An apparatus comprises an inlet to convey a first fluid comprising first and second components having different density into a rotating chamber such that an interface forms between at least portions of such components at a first location. At least one outlet removes at least one of the first and second components. A controller is adapted to introduce the first fluid and is operable to move the interface from the first location to a second location and 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 controller is operable to determine a flow rate of the first or second component based, at least in part, on the time interval between when the interface moves from and returns to the second location. At least one sensing assembly is adapted to sense the location of the interface.