Armored high-temperature thermocouples are important tools for measuring temperature in high-temperature environments, but various malfunctions may occur during their use due to factors such as high temperature, corrosion, and mechanical stress. Here are some common diagnostic methods and solutions for faults, as well as key tips for extending their lifespan.

1、 Common fault diagnosis

（1） Measurement error increases

Fault manifestation

The deviation between the measured temperature and the actual temperature is significant, or the measured value fluctuates frequently.

Possible reasons

Thermocouple aging: In high-temperature environments, the thermoelectric electrode material may degrade due to long-term high temperatures, leading to changes in thermoelectric properties.

Insulation damage: High temperatures may cause aging or damage to the insulation layer of armored thermocouples, resulting in short circuits between the hot electrodes or with the outside world.

Thermocouple wiring error: Loose or incorrect wiring may cause unstable measurement signals.

diagnostic method

Check the appearance of the thermocouple for any obvious signs of damage or aging.

Measure the insulation resistance of a thermocouple using a megohmmeter, which should be greater than 100M Ω under normal circumstances.

Check if the wiring terminals are secure and recalibrate the wiring.

Solution

Replace aging or damaged thermocouples.

Repair or replace the insulation layer to ensure the insulation performance between the thermoelectric electrodes.

Reconnect the wiring correctly and perform calibration testing.

（2） Thermocouple fracture

Fault manifestation

The measured value suddenly disappears or displays as an open circuit state.

Possible reasons

Mechanical stress: In high-temperature environments, thermocouples may experience mechanical stress due to frequent thermal expansion and contraction, leading to fracture.

Corrosion: Corrosive gases or media in high-temperature environments may accelerate the corrosion of thermoelectric electrodes, leading to fracture.

Improper installation: The thermocouple installation position is not reasonable and may be subject to external mechanical impact or vibration.

diagnostic method

Check the installation position of the thermocouple for any obvious mechanical damage or corrosion marks.

Check the fixing method of the thermocouple to ensure that it is securely installed and not affected by external stress.

Solution

Choose thermocouple materials that are more corrosion-resistant, such as high-temperature alloys or ceramic protective tubes.

Optimize the installation position of thermocouples to avoid mechanical stress and vibration.

Regularly check the fixing of thermocouples to ensure their secure installation.

（3） Slow response

Fault manifestation

The measurement of temperature changes lags behind and cannot reflect the actual temperature changes in a timely manner.

Possible reasons

Excessive thickness of thermocouple protection tube: Excessive thickness of protection tube will increase the delay of thermal conduction.

Dust accumulation inside the thermocouple: After long-term use, dust may accumulate inside the protective tube, affecting the thermal conductivity efficiency.

Thermocouple aging: The aging of the thermoelectric electrode material leads to a decrease in thermoelectric properties.

diagnostic method

Check the thickness and material of the protective tube to ensure it meets the design requirements.

Remove the protective tube and check for any dust or other impurities inside.

Compare the response speed of new and old thermocouples to determine if it is caused by aging.

Solution

Choose a thinner protective tube or replace it with a material with better thermal conductivity.

Clean the inside of the protective tube to remove dust and impurities.

Replace aging or degraded thermocouples.

2、 Key techniques for extending the service life of armored high-temperature thermocouples

（1） Reasonable selection

Choose the appropriate material

Choose thermocouple materials that are resistant to high temperatures and corrosion according to the usage environment. For example, in high-temperature oxidative environments, nickel chromium nickel silicon (K-type) thermocouples can be selected; In a reducing environment, platinum rhodium platinum (S-type) thermocouples can be selected.

Consider the material of the protective tube

The protective tube should have good high temperature resistance and corrosion resistance, and materials with good thermal conductivity should be selected as much as possible to reduce thermal conduction delay.

（2） Correct installation

Installation location

Thermocouples should be installed in locations that are sensitive to temperature changes, avoiding installation in blind spots or areas affected by mechanical stress.

fixation method

Thermocouples should be installed firmly to avoid damage caused by vibration or impact. At the same time, the bending radius should be minimized as much as possible during installation to avoid applying additional mechanical stress to the thermocouple.

Sealing treatment

In high-temperature environments, the joints and protective tubes of thermocouples should be sealed to prevent corrosive gases or media from entering.

（3） Regular maintenance

Visual inspection

Regularly inspect the appearance of thermocouples to see if there are any signs of damage, corrosion, or aging. If there is any abnormality, it should be replaced in a timely manner.

insulation detection

Regularly use a megohmmeter to test the insulation resistance of thermocouples to ensure their good insulation performance.

Cleaning and Calibration

Regularly clean the inside of the thermocouple protection tube to remove dust and impurities. At the same time, regular calibration is carried out to ensure measurement accuracy.

（4） Environmental control

temperature control

Try to avoid keeping the thermocouple in a high-temperature environment for a long time and control the working temperature range reasonably.

corrosion prevention

In corrosive environments, thermocouples can be surface coated or protected with more corrosion-resistant tubes.

Avoid mechanical stress

Thermocouples should be kept away from mechanical vibration sources, and shock absorbers can be added if necessary.

By using the above fault diagnosis methods and maintenance techniques, the service life of armored high-temperature thermocouples can be effectively extended, ensuring their reliability and accuracy in high-temperature environments.