Abstract: A vacuum priming system for drawing liquid into the pumping chamber to prime a vertical shaft centrifugal pump for pumping sewage or the like. The vacuum priming system includes a priming pump for developing a partial vacuum in the pumping chamber and causing liquid to be drawn into the pumping chamber to evacuate air therefrom. A sensor is provided for detecting the interface between the liquid and air in an area in or above the pumping chamber for activating and deactivating the priming pump. The sensor comprises a low sensitivity capacitive proximity sensor having a generally vertical sensing face for sensing the relative movement of the interface between the liquid and air.