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.
