industrial electric furnace on improper use of thermocouple

1. Matching of thermocouple and compensation wire

1.1 Fault instance 2

When the actual temperature in the high temperature area is above 1000℃, the temperature error of the instrument indicating temperature is large. When the actual temperature is 800℃ or below, the temperature value displayed by the instrument is also positive.

1.2 On-site fault treatment and cause analysis

(1) Processing method

The thermocouple and display instrument here have been calibrated and calibrated and the installation program is normal. In this case, the thermocouple used on the electric furnace is platinum rhodium 10- platinum. Keeping the temperature of the instrument shows in the furnace temperature 800 ℃, use a millivoltmeter measure indicator input voltage of millivolt values and the thermocouple output voltage of millivolt values, while indicating meter and 800 ℃ temperature values is consistent, but controlled indexing table identified 10 - platinum and platinum rhodium thermocouple index value of accumulation of cold end temperature correction theory millivolt values comparatively large difference. Check the compensation wire of the thermocouple and find that the material is "copper-Constantan"; This type of compensating conductor is suitable for "nickel-Chrome-nickel silicon" thermocouples. The "platinum-Rhodium-10-platinum" thermocouple shall use the "copper-Nickel" compensation wire. Replace "copper - copper nickel" compensation to successively, trouble shooting.

(2) Cause analysis

Failure to comply with the matching of compensation wire and thermocouple during field installation resulted in failure. The temperature measurement principle of thermocouple is based on the constant temperature at the cold end. The negative pole of the thermocouple should be set in the area where the temperature is constant, and the accurate measurement can be achieved by a certain temperature compensation. This type of temperature compensation is known as cold end compensation. The function of the compensating conductor is not to compensate the potential of the cold end; In order to keep the temperature of the cold end of thermocouple temperature measuring electrode constant, the electrode must be extended. However, the electrodes of the thermocouple are all precious metals. For example, the platinum rhodium-10-platinum thermocouple in this case, the extension of the electrodes will increase the investment cost of the equipment. So, it is usually used in room temperature (0-100 ℃) and thermocouple electrode materials thermoelectric properties of similar metal wires in the cold end extends to the hot electrode away from heat source, temperature stability of the region, the material of conductor is called extension wire or compensation conductor, obviously, the function of the compensating conductor is "insufficient compensation hot electrode length". According to the compensation wire and thermoelectric thermoelectric characteristics at room temperature close to the train of thought, compensation wire and thermocouple are used together.

1.3 Key points revealed by Fault 2

Point 4: The compensation wire should be configured when the thermocouple works. The compensation wire only plays the role of extending the electrode, and its length does not affect the measuring effect within the distance of the thermocouple's allowed device (because the thermoelectric potential is only related to the material).

Point 5: Compensation wire should be used with the thermocouple used.

Point 6: The best working condition of compensation conductor is 0-100℃.