Open-Circuit Breathing Systems

For most tasks, the professional diver is working in a small area for long periods. Because of this, he or she does not need the mobility of the scuba diver. The breathing gas normally comes from the surface in a hose, either supplied from storage cylinders or compressed as needed by a motor-driven compressor. The cable for the communication system and a hose connected to a depth measuring system are often bound to the gas supply hose. Another hose with a flow of hot water may also be used to warm the diver. It is normal for the diver to have an alternative supply of breathing gas in a cylinder on his or her back. This supplies the diver with breathing gas if the main supply should fail.

Free-flow systems were used in the first commercial air diving apparatus. The diver was supplied with a continuous flow of air that was pumped down a hose by assistants turning a hand-operated pump. The hand-operated pumps have long gone, but the same principle is still in use. In the most common system, called standard rig, the diver’s head is in a rigid helmet, joined onto a flexible suit that covers the body. The diver can control buoyancy by controlling the amount of air in the suit. The main problem with the system is that the flow of fresh breathing gas must be sufficient to flush carbon dioxide from the helmet. The flow required to do this is about 50 litres/minute (lpm), measured at the operating depth; this is well in excess of that needed with a demand system.

The other problem associated with free-flow systems and the high gas flow is the noise this generates. In the early days, the diver was also exposed to the risk of a particularly unpleasant form of barotrauma. If the pump or air supply hose breaks, the pressure of the water tends to squeeze the diver’s soft tissues up into the helmet. This is prevented by fitting a one-way valve that stops flow back up the hose. For deep dives, where oxygen-helium mixtures are used, the cost of gas becomes excessive. A method of reducing the gas consumed may be fitted. For example, some units incorporate a canister of carbon dioxide absorbent to purify the gas. The gas flow round the circuit is generated by a Venturi system that does away with the need for valves to control gas flow. The rig is converted into a rebreathing system, which has a separate set of problems that are considered in a later section.

Demand systems were developed to gain a reduction in gas consumption compared with free-flow systems. They also enable the diver to talk underwater. Several types of equipment are in common use. One type uses a full-face mask that seals round the forehead, cheeks and under the chin. The back of the diver’s head may be exposed to the water or covered with a wetsuit hood that is joined onto the face mask.

Another type is fitted in a full helmet. An oronasal mask in the helmet reduces rebreathing of exhaled air. The helmets are often less comfortable than the face masks, but they give better thermal and impact protection.

These helmets may also be used at greater depths, where helium mixtures are used. A return hose may be used to allow collection of the exhaled gas at the surface for reprocessing.

When compared with a demand valve held in the mouth, all the systems mentioned earlier have the major advantage of reducing the chance of the diver’s drowning. This is important if the diver becomes unconscious and/or has a convulsion while breathing high partial pressures of oxygen (PO2). The increased safety and the advantages of a clear verbal communication system have led to the adoption of helmets by most diving firms.

Sets that use a helmet and a full-face mask reduce the risk of drowning and can allow the diver to converse with people on the surface.