A portable object includes a case forming a housing in which is arranged a device requiring air to operate. The case includes at least a first opening. The portable object further includes a closure device provided with a valve system arranged to have a first stable, open position in which the orifice is open and allows air to pass through, and a second stable, closed position, in which the orifice is closed so that the portable object is impermeable to gases and to liquids. Switching from the open position to the closed position is automatic in case of immersion.