a gas release device that includes a reservoir of pressurised gas and a passageway from said reservoir to an outlet . there is an elastic gas permeable membrane in the passageway that allows gas release only via the membrane . a gas impermeable member is provided in the passageway that includes an interface positioned to contact said gas permeable membrane . displacement of the gas permeable membrane is responsive to the pressure of the gas in the reservoir such that there is a greater interfacial contact between the gas impermeable member and the gas permeable membrane at a higher gas pressure than at lower pressure to thereby vary the exposed cross sectional area of the permeable membrane available to the flow in an inverse relationship to pressure .