What are the common failure phenomena of thermocouples?

The common failure phenomena of thermocouples are mainly the following:
Inaccurate measurement value
Temperature deviation: There is a large deviation between the measured temperature and the actual temperature. This may be due to improper selection of thermocouples, such as using a thermocouple that is not suitable for high temperature measurement in a high temperature environment; or the installation position of the thermocouple is unreasonable, and the target temperature is not accurately measured, such as installing it in a place with unstable airflow, resulting in large fluctuations in the measured value.
Degradation of accuracy: After long-term use, the accuracy of the thermocouple may decrease. This may be because the material of the thermocouple is aged, contaminated or corroded, affecting its thermoelectric properties. For example, when used in an environment with corrosive gases, the electrodes of the thermocouple may be corroded, resulting in increased measurement errors.
Unstable output signal
Signal fluctuation: The thermoelectric potential signal output by the thermocouple fluctuates irregularly. This may be due to the loosening of the thermocouple terminals, resulting in unstable contact resistance, which causes signal fluctuations; or the compensation wire is damaged, generating unstable signals under external interference.
Signal jump: The output signal suddenly jumps by a large amplitude. There may be a local short circuit or open circuit inside the thermocouple, such as damage to the insulation layer of the thermocouple, resulting in a short circuit between the electrodes, or the electrode is broken, resulting in an open circuit, which will cause abnormal signal jumps.
Response time is too long
Thermal inertia is large: the thermocouple's response to temperature changes becomes slow and cannot reflect the rapid temperature changes in time. This may be due to the thermocouple's protective sleeve being too thick or the material's poor thermal conductivity, which increases the heat transfer time; or the thermocouple is improperly installed, such as insufficient insertion depth, which causes it to be unable to fully sense the temperature changes of the measured medium.
Abnormal thermoelectric potential output
No thermoelectric potential output: The thermocouple has no thermoelectric potential output within the normal operating temperature range. This may be because the hot and cold ends of the thermocouple have the same temperature, resulting in no temperature difference to generate thermoelectric potential; or the thermocouple has been damaged, such as the electrode burning out, the internal circuit breaking, etc.
Thermoelectric potential reversal: The output thermoelectric potential is opposite to the normal situation. This may be because the positive and negative poles of the thermocouple are connected in reverse, or there are other interference sources in the measurement circuit, resulting in signal reversal.
Appearance damage
Damage to the protective sleeve: The protective sleeve of the thermocouple is cracked, broken or deformed. This may be due to mechanical shock, high temperature thermal stress and other factors during installation or use. When the protective sleeve is damaged, the thermocouple electrode is exposed to the external environment, which is susceptible to corrosion and contamination, thus affecting the measurement accuracy and service life.
Electrode discoloration or deformation: The surface of the thermocouple electrode changes color, oxidizes or deforms. This is usually due to long-term operation in a high temperature environment, oxidation or chemical reaction of the electrode material, or deformation of the electrode due to excessive external force, which will affect the performance of the thermocouple.