The main factors affecting the measurement error are: thermocouple insertion depth, response time, thermal radiation, and thermal impedance.
1, the impact of response time
The basic principle of temperature measurement by contact method is that the temperature measuring element must reach thermal equilibrium with the measured object. Therefore, it takes a certain period of time for the temperature measurement to reach a thermal equilibrium between the two. The length of the holding time is related to the thermal response time of the temperature measuring element. The thermal response time mainly depends on the sensor's structure and measurement conditions. For gas media, especially stationary gases, at least 30 min should be maintained to achieve equilibrium;
For liquids, it should be faster than 5 minutes. For a test site where the temperature is constantly changing, especially during the transient process, the entire process is only 1 second, and the response time of the sensor is required to be in the order of milliseconds. Therefore, an ordinary temperature sensor not only fails to keep up with the lag of the temperature change of the measured object, but also causes measurement error because the heat balance is not reached. It is best to choose a sensor that responds quickly. For the thermocouple, except for the effect of the protective tube, the diameter of the measuring end of the thermocouple is also the main factor. That is, the finer the twin wire, 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.
Where tâ€”measurement time S,
Î”Î¸â€”The error caused by the temperature measuring element at time t, K or Â°C
Î”Î¸0â€”â€œt=0â€ time, error caused by temperature measuring element, K or Â°C
Ï„ - time constant S
Eâ€”the bottom of the natural logarithm (2.718)
Therefore, when t=Ï„, then Î”Î¸=Î”Î¸0/e is 0.368,
If t=2Ï„, then Î”Î¸=Î”Î¸0/e2 is 0.135.
When the temperature of the measured object rises or falls at a certain speed Î± (k/s or Â°C/s), after a sufficient time has elapsed, the resulting response error can be expressed by the following equation:
In the formula, Î”Î¸âˆžâ€”after a sufficient time, the error caused by the temperature measuring element.
As can be seen from the above, the response error is proportional to the time constant (Ï„). In order to increase the verification efficiency, many companies use automatic verification devices to check the incoming thermocouples. However, this device is not perfect. The heat treatment workshop of the No. 2 transmission plant found that misalignment can easily occur if the constant temperature at the 400Â°C point is not enough to reach the heat balance.
2, the impact of insertion depth
a choice of temperature points
b insertion depth
When the thermocouple is inserted into the location to be measured, heat flow will occur along the length of the sensor. When the ambient temperature is low, there will be heat loss. Causes the thermocouple and the measured object's temperature to be inconsistent and produce the temperature error. In short, the error caused by heat conduction depends on the insertion depth.
The insertion depth is related to the protection tube material. Metal protection tube because of its good thermal conductivity, its insertion depth should be deeper (about 15-20 times the diameter), ceramic material, good thermal insulation, can be inserted slightly (about 10-15 times the diameter). For engineering temperature measurement, the insertion depth is also related to the state of the measurement object being stationary or flowing. For example, the measurement of the flowing liquid or high-speed airflow temperature will not be limited to the above, and the insertion depth can be lighter. The specific value should be determined experimentally.
3, the impact of heat radiation
Thermocouples inserted into the furnace for temperature measurement will be heated by the heat radiation emitted by high temperature objects. It is assumed that the gas in the furnace is transparent, and if the temperature difference between the thermocouple and the furnace wall is large, temperature measurement errors will occur due to energy exchange.
In unit time, the radiative energy exchanged between the two is P, which can be expressed as:
In the formula, the Ïƒ-Stop-Boltz constant
Ttâ€”The temperature of the thermocouple, K
Twâ€”The temperature of the furnace wall, K
In the unit time, the thermocouple is surrounded by the same gas (temperature T), and the energy that will exchange heat through convection and heat conduction is Pâ€².
Aâ€”The surface area of â€‹â€‹the thermocouple
In the normal state, P=P', the error is:
The unit error is
Therefore, in order to reduce the heat radiation error, the heat conduction should be increased and the furnace wall temperature Tw should be as close as possible to the thermocouple temperature Tt.
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