When non-circular gears rotate, they can pump fluids because their lobes leave empty volumes within the disk of revolution. A circular gear completely fills its disk of revolution, thus leaving no space for a fluid to enter. As the lobes turn, the hollow volume goes through a revolution as well, carrying along any fluid within. This allows for moving a known amount of fluid per revolution. In this way flow can be easily measured or regulated. Just count or control the number of revolutions and the amount of transferred fluid is known.