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
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.
BUOYANCY COMPENSATOR DEVICES
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.
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.
DIVER LOCATION DEVICES
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.
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.