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Rapid control response can not only ensure that the equipment reaches the set value quickly, but also maintain a stable flow to avoid the impact of pressure fluctuations. The response speed of a device plays a vital role in measuring accuracy and stable flow control.
Physical time constant
The physical time constant is represented by the Greek letter Tau, which is one of the few standards used to describe the response time nowadays. That is, the time required for the mass flow 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 the natural logarithm, and 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 explain any problem, because the remaining 36.8% of the set value may take more time than or less than the previous 63.2%. The mass flow controller reached 63.2% of the set value in 7.4 milliseconds.
Any specified set point percentage change area
Many technical parameters of mass flow controllers use the change in a certain percentage range of the set value to define the equipment response speed, such as the time required to reach 90% from 10% of the set value. Because the 10% control response curve changes greatly, the resulting control response speed may not be equal to the actual control performance. It took 11.4 milliseconds for the mass flow controller to reach the set value. The span of the two segments is large, and the span of the two segments below 10% and above 90% is much smaller. For this figure, if the response speed of the device is defined according to the time taken from 10% to 90% of the set value, the conclusion is 6~7 milliseconds.
Adopting the principle of thermal diffusion, thermal diffusion technology is a technology with good performance and high reliability under harsh conditions. Its typical sensing elements include two thermal resistors (platinum RTD), one is a speed sensor, and the other is a temperature sensor that automatically compensates for the change of gas temperature. When two RTDs are placed in the medium, the speed sensor is heated to a constant temperature above the ambient temperature, and the other temperature sensor is used to sense the medium temperature. The gas mass flow through the speed sensor is calculated by the heat transfer of the sensor element.
As the gas flow rate increases, the heat carried by the medium increases. Lower the temperature of the sensor. In order to keep the temperature constant, the working current through the sensor should be increased. The increased current is proportional to the flow rate of the medium. If the pressure drop of the filter is found to increase after the gas flowmeter is put into operation for a period of time, the filter should be cleaned or the filter medium should be replaced. If the pressure drop of the flowmeter is found to increase, it is strictly prohibited to vent suddenly to avoid damaging the gear in the meter.
