Abstract: A flow sensor for producing an electrical signal dependent upon fluid flow has a housing insertable in a bore extending transversely to and intersecting a flow passage in a port plate. A flap pivoted to the housing about an axis transverse to the flow passage is biassed by spring means into a resting position in which the flow passage is obturated. Pivotal movement of the flap from its resting position by fluid flow in the flow passage is converted into an electrical signal by a transducer, such as a potentiometer whose slider is coupled to the flap.