconsistent, resulting in measurement errors. The error caused by heat conduction is related to the insertion depth and the insertion depth is related to the material of the protective tube. The insertion depth for metal protection tube should be deeper (about 15-20 times the diameter) because of its good thermal conductivity; the insertion depth for ceramic material is can be shallower(about 10-15 times the diameter). For engineering temperature measurement, the insertion depth is also related to whether the measurement object is static or flowing. For example, the measurement of the temperature of flowing liquid or high-speed air flow will not be subject to the above restrictions,the insertion depth can be shallower. (2) .Response Time The basic principle of contact temperature measurement is that the temperature measuring element should reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain period of time to make the two reach thermal equilibrium when measuring the temperature. The holding time is related to the thermal response time of the temperature measuring element, and the thermal response time mainly depends on the structure of the sensor and the measurement conditions. For gaseous, especially static gases, it should be maintained for at least 30 minutes to reach equilibrium; for liquids, the fastest time should be more than 5 minutes. For the measured temperature field with constantly changing temperature, especially the instantaneous change process, the response time of the sensor is required to be at the millisecond level. Otherwise the ordinary temperature sensor will not only lag because it cannot keep up with the speed change of the measured object, but also measurement errors occur due to failure to reach thermal equilibrium. Therefore, it is best to choose a sensor with a fast response speed. For thermocouple, in addition to the influence of the protective tube, the diameter of the thermocouple’s measuring end is also the main factor, the smaller the diameter of the measuring end and conductor wire, the shorter the thermal response time. (3) .Heat Radiation The thermocouple inserted into the furnace is heated by the heat radiation emitted by the high temperature object. If the gas in the furnace is transparent and the temperature difference between the thermocouple and the furnace wall is large, there will be a temperature measurement error due to the thermal radiation between the thermocouple and the furnace wall. Therefore, in order to reduce the thermal radiation error, the temperature of the furnace wall should be as close as possible to the thermo...

The principle of thermocouple temperature measurement is based on the thermoelectric effect, two different conductors are welded at one end and connected at the other end to form a closed loop. When there is a temperature difference between the two ends, the circuit generates thermal electromotive force (also known as thermal electromotive force). The phenomenon that thermoelectric potential is generated by temperature difference is called "thermoelectric effect". These two different conductors combine to form a thermocouple. According to the above principle, the following conditions must be met for thermocouple to generate thermoelectric potential: thermocouple must be composed of two conductor materials with different properties but meeting certain requirements; there must be a temperature difference between the working end of the thermocouple and the reference end. The greater the temperature difference between the working end and the reference end of the thermocouple, the greater the total thermoelectric potential. The materials required to select two different conductors of thermocouple must meet the following conditions: high physical stability and chemical stability; small temperature coefficient of resistance and high conductivity; the generated thermoelectric potential and temperature should be in a linear relationship, the reproducibilityandprocessing performance is good. After understanding the basic principle and structure of thermocouple, it can provide a shortcut for troubleshooting when thermocouple is repaired. During the visual inspection of the thermocouple, the following aspects are mainly checked: check whether the junction box is in good condition, whether the protective tube is bent or burned; shake the thermocouple gently, and observe whether there is any abnormal sound in the tube. When disassembling and inspecting, check whether the terminal is clean or the screws are loose; pull the thermal electrode out of the protective tube and observe whether the insulating tube is wet, whether the thermal electrode has transverse cracks (inferior thermocouple) or whether the surface has small deep marks like a knife cut (advanced thermocouple); observe whether the color of the thermocouple junction is normal. The failure of thermocouple in use is generally that the output potential is lower or higher than the actual value, and the thermoelectric potential is sometimes absent or insensitive. 1. Potential is lower than actual value (meter indication is low) (1) Deteriorated thermocouple electrode or moldy working end Method: cut off the deteriorated part, re-weld the working end or replace with a new electrode. (2) Moisture leakage inside the thermocouple Method: Take out the thermal electrode, dryit and insulate porcelain tube. Check the leakage of the protective tube. Those who fail should repair welding or replace the new protective tube; (3) Dust between the terminals in the thermocouple jun...