Method and apparatus for separating a volume of composite liquid into at least a first component and a second component including centrifuging a separation bag (1) containing a volume of composite liquid and a volume of gas so as to sediment at least a first component and a second component separation bag (1); displacing a volume of hydraulic fluid against the separation bag to cause a transfer of at least one fraction of the content of the separation bag (1) into at least one component bag (2, 3, 4, 5) connected to the separation bag (1); determining the volume of hydraulic fluid displaced and determining the volume of the composite liquid or the transferred components from the determined volume of displaced hydraulic fluid.