Fluid flow switches with low flow resistance

A flow switch comprising:

• a body with a bore extending therethrough, the bore having relatively narrower and wider sections;
• a member movable within the narrower and wider sections of the bore such that the member is a relatively close fit in the narrower bore section, the member having a first portion which permits only limited fluid flow in the close fit clearance between the first portion and the narrower bore section, and a second portion having a flow bypass arrangement whereby substantial fluid flow is possible with the second portion located in the narrower bore section, the member being movable by fluid flow from a closed position to an open position, wherein in the closed position, both the second portion and at least part of the first portion are within the narrower bore section, whereas in the open position, the first portion and a part of the second portion are within the wider bore section thereby allowing fluid to flow;
• and
• means for detecting whether the member is in the open position or the closed position;
• and
• wherein at least the narrower bore section and the movable member are both generally cylindrical;
• the first portion of the member comprises a first solid cylinder;
• the flow bypass arrangement defines flow-enabling spaces in the second portion of the member;
• and the second portion of the member comprises a second solid cylinder of smaller diameter than that of the first solid cylinder, and a plurality of vanes extending from the second solid cylinder such that the effective external width of the second portion is the same as the diameter of the first portion.