Abstract: A device for venting gas from an expansible gas chamber (lifejacket, lifting bag, etc.) of an underwater buoyancy apparatus. The device comprises a monitoring device which monitors the rate of ascent of the expansible gas chamber and provides a control signal when the ascent velocity exceeds a predetermined speed. A venting valve assembly controls the venting of gas from the expansible gas chamber and is responsive to the control signal to vent gas reducing the buoyancy of the expansible gas chamber.

Abstract: A device for automatically opening a gas supply valve when the device reaches a predetermined depth, e.g. when diving, has a gas reservoir, a pressure sensing means providing a control signal when the ambient pressure exceeds that of the gas reservoir, and a supply valve which is opened upon receipt of that signal. A preferred embodiment has a flexible diaphragm exposed at its faces respectively to the ambient pressure and to the gas reservoir pressure, and arranged to move an actuator which acts directly or indirectly on the gas supply valve. Underwater buoyancy apparatus, for use for example by a diver, has a buoyancy chamber to which as can be supplied to alter buoyancy, and the device is incorporated between a gas supply and the buoyancy chamber to give automatic increase of buoyancy when a predetermined depth is reached.