Common failure causes and treatment methods of thermocouples

Fault phenomenon:

A Thermoelectric potential is smaller than the actual value.

Cause analysis:

(1) Short circuit.

(2) Short circuit between the terminals in the thermocouple junction box.

(3) The compensation wire is short-circuited due to insulation burnout.

(4) The compensation wire does not match the thermocouple.

(5) The polarity of the compensation wire and the thermocouple is reversed.

(6) Insufficient insertion depth and incorrect installation position.

(7) The temperature of the cold end of the thermocouple is too high.

Solution:

(1) If it is caused by moisture, it can be dried; if it is caused by poor insulation of the porcelain tube, it should be replaced.

(2) Open the junction box and brush the terminal board clean.

(3) Re-insulate the short-circuit or replace the new compensation wire.

(4) Replace with the same type of compensation wire.

(5) Reconnect correctly.

(6) Change the installation position and insertion depth.

(7) Replace the connecting wire of the thermocouple with a compensation wire to move the cold end away from the high temperature area.

B The thermoelectric potential is larger than the actual value.

Cause analysis:

(1) The compensation wire does not match the thermocouple model.

(2) The insertion depth is insufficient or the installation position is incorrect.

(3) The thermocouple is deteriorated.

(4) Interference signals enter.

(5) The temperature of the reference end of the thermocouple is too high.

Treatment method:

(1) Replace the compensation wire of the same model.

(2) Change the installation position or insertion depth.

(3) Replace the thermocouple.

(4) Check the interference source and eliminate it.

(5) Adjust the reference end temperature or make corrections.

C, The indication of the measuring instrument is unstable, sometimes there is no interference, sometimes high and sometimes low.

Cause analysis:

(1) The thermocouple has poor contact at the terminal.

(2) The thermocouple has intermittent short circuit or intermittent grounding.

(3) The thermocouple is broken or seems to be broken.

(4) The thermocouple is not installed firmly and swings.

(5) The compensation wire is grounded or intermittently short-circuited.

Treatment method:

(1) Reconnect.

(2) Remove the thermocouple's thermocouple from the protection tube, find the fault point and eliminate it.

(3) Replace the new electrode.

(4) Install firmly.

(5) Find the fault point and eliminate it.

D The thermocouple potential error is large.

Cause analysis:

(1) The thermocouple is deteriorated.

(2) The installation position and installation method of the thermocouple are improper.

(3) There is too much dirt on the surface of the thermocouple protection tube.

(4) The measuring line is short-circuited (thermocouple and compensation wire).

(5) Thermocouple circuit is broken.

(6) The terminal is loose.

Solution:

(1) Replace the thermocouple.

(2) Change the installation location and installation method.

(3) Clean it.

(4) Replace the insulation of the short circuit.

(5) Find the broken wire and reconnect it.

(6) Tighten the terminal.

Fault diagnosis method of thermocouple input

After the instrument is correctly wired according to the instrument wiring diagram and powered on, the instrument first displays the thermocouple graduation number, then displays the instrument range, and then the digital tubes on the lower row of the instrument display the set temperature, and the digital tubes on the upper row of the instrument display the measured temperature. If the digital tube on the instrument does not display the temperature of the heating element, but displays "OVER", "0000" or "000", it means that the input part of the instrument is faulty and the following test should be performed:

A) Remove the thermocouple from the thermocouple input end of the instrument, and then use any wire to short-circuit the thermocouple input end of the instrument. When the power is on, if the value displayed by the digital tube on the instrument is about room temperature, it means that the internal connection of the thermocouple is open, and the same type of thermocouple should be replaced. If the above situation still exists, it means that the input end of the instrument was damaged during transportation, and the instrument should be replaced.

B) Remove the thermocouple of the above-mentioned faulty instrument and replace it with a thermocouple connected to the instrument of the same type and graduation number that is running normally next to it. After power is on, when the digital tube on the original faulty instrument displays the temperature of the heating element, it means that the internal connection of the thermocouple is open, and the same type of thermocouple should be replaced. If the above situation still exists, it means that the input end of the instrument was damaged during transportation, and the instrument should be replaced.

C) Remove the faulty thermocouple from the instrument, set the multimeter to the ohm (R) * 1 position, and use the two probes of the multimeter to measure the two ends of the thermocouple. If the resistance value displayed on the multimeter is large, it means that the internal connection of the thermocouple is open, and replace the thermocouple of the same type. Otherwise, there is a certain resistance value, which means that there is a problem with the input end of the instrument and the instrument should be replaced.

D) The wiring is correct according to the instrument wiring diagram. If the instrument is powered on, the upper row of digital tubes on the instrument shows negative values, etc., which means that the thermocouple " " and "-" connected to the instrument are connected incorrectly. Just replace them again.

E) When the wiring is correct, the temperature displayed on the upper row of digital tubes on the instrument differs from the actual measured temperature by 30ºC~60ºC. Even more difference, it means that the graduation number of the instrument is wrong with the graduation number of the thermocouple. According to the corresponding relationship between the temperature (ºC) and the millivolt (MV) value of thermocouples such as thermocouple graduations B, S, K, and E, under the same temperature (ºC), the millivolt value (MV) generated by the B graduation is the smallest, the S graduation is the second smallest, the K graduation is the largest, and the E graduation is the largest. The judgment is made based on this principle.