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Main influence factors of measurement error of thermocouple temperature sensor

Published on: September 3, 2019 by Zetian sensing

The influence of insertion depth

1) The selection of temperature measurement point and the installation position of thermocouple temperature sensor are the most important. In terms of temperature measurement, it will lose its representative position and process control.

2) When the thermocouple temperature sensor is inserted into the measured site, heat flow will be generated along the length direction of the temperature sensor. When the ambient temperature is low, there is heat loss. As a result, the temperature of the thermocouple and the measured object is inconsistent, resulting in temperature measurement error. In short, the error caused by heat conduction is related to the depth of insertion. The insertion depth is related to the material of the protective tube. Because of its good thermal conductivity, the insertion depth of metal protective tube should be deeper (about 15-20 times of its diameter), and the ceramic material has good thermal insulation performance. It can be inserted shallowly (about 10-15 times of the diameter). For engineering temperature measurement, the insertion depth is also related to the static or flowing state of the measured object. For example, the measurement of the temperature of flowing liquid or high-speed air flow will not be subject to the above restrictions. The insertion depth can be shallower, and the specific value should be determined by experiments.

2 Effect of response time

The basic principle of contact temperature measurement is that the temperature measuring element should reach the thermal balance with the measured object. Therefore, it is necessary to keep a certain time in the temperature measurement to make the two reach the thermal balance. The holding time is related to the thermal response time of the temperature measuring element. However, the thermal response time mainly depends on the structure and measurement conditions of the sensor, and the difference is great. For the gas, especially for the gas, it should be kept at rest for at least 5 minutes.

When the temperature changes continuously, especially the transient process, the response time of the sensor is required to be in millisecond. Therefore, the ordinary Zetian temperature sensor not only can not keep up with the temperature change speed of the measured object, but also can not reach the thermal balance and produce measurement error. It is better to choose the sensor with fast response. For the thermocouple, in addition to the influence of the protection tube, the diameter of the measuring end of the thermocouple is also the main factor, that is, the thinner the wire is, the smaller the diameter of the measuring end is, the shorter the thermal response time is. The thermal response error of temperature measuring element can be determined by the following formula.

Δθ=Δθ0exp(-t/τ)               (1)

Where T - measurement time s, Δθ - error caused by temperature measuring element at time t, K or ℃; Δ θ 0 - error caused by temperature measuring element at time t, K or ℃; τ - time constant s; e - base of natural logarithm (2.718); therefore, when t = τ, then Δθ = Δθ 0 / E is 0.368; if t = 2 τ, then Δθ = Δθ 0 / E2 is 0.135.

When the temperature of the measured object rises or falls at a certain speed α (K / s or ℃ / s), after enough time, the response error can be expressed as follows:

Δθ∞=-ατ                            (2)

Where Δ θ∞ - error caused by temperature measuring element after enough time.

It can be seen from equation (2) that the response error is directly proportional to the time constant (τ). In order to improve the verification efficiency, many enterprises use Zetian sensor automatic verification device to verify the thermocouple, but the device is not perfect. The heat treatment workshop of No.2 steam transmission plant found that if the constant temperature at 400 ℃ is not enough and the heat balance cannot be reached, it is easy to misjudge.

3 Effect of thermal radiation

The thermocouple inserted into the furnace for temperature measurement will be heated by the thermal radiation from the high-temperature object. Assuming that the gas in the furnace is transparent and the temperature difference between the thermocouple temperature sensor and the furnace wall is large, the temperature measurement error will be caused by energy exchange.

In unit time, the radiation energy exchanged between them is p, which can be expressed as follows:

P=σε(Tw4 - Tt4)            (3)

Where σ - Stefan Boltz constant; ε - emissivity; TT - temperature of thermocouple, K; TW - temperature of furnace wall, K

In unit time, the thermocouple will exchange heat with the surrounding gas (temperature is t) through convection and heat conduction, and the energy exchange is p ′

P′=αA(T-Tt)                  (4)

Where α is the thermal conductivity; a is the surface area of thermocouple; under normal condition, P = P ', and the error is as follows:

Tt-T=σε(Tt4-Tw4)/αА     (5)

For unit area, the error is

Tt-T=σε(Tt4-Tw4)/α       (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 close as possible to the temperature TT of the thermocouple. In addition, attention should also be paid to the installation of thermocouple: the thermal radiation from solid should be avoided as far as possible, so that it can not be radiated to the thermocouple surface; the thermocouple should be equipped with thermal radiation shield.

The thermal impedance increases by 4

Thermocouple used at high temperature Temperature sensor If the measured medium is gaseous, the dust deposited on the surface of the protective tube will be burned and melted on the surface, which will increase the thermal impedance of the protective tube; if the measured medium is melt, there will be slag deposition during the use process, which not only increases the response time of thermocouple, but also makes the indicated temperature lower. Therefore, in addition to regular verification, in order to reduce the error, it is also necessary to conduct regular sampling inspection. It is not only used to measure temperature in time, but also used to measure temperature in time.