What is the reason for the unstable thermoelectric potential of thermocouple?

The instability of thermoelectric potential of thermocouples may be caused by many reasons, as follows:
Problems with the thermocouple itself
Uneven thermoelectric material: The thermoelectric material may have uneven composition or inconsistent structure during the production process. This will cause differences in the thermoelectric properties of different parts of the thermocouple. When the temperature changes, the thermoelectric potential generated in each part is inconsistent, making the overall thermoelectric potential unstable.
Aging of thermocouples: Thermocouples used for a long time will gradually age. For example, the thermocouple may oxidize and grow grains. These changes will change the physical and chemical properties of the thermocouple, thereby affecting the stability of the thermoelectric potential.
Impurities inside the thermocouple: If impurities are mixed in the thermocouple manufacturing process, or if impurities invade the thermocouple during use, the impurities may react chemically with the thermocouple material, or affect the electron transmission of the thermocouple, resulting in unstable thermoelectric potential.
Installation and connection problems
Improper installation position: If the thermocouple is installed in a place with a large temperature gradient, or the installation position is close to a heat source, a cold source, a vent, etc., the temperature measured by the thermocouple will be inaccurate, resulting in unstable thermoelectric potential.
Loose wiring: If the terminals between the thermocouple and the compensation wire and the measuring instrument are loose, the contact resistance will change. When current passes through, the change in contact resistance will cause the voltage drop to change, which will make the measured thermoelectric potential unstable.
Environmental factors
Electromagnetic interference: If there is a strong electromagnetic field around the thermocouple, such as large motors, transformers, high-frequency equipment, etc., the electromagnetic field will generate an induced electromotive force in the thermocouple circuit, which will be superimposed on the thermoelectric potential of the thermocouple, resulting in unstable measured thermoelectric potential.
Humidity influence: When the ambient humidity is too high, the insulation performance of the thermocouple may be reduced, resulting in partial leakage of the thermoelectric potential, making the measured thermoelectric potential unstable. In addition, humidity changes may also affect the chemical state of the thermoelectrode surface, indirectly affecting the thermoelectric potential.
Measurement circuit problems
Compensation wire problems: If the model of the compensation wire does not match the thermocouple, the compensation potential generated at different temperatures will be inaccurate, making the measured thermoelectric potential unstable. In addition, if the compensation wire is damaged, such as the outer skin is damaged, the internal wire is broken, etc., it will also affect the signal transmission, resulting in unstable thermoelectric potential measurement.
Measuring instrument failure: Aging or damage of the electronic components of the measuring instrument, or problems with the software algorithm of the instrument may lead to inaccurate measurement of the thermocouple thermoelectric potential and unstable thermoelectric potential.
Temperature change problem
Too fast temperature change: When the temperature change rate of the measured object is too fast, the thermal inertia of the thermocouple may cause it to fail to track the temperature change in a timely and accurate manner, so that the output thermoelectric potential fluctuates and cannot truly reflect the change of the measured temperature.
Over-temperature use: If the thermocouple works in an environment that exceeds its maximum operating temperature for a long time, the performance of the thermocouple material will change dramatically, the thermoelectric potential will become unstable, and even the thermocouple may be damaged.