Abstract: An indication device indicates an undesired operation condition of a centrifugal separator and comprises an inlet valve situated in a supply conduit to the separator, and an outlet valve situated in an outlet conduit from the separator. During normal operation of the separator the valves are adjusted in open normal valve positions for passing normal flows through the supply and outlet conduits. A control unit is adapted to activate a signal generator to generate an error signal in response to a pressure sensor if the pressure sensor during a predetermined period of time during operation of the separator senses a course of pressure change in the outlet conduit different from an expected normal course of pressure change, when the inlet and outlet valves during predetermined period of time are adjusted in valve positions which at least substantially decreases the flows in the supply and outlet conduits from the normal flows.