Mixing your gases - Why would you bother? Part 2

 
The major drawbacks with air diving occur with increasing depth. At depths of less than 30m air is hard to beat for no other reason than for its ease of preparation (ie simply pumping it into a tank), its low cost and universal availability, and for the massive amount of data behind the decompression schedules that we follow. No particular preparation is required for the components of the breathing apparatus we use for air aside from the usual regular maintenance.

As we pass depths of 30m however air places considerable constraints on our diving. Nitrogen narcosis is more likely to become an issue although individual susceptibility makes it hard to put a strict depth limit on air diving from this point of view. Past 50m most divers are suffering significant narcosis which will decrease their ability to extricate themselves from a problem arising at depth. As a depth of 60-70m is reached, oxygen toxicity becomes an issue. A more practical problem that air diving poses at commonly dived depths (20-40m) is the decrease in no decompression limits (21m 35min, 30m 15min, 39m 8min).

These are the depths where EANx diving comes into its own by extending the NDLs by a useful amount. So for EANx 32%:

Alternatively some divers will use EANx and follow the shorter air NDLs for a perceived decrease in DCS risk. This is less valid as an approach as the old saying goes, "half of bugger all is still bugger all!". I.e. the tiny risk of DCS following the air tables is only made a little smaller by using nitrox. The trade-off for the longer bottom time using EANx is a significant increase in complexity of dive management/planning, equipment (some components need to be "oxygen clean" to avoid combustion), cost of air fills and a risk of oxygen toxicity at shallower depths than with air. No benefit from the point of view of narcosis is gained by breathing EANx. In summary, the main benefit of EANx is to prolong bottom time at shallow depths. It is not suitable for deep diving.

Gas mixing adds significant expense to a very expensive past-time! Analysers, whips and gauges cost big dollars. But aint it fun!

So which gas to use if we want to 'safely' go beyond the 37m recommended by the recreational agencies? The two problems which loom at increasing depth are inert gas narcosis, and oxygen toxicity:

Narcosis is combated by exchanging the narcotic gas nitrogen for the less narcotic gas helium. This can be in the form of heliair, trimix or heliox as discussed last month. The term "Equivalent Nitrogen Depth" or END has been coined to suggest what narcotic effect a particular strength of helium mix would have compared to that dive done on air. E.g. to plan a dive to 70m with an END or narcotic equivalent of 30m, one would use a trimix 17/50 (17% O2, 50% He, 33% N2). The 17% oxygen gives a PO2 on the surface of 0.17 which may be dangerously low, however at 70m this gives a PO2 of 1.36 which is within the safe limit for oxygen toxicity. To eliminate the risk of narcosis altogether one could replace all the N2 with helium, however this would be very expensive and may incur a larger decompression penalty than a N2 containing mix (AIR dive to 70m 20mins gives 85 mins total to surface after deco, Heliox 17/83 to 70m for 20mins gives 150mins total to surface after deco). So the use of helium in dive gases gives a significant improvement in narcosis, but may increase decompression penalties.

One can see that the complexities of such diving involve very advanced training, a thorough knowledge of the problems involved, not to mention a large outlay in time and money! However for the small group of divers to wish to dive longer, deeper or with more tanks than the Desert Fox, technical and mixed gas diving might well appeal!