Device for monitoring and adjusting, in particular for fixing and securing, the radial position of an interface layer in a nozzle centrifuge for separating a light phase having a relatively low density and a heavy phase having a relatively high density from a mixture containing these two liquids and a solid, the nozzle centrifuge including a rotor (1-3) which is rotatable around a rotational axis (R) and which forms an inlet (9) for said mixture, a separating chamber (7) communicating with said inlet (9) and having a radially inner part (7a) and a radially outer part (7b), nozzles (12) at the separating chamber distributed around the rotational axis (R) for throwing out the heavy phase and the solid, an outlet (13a, 13b, 14) for discharging the light phase, an overflow outlet (19) being radially delimited by a cover or level ring (20) for discharging the heavy phase, and an inlet (9; 25, 26) for supplying additional heavy phase to the centrifuge, characterized in that a sensor (24) is arranged in the flow path of the heavy phase (23) leaving the centrifuge for monitoring the magnitude of the flow and transmitting a corresponding signal such that the supply of additional heavy phase via the inlet (9) or (25, 26) can be adjusted in reply to the signal.