Many training agencies are now moving away from deep air courses. GUE, IANTD, and TDI are all moving over to Trimix gases for diving beyond 30m. PADI & BSAC still respectively offer the Deep Diver and Extended range course, though ERD is being replaced in BSAC by the Sport Mixed Gas programme.

In my experience those students that want to dive safely beyond recreational depths are now electing to take either Advanced Nitrox or Advanced Recreational Trimix as their foundation course for diving to develop the necessary skills and training for this type of diving.

Trimix is a combination of three gases- oxygen, helium and nitrogen. The idea behind trimix is to displace nitrogen with helium so that the drawbacks of breathing high partial pressures of nitrogen- inert gas narcosis-can be reduced.Typically the air we breath is composed of about 20.9% oxygen, 79.1% nitrogen and .033% carbon dioxide plus various intert trace gases. This is fine until we start breathing air under water. As we go deeper the partial pressure of these gases increases and we begin to suffer side effects such as mental impairment, mild euphoria,task fixation and reduced intellectual capacity. Divers can experience these effects from depths as shallow as 25metres.

In order to combat inert gas narcosis we can utilise different gas mixtures to reduce our tissue loading of nitrogen and hence our level of narcosis. Nitrox is the gas of choice down to 30m as it extends no-decompression limits through the use of oxygen and reduced nitrogen tissue absorption. Beyond 30m nitrox has issues surrounding oxygen toxicity, for example utilising 32% nitrox the MOD at 1.4ppo2 is 34m. Most training agencies suggest that oxygen exposure should be kept to 1.4ppo2 for bottom gas in cold waters such as the UK where we may be working against the conditions.

If we are diving deeper than 30m then trimix has substantial advantages as the narcotic effects are nil. Helium is less dense and breathing resistance is significantly reduced and helium off-gases rapidly and its does not enter the slow tissues as readily as nitrogen. There are some disadvantages in that helium conducts heat 5 times faster than air and that ascents need to be carefully managed in order to comply with decompression requirements.

Deep air used to be the only solution before Trimix training become available. Nowadays there are better options for divers wishing to go beyond 30m. Inert Gas narcosis becomes an issue beyond 30m and although divers believe that they can develop a resistance to narcosis, in truth this is more fiction than fact. Regular deep air divers can get used to dealing with the reduction in mental capacity but the sad fact is that when they need to react to a potentially dangerous situation then inert gas narcosis can prevent them reacting quickly and correctly. Whilst training and basic muscle memory can help the air diver, a diver on a Trimix will always be better able to react due to the reduced level of narcosis.

Some agencies continue to offer deep air diver training without the requirement for the use of a twinset. If a diver calculates their own level of air consumption it soon becomes apparent that 3ltr ponies are insufficient to safely get a diver back to the surface should something go wrong with their air supply. Air consumption can rise to 100 litres per minute under stress, and sustained rates in excess of 35 litres per minute have been recorded. A typical pony has a total capacity of 600 litres and at 40 msw (5 bar) then the diver could be using 175 litres per minute whilst at depth. During the ascent the diver could maintain 35 litres per minute and in that event have insufficient air remaining to safely reach the surface. Using a twinset would enable the diver to enjoy a larger gas reserve if observing rule of thirds or minimum gas levels. Additional using Trimix would enable our diver to react more appropriately and seek the assistance of his team to rectify the problem.

With the availability of Trimix courses and training then today there is little need for deep air diving in the Uk. Granted in isolated areas without the benefit of Helium it may remain the only solution but in the UK divers are steadily moving toward the safer options of Trimix for beyond 30 msw.Whilst any dive beyond 30m carries a degree of risk by using the appropriate equipment and training together with the correct gas then divers are able to enjoy diving safely beyond these limits.

If you would like more information about developing you diving to take advantage of either Nitrox or Helium based mixtures then please contact me via email or the above telephone number