Pneumatic pneumatic valve size of the interface

interface of pneumatic pneumatic valve size should be smaller than the corresponding pipe, that is to say, when I answer the pneumatic pneumatic valve plus two size head, is this why? Main purpose is to pressure fluctuations as small as possible, make the adjustment under the steady state as soon as possible. May not like the pipe size of pneumatic valve size, only increase the capitulum connection. 。 Regulating pneumatic valve if interface can make control pneumatic valve working point is low, often working in the small open degree, the pneumatic control valve is bad; 。 Small diameter will increase the adjusting adjustment precision of the pneumatic valve; 。 Small diameter of conventional pneumatic valve to cheaper. But that is not absolute, and some also same size!