The main factors affecting the measurement error of thermocouples

The main factors affecting measurement errors include thermocouple insertion depth, response time, thermal radiation, and thermal impedance.
1. The impact of response time
The basic principle of contact temperature measurement is that the temperature measuring element must reach thermal equilibrium with the object being measured. Therefore, it is necessary to maintain a certain period of time during temperature measurement in order to achieve thermal equilibrium between the two. The length of holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the structure and measurement conditions of the sensor, with significant differences. For gas media, especially stationary gases, equilibrium should be maintained for at least 30 minutes or more;
For liquids, the fastest time should be at least 5 minutes. For the temperature constantly changing test site, especially the instantaneous change process, if the entire process is only 1 second, the response time of the sensor is required to be in the millisecond level. Therefore, ordinary temperature sensors not only fail to keep up with the temperature change rate of the measured object, but also produce measurement errors due to the inability to achieve thermal equilibrium. It is best to choose a sensor with a fast response. For thermocouples, in addition to the influence of protective tubes, the diameter of the measuring end of the thermocouple is also the main factor, that is, the thinner the thermocouple wire, the smaller the diameter of the measuring end, and the shorter its thermal response time. The thermal response error of the temperature measuring element can be determined by the following equation.
Δθ=Δθ 0exp (- t)/ τ)
In the formula, t - measurement time S,
Δθ- The error caused by the temperature measuring element at time t, K or ℃
Δθ Error caused by temperature measuring element at time 0- "t=0", K or ℃
τ- Time constant S
E - The base of natural logarithms (2.718)
Therefore, when t= τ When, then Δθ=Δθ 0/e is 0.368,
If t=2 τ When, then Δθ=Δθ 0/e2 is 0.135.
When the temperature of the tested object is at a certain speed α When (k/s or ℃/s) rises or falls, the response error generated after sufficient time can be expressed as follows:
Δθ∞=-ατ
In the formula Δθ∞- The error caused by the temperature measuring element after sufficient time.
As can be seen from the above, the response error is related to the time constant( τ) Directly proportional. In order to improve the efficiency of calibration, many enterprises use automatic calibration devices to calibrate incoming thermocouples. However, this device is not very complete. The heat treatment workshop of the Second Automobile Transmission Factory has found that if the constant temperature time at 400 ℃ is not enough to achieve thermal equilibrium, it is easy to make misjudgments.

2. The impact of insertion depth
Selection of temperature measurement points
Insertion depth of b
When the thermocouple is inserted into the tested location, heat flow will be generated along the length direction of the sensor. When the ambient temperature is low, there will be heat loss. Causing temperature measurement errors due to temperature inconsistency between the thermocouple and the measured object. In short, the errors caused by heat conduction are related to the insertion depth.
The insertion depth is related to the material of the protective tube. Due to its good thermal conductivity, the insertion depth of metal protective tubes should be deeper (about 15-20 times the diameter). Ceramic materials have good insulation performance and can be inserted shallower (about 10-15 times the diameter). For engineering temperature measurement, the insertion depth is also related to whether the measurement object is in a static or flowing state. For example, the measurement of the temperature of a flowing liquid or high-speed airflow will not be subject to the above limitations. The insertion depth can be shallower, and the specific value should be determined by experiments.
3. The impact of thermal radiation
A thermocouple inserted into the furnace for temperature measurement will be heated by the thermal radiation emitted by high-temperature objects. Assuming that the gas inside the furnace is transparent, and when the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange.
The radiation energy exchanged between the two within a unit time is P, which can be expressed as follows:
P= σε (Tw4 Tt4) (2-3)
In the formula σ- Stefan Boltz constant
ε- Emissivity
Tt - Temperature of thermocouple, K
Tw - Temperature of furnace wall, K
In a unit of time, the energy exchange between the thermocouple and the surrounding gas (temperature T) through convection and heat conduction is P ′
P ′= α A (T-Tt) (2-4)
In the formula α- Thermal conductivity
A - Surface area of thermocouple
Under normal conditions, P=P ′, with an error of:
Tt-T= σε (Tt4 Tw4)/ αА (2-5)
For a unit area, the error is
Tt-T= σε (Tt4 Tw4)/ α (2-6)
Therefore, in order to reduce thermal radiation errors, it is necessary to increase heat conduction and make the furnace wall temperature Tw as close as possible to the temperature Tt of the thermocouple.