This paper summarizes the treatment of the three most common fault phenomena of thermocouples without thermoelectric potential, thermoelectric potential change and instrument display instability.
Thermocouple without thermoelectric potential
The thermocouple has no thermoelectric potential. The common saying is that the thermocouple is broken. The main performance is that the meter displays the maximum value or remains in place. In this case, check whether the thermocouple internal short wire is short-circuited. The break of the thermocouple wire may be caused by mechanical damage to the electrode or long-term deterioration of the thermocouple at high temperatures.
Approach:
1. If the thermocouple wire is lost or broken, the solder joint can be removed and re-welded. After passing the test, it can be installed and used (the length of the thermocouple made by re-welding will be shorter, pay attention to the change of the insertion length).
2. If the bump is serious or the filament is deteriorated, the new thermocouple should be replaced in time, and the original thermocouple is discarded.
Thermocouple thermoelectric potential change
The thermocouple thermoelectric potential change is mainly manifested in the fact that the thermocouple output signal does not match the actual value. At this time, check whether the thermocouple insertion length meets the site temperature measurement requirements (too long or too short); whether the installation position and installation method are proper; whether the thermocouple protects the surface of the tube from fouling; whether the thermocouple is wet or not inside the thermocouple; There is a phenomenon of diameter reduction; whether the solder joint of the thermocouple is spherical in shape, whether the surface is smooth, whether there are bubble pores or slag inclusion; whether the thermocouple filament is discolored or deteriorated.
Approach:
1. Take out the thermocouple wire and say that the protective tube and the wire are dried separately (do not use fire to bake).
2. Check the geometry of the thermocouple with a vernier caliper and replace it if it does not meet the requirements.
3. The thermocouple solder joint is not spherical, the surface is uneven, and there are no bubble pores or slag inclusions. The solder joint is re-welded and used after passing the test.
4. If the thermocouple is discolored, replace it with a new one.
5. Change the thermocouple insertion length or mounting position to the best measurement requirements and install it securely.
Thermocouple input meter display value is unstable
In the case that the display instrument has been tested and has no fault, the display value of the instrument is floating. In this case, check whether the thermocouple terminal is in good contact with the thermocouple wire; whether the thermocouple is firmly installed; whether the thermocouple has a swing phenomenon; There are conductive liquid, moist dust and metal impurities; whether the electrode is grounded, short-circuited or open circuit; whether the connection with the meter is firm; whether the thermocouple wire is broken or not welded.
Approach:
1. Clean the thermocouple junction box and install it firmly after drying.
2. Re-weld the thermocouple and use it after passing the test.
3. Use a multimeter to measure the resistance value of the thermocouple wire, and replace it if it is unqualified.
4. Find the grounding, short circuit, and open circuit of the thermocouple compensation wire, and repair or replace the new compensation wire.
In addition, due to work-related human error, such as the thermocouple indexing number does not match the meter, the thermocouple compensation wire and the thermocouple type do not match, the thermocouple compensation wire and the thermocouple polarity reverse connection, the thermocouple insertion length is not, etc. It is manifested that the thermocouple output is larger or smaller than the actual value, causing the meter display value to be higher or lower. The treatment method of such faults is relatively simple. As long as the responsibility of the instrument installer is strengthened, the contents of the manual can be read in detail, and the thermocouple installation specifications and instrument requirements can be correctly followed.
