A separation method comprises introducing a first fluid comprising first and second components having different density into a centrifugal field and allowing an interface to form between at least portions of such components at a first location. The method also comprises moving the interface from the first location to a second location and further comprises introducing the first fluid and removing one of the first and second components at known controlled flow rates so as to move the interface from the second location in a direction toward the first location and to return the interface to the second location. The flow rate of the first or second component may be determined based, at least in part, on the time interval between the interface moving from and returning to the second location. The flow rate may also be based on the weight of the one component removed during the time interval.