A centrifugal separator (1) for separating a fluid mixture into components, comprising a non-rotating part (2, 3), a rotor (5) which is attached to a shaft (6) which is rotatably supported in the non-rotating part (2, 3) around a rotational axis (x), which rotor forms within itself a separation space (8) delimited by a rotor wall (7). The separator comprises an inlet (8) extending into the rotor (5) for supply of a fluid mixture to be separated in the separation space (8), at least one sensor measuring unbalance conditions in the frame; a level determining arrangement comprising two or more space defining elements (16, 17) of arbitrary form arranged on the interior surface of, or close to, the rotor wall (7), where each space defining element (16, 17) defines a space which communicates with the separation space (8) or another of said space defining elements through at least one inlet opening (20) arranged at a certain radius (a, b) from the rotational axis (x) and not outside that radius and where that certain radii (a, b) of the space defining elements (16, 17) are different. Methods for determining when a predetermined amount of heavy phase fluid (purification) or sludge (clarification) has been separated. The aim of the invention is to provide separator and methods with which it is possible to determine when the level of separated heavy phase fluid or sludge is high enough for emptying or discharge of the separator.