Control response speed and stable control time of mass flow controller1

Mass flow controller control response speed and stable control time Fast control response not only ensures that the equipment reaches the set point quickly, but also maintains a stable flow rate to avoid being affected by pressure fluctuations. There is no doubt that the response speed of a device plays a vital role in the measurement accuracy and stable flow control. Physical time constant (Tau) The physical time constant is represented by the Greek letter Tau, which is one of the few standards used to describe the response time, that is, the time required for the controller to reach a certain percentage of the set value, which is defined as 1 -1/e, where e is an irrational number as the base of natural logarithm, its value is 2.72, so 1-1/e is 63.2% of the set value. Although this definition is a physical standard, it may not be telling, because the remaining 36.8% of the set value may take more or less time than the previous 63.2%. Arbitrary set value percentage change area Many mass flow controller technical parameters use the change within a certain percentage range of the set value to define the response speed of the equipment, for example: the time required to reach 90% from 10% of the set value. Due to the large variation of the initial and subsequent 10% control response curves, the resulting control response speed may not be equivalent to the actual control performance. Stabilization time Since the controller may overshoot after reaching the set value, the manufacturer usually uses the time required to reach the set value and stabilize as the response speed of the equipment, and gives a tolerance range for the waveform oscillation amplitude. For example, the allowable range of 2% means that the flow fluctuation is stable within 2% of the set value. Flow Controllers Generally, the accuracy of mass flow controllers is between 0.5% and 2%, but some manufacturers have extended the stability tolerance range to 10%. We believe that if the user wants to control the flow rate at the expected set value, then its stable allowable range must also fall within the range of equipment accuracy. As shown in the oscilloscope screenshot below, the controller reached a stable tolerance equivalent to the device's accuracy (1% of full scale) at 27.6 ms.