What factors mainly affect the temperature measurement error of thermocouples?

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1. Errors caused by unstable thermoelectric properties of the thermocouple. After being used for a period of time, and under the influence of the environment in which it is used, the thermoelectric properties of the thermocouple will change to some extent; as a result, the temperature measured by this thermocouple deviates from the actual temperature. The main factors affecting the stability of thermocouples are: (1) contamination and corrosion of the thermocouple electrodes by the substance being measured ； (2) Deformation stress caused by the deformation of the thermoelectric electrode under external forces ； (3) The microstructural grains of the temperature-sensitive electrode change at high temperatures ； (4) Thermal electrodes undergo oxidation and other reactions due to exposure to air. 2. Effect of thermocouple non-uniformity: The uniformity of a thermocouple refers to the degree of uniformity of the material constituting its thermal electrodes. If the materials of the two thermal electrodes in a thermocouple are uniform, the thermoelectric potential generated by the thermocouple circuit is directly proportional to the temperature difference across its ends, and is independent of the temperature gradient along the length of the thermal electrodes. 3. Selection of temperature measurement points: Choosing the appropriate temperature measurement points is a crucial factor when installing thermocouples. Selecting representative points for temperature measurement is of particular importance for the entire production environment. If the location is chosen improperly, control and measurement will be meaningless. At the measurement point, the depth to which the thermocouple is inserted also affects temperature measurement. When there is a temperature difference between the external environment and the temperature of the object being measured, the temperature read by the thermocouple will undergo heat exchange with the external environment as the sensor extends, resulting in measurement errors. Due to the constantly changing ambient temperature, this temperature error is immeasurable. In addition to the errors introduced by heat conduction due to the depth of insertion, the material of the protective tube surrounding the thermocouple can also cause measurement errors. For example, protective tubes made of metal have good thermal conductivity; therefore, during measurement, they need to be inserted deeper to prevent heat loss. For ceramic protective tubes, they need to be inserted at a shallower depth. 4. Influence of response time: The temperature of the temperature sensing element can remain stable only when the temperature of the object being measured is the same as that of the sensing element; at this point, the two reach thermal equilibrium, which is the basic principle behind contact-based temperature measurement. It takes some time for the temperature of the object being measured to reach that of the temperature sensing element; the length of this time is primarily determined by the thermal response capability of the temperature sensing element, that is, its thermal response time. The thermal response time is affected by the structure of the sensor and the measurement conditions; experiments show that the response times vary significantly under different conditions. In a stationary gaseous medium, continuous contact for over 30 minutes is required to achieve thermal equilibrium; for stationary liquids, this time is reduced to 5 minutes. In practical engineering applications, the temperature of the object being measured is often in a state of constant change, with short periods of rapid variation; therefore, higher requirements are placed on sensors, which generally need to have a response time in the range of milliseconds. If the response capacity is insufficient, measurement lag occurs. Therefore, when choosing a sensor, one should select one with a fast response speed. The diameter at the measuring end is an important factor affecting response speed. If the filaments are thinner, the response time is shorter. 5. Effect of thermal radiation: When using a thermocouple to measure the temperature in a furnace, the thermal radiation from the hot objects inside the furnace toward the thermocouple can cause its temperature to rise. If the gas inside the furnace is assumed to be transparent, and there is a large temperature difference between the thermocouple and the furnace wall, then energy exchange will also lead to errors in temperature measurement. Increasing heat conduction can effectively reduce this error, bringing the temperature of the furnace wall closer to that of the thermocouple. 6. Effect of increased thermal impedance: When the thermocouple is in a high-temperature operating environment, the gaseous medium being measured can melt the dust on the surface of the protective tube, resulting in an increase in the thermal impedance of that protective tube. If the medium is a melt, slag will precipitate during operation; this precipitated slag increases the response time of the thermocouple, resulting in a lower indicated temperature. Therefore, regularly checking or conducting occasional random inspections of the operating condition of thermocouples enables the timely detection of any abnormalities in them, thereby reducing measurement errors. 7. Errors caused by the heterogeneity of the thermocouple wire: According to the aforementioned measurement principle, when the wire of a thermocouple is homogeneous, the length has no effect on the measurement results, as per the rule for homogeneous circuits. In reality, when manufacturers produce twisted yarns, factors such as temperature often lead to uneven thickness of the yarn, resulting in a bamboo-like pattern, or even severe curling. Furthermore, other problems arise during subsequent processing; repeated processing of the thermocouples causes them to deform and lose their homogeneity. During temperature measurement, many wire pairs are located in high-temperature areas; if there is no uniformity and the ambient temperature changes, parasitic thermoelectromotive forces will arise locally in the thermocouples, thereby introducing errors. Through an in-depth analysis of the basic working principle of thermocouples, their specific methods of use, and the problems that arise during their production, the main causes of errors in thermocouple-based temperature measurement were identified. In the manufacturing and use of thermocouples, factors such as the insertion depth of the sensor, the response time of the thermocouple to temperature changes, thermal radiation from the surrounding environment, changes in thermal impedance, and the heterogeneity of the thermocouple wire are all key contributors to measurement errors. Therefore, improving the accuracy of temperature measurement requires reducing or avoiding these factors. Analyzing the causes of errors in thermocouple temperature measurement has certain guiding significance for practical engineering applications.