In the preferred embodiment of the invention, the controller 40 lowers the rotational speed of the rotor 12 so that a centrifugal force at dam 50 remains above 850G. Then, the controller calculates a value for K=Qi /Ni 2, where K is a constant, Qi is the current flow rate into the fluid chamber 22, and Ni is the current rotational speed of the centrifuge rotor 12.