Dive Atlas of the World. Jack Jackson

from Nitrox tables, but have a greater safety factor if calculated from air tables. Many divers feel less fatigued after diving on Enriched Air Nitrox, though there is no scientific proof of this, and many experience a lower rate of gas consumption.

      Another way in which Nitrox can be used to divers’ advantage is that divers, who have been deep while breathing air or other gas mixtures, can shorten their decompression times at shallow depths by changing to a mixture containing 50–80 per cent oxygen. This mixture enables faster elimination of excess nitrogen (if using air) or helium (if using mixed gases). However, due to oxygen toxicity, the depths to which divers can descend depend on the percentage of oxygen in the Nitrox mix used. The higher the percentage of oxygen, the shallower will be the maximum depth to which they can go. Divers should not descend to depths where the partial pressure of oxygen exceeds 1.4ata. Atmospheres absolute (ata) is the sum of atmospheric pressure and the hydrostatic pressure – the total weight of water and air above us.

      There may be circumstances when a diver breathing Enriched Air Nitrox has to go deeper than oxygen toxicity allows on that particular Nitrox mixture. In this situation, if the diver has a separate small cylinder of normal air fitted with its own regulator, it is possible to switch to breathing from this cylinder for a brief foray deeper than the depth allowed on the Nitrox mixture. The diver can then switch back to breathing Enriched Air Nitrox after returning to a depth where oxygen toxicity is no longer a problem.

      Another problem with oxygen breathed at higher than normal partial pressures, is that when used over long periods it affects the central nervous system. Divers must be careful not to exceed the recommended oxygen tolerance units (OTUs), particularly on repetitive dives.

      High concentrations of oxygen cause combustion on contact with oils and greases. Scuba cylinders and their valves come in contact with pure oxygen during filling, so they must be scrupulously clean. Standard regulators should be suitable for Nitrox mixtures of less than 40 per cent oxygen, but for higher concentrations, their O-rings must be replaced with ones that do not require lubricants.

      HELIOX AND TRIMIX

      For deeper diving, one must lower the oxygen content to reduce oxygen toxicity as well as reducing the nitrogen content. This is done by replacing some of the nitrogen with helium (Trimix) or all the nitrogen with helium (Heliox). Helium has the advantage of reducing problems with Nitrogen Narcosis, but gives no advantage with decompression times. It is a lighter element than nitrogen and more of it is absorbed by the body, which then has to be eliminated on ascent. It also conducts heat away from the body more quickly during respiration.

      As divers go deeper they must reduce the oxygen content still further. There is almost an optimum mix for each depth. Divers use a ‘travel-mix’ suitable for breathing from the surface down to a calculated depth and then switch over to a ‘bottom-mix’ with an even lower oxygen content. However, bottom-mixes have too low an oxygen content to be breathed safely at shallower depths. During ascent, there will be a depth at which the divers must switch back to the travel-mix; shallow decompression stops will be shorter if they switch to mixes high in oxygen when close to the surface. Deep dives using Heliox or Trimix involve several clearly-marked cylinders of different gas mixtures and the diver has to identify the correct regulator attached to the correct cylinder, for each phase of the dive. This has led to the development of modern rebreathers, in which the gas mixture can be modified as one changes depth.

       Divers pass the entrance of a cavern in the Amirantes in the Indian Ocean. Black corals are often found in the dim light under overhangs.

      REBREATHERS

      The acronym SCUBA stands for Self-Contained Underwater Breathing Apparatus. With traditional SCUBA we waste most of the oxygen we breathe by exhaling into the water. This is termed an open-circuit system. Some companies have modernized, closed or semi-closed circuit scuba equipment for recreational use and these are termed rebreathers. When using rebreathers, divers breathe a gas mixture containing oxygen and when they exhale, the carbon dioxide in their exhaled gases is chemically removed by passing the gases through Soda-Lime. The Soda-Lime is referred to as a scrubber. Some additional oxygen is added to the cleaned, exhaled gases and that mixture is breathed again, hence the name rebreather. The closed-circuit system does not dump any gas into the water until the diver ascends, while the semi-closed circuit system only dumps a small portion of each exhalation. In this way divers get long diving times out of a relatively small amount of breathing gas.

      Rebreathers can be based on Nitrox or, for deeper diving, Trimix or Heliox. They require considerable maintenance and a constant eye must be kept on gauges to ensure that everything is working correctly.

      TRAVELLING TO DIVE

      The terrorist attacks on the USA of 11 September 2001 mean that the security for air travel will never be the same again, with underwater photographers and divers being the hardest hit. Many airlines now have strict rules and limits on checked and carry-on baggage.

      Apart from the necessary paperwork, passport, visa, vaccinations/health certificates, travel and diving insurance, necessary prescription medications and ‘C’-card, the most important part of travelling to dive is to have these and other essentials such as cameras, film or memory cards, diving computers, prescription masks and spectacles in your carry-on baggage. Flights are notorious for losing or delaying baggage.

      Your passport should be valid for six months longer than the expected duration of the trip and have at least six empty pages. If you carry local banknotes, these should be clean and unmarked. Have photocopies of all paperwork, passport photographs for local permits and your driving license if intending to hire a vehicle.

      Keeping diving equipment within most airlines’ check-in baggage limit of 20kg (44 lb) is a problem. Some airlines will allow an extra 10kg (22 lb) for divers on presentation of a C-card. American airlines have more sensible baggage limits based on size.

       DIVE PLANNING – ‘PLAN THE DIVE AND DIVE THE PLAN’

      All dives should be planned. The leader of the dive should give a detailed briefing that covers the expected time in the water, what the current is doing, what should be seen on the dive, what depth divers should expect to dive to and when they should ascend.

      However, divers should also take into account their own health and fitness, and normal rate of air consumption. They should consider the depths and times of their last few dives, the surface intervals between them and the time that has elapsed since the last dive. There is now some debate over whether the first dive of the day should always be the deepest and all other dives on the same day progressively shallower, but it is best to keep to the standard practice.

      Diving bags or rucksacks are preferred for easy stowage by live-aboard boat skippers, but they attract attention as containing expensive equipment and do not stand up well to airport baggage handlers. Pelican cases are also prime targets for airport thieves. Shabby cases are a better option.

      Check out what equipment is available for rent at your destination so that you can minimize your checked baggage. However, remember that at Third World destinations rental equipment may be in poor condition and fins and wet suits are often too small for large Caucasians.

      TRAVELLING DIVER’S CHECK LIST

      ■ Clothes and wash kit for surface use

      ■ Mask, snorkel and fins (either full-foot or adjustable with bootees)

      ■ Regulator with contents gauge (manometer) and alternate air supply

      ■ Buoyancy Compensator Device (stabilizing jacket)

      ■ Weight belt and weights if not