A method and apparatus for measuring gas flow volume in a system (of the kind in which the amount of gas flowing in a duct can vary in response to the variation of area of a variable area orifice in a duct and/or in response to changes in the total pressure of the gas upstream of the orifice) references...http://www.google.com/patents/US4796651?utm_source=gb-gplus-sharePatent US4796651 - Variable gas volume flow measuring and control methods and apparatus
