Influence of Response Speed ​​and Stable Control Time on Measurement Accuracy of Mass Flow Controller

Control response speed and stable control time of the mass flow controller Fast control response not only ensures that the equipment reaches the set value 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 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%. From the oscilloscope screenshot below, it can be seen that the ALICAT® mass flow controller reached 63.2% of the set point in just 7.4 milliseconds. Arbitrary set value percentage change range 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. As shown in the figure below, the ALICAT® mass flow controller took 11.4 milliseconds to reach the set value. It can be clearly observed from the waveform diagram of the oscilloscope: the 50% section has a large span, and the two sections below 10% and above 90% The span is much smaller. As far as the figure is concerned, if the response speed of the equipment is defined according to the time taken to reach 90% from 10% of the set value, then the conclusion is 6~7 milliseconds. 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 allowable 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%. At ALICAT we believe that if the user wants to control the flow rate at the desired set point, 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.