The controller of the plural component proportioner (10) dispenses both components (or all three if a three component material) as necessary to maintain the correct mix ratio. The fluid displacement measuring technique described allows this proportioning to take place with the “A” or main component pump always able to supply fluid to the spray gun without being stopped. The “B” component, at a higher pressure, is added as necessary to control ratio. The control can determine whether the pumps were properly loaded with fluid, and the fluid was properly compressed for accurate measurement. This technique allows for the proportioned fluids to be provided to the spray gun(s) at high flow rates and consistent spray pressure without interruption.