Abstract: This invention relates to a fluid transfer system including two elongated fluid transfer chambers, fluid inlet and outlet arrangements at each end of each chamber, oppositely directed one-way inlet and outlet valves in the inlet and outlet arrangement at a first end of each chamber for controlling the flow of a driven fluid into and out of the chamber, oppositely direted inlet and outlet controlled valves in the inlet and outlet arrangement at the second end of each chamber for controlling the flow of a drive fluid into and out of the chamber, a pressure balancing arrangement including a port in each of the controlled valves, an actuator on each controlled valve which is adapted to open and to close the valve and the pressure balancing port in the valve, and a control system which is connected to the actuators of each of the controlled valves for proportionally opening and closing the controlled inlet valves of each chamber in exact opposite phase to each other and for opening and closing the controlled chamb