A device includes a case having a surface with a perforation and a first cavity containing a gas generating fuel. A first membrane is supported by the case inside the first cavity. The first membrane has an impermeable valve plate positioned proximate the perforation. The first membrane is water vapor permeable and gas impermeable and flexes responsive to a difference in pressure between the cavity and outside the cavity to selectively allow water vapor to pass through the perforation to the fuel as a function of the difference in pressure. A second membrane that is water vapor permeable gas impermeable is coupled between an outside of the case exposed to ambient atmospheric gas and the valve plate creating a reference pressure second cavity configured to reduce the effects of ambient pressure transients on the difference in pressure. A fuel cell membrane may be included in the device to produce electricity.