Abstract: 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 the total pressure upstream of the orifice to the static pressure downstream of the orifice to produce a first signal representative of the differential pressure between the total pressure upstream of the orifice and the static pressure downstream of the orifice. The method and apparatus produce a second signal corresponding to the area of the opening in the orifice. The first and second signals are supplied to a gas flow volume database which is correlated to the sensors and to the orifice in the duct. The gas flow volume occurring in the duct at the first and second signals is read out from the database.