Abstract: A two-stage breathing apparatus comprises a compressed air supply cylinder (1) provided with a cylinder valve (2) communicating through a high-pressure duct (3) with a manifold (4). A first high-pressure branch (5) of the manifold (4) communicates with a pressure indicator warning unit (6) and a second high pressure branch (7), comprising a high pressure flexible hose, connects the manifold (4) via a non-return valve (8) with a high pressure quick release connector (10). The manifold (4) contains a pressure reducer (11) which feeds a demand valve (12) provided on a breathing mask (14) via a further flexible hose (13).The non-return valve (8) permits flow in the direction from the high-pressure quick release connector (10) to the supply cylinder (1), but prevents flow in the direction from the supply tank (1) to the high-pressure quick release connector (10).

Abstract: A network provides communication between end units (1). Each end unit (1) received an entitlement to transmit in the form of a go-ahead. It is then able to transmit a frame of user data, which is broadcast by a central unit (7) to all other end units (1). When the end unit (1) transmitting the frame wishes to relinquish its entitlement to transmit it regenerats the go-ahead, which is passed by the central unit (7) to the next end unit (1) taking them in a predtermined cyclic sequence. The system allows frames to be acknowledged: the central unit (7) collects acknowledgements from end units (1) receiving the frame successfully, and when every expected station has acknowledged, returns an acknowledgement to the source end unit (1). Central units (1) may be combined to form larger systems.