What is the effect of thermocouple insertion depth on temperature measurement?
First, the effect of thermocouple insertion depth
(1) the choice of temperature measurement thermocouple installation location, that is, the choice of temperature measurement point is the most important. The location of the temperature measurement point, for the production process, must be typical, representative, otherwise it will lose the significance of measurement and control.
(2) Insert the depth of the thermocouple into the measured place, along the length of the sensor will produce heat flow. When the ambient temperature is low, there will be heat loss. Resulting in thermocouple and the measured object temperature is not consistent with the temperature error. In short, the error caused by heat conduction is related to the depth of insertion. And the insertion depth is related to the protective tube material. Metal protection tube because of its good thermal conductivity, the depth of the insert should be deep (about 15-20 times the diameter), ceramic insulation properties, and can be inserted into the shallow (about 10-15 times the diameter). For the engineering temperature measurement, the depth of insertion is also related to the state of measurement of static or flow, such as the flow of liquid or high-speed air temperature measurement, will not be limited, insert the depth can be shallow, the specific value should be experimentally determined.
Second, the impact of response time
The basic principle of the contact method is to measure the thermal balance of the temperature measurement element. Therefore, the temperature required to maintain a certain time in order to make the two to achieve heat balance. While maintaining the length of time, with the thermal response time of the temperature components. The thermal response time depends mainly on the structure of the sensor and measurement conditions, the difference is great. For gas medium, especially static gas, at least should be maintained for more than 30min to achieve balance; for liquid, the fastest also in more than 5min. For the ever-changing temperature of the test site, especially the instantaneous process of change, the whole process is only 1 second, then the sensor response time required in milliseconds. Therefore, the ordinary temperature sensor not only can not keep up with the measured object temperature change speed lag, but also because of the heat balance can not produce measurement error. It is best to choose a responsive sensor. In addition to the protective effect of the thermocouple, the diameter of the measuring end of the thermocouple is also the main factor, that is, the finer the finer, the smaller the diameter of the measuring end, the shorter the thermal response time. Temperature measurement element thermal response error can be determined by the following formula [1]. Δθ = Δθ0exp (-t / τ) (2) where t - measurement time S, Δθ - at time t, error caused by temperature measurement element, K or ° Δθ0- "t = 0" (2) Therefore, when t = τ, Δθ = Δθ0 / e is 0.368, and when t = 2τ, then Δθ = Δθ0 / e is t = τ τ - time constant S e - the bottom of natural logarithm (2.718) Δθ0 / e2 is 0.135. When the temperature of the object to be measured is increased or decreased at a certain speed α (k / s or ° C / s), the response error can be expressed by the following equation after a sufficient time: Δθ ∞ = 2) where Δθ ∞ - after enough time, the error caused by the temperature measurement element. It can be seen from (2-2) that the response error is proportional to the time constant (τ). In order to improve the efficiency of the test Many enterprises use automatic verification device, the test thermocouple to the factory, but the device is not very perfect. 2 steam gearbox plant heat treatment workshop found that if the 400 ℃ point of the temperature is not enough time to reach the heat balance, it is prone to miscarriage of justice.
Third, the impact of thermal radiation
The thermocouple inserted into the furnace for temperature measurement will be heated by the heat radiation emitted by the high temperature object. Assuming that the furnace gas is transparent, and the thermocouple and the furnace wall temperature difference is large, due to energy exchange and produce temperature error. In the unit time, the radiant energy exchanged between the two is P, which can be expressed by the following equation: P = σε (Tw4 - Tt4) (2-3) where σ - Stefan - Boltz constant ε - emissivity Tt - The temperature of the thermocouple, the temperature of the K Tw-furnace wall, and the energy of the heat exchange between the thermocouple and the surrounding gas (temperature T) through convection and heat conduction in the unit time, P 'P' = αA ( T-Tt) (2-4) where α-thermal conductivity A-thermocouple surface area in the normal state, P = P ', the error is: Tt- T = σε (Tt4-Tw4) / αА (2 -5) For the 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, as far as possible Close to the thermocouple temperature Tt. In addition, the installation should also pay attention to: ① thermocouple installation location, should be as far as possible to avoid the heat emitted from the solid, so that it can not be radiated to the thermocouple surface; thermocouple is best with a heat radiation cover.
Fourth, the impact of increased thermal impedance
In the use of high temperature thermocouple, if the measured medium is gaseous, then the protective tube surface deposition of dust and so on will be burned on the surface, so that the thermal resistance of the protective tube increased; if the measured medium is melt, There will be slag deposition, not only increased the response time of the thermocouple, but also to indicate the temperature is low. Therefore, in addition to regular verification, in order to reduce the error, often sampling is also necessary. For example, imported copper smelting furnace, not only installed a continuous thermocouple thermocouple, also equipped with consumption thermocouple temperature device for the timely calibration of thermocouple for continuous temperature accuracy.
