A system for controlling the flow of fluid through a line (3). The system may include first and second valves (A, B; and 6, 7) in the line (3). Between the first and second valves is located a chamber (52, 82), part of which may be filled with fluid having a variable volume (52), and another part of which is filled with a measurement gas (82, V.sub.1), such as air. The chamber may be isolated from the pressure effects in the rest of the line by closing both valves. The valves may also permit fluid to flow into or out of the chamber. The second portion of the chamber has a common boundary with the first part in such a way that the combined volume of the first and second parts is constant. A loudspeaker (22, 33, 571) creates sound waves in the gas in the second part of the chamber, in order to measure the volume of the fluid in the first part. A controller directs the operation of the valves and controls positive and negative pressure sources, which may be used to create subsonic pressure variations to produce the desired flow. The volume measurement is achieved by acoustic volume measurement techniques.