What are the factors that affect the stability of thermocouples?

Thermocouple temperature measurement principle is based on thermoelectric effect. It is an instrument that converts temperature change into thermoelectric potential change for temperature measurement. It is a widely used temperature sensor. Two conductors of different components (called thermocouple wires or thermoelectrodes) are connected at both ends to form a loop. When the temperature of the junction is different, an electromotive force will be generated in the loop. Among them, the end directly used to measure the temperature of the medium is called the working end (also called the measuring end), and the other end is called the cold end (also called the compensation end); the cold end is connected to the display instrument or the matching instrument, and the display instrument will indicate the thermoelectric potential generated by the thermocouple.
Thermocouple thermometer consists of thermocouple, electric measuring instrument and connecting wire. It is widely used to measure the temperature in the range of 100-1600℃. Thermocouples made of special materials can also measure higher or lower temperatures. There are many types of thermocouple structures, and the most widely used ones are ordinary thermocouples and armored thermocouples. When measuring temperature with thermocouples, the cold end must be 0℃, otherwise measurement error will occur. When used in the process, it is difficult to keep the cold end at 0℃, so the cold end of the thermocouple must be compensated.
Connection Problems. Many measurement errors are caused by unintended thermocouple junctions. Remember that any junction will result in the junction of two dissimilar metals. If you need to increase the length of your thermocouple wire, you must use the correct type of thermocouple extension wire (such as Type K for a Type K thermocouple). Using any other type of wire will result in a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Lead Resistance. To minimize thermal shunting and improve response time, thermocouples are made from thinner wire (in the case of platinum types, cost is also a consideration). This can result in a thermocouple with higher resistance, which can make it sensitive to noise and can also cause errors due to the input impedance of the measurement instrument. A typical junction of a thermocouple with exposed 32AWG wire (0.25mm diameter) will have a resistance of about 15 ohms/meter. The PicoTC-08 has a 2MΩ input impedance, so an error of less than 0.01% will be produced for 12 meters of this cable. If a thermocouple with thin wire or long cable is required, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker and therefore has a lower resistance) to run between the thermocouple and the measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.