Do You Know About The Role Of Thermocouple Insertion Depth?

(1) Selection of temperature measurement point The installation position of the thermocouple, that is, the selection of the temperature measurement point is the most important. The position of the temperature measurement point must be typical and representative for the production process, otherwise the meaning of measurement and control will be lost.

(2) Insertion depth When a thermocouple is inserted into the site to be measured, heat flow will occur along the length of the sensor. There is heat loss when the ambient temperature is low. The thermocouple is inconsistent with the temperature of the object to be measured and a temperature measurement error is generated. In summary, the error caused by heat conduction is related to the depth of insertion. The depth of insertion is related to the material of the protective tube. Due to its good thermal conductivity, the metal protection tube should have a deeper insertion depth (about 15-20 times the diameter). The ceramic material has good thermal insulation properties and can be inserted shallowly (about 10-15 times the diameter). For engineering temperature measurement, the insertion depth is also related to the state in which the measurement object is stationary or flowing. For example, the measurement of the flowing liquid or the high-speed airflow temperature will not be limited by the above, and the insertion depth may be shallower, and the specific value should be determined experimentally. 2.2 Influence of response time The basic principle of contact temperature measurement is that the temperature measurement component should reach thermal equilibrium with the measured object. Therefore, it is necessary to maintain a certain time during temperature measurement in order to achieve thermal equilibrium between the two. The length of the hold time is related to the thermal response time of the temperature measuring element. The thermal response time depends mainly on the structure of the sensor and the measurement conditions, which are extremely different. For gaseous media, especially stationary gases, at least 30 minutes should be maintained to reach equilibrium; for liquids, the fastest should be more than 5 minutes. For the measured location where the temperature is constantly changing, especially the instantaneous change process, the whole process is only 1 second, and the response time of the sensor is required to be in the order of milliseconds. Therefore, the ordinary temperature sensor not only fails to keep pace with the temperature change rate of the object to be measured, but also causes measurement errors due to the inability to achieve thermal balance. It is best to choose a sensor that responds quickly. For the thermocouple, in addition to the influence of the protective tube, the diameter of the measuring end of the thermocouple is also the main factor, that is, the finer the filament, the smaller the diameter of the measuring end, and the shorter the thermal response time. The thermal response error of the temperature measuring element can be determined by the following formula [1]. Δθ=Δθ0exp(-t/τ) (2-1) where t-measurement time S, Δθ-the error caused by the temperature measuring element at time t, K or °C Δθ0-"t=0" time, temperature measuring element The resulting error, K or °C τ - time constant S e - the base of the natural logarithm (2.718) Therefore, when t = τ, then Δθ = Δθ0 / e is 0.368, if t = 2τ, then Δθ = Δθ0/e2 is 0.135. When the temperature of the measured object rises or falls at a certain speed α (k/s or °C/s), after a sufficient time, the generated response error can be expressed by the following formula: Δθ∞=-ατ (2- 2) where Δθ∞-the error caused by the temperature measuring element after sufficient time. It can be seen from equation (2-2) that the response error is proportional to the time constant (τ). In order to improve the verification efficiency, many enterprises use automatic verification devices to check the thermocouples entering the factory, but the device is not perfect. The heat treatment workshop of the Second-Vehicle Transmission Plant found that if the constant temperature at 400 °C is not enough to reach the thermal equilibrium, misjudgment is easy.

2.3 Influence of heat radiation Thermocouples inserted into the furnace for temperature measurement will be heated by the heat radiation emitted by the high temperature object. It is assumed that the gas in the furnace is transparent, and when the temperature difference between the thermocouple and the furnace wall is large, a temperature measurement error will occur due to energy exchange. In unit time, the radiant energy exchanged between the two is P, which can be expressed by the following formula: P = σ ε (Tw4 - Tt4 ) (2 - 3) where σ - 忒藩 忒藩 - Boltz constant ε - emissivity Tt - The temperature of the thermocouple, K Tw-the temperature of the furnace wall, K is the temperature of the thermocouple and the surrounding gas (T is T) per unit time, and the energy of heat exchange through convection and heat conduction is P′ P′=αA ( T-Tt) (2-4) where α-thermal conductivity A-the surface area of the thermocouple is under normal conditions, P=P′, and the error is: Tt- T=σε(Tt4-Tw4)/αА (2 -5) For unit area, the error is Tt-T=σε(Tt4-Tw4)/α (2-6). Therefore, in order to reduce the thermal radiation error, the heat conduction should be increased and the furnace wall temperature Tw should be as much as possible. The temperature Tt close to the thermocouple. In addition, should also pay attention to the installation: 1 thermocouple installation position, should avoid the heat radiation from the solid as much as possible, so that it can not radiate to the surface of the thermocouple; 2 thermocouple preferably with a thermal radiation shielding sleeve.

2.4 Influence of increased thermal impedance The thermocouple used at high temperature, if the measured medium is in a gaseous state, the dust deposited on the surface of the protective tube will be melted on the surface, so that the thermal resistance of the protective tube is increased; The medium is a melt, which will deposit slag during use, which not only increases the response time of the thermocouple, but also lowers the indicated temperature. Therefore, in addition to periodic verification, in order to reduce errors, frequent sampling is also necessary. For example, imported copper smelting furnaces are equipped with not only continuous temperature measuring thermocouples, but also consumption-type thermocouple temperature measuring devices for timely calibration of thermocouples for continuous temperature measurement.