A gas generating system comprises an air separation module (ASM) in fluid
communication with a nitrogen enriched air (NEA) line and an oxygen
enriched air (OEA) line. A valve is positioned in the OEA line and an
oxygen sensor is positioned in the NEA line. The valve can be modulated
in response to the oxygen sensor to create a backpressure forcing more
airflow into the NEA line. The OEA line can be backpressured until a
predetermined NEA oxygen concentration is reached.