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People engaged in laboratory and pharmaceutical factory air conditioning and ventilation system engineering should be familiar with the names "constant air volume valve" and "variable air volume valve". However, for those who have just been exposed to these two types of valves, it is impossible to guess their functions through literal interpretation - "Is a constant air volume valve only having one air volume setting value?" "Does a variable air volume valve have multiple air volume setting values?" In fact, these understandings are not correct. The name of the constant air volume valve is named based on the control objective of the air valve. In the construction industry standard JG/T436-2014 Building Ventilation Air Volume Control Valve, the definition of a Constant Volume Valve is "an air valve that maintains a constant air volume within the specified static pressure range in front of the valve.". There is no definition of variable air volume valve in the standards related to air valves. In the national standard GB/T16803-2018 Terminology for Heating, Ventilation, Air Conditioning and Purification Equipment, there is a definition of a variable air volume terminal device, which automatically adjusts the supply air volume to maintain the required indoor parameters based on changes in the load of the air-conditioned room. The variable air volume terminal device used for indoor temperature control usually comes with an air volume measurement device to solve the problem of room temperature changes lagging behind the adjustment of air supply volume. However, in controlled environments such as laboratories and pharmaceutical factories, variable air volume valves for pressure differential control can be used without air volume measurement devices. It is obvious that the definition of this national standard is not entirely suitable for our understanding of the function of variable air volume valves in the scenario we are concerned about. Although there is no official definition for variable air volume valves, it is not difficult to see that the name of the variable air volume valve is named based on the adjustment method, which means that the variable air volume valve controls a certain parameter, such as room temperature or pressure difference, by changing the air volume. It can be seen that fixed air volume valves and variable air volume valves are not classified in the same way. The former is defined according to control objectives or controlled parameters, while the latter is defined according to adjustment methods. So, these two terms often confuse people& Nbsp; For the convenience of technical communication without ambiguity, we refer to the flow control valve (FCV) as the air valve that aims to control the air volume. Regardless of whether the air volume control valve has a set value or multiple set values, it belongs to the air volume control valve. For example, in a GMP workshop that adopts a constant air volume system, the room is constantly at a certain supply air volume during normal production operation. During the time when there is no production after work, it enters the duty mode, and the air volume decreases slightly. In this way, the air volume control valve has two air volume settings. For example, the air volume control valve used for ventilation hood exhaust has an air volume setting value determined based on the height of the sliding door, and theoretically there are countless air volume setting values. The pressure control valve (PCV), which aims to control the pressure difference in the room, is no longer referred to as a variable air volume valve. The terms FCV and PCV make it easier for people to understand the function of air valves, avoid ambiguity in terminology, and provide great convenience for communication between air conditioning, ventilation, and control engineering personnel. < Br/>