The accurate and standardized application of sensors is the key to ensuring the normal operation of measurement and control equipment and realizing process technology in industrial production. Equipment operation failures and production accidents caused by improper configuration and use of temperature measurement sensors and measurement control instruments on the industrial heating furnace operation site. These failures and accidents make the temperature control of the thermometer inaccurate, which will affect the process of thermal processing production and work efficiency, resulting in a reduction in product quality. In severe cases, the product batch will be unqualified, and even cause damage to the electric furnace equipment, resulting in shutdown of the electric furnace &damage accident. Therefore, in the industrial heating production site, it is necessary to ensure that the use and supporting of temperature sensors, devices and instruments are standardized.Thermocouple is a sensor device for the temperature display and control system of an electric furnace for industrial heating.
1. Compensation wire and thermocouple mismatch use
The temperature indication above 1000℃ in the high temperature zone in the thermal processing workshop is too high. When the temperature is 800℃and below, the temperature display value is also too large.
1.1. Treatment method and cause analysis
(1) Method
The wrong selection of compensation wire matching leads to inaccurate temperature measurement. Replace the corresponding compensation wire, and the fault is eliminated.
(2) Cause
When the instrument was installed on site, the matching relationship between the compensation wire and the thermocouple was ignored, resulting in failure. The principle of thermocouple temperature measurement is based on the constant temperature of the cold junction. Therefore, the negative electrode of the thermocouple should be set in an area with constant temperature at the installation site, and then achieve the purpose of accurate measurement through a certain temperature compensation; this type of temperature compensation is cold junction compensation. It should be noted that the function of the compensation wire is not to compensate the potential of the cold junction: in order to keep the temperature of the cold junction of the thermocouple temperature measuring electrode constant, the electrode must be extended, but the electrodes of the thermocouple are all precious metals.Therefore, the usual practice is to extend the cold end of the thermoelectric electrode with a metal wire with similar thermoelectric properties to the thermoelectric electrode material at room temperature (0~100℃). To the area away from the heat source and the temperature is stable, the wire of this material is called an extension wire or a compensation wire. Obviously, the function of the compensation wire is to "compensate for the lack of length of the thermode". According to the idea that the thermoelectric characteristics of the compensation wire and the thermode are similar at room temperature, the compensation wire and the thermocouple are used together.
2. Misuse of positive and negative poles of compensation wire and thermocouple
2.1. There is no temperature indication after reinstalling in place
2.2. Treatment method and cause analysis
(1) Method
If the instrument and thermocouple are running normally, the verification is qualified. Such problems are mainly considered from the perspective of installation. The pointer of the meter deflects in the opposite direction, indicating that there is no problem with the line connection. Use a millivolt meter to measure the thermocouple and the input terminal of the instrument. If it is found that the positive and negative poles of the thermocouple are reversed, adjust the positive and negative poles. The instrument indicates normal but finds that the indicated temperature value is smaller than the actual temperature value, excluding cold junction compensation and zero point calibration. Observe whether the positive and negative poles of the thermocouple compensation wire are reversed, adjust to the correct and the equipment returns to normal.
(2) Cause
The reason why thermocouple and compensating wires can convert temperature signals into potential signals is that different metals produce different potentials under the same temperature difference. This allows the different electrodes of the thermocouple and its compensating wire to have positive and negative polarity with respect to the output. Reversed polarity naturally makes the meter unable to display.
3. Insufficient cold junction compensation
3.1. A large number of temperature controllers now indicate low faults.
3.2. Treatment method and cause analysis
(1) Method
The millivolt value at the input end of the thermostat was actually measured, and the theoretical temperature was calculated. Compared with the actual temperature of the electric furnace, it was found that the temperature transmitter had a low zero point adjustment problem. Calibrate the zero point of the thermostat, and the fault is eliminated.
(2) Cause
This is a cold junction compensation problem. The theoretical value of the cold junction temperature of a thermocouple is 0℃. But in the industrial field, the cold junction of the thermocouple is usually at room temperature. The potential received by the temperature transmitter input is equal to the potential E1 generated by the thermocouple minus the cold junction potential (ambient temperature potential) E2,(V=E1-E2).
In the instrument calibration room with low room temperature , and the thermoelectric potential generated by the cold end is E2d; at the equipment installation site, the room temperature is higher than the calibration room, and the thermoelectric potential of the cold end at this time is E2h. This makes the instrument's input potential V lower than that of the instrument calibration site at the production site. After using the compensation wire to make the temperature of the cold junction of the thermocouple reach a relatively stable temperature, the temperature compensation of the cold junction should be carried out according to the actual operating temperature of the cold junction. The common and convenient method is to adjust the zero point of the instrument to achieve the purpose of compensation. The compensation wire only serves to extend the cold junction, and cannot compensate the temperature measurement error caused by the cold junction temperature not being 0℃(because the division value of the thermocouple is set at the cold junction 0℃).
4. Installation fault of compensating wire
4.1. The temperature indicator pointer swings up and down, and the pointer points to the maximum when it is stable.
(1) Check the power supply, electric furnace body, indicating control instrument and thermocouple body are all normal, observe the compensation wire, and find that the compensation wire and the indicator wire are only connected to the wire directly on the wiring screw. After operation, heat and ablation cause contactpoor contact. Re-process the joint, the original compensation wire is copper-copper-nickel, crimp the steel wire lug (terminal) at both ends, and apply neutral Vaseline paste on the connection to prevent corrosion. The equipment returned to normal and has been running without failure for a long time.
(2) The contact resistance between the compensation wire and the input terminal of the instrument increases, which makes the potential of the input terminal of the instrument increase.
4.2. The pointer of the temperature indicator swings up and down, sometimes it indicates that the temperature is normal, and sometimes there is no temperature display.
(1) Method:The positive electrode of the compensation wire is damaged and grounded at the contact point with the equipment shell.
(2) Cause:When the compensating wire is grounded, there is no signal output to the indicating instrument; when the ground is temporarily disconnected from the ground due to some reason (pulling or moving), the device is normal; when intermittent grounding occurs, the display pointer fluctuates up and down.
4.3. The pointer of the indicating instrument fluctuates up and down unstable, but the fluctuation range is not large. The fluctuation is obvious during the heating process; the electric furnace is stable or occasionally fluctuates during heat preservation.
(1) Method:The pointer fluctuation of the temperature display instrument is related to the change of the heating current. The compensation wire is laid in the same direction as the main wire of the resistance furnace heating power supply and bundled together. Lay the compensation wire and instrument connection wire separately to isolate the compensation wire from the main power line, and the fault is eliminated.
(2) Cause:The compensating wire is subject to electromagnetic interference from the mains.
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