a reliable and economical apparatus for relieving pressure in a large aircraft cabin oxygen supply , where multiple oxygen cylinders are used concurrently . a series - parallel array of valves actuated by changes in differential pressure between the oxygen supply and the ambient cabin atmosphere provides overpressure relief the series connection of the valves reduces the risk of open - valve failures , while the parallel connection of sets of series - connected valves reduces the risk of closed - valve failures . the small number of valves used in its design reduces the cost of the apparatus . the series - parallel structure is optionally extended to larger numbers of valves to facilitate the use of less - expensive valves and supporting components without loss of reliability .