Apparatus features a signal processor or signal processing module configured to: receive signaling containing information about a central air-core of an overflow pipe of a hydrocyclone where fluid flow is concentrated in an outer annular region of the overflow pipe that is against an inner wall of the overflow pipe during a normal operation of the hydrocyclone; and determine corresponding signaling containing information about a collapse of the central air-core of the overflow pipe of the hydrocyclone during an abnormal operation of the hydrocyclone, based upon the signaling received. The signaling contains information about a fluid flow rate of the fluid flow by detecting a change in the magnitude of a force and/or a moment on the probe.