The selection of the installation location for the temperature measurement point, i.e., the choice of the temperature measurement point, is the most important. The location of the temperature measurement point must be typical and representative for the production process; otherwise, the meaning of measurement and control will be lost.
Insertion Depth When a thermocouple is inserted into the measured location, heat flow occurs along the length of the sensor. When the ambient temperature is low, heat loss occurs. This causes a temperature difference between the thermocouple and the measured object, resulting in measurement errors. In summary, errors caused by heat conduction are related to the insertion depth. The insertion depth is also related to the material of the protective tube. Metal protective tubes, due to their good thermal conductivity, require a deeper insertion depth (about 15-20 times the diameter). Ceramic materials have good thermal insulation properties and can be inserted shallower (about 10-15 times the diameter). For engineering temperature measurement, the insertion depth also depends on whether the measured object is stationary or flowing. For example, measurement of flowing liquid or high-speed gas temperature is not subject to the above restrictions, and the insertion depth can be shallower; the specific value should be determined by experiment.
Effect of Response Time The basic principle of contact temperature measurement is that the temperature sensing element must reach thermal equilibrium with the measured object. Therefore, a certain time must be maintained during temperature measurement to achieve thermal equilibrium. The duration of this holding time is related to the thermal response time of the temperature sensing element. The thermal response time mainly depends on the sensor structure and measurement conditions, and varies greatly. For gaseous media, especially stationary gases, at least 30 minutes should be maintained to achieve equilibrium; for liquids, at least 5 minutes is required. For measured locations with constantly changing temperatures, especially transient processes where the entire process lasts only 1 second, the sensor's response time must be on the order of milliseconds. Therefore, ordinary temperature sensors not only lag behind the temperature change rate of the measured object but also cause measurement errors due to failure to reach thermal equilibrium. It is best to choose a sensor with fast response. For thermocouples, in addition to the influence of the protective tube, the diameter of the measuring junction is also a major factor; that is, the thinner the thermocouple wire and the smaller the measuring junction diameter, the shorter the thermal response time. The thermal response error of the temperature sensing element can be determined by the following formula [1]. Δθ=Δθ0exp(-t/τ) (2-1) Where t—measurement time S, Δθ—error caused by the temperature sensing element at time t, K or ℃ Δθ0—error caused by the temperature sensing element at time “t=0”, K or ℃ τ—time constant S e—base of natural logarithm (2.718) Therefore, when t=τ, then Δθ=Δθ0/e, which is 0.368; if t=2τ, then Δθ=Δθ0/e2, which is 0.135. When the temperature of the measured object rises or falls at a certain rate α (k/s or ℃/s), after sufficient time, the resulting response error can be expressed by the following formula: Δθ∞=-ατ (2-2) Where Δθ∞—error caused by the temperature sensing element after sufficient time. It can be seen from formula (2-2) that the response error is proportional to the time constant (τ). To improve calibration efficiency, many enterprises use automatic calibration devices to calibrate incoming thermocouples. However, these devices are not perfect. The heat treatment workshop of the Second Automobile Factory's Gearbox Plant found that if the constant temperature time at 400℃ is insufficient and thermal equilibrium is not achieved, misjudgment is prone to occur.
Effect of Thermal Radiation A thermocouple inserted into a furnace for temperature measurement will be heated by thermal radiation emitted from high-temperature objects. Assuming the gas in the furnace is transparent and the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange. The radiant energy exchanged between the two per unit time, P, can be expressed by the following formula: P=σε(Tw4 - Tt4) (2-3) Where σ—Stefan-Boltzmann constant ε—emissivity Tt—thermocouple temperature, K Tw—furnace wall temperature, K Per unit time, the thermocouple also exchanges heat with the surrounding gas (temperature T) through convection and conduction, with energy P′: P′=αA(T-Tt) (2-4) Where α—thermal conductivity A—surface area of the thermocouple Under normal conditions, P = P′, and the error is: Tt-T=σε(Tt4-Tw4)/αA (2-5) For unit area, the error is: Tt-T=σε(Tt4-Tw4)/α (2-6) Therefore, to reduce thermal radiation errors, heat conduction should be increased, and the furnace wall temperature Tw should be as close as possible to the thermocouple temperature Tt. Additionally, during installation, attention should be paid to: ① The thermocouple installation position should avoid thermal radiation from solids as much as possible, preventing it from radiating onto the thermocouple surface; ② The thermocouple should preferably be equipped with a thermal radiation shielding sleeve.
Effect of Increased Thermal Impedance For thermocouples used at high temperatures, if the measured medium is gaseous, dust and other deposits on the surface of the protective tube will melt and adhere to the surface, increasing the thermal impedance of the protective tube; if the measured medium is a melt, slag will deposit during use, not only increasing the response time of the thermocouple but also causing the indicated temperature to be lower. Therefore, in addition to regular calibration, frequent spot checks are also necessary to reduce errors. For example, imported copper smelting furnaces are not only equipped with continuous temperature measurement thermocouples but also with consumable thermocouple temperature measurement devices for timely calibration of the accuracy of the continuous temperature measurement thermocouples.
