let's say I have two points in a line, A and B. Between them there is a valve that can regulate the amount of the stream from A to B. Considering that there is a steady flow of the stream between A and B, and is to be always maintained. Is it better to regulate the valve in accordance to the pressure or with the mass flow? what is the difference of using pressure regulator instead of the mass flow meter if the aim is only to maintain the pre-set steady flow?