The relationship between fast thermocouple insertion depth and response time

The correlation between the depth of rapid thermocouple extraction and the corresponding time

For gas media, especially moving gases, it is necessary to maintain equilibrium for at least 30 minutes; For liquids, the fastest time should be at least 5 minutes. For situations where the temperature is constantly changing, especially in the case of sudden changes, if the entire process takes only 1 second, the response time of the sensor request should be in milliseconds.

Therefore, typical temperature sensors not only lag behind the temperature change rate of the measured tool, but also experience measurement deviations due to the inability to achieve thermal equilibrium. Choose sensors that respond quickly.

For thermocouples, in addition to the influence of maintenance tubes, the measuring end diameter of the thermocouple is also an important factor, that is, the thinner the thermocouple wire, the smaller the measuring end diameter, and the shorter the thermal response time.

The thermal response deviation of the temperature measuring element can be determined by the following equation: Δθ=Δθ0exp(-t/τ)

In the formula, t represents the measured time S, Δ θ - the deviation caused by the temperature measuring element at time t, K or ℃ Δ θ 0- "t=0" time, the deviation caused by the temperature measuring element, K or ℃ τ - the time constant S e - the base of the natural logarithm (2.718)

Therefore, when t=τ, then Δ θ=Δ θ 0/e is 0.368. If t=2 τ, then Δ θ=Δ θ 0/e2 is 0.135.

When the temperature of the tested tool rises or falls at a certain rate α (k/s or ℃/s), the corresponding deviation that occurs after sufficient time can be expressed by the following equation: Δθ∞=-ατ

In the formula, Δ θ ∞ - the deviation caused by the temperature measuring element after sufficient time. From the above equation, it can be seen that the response deviation is directly proportional to the time constant (τ).

In order to improve the effectiveness of calibration, many enterprises take the initiative to calibrate and install thermocouples, stopping the calibration of incoming thermocouples. However, this installation is not very strict. The heat treatment workshop of the Second Automobile Transmission Factory has invented that if the constant temperature at 400 ℃ is insufficient to achieve thermal equilibrium, misjudgment can easily occur.