An apparatus for controlling the volume of flow of a gas. A reference volume has an input conduit connected to the source of gas and an output conduit connected to a utilization device. Valves in the input and output conduits are cyclically controlled to permit pressure in the reference volume to reach a high and then to attain a low. The reference volume pressure is measured twice during each cycle to provide voltages representative of the high and low pressures. Circuit means control the time and duration of operation of the valves to cause the difference between the high and low pressures to equal a predetermined value.