Chambers, Habitats and Underwater Vehicles

Divers may use several special types of vehicles and living facilities. These include vehicles that are hoisted and lowered to transport divers to and from deep dive sites, propelled vehicles to increase the diver’s range and endurance (i.e. diver propulsion vehicles [DPVs], often used by technical divers) and machines to carry underwater equipment. The accommodations to be considered include underwater houses and pressurized houses at the surface.

Submersible decompression chambers (SDCs), often called personnel transfer capsules, are used to transport divers and any attendants from the surface to the work site, and they may also be used as a relay station and store for gas and equipment. The most complex SDC may carry the diver at constant pressure from a deck decompression chamber (DDC) to the work site and back. The simplest SDC consists of a bell chamber that is open at the bottom and allows the diver to decompress in a dry environment, exposed to the same pressure as the surrounding water.

Habitats are underwater houses that accommodate divers in air- or gas-filled environments. They are used by divers to rest between excursions. Divers have lived in some of these habitats for weeks at a time.

Deck decompression chambers (DDCs) can be small and used for surface decompression, a procedure that allows a diver to be decompressed in a dry chamber instead of in the water. Larger chambers can be used to treat divers with decompression illness and other diseases that respond to compression, in which case the chamber may be called a recompression chamber.

DDCs are also used to house divers for prolonged periods under elevated pressure. In this case, divers are carried to their work by an SDC or a small submarine that keeps the diver in a pressurized environment. At the end of the job, possibly after several weeks, the pressure in the DDC is lowered slowly to return the diver to atmospheric pressure.

Transport vehicles can carry the divers at normal atmospheric pressure, at ambient pressure in a dry environment or in a wet environment. These include vehicles towed by a boat. A small motor and propeller that pulls the diver along gives increased speed with reduced effort. Some submarines have a lock system to allow divers to leave and enter underwater.

One atmosphere diving equipment, such as the JIM suit, seals the diver in a pressure-resistant compartment. It has flexible arms with tools on the ‘hands’ for the diver to work underwater. The early types of suit had legs that gave the diver the ability to walk on firm surfaces if there was little current. The diver had no control in mid-water and had to be lowered and hoisted from the surface. In other designs, such as the Newtsuit and WASP system, the diver controls a set of propellers that make him or her a cross between a diver and a one-person submarine.

Life support systems are required to provide the occupants of all these vehicles, habitats and chambers with a respirable atmosphere. These work on the same principles as a diver’s breathing apparatus, and in some vehicles the diver may even be wearing a breathing apparatus. The system must be self-contained for transport vehicles, but for habitats and SDCs the gas is generally supplied from the surface.

Gas from the surface can be supplied in a free flow and escape out the bottom or be recirculated through a purifying system. Simple gas purifying systems can involve a hand-powered pump to force gas through a carbon dioxide absorption canister with a manually operated system for adding oxygen. The most complex systems are those found on large submersibles, nuclear submarines and chambers used for deep saturation dives. These have automatic closed systems with provision for removing trace contaminants and odours, and they also regulate temperature, pressure and humidity.

Gas reclaimers are mainly used to recover helium to be used again. They help to lower costs by reducing the amount of gas used. One type cools the gas until the other gases are liquefied, leaving pure helium to be stored and used again. Other types use a chromatographic technique to separate the gases.

Diving Rebreathing Systems

Semi-closed rebreathing system.

Respiration is designed to provide our tissues with oxygen and to eliminate carbon dioxide produced by metabolism. When we breathe on the surface, we consume about 25 per cent of the oxygen that we inhale with each breath. Thus, if our respiratory minute volume (RMV) were to be 20 lpm, we would breathe in 4 litres of oxygen each minute, of which 1 litre would be consumed and 3 litres would be exhaled back into the surrounding atmosphere. Although this may not seem very efficient, the situation becomes substantially worse when we descend on open-circuit scuba equipment.

As the depth and pressure increase, the amount of gas we inhale with each breath must also increase to compensate. Thus, at 40 metres (5 ATA), we would need to breathe 100 lpm from our cylinder to achieve the same 20 lpm surface RMV. This 100 litres of air would contain about 20 litres of oxygen, of which 19 litres are being exhaled into the ocean unused!

One solution to this inefficiency of gas consumption is to recirculate the gas, removing the carbon dioxide and adding only the oxygen that is consumed by the diver back into the circuit. This is called a ‘rebreather’, and such breathing apparatus can offer substantial reductions in gas consumption over open-circuit systems. The following is a summary of some advantages and disadvantages of rebreather systems, which are expanded upon in the following paragraphs:


  • Vastly reduced gas consumption, especially during deep diving.
  • Reduction of cold stress and dehydration by the breathing of warm, humidified gas.
  • Lack of bubbles good for photography, covert operations, fragile environments such as caves.
  • Improved decompression efficiency because of maintenance of ‘optimal PO2’.
  • Excellent duration in relatively small unit.


  • Significant initial cost.
  • Greater complexity and vastly increased need for training, vigilance and maintenance.
  • Different hazards to diver, higher overall risk.

Rebreathers fall into one of two main types – closed-circuit rebreathers (CCRs) and semi-closed- circuit rebreathers (SCRs). Although both types recirculate all, or part, of the breathing gas, the main difference lies in the way that the oxygen level is controlled and added into the circuit.

In general, SCRs are less complex but less efficient and have depth limitations dependent on the gas selection. CCRs are the most complex but also the most efficient and most capable with regard to depth and duration.

Because of the similarity between SCR and CCR sets, their common features are discussed first, and features peculiar to each type are then highlighted separately.

The usual gas flow pattern found in a rebreathing set is shown in Figure 4.5. The movement of inhaled and exhaled gas is controlled by one-way valves at the mouthpiece as the gas flows round the circuit. For largely historical reasons, rebreathers of UK or European origin usually have a clockwise gas flow pattern, whereas those of US origin have an anticlockwise pattern. However, this is not universally so.

A stylised rebreather layout.
Figure 4.5 A stylised rebreather layout.

As the diver descends, gas must be added from a high-pressure cylinder into the breathing loop so that a constant volume is maintained within the system. In most units, the gas is automatically added via a regulator-type valve (automatic diluent valve [ADV]). A manually controlled valve allows the diver to add extra gas if it is required. This addition of gas will affect buoyancy.

The counterlung acts as a gas storage bag that expands and contracts as the diver breathes. It normally incorporates a relief valve (over-pressure valve [OPV]) that releases surplus gas into the water and prevents excess pressure building up. Venting of excess gas is needed in CCR sets when the diver ascends and the gas in the counterlung expands. In SCR sets, excess gas vents regularly through the relief valve.

The carbon dioxide absorbent is usually a mixture of calcium and sodium hydroxides. These chemicals react with carbon dioxide to form carbonates and water, as shown:

as shown:

Closed-circuit oxygen systems are the simplest CCR sets. The counterlung is filled with oxygen from the cylinder. As oxygen is consumed, the volume of the bag decreases. In some units, a trigger mechanism that operates like a demand valve releases more gas into the bag. In other units, there is a mechanism that releases a continuous flow of oxygen into the circuit. A manually operated method of adding oxygen to the breathing bag is also usually fitted. This will be needed when the diver puts the unit on, when he or she goes deeper and the gas in the breathing bag is compressed, or when the diver needs to increase buoyancy.

The unit can be operated as a closed system because, unless something goes wrong, the gas in the breathing bag will contain a high concentration of oxygen, diluted with nitrogen that was in the lungs and body of the diver when he or she put the unit on. It is standard practice to flush the counterlung with oxygen at set intervals to ‘denitrogenate’ before starting the dive to prevent a build-up of diluting gases.

Possible problems with these units include carbon dioxide toxicity if the absorbent fails, dilution hypoxia if the oxygen is impure or the diver neglects to flush nitrogen from the lungs and the counterlung and oxygen toxicity if the diver descends too deep.

To reduce the risk of oxygen toxicity, a depth limit of about 6 to 8 metres is often imposed on the use of these units to limit the PO2 to 1.6 to 1.8 ATA, a range generally deemed acceptable for military operations, although too high for recreational technical diving, where a lower risk is appropriate and consequently a PO2 significantly lower than 1.6 ATA is usually maintained.

Closed oxygen rebreathing apparatus has the particular advantage that a small unit may give a long endurance. A unit weighing less than 15 kg can allow dives of more than 2 hours. The lack of bubbles and quietness of this unit are also important in some specialized roles such as clandestine operations.

Rebreathing units are quieter and have a greater endurance than scuba units. The extra hazards and costs involved restrict their use and demand significant extra training, maintenance and vigilance.

In closed-circuit mixed gas systems, oxygen and a diluting gas are fed into the breathing loop at rates required to keep the PO2 within safe limits and to provide an adequate volume of the mixture.

Electronic closed-circuit mixed gas rebreather layout.
Figure 4.6 Electronic closed-circuit mixed gas rebreather layout.

Figure 4.6 shows the fundamental features of this system.

As with the closed-circuit oxygen unit, the diver inhales breathing gas from the counterlung and exhales through the carbon dioxide absorber back into the counterlung. As the diver consumes oxygen, the PO2 in the counterlung falls, and this fall is detected by oxygen sensors. At a certain level, a valve injects more oxygen into the circuit. Both mechanically and electronically controlled units are commonly seen in recreational diving, and there has been much controversy as to which arrangement is safer.

Although all rebreather divers should know their PO2 at all times, the above argument hinges on the requirement for the diver in the manual system to be forced to know his or her PO2 at all times (although in most systems a basal flow of oxygen is continually bled into the unit). However, the requirement to manage the PO2 in this system can create problems during times of high task loading. In contrast, there is less obvious compulsion for the diver using an electronic rebreather to know the PO2 at all times, and should the controlling computer fail, the diver would be at risk, although the chances that the computer will fail in an electronic rebreather are very low. The reality is that, to date, neither system has been shown to offer a survival advantage and all rebreather divers should make a habit of knowing their PO2 at all times.

Most modern mixed gas CCRs use a series of three redundant oxygen sensors (galvanic fuel cells) to track the PO2 in the loop. This allows for the comparison of the outputs of the sensors because they are relatively fragile and prone to failure, as well as having a limited life (usually ~18 months).

If the volume of gas in the bag falls, this triggers a second valve that adds diluting gas (diluent) from a separate cylinder. Air, trimix (helium, nitrogen and oxygen) or heliox (helium and oxygen) may be used as the diluent depending on the planned depth and profile of the dive. The selection of the gas is determined largely by oxygen toxicity and work of breathing issues. For the former, the diluent gas should not have a PO2 greater than a predetermined set-point at depth (preferably a little lower so that an ‘oxygen spike’ does not happen during descent when diluent is added to the loop). To manage the latter, the diver should calculate the density of the gas at the proposed maximum depth, such that it does not exceed the manufacturer’s recommendation and maintains the work of breathing within the specifications of the unit.

Manual controls and displays indicating the oxygen concentration are often fitted to allow the diver to override the controls if the automatic control fails. In many cases, divers also either raise the ‘set-point’ or flush the unit with oxygen during the final decompression stop at 6 metres to shorten decompression time.

This system would appear to be the most efficient breathing system. It is more economical in terms of gas usage than any other gear apart from the oxygen-breathing apparatus. It enables a diver to go deeper for longer and with fewer encumbrances than other equipment.

As an example of the efficiency of this type of equipment, it has been estimated that in a helium saturation dive program involving a prolonged series of dives to 180 metres, the cost of helium for a CCR apparatus was about 2.5 per cent of the cost of an SCR diving apparatus. These advantages must be balanced against the greater initial cost and complexity of the system. This complexity can lead to fatal malfunctions.

SCRs offer some of the saving in gas obtained in the closed systems while avoiding the depth limitations of the oxygen sets and the greater complexity of the closed mixed gas sets. The basic system is shown in Figure 4.7.

Semi-closed rebreathing system.
Figure 4.7 Semi-closed rebreathing system.

SCRs typically use oxygen-enriched air (nitrox) as the breathing mix instead of oxygen. Two major types of SCR are in common use. The most common is the constant mass flow (CMF) type, but the keyed respiratory minute volume (RMV keyed) type has some advocates, especially with US cave divers.

In a typical CMF SCR system, the gas flow and composition are chosen for maximum efficiency for the proposed dive. First, the composition of the gas is chosen, with as high an oxygen concentration as possible without creating an unacceptable risk of oxygen toxicity at the planned maximum depth. This level may be changed depending on the duration of exposure. The flow is then chosen so that the diver will receive sufficient oxygen while working on the surface.

The oxygen concentration in the diver’s inspired gas is determined by the flow into the system, the diver’s consumption and loss through the relief valve. It ranges from close to that in the supply bottle when the diver is resting down to about 20 per cent when the diver is working at the maximum expected rate.
In the RMV keyed sets, a fixed volume of gas is dumped from the circuit with each breath, with ‘new’ gas added via a demand valve.

As a safety precaution with an SCR, almost invariably an excess of gas is added to the loop and is vented through the relief valve, thus making these devices less efficient on gas than CCRs. However, an SCR system with a flow of 12 lpm gives an eightfold saving of gas compared with a demand system when the diver is consuming 1 lpm of oxygen. This saving would increase if the scuba diver was working harder and consuming more air.

The high oxygen concentrations in both SCR and CCR systems mean that the diver may not absorb as much nitrogen as he or she would if breathing air. This can give a decrease in the decompression needed, but unless the PO2 in the loop is actually measured, the diver must ‘guesstimate’ the PO2 for decompression purposes. On occasion, this has resulted in problems.

Military divers have traditionally been the main users of SCR sets, although they have become increasingly popular in recreational diving. The reduced gas flow with these sets means that they can be designed to make little noise. If they are constructed from non-magnetic materials, they can be used for dives near mines, although CCRs are now more often used for mine countermeasures.

Problems with rebreathers

Both CCR and SCR systems introduce a variety of potential hazards (see Chapter 62):

Carbon dioxide accumulation can occur if the scrubber fails. This can occur if the scrubber material is used for too long, if the scrubber is incorrectly attached or if the scrubber’s instantaneous capacity to remove carbon dioxide is exceeded because of a diver’s high demand, such as with exertion.

Oxygen toxicity can occur if the diver exceeds his or her depth limit, descends too quickly when diving at a particular PO2 (set-point) or uses a mixture with too much oxygen in it. This can occur if an excessively oxygen-rich mix was added to the diluent cylinder or if the solenoid or manual oxygen injection valves jam open.
Hypoxia may result if the gas flow decreases. This can be caused by omitting to turn on the gas supply before descending, exhausting the oxygen supply, solenoid or electronics failure, and ascending too rapidly to enable the solenoid to add sufficient oxygen to the loop. Hypoxia can also occur if the diver works harder than expected or if a mix with too little oxygen is used.

A review of the 181 reported recreational CCR-related deaths that occurred between 1998 and 2010 estimated that the fatality rate for CCR users was about 10 times that of recreational open-circuit divers. The author also suggested that CCRs have a 25-fold increased risk of component failure compared with manifolded twin-cylinder open-circuit systems. It was suggested that this risk could be partly offset by carrying a redundant bail-out system.

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.

Professional or Technical Diving Equipment

This section deals with the more specialized equipment used by professional and military divers, as well as some recreational technical divers. Many of the military diver’s tasks, and some of those of the professional diver, involve comparatively shallow depths. Such tasks could be conducted with scuba gear of the type described earlier. Equipment fitted with communication devices allows the diver to confer with the surface support. Communication devices operate better in air, so they are commonly fitted into a helmet or full-face mask. In these devices, the airflow may either be continuous or on demand.

More specialized equipment is used for some military diving where an element of stealth is required. For these tasks, an oxygen rebreathing system that can be operated with no telltale bubbles may be used. In dealing with explosive mines, stealth is again required to avoid activating the noise- or magnetically triggered circuits. If the mine may be too deep for an oxygen set, a rebreathing system with an oxygen-nitrogen mixture may be used.

For even deeper tasks, for which oxygen-helium mixtures are used, some method of reducing the gas loss gives cost and logistical savings. This can be achieved by the diver’s using a rebreathing system or returning the exhaled gas to the surface for reprocessing.

Dive Boats

Boats used for diving range from kayaks and canoes to large, specialized vessels that support deep and saturation diving. The facilities required depend on the nature of the diving, but there are minimum requirements. In some conditions, a second safety boat or tender may be needed. Divers may need to be picked up after drifting away from the main vessel.

Propellor guards, or a safe propulsion system such as a water jet, is desirable if there is any chance that the engine will be engaged during diving operations.

A diving platform or ladder is needed on most boats to facilitate the diver’s return from the water. Consideration should be given to the recovery of an unconscious or incapacitated diver, which is ideally done with the diver positioned horizontally. This can be very difficult with both large and small boats, and an appropriate system should be established and practised. Recovery into an inflatable craft is often an easier alternative because the diver can be dragged, rather than lifted, into the boat. Also, the softer air-filled hull is less likely than a rigid hull to injure a diver.

Diving flags, lights or other signals as required by the local maritime regulations should be available. These are designed to warn boat operators to slow down or keep clear. In some places they can offer legal, if not physical, protection from the antics of other craft. Unfortunately, in many places the flag is not recognized or is ignored, and in most areas ‘boat propeller attacks’ cause more deaths than shark attacks.

The first aid kit and emergency medical equipment should be chosen depending on local hazards and the distance from assistance.

Diving Safety and Protective Equipment

Two of the more sophisticated current model recreational dive computers (a) Galileo Sol (Scubapro, USA); and (b) Vytec (Suunto, Finland).

The best safety measures available to a diver are adequate health and fitness, proper training, appropriate and functional equipment and common sense. Almost all accidents are preventable, and the authors do not ascribe the popularly held belief that these accidents are attributable to an ‘act of God’. Many accidents involve human, often predictable and thus correctable, mistakes. This point is developed in Chapter 46, in which deaths and accidents are considered. Several items of equipment that reduce the hazards of diving, or assist with coping with them, are discussed here.


Emergency air supplies can take a variety of forms. In the early days it was common to rely on buddy-breathing, a procedure in which two divers shared an air supply in the event one of them had an air supply failure. Both anecdote and analysis of diving accident statistics showed that this procedure often did not work in an emergency. The use of a second regulator attached to the scuba set, often called an octopus rig, has now become standard fare, and its introduction and widespread use have helped to avoid many serious diving accidents. However, neither buddy-breathing nor an octopus rig will be of use if the diver with gas is not available or is unwilling to cooperate. For this reason, a second source of air (redundant supply) that is available to each diver without external assistance is now favoured. For cave divers this may essentially be a second scuba set. For technical divers with substantial mandatory decompression obligations, a redundant gas supply is also essential, and they often carry what is known as a stage cylinder.

For most divers, who have relatively ready access to the surface, a smaller cylinder with an independent regulator can be used. One commercially available device, known as Spare Air (Submersible Systems, Inc.), is carried by some divers. However, the air supply is very small, enabling only a few breaths for ascent. For this reason, these devices are not commonly used and are not sufficient for deep dives or dives requiring decompression. It is important that a redundant supply provides adequate gas for a relatively safe ascent.

It is also sometimes possible for a diver to breathe air from the BCD for a short period of ascent. However, this has potential hazards, including aspiration of water, infection and buoyancy control problems. A BCD with an independent air supply is available but not commonly used.


Thermal protection is needed in cold water or on prolonged dives to minimize the risk of hypothermia. This protection is normally provided by insulated clothing, which reduces heat loss. The most common protection is a wetsuit, made from air-foamed Neoprene rubber. The water that leaks into spaces between the suit and the diver soon warms to skin temperature. Foamed Neoprene has insulation properties similar to those of woollen felt. Its effectiveness is reduced by loss of heat with water movement and increasing depth. Pressure decreases insulation by reducing the size of the air cells in the foam. At 30 metres of depth, the insulation of a wetsuit is about one third of that on the surface (see Figure 27.1). The compression of the gas in the foam also means that the diver’s buoyancy decreases as he or she goes deeper. The diver can compensate for this by wearing a BCD. If the diver does not, he or she needs to limit the weights, but this will mean that the diver is too buoyant when closer to the surface. The buoyancy and insulation of a wetsuit decrease with repeated use.

Another other common form of thermal protection is the drysuit. This is watertight and has seals round the head, feet and hand openings. There is an opening with a waterproof seal to allow the diver to get into the suit. The drysuit allows the diver to wear an insulating layer of warm clothes. A gas supply and exhaust valve are needed to allow the diver to compensate for the effect of pressure changes on the gas in the suit. The gas can be supplied from the scuba cylinder or a separate supply.

The diver needs training in the operation of a drysuit or he or she may lose control of buoyancy by excessive addition of air into the suit. This can lead to an uncontrolled ascent, sometimes inverted, when the excess of gas expands, speeding the ascent. If the diver tries to swim downward, or otherwise becomes inverted in the water, the excess gas may accumulate around the legs, from where it cannot be vented through the exhaust valve. The excess gas can also expand the feet of the suit and cause the diver’s fins to pop off. The diver can find himself or herself floating on the surface with the suit grossly overinflated, a most undignified and potentially dangerous posture.

Heat can also be supplied to a diver to help him or her keep warm. The commonly used systems include hot water pumped down to the diver through hoses. Various chemical and electrical heaters are also available. External heat supplies are more often used by commercial divers.

Semi-drysuits are essentially wetsuits with enhanced seals at the neck, hands, feet and zippers. These seals help to reduce the amount of water entering and leaving the suit and so reduce heat loss. They are not as effective as drysuits in keeping the diver warm, but they can provide thermal protection similar to that of a significantly thicker wetsuit and so increase the level of comfort for the wearer, as well as reducing the amount of weight carried.


BCDs consist of an inflatable vest (or back-mounted bags [wings]) worn by the diver and attached to a gas supply from the regulator. The BCD allows the diver to adjust buoyancy underwater or helps bring the diver to the surface and/or support him or her there. The ability to change buoyancy allows the diver to hover in the water and adjust for any factor that causes density to increase (e.g. wetsuit compression, picking up a heavy object on the bottom).

Most BCDs can be inflated via a hose from the regulator. Some have a small separate air bottle that can also be used as an emergency air supply, although these are now rare. Several valves to release gas are fitted so the diver can reduce buoyancy by venting gas from the compensator.

Divers can lose control of their buoyancy while ascending. As the diver starts to ascend, the expanding gas in the BCD increases its lift and in turn increases the rate of ascent. Such a rapid, uncontrolled ascent can lead to a variety of diving medical problems including pulmonary barotrauma and DCS.

In the past, BCDs were also designed to float an unconscious diver face-up on the surface. However, with the current designs this useful benefit has been largely foregone.


A depth gauge, timer and a means of calculating decompression are needed if an unsupervised diver is operating in a depth or time zone where decompression stops may be needed. Electronic, mechanical and capillary gauges have been used as depth gauges by divers. Capillary gauges, although now rarely used, measure pressure by the reduction in volume of a gas bubble in a graduated capillary tube and were useful only at shallower depths. Most gauges record the maximum depth reached by the diver during the dive, an important feature for tracking decompression status if using tables. Although the modern digital gauges are relatively accurate, there can occasionally be problems (as there often were with mechanical gauges), and the need to check the accuracy of gauges is often overlooked. Faulty gauges have caused divers to develop DCS.


Dive computers use a depth (pressure) sensor, timer, microprocessor, display and various other features. They are encoded with a decompression algorithm – a set of mathematical equations designed to simulate the uptake and elimination of inert gas within a diver’s body. By sampling the depth and recalculating every few seconds, these computers enable dive times well beyond those permitted by tables on most dives, especially on multi-level and repetitive dives. Some of the more sophisticated models take into account ambient temperature and/or gas consumption, and some even measure heart rate (Figure 4.3). However, they can still only ‘guesstimate’ a diver’s actual saturation, and DCS remains a significant concern with computer users. In fact, most people diagnosed with DCS these days have been diving within the limits indicated as theoretically safe by their devices. Users are well advised to use more conservative limits than the ‘factory settings’. Some models enable the user to adjust the computer to more conservative modes.

Two of the more sophisticated current model recreational dive computers (a) Galileo Sol (Scubapro, USA); and (b) Vytec (Suunto, Finland).
Two of the more sophisticated current model recreational dive computers (a) Galileo Sol (Scubapro, USA); and (b) Vytec (Suunto, Finland).

Despite this, dive computers have revolutionized diving because of their flexibility and the vastly increased underwater times enabled. Possibly their greatest contribution to diving safety is the incorporation of ascent rate warnings to caution the wearer when he or she ascends faster than the recommended rate, which is usually substantially slower than traditional rates used with most decompression tables.


The role of this gauge is discussed earlier. The contents gauge indicates the pressure and, by extrapolation, the amount of gas remaining in the supply cylinder.


Because of the risks in diving, it is generally considered foolhardy to dive without some method of summoning assistance. Most commercial divers do this with an underwater telephone or signal line. Divers who do not want the encumbrance of a link to the surface can dive in pairs, commonly called a ‘buddy pair’. Each diver has the duty to aid the other if one gets into difficulty. The common problem in the use of the buddy system is attracting the attention of the buddy if he or she is looking elsewhere or if separation has occurred, whether intentional or otherwise.

Underwater audible signalling devices are commercially available and are useful in such circumstances. These are generally driven by breathing gas and are attached to the low-pressure hose in series with the BCD inflator.


Sometimes divers can be difficult to sight on the surface after a dive because of the sea conditions and/or divers surfacing distant from the boat, often swept away by current. This can lead to stranding of divers at sea for extended periods, with some lost forever.

Various devices are available to try to prevent this problem. Commonly used location devices include horns, whistles, mirrors, safety sausages and other surface marker buoys (SMBs). There are also commercially available electronic diver location devices. Some consist of a receiver and a number of transmitters. The receiver is located on the boat (or can be elsewhere), and individual transmitters are issued to divers. This system enables a charter operator to track its divers continuously. Suitable electronic position-indicating radio beacons (EPIRBs) have been developed or adapted for use by divers, and these are becoming more frequently used. One such device is shown in Figure 4.4. They can be especially helpful when diving in remote locations. However, rescue depends on adequate monitoring of distress signals, as well as the willingness and ability of local authorities to perform a search and rescue. This can be a problem in some developing countries.

Figure 4.4 Nautilus Lifeline, BC, Canada.
Figure 4.4 Nautilus Lifeline, BC, Canada.


A ‘mermaid’ line is attached to the stern of the boat and extends down-current. It aids recovery of divers when they surface downstream. (Some call this the ‘Jesus line’ as it saves sinners – i.e. divers who have erred and surfaced down-current from the dive boat!) This is not needed if a lifeline or pickup boat is being used, or if the current is insignificant.

A shot line is a weighted line that hangs down from the dive boat or from a buoy. It is often used to mark the dive site and as a descent and ascent line. It can also be the centre for a circular pattern search. It can be marked with depth markers that can be used to show the decompression stop depths. The diver can hold onto the line at the depth mark. A lazy shot line is a weighted line that does not reach the bottom and is used for decompression stops.

A lead line is often used to assist the diver on the surface. It leads from the stern of the boat to the anchor chain. It allows the diver, who has entered the water at the stern of the boat, to reach the anchor when the current is too strong to swim to it.

When diving in caves or some wrecks, a ‘guide line’ should be use. This is a continuous line to the entrance is needed so that it can be followed if the divers become disorientated or when visibility is lost because of torch failure or formation of an opaque cloud by disturbed silt. Each diver should be within arms reach of the main line.

Surface-Supply Breathing Apparatus

A diver can also use a demand valve with air supplied by a hose from the surface. This equipment, surface-supply breathing apparatus (SSBA), restricts the diver’s range and depth to the length of the air supply hose. Its advantages are that the diver is freed from the cumbersome air cylinders and the air supply can be as large as needed, instead of being restricted by the diver’s carrying capacity and need for mobility. The air for SSBA may be stored in large tanks or compressed as required. The use of a compressor, often called a ‘hookah’ system, is economically attractive because the air is compressed to a lower pressure than that required for storage tanks. However, the compressor needs to be reliable and there needs to be an observer to monitor the compressor during operation.

Two modified forms of SSBA have found support in some circles. In one, a small motor and air compressor are supported on a float on the surface. This apparatus supplies air to one or two divers. In the other, the divers tow a float that supports an air cylinder. An advantage of these systems is that, if the hoses are short, the divers are unable to reach the depth needed to develop decompression sickness (DCS). A significant problem is that the user has no indication of when the gas supply will fail. Therefore, it is prudent for the diver to carry a small bail-out cylinder and regulator. Also, some novice users may forget that they are still exposed to the other hazards of scuba diving, such as pulmonary barotrauma.

In some resort areas, these devices are hired by novices who have had no training and who may be medically unfit to dive. Such use should be controlled and monitored in a similar manner to normal scuba instruction and equipment hires.

Self-Contained Underwater Breathing Apparatus – Scuba

SCUBA demand valve (a) during inspiration and (b) exhalation.

The simplest form of breathing apparatus consisted of a gas source and a tap that the diver turned on to obtain each breath of air. This system was in use until the 1930s, but much of the diver’s time and concentration were taken up in operating the tap. In the most common breathing apparatus, the Aqualung or self-contained underwater breathing apparatus (scuba), the tap is replaced by a two-stage valve system. The flow of gas to the diver is triggered by the diver’s inspiratory effort and is closed by expiration or cessation of inspiration.

Figure 4.1 and Figure 4.2 (a) and (b) show the operating principles of a simple regulator and demand valve system. The air is stored in a cylinder at a maximum pressure that is determined by the design of the cylinder. For most cylinders this pressure, called the working pressure, is 160 to 300 bar (2300 to 4350 psi).

First stage reducer valve
Figure 4.1 First stage reducer valve: the gas escapes from the cylinder until the pressure above the piston increases to a level where the force on the piston can compress the spring, pushing the first stage valve seat down and shutting the gas flow off. The valve opens again when the pressure above the piston (and in the hose to the second stage valve) falls. This is normally because the diver has taken another breath.

The first stage of the valve system (see Figure 4.1) reduces the pressure from cylinder pressure to the equivalent of about 10 ATA greater than the pressure surrounding the diver, and it regulates its outlet pressure at this value. The valve is held open by the force of a spring until the pressure above the first stage piston builds up and forces the valve seal down on the seat, thus shutting off the gas. The first-stage valve opens and closes as gas is drawn from the system by the diver. In some regulators, the water can enter the water chamber and helps the spring to hold the valve open. In others, the ambient water pressure is transmitted indirectly. This adjustment of the supply pressure with water pressure is designed to prevent the flow decreasing as the diver descends.

When the diver inhales, he or she reduces the pressure in the mouthpiece chamber, or second-stage valve. As the diver does so, the diaphragm curves inward and depresses the lever (see Figure 4.2 (a)). The inlet valve opens and remains open until inhalation ceases. At this stage, the diaphragm moves back into the position shown in Figure 4.2 (b). The second-stage valve is usually called the demand valve.

SCUBA demand valve (a) during inspiration and (b) exhalation.
Figure 4.2 SCUBA demand valve (a) during inspiration and (b) exhalation. The arrows indicate air flow. During inspiration the diver decreases the pressure in the mouthpiece. This causes the diaphragm to curve in and tilt the air supply valve open. At the end of inspiration air continues to flow until the pressure in the mouthpiece equals the pressure in the water chamber, at this stage the diaphragm will return to the position shown in 4.2(b), and the air supply valve shuts. During exhalation the pressure in the mouthpiece is greater than that in the surrounding water. This pressure difference forces the exhaust valve open and allows exhaled air to escape. The purge button is used to trigger a flow of gas from the supply without the need to inhale from the regulator.

Expired gas passes out of the second stage through an exhaust valve. In the demand valve, gas flow increases with respiratory effort because the valve opens more, allowing the diver to breathe normally. The purge button allows the diver to open the inlet valve to force any water out of the regulator. The diver may need to do this if he or she takes the regulator from the mouth while underwater or if the seal around the mouthpiece is poor.

The scuba regulator is designed to provide the diver with a gas supply matched to his or her respiratory needs.

Most divers have little difficulty using scuba. However, when they first don it, the weight and bulk will make them awkward, and may aggravate back problems. In the water, the buoyancy of the set offsets its weight.

The diver’s lips should be sealed around the mouthpiece to prevent the entry of water. Water can enter through a hole in the mouthpiece if the mouthpiece is poorly attached or through the diaphragm or exhaust valve if either is faulty. A leak can generate an aerosol if the water reaches the inlet valve of the second-stage valve. The aerosol can cause distress to the diver and may sometimes cause a syndrome called salt water aspiration syndrome, or it may trigger other medical conditions such as asthma or possibly a cardiac dysrhythmia.

Another problem associated with demand valves is that they may cause pain in the temporomandibular joint. This condition is considered in Chapter 42.

In very cold water, the first stage of the regulator may ‘freeze up’. This occurs because the air cools as it passes through the first stage and can ice up with the piston frozen in the open position. The problem can be reduced by using a first stage that is designed for operation in cold water.

Because the first stage regulates the pressure to the second stage, the inspiratory effort required to cause a flow does not vary until the cylinder is almost empty. Then the pressure in the hose to the second stage falls and the flow decreases. The diver’s first warning that the cylinder is almost empty is increased resistance on inhalation. However, this warning may be minimal or absent with modern regulators.

Most divers have a console that includes a pressure gauge connected to the cylinder by a hose. Some modern systems transmit the cylinder pressure via radiofrequency signals rather than via a hose. The pressure gauge provides the diver with a measure of the remaining air supply. The contents of the cylinder are proportional to the pressure, so the gauge is often called the ‘contents gauge’. Divers tend to say they have 50 bar left, rather than the volume this represents. A major problem is that a diver who is entranced by the scenery, concentrating on a task or distracted may run out of air because he or she forgets to check the gauge. An audible low-air warning is incorporated into some systems and is valuable. A diver needs to ensure that he or she has a cylinder or cylinders with adequate gas supply for the planned dive, and an additional reserve.

A traditional and almost obsolete system to prevent divers from running out of air is a reserve valve. In operation it resembles a boiler safety valve; the air escapes to the diver until the cylinder pressure falls to the level at which the reserve valve seats. The remainder of the air can be released by pulling a lever that opens the reserve valve. One problem with this is that the valve lever may be inadvertently put into the ‘on’ position, causing the diver to use the reserve of gas without being aware of this. Another common problem is valve failure.

Snorkeling/Breath-Hold Diving Equipment

The simplest assembly of diving equipment is that used by snorkelers – a mask, snorkel and a pair of fins. In colder climates, a wetsuit may be added for thermal insulation and a weight belt to compensate for the buoyancy of the suit. In tropical waters, a ‘stinger suit’ provides not only a little thermal comfort but also some protection from box jellyfish and other stings.


A mask is needed to give the diver adequate vision underwater. The mask usually covers the eyes and nose. Traditionally, masks were made from rubber, although now most are made from silicone. The mask seals by pressing on the cheeks, forehead and under the nose with a soft silicone edge to prevent entry of water. Swimming goggles, which do not cover the nose, are not suitable for diving. The nose must be enclosed in the mask so that the diver can exhale into it to allow equalization of the pressure between the face and mask with the water environment. It should be possible to block the nostrils without disturbing the mask seal to enable the wearer to perform a Valsalva manoeuvre. Full-face masks that cover the mouth as well as the eyes and nose, or helmets that cover the entire head, are more commonly used by professional divers and are considered in the section on professional diving equipment.

The faceplate of the mask should be made from hardened glass. A diver with visual problems can choose from a selection of corrective lenses that are commercially available. These are designed to attach directly to certain masks.

Alternatively, prescription lenses can be ground and glued to a variety of masks. Ocular damage can occur if hard corneal lenses are used for diving (see Chapter 42). Certain contact lenses may be lost if the mask floods and the diver fails to, or is unable to, take preventive action. Some people with allergy problems react to the rubber of the mask, although this is rarely an issue with silicone.

All masks cause a restriction in vision. With most masks, the diver can see about one third of his or her normal visual field. The restriction is most marked when the diver tries to look down toward the feet. This restriction can be a danger if the diver becomes entangled. However, there are some masks available with a tilted lens to provide a better downward field of vision.

The more nervous beginner may find the visual restriction worrying and may possibly fear that there is a lurking predator just outside the field of vision. The visual field varies with the style of mask. Experimentation is also needed to find which mask gives a good seal, to minimize water entry. The diver needs to master a technique to expel water from the mask. If it is not learned and mastered, a leaking mask can become a major problem, sometimes leading to panic.


The typical snorkel is a tube, about 40 cm long and 2 cm in diameter, with a pre-moulded or creatable U-bend near the mouth end. A mouthpiece is fitted to allow the diver to grip the tube with the teeth and lips. The tube is positioned to pass upward near the wearer’s ear to enable him or her to breathe through the tube while floating on the surface and looking down. Any water in the snorkel should be expelled by forceful exhalation before the diver inhales through the snorkel.

Many attempts have been made to ‘improve’ the snorkel by lengthening it, adding valves, modifying its shape and some other means. There is little evidence of the success of most of these attempts.

All snorkels impose a restriction to breathing. A typical snorkel restricts the maximum breathing capacity to about 70 per cent of normal. The volume of the snorkel also increases the diver’s anatomical dead space. Because of this, increasing the diameter substantially to reduce the resistance is not a viable option. These problems add to the difficulties of a diver who may be struggling to cope with waves breaking over him or her (and into the snorkel) and a current that may force the diver to swim hard. There have also been anecdotal reports of divers inhaling foreign bodies that have previously lodged in the snorkel.


Fins (or flippers) are mechanical extensions of the feet. Fins allow the diver to swim faster and more efficiently, and they free the diver’s arms for other tasks. The fins are normally secured to the feet by straps or are moulded to fit the feet. Various attempts have been made to develop fins that give greater thrust with special shapes, valves, controlled flex, springs and materials, all competing for the diver’s dollar. Some of these fins can improve the thrust, but the wearer needs to become accustomed to them. Others have little effect.

Divers often get cramps, either in the foot or calf, if fins are the wrong size, if the diver has poor technique or if the diver has not used fins for an extended period. The loss of a fin may also cause problems for a diver, especially if he or she has to a swim against a current, or fails to attain appropriate orientation underwater or buoyancy on the surface.


Even without the buoyancy of a wetsuit, some divers require extra weights to submerge easily. The weights are made from lead, and most are moulded to thread onto a belt. Some weights are designed to fit into pouches, either on a belt or, for scuba divers, attached to a buoyancy compensator device (BCD). Whatever weighting mechanism is used needs to be fitted with a quick-release buckle or other mechanism to allow a diver to drop the weights quickly and so aid his or her return to, or enable the diver to remain on, the surface. The situations in which a quick-release buckle may not be fitted (or may be de-activated) are those where it would be dangerous to ascend, such as in caves where there is no air space above the water.

In some circumstances, it is necessary for a diver to ditch the weight belt to reach, or remain on, the surface in an emergency. Such situations include an emergency in which the scuba diver cannot inflate the BCD, for example, if the diver is out of breathing gas. Unfortunately, divers often fail to release the belt if they are in difficulty. The reason for this omission is not clear, but it is likely often the result of stress or panic. Adequate initial training and practice help to reinforce the skill so that it will become more automatic when required. It also needs to be reinforced periodically. Unfortunately, much of the current training fails to focus adequately on this important emergency drill.

An alternative drill of taking the belt off and holding it in one hand (preferably away from the body) is useful in some situations in which the diver is likely to become unconscious and inflating the BCD is not an option or may not be sufficient (e.g. when deep). In the event of unconsciousness, the belt will hopefully fall away, causing the diver to rise to the surface. Holding the belt away from the body should reduce the chance of entanglement with the diver if it is dropped.

In many fatal diving accidents the diver did not release his or her weights.

This basic free diving equipment is adequate for diving in shallow, relatively warm water. Experience with this gear is excellent training for a potential scuba diver. The diver can gain the basic skills without the extra complications caused by scuba gear. It allows a more realistic self-assessment of the desire to scuba dive and the subsequent rewards. With the confidence gained in snorkeling and breath-hold diving and the associated aquatic skills, the diver is also less likely to become as dependent on the breathing apparatus. In cold climates, a snorkel diver needs a suit to keep warm. Suits are discussed in Chapter 27.


About Diving Equipment

The first part of this chapter deals with the equipment used by most recreational divers. The more complex and unusual types of diving equipment that are used by technical, commercial or military diving operations are dealt with in the second part of the chapter. Attention is paid to the problems the equipment can cause, particularly for the student or novice. This is of importance in understanding the medical problems that are related to diving equipment. It may also help the reader to understand the stresses experienced by the novice diver.