A system is provided for controlling flow of a first fluid through a line. Dispensing means isolates a region of the first fluid in the line from effects of pressure in the line outside of the region and repetitively dispenses into and out of the region volume increments of first fluid. Measurement fluid housing means houses measurement fluid in communication with the region such that the dispensing of increments of first fluid into or out of the region causes a change in the measurement fluid pressure. Displacement means displaces a predetermined volume increment of measurement fluid into and out of the measurement fluid housing means, such that displacement of the predetermined volume increment causes a change in the measurement fluid pressure when the region is isolated by the dispensing means. Pressure measurement means measures changes in measurement fluid pressure. Control means, in communication with the pressure measurement means, the displacement means, and the dispensing means, causes the dispensing means to dispense first fluid in increments based on data from the pressure measurement means, such that at least one of the predetermined volume increments of measurement fluid displaced serves as a reference for the volume increments of first fluid outputted.