To control the flow of air into and/or out of a process chamber, such as a clean room, a two-stage system is disposed in conduits leading into and/or out of the process-chamber. One stage includes a regulator, which has a plenum disposed between the environment and the air source or the vacuum source. The regulator maintains in the plenum a pressure that is between the pressures of the environment and the source and that is a constant amount different from the environment's pressure. The regulator includes a piston having a frontal face exposed to air in the plenum flowing between the environment and the source, and a distal face exposed to the environment's pressure. In some embodiments, a gate is attached to the piston. Either the gate or the piston is mounted so as to variably impede the flow of air through the regulator and so that the weight of the piston tends to move the piston in a direction so as to lessen the impedance on the air flow. The other stage includes an adjustable valve, located between the regulator and the environment, for further impeding the flow of air.