Abstract: A method and apparatus are provided for adsorbing a first gas from a mixture thereof with a second gas, to reduce the concentration of the first gas in the mixture to below a desired maximum concentration, in which the adsorption cycle is timed according to the rate of fill through an orifice of a space of predetermined volume by withdrawing a portion of system gas, feeding such system gas portion into a space of predetermined volume through an adjustable orifice, accumulating the withdrawn system gas in the space, and discontinuing the adsorption cycle when the volume and/or pressure of accumulated system gas in the space reaches or exceeds a predetermined minimum, thereby making it possible to control the cycling independently of any external control or power source.