Abstract: An input valve having an input that can be coupled to a source of pressurized gas is open during first portions of successive cycles and closed during the remainder. A volume V is connected between the output of the valve and a load, and gas flows through the volume V to a load during the entire cycle. Pressure in the volume V is measured at the end of the first portion of each cycle and/or at the end of each cycle, and means responsive to a pressure can be used to control the flow of gas through the input valve so as to maintain the pressure on the load at a predetermined value or the difference in the pressures can be used to maintain a constant flow to the load. An output valve can be inserted between the volume and the load and closed while pressure measurements are being made so that the pressure is constant during the measurement.