A method and apparatus for sensing the velocity of a fluid flow, especially within the boundary layer of a fluid flow path through a compressor. A reference flow under constant pressure is established through an orifice dividing two plenum chambers, and directed through a flow sensing opening in a probe...http://www.google.ca/patents/US3930742?utm_source=gb-gplus-sharePatent US3930742 - Velocity probe for compressor surge control
