In fuel dispensing systems, such as these used for delivering gasoline to the fuel tank of a vehicle, environmental protection laws require that vapors emitted from the tank during the fuel dispensing process be recovered. Fuel is customarily delivered through a nozzle via a fuel hose and vapors are recovered from the nozzle via a vapor hose that conveys the vapor to the main supply tank from whence the fuel came. In what is referred to as a balanced system, the vapors are forced through the vapor hose by the positive pressure created in the vehicle tank as the fuel enters it. In other systems the vapor is sucked from the vehicle tank by a vapor recovery means connected to the vapor hose and forced into the supply tank.
In some systems, such as that described in the U.S. Pat. No. 4,223,706 to McGahey, mechanical means are used for controlling the relative volumetric flows of the fuel delivered to a nozzle by a fuel delivery means and of the vapor recovered by the suction of a vapor recovery means. In McGahey, the liquid fuel flow itself is used to drive a vapor pump. This usually causes a reduction in the fuel delivery rate which is inconvenient for the user. In U.S. Pat. No. 3,826,291 to Steffens, the vapor pump is calibrated so as to move a volume of vapor essentially equal to the volume of liquid flowing through the meter which measures the flow of fuel.
Unfortunately, these and other systems using mechanical controls generally cannot be adjusted so as to adapt to changing environmental conditions such as changes in temperature and/or to changes in operation. It is also difficult for such systems to accommodate additional controls or features, some of which may not even be known at the time of their manufacture.