Molecular sieve-type gas separation systems

An on-board oxygen generating system for an aircraft, comprising a molecular sieve-type gas separation system arranged to deliver oxygen-enriched air as product gas to an outlet by adsorbing nitrogen from air fed to the system, the system including at least two sieve beds and control means for cyclically subjecting each said sieve bed to a charge/adsorption on-stream phase followed by a purge/desorption regeneration phase, the control means providing for a range of selectable cycle times and including means sensitive to the oxygen content of the product gas, means responsive to aircraft altitude to display a desired oxygen content of said product gas at an altitude within an operating range, and comparator means for comparing the displayed desired oxygen content with the sensed oxygen content of the product gas and adapted to adjust the cycle time in the sense required to null the difference between said displayed and sensed oxygen contents of the product gas.