Abstract: Light-scatter smoke detector, depending from the ceiling of a room, has a base unit with a battery to power an infra-red light source and infra-red light sensor, both of which are located in an upper element. A lower element has a surface corresponding to the exterior of a cone, and forming the base of a chamber. A surface makes, with a frusto-conical surface of the upper element, an annular wedge-shaped recess to chamber, this recess facing both source and sensor. The surfaces have a matt-black coating to promote energy absorption, so that the recess inhibits any light entering it from ever re-emerging. The stream of air flowing through the detector is monitored in the central chamber for the presence of smoke particles, by the sensor watching for light which originated from the source being scattered by smoke particles in the chamber and arriving at the sensor.

Abstract: A power assisted respirator comprises a facepiece for covering at least the nose and mouth of the wearer which has an outlet provided with a one-way exhale valve which is openable to permit air to flow out of the facepiece when a predetermined pressure P is established within the facepiece. A pump unit supplies air to the space within the facepiece. The pump unit may be connected to an inlet of the facepiece by a flexible hose or may be mounted directly on or in the facepiece. A filter canister is connected to the inlet of the pump means for filtering air supplied to the facepiece. A one-way inlet valve is provided in the path of air flowing from the pump unit to the facepiece and a pressure sensor is provided for sensing the pressure of air in the region of the pump unit inlet for causing deenergisation of the pump unit when the pressure in the region of the pump unit inlet exceeds a predetermined level.