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Although air has been the standard diving gas for many years, the military and occupational dive sectors have been using other gas mixtures for a long time. Technical divers (recreational divers who do deco diving, operate in overhead environments or use gases other than air), have taken up the use of "gas" over the last 20 years and now the recreational agencies are pushing "nitrox" as the only way to dive in certain situations. So what's all the buzz about? Is it for you?
 Gas mixing at Aquamarine, Vanuatu. Jason Blackwell mixing it up.
Firstly some definitions:
Air 21% oxygen 79% nitrogen (roughly)
Nitrox any other combination of O2 and N2
EANx Enriched Air Nitrox. Any O2 N2 mix of greater than 21% oxygen. Two commonly used mixtures tested initially by NOAA are 32% and 36% O2 (EANx 32, EANx 36)
Trimix A mixture of O2, He, and N2. Written e.g. Trimix 18/30 which means 18% oxygen, 30% helium and (implied 52% nitrogen).
Triox Basically trimix, but with 21% O2 or more.
Heliair A form of Trimix made by adding helium to air.
Heliox He plus O2.
Other gases which may be used in unique situations include hydrogen, argon, neon and xenon.
 The mix master himself; Richard Taylor from TDI.
Each of these gases (O2, N2 and He) has unique physical and biological properties which must be considered when planning their use.
Oxygen. Oxygen is the life sustaining element carried by the other inert gases I will discuss. At 1 atmosphere the 21% we breathe gives a partial pressure (PO2) of 0.21 bar. If we breathe less than about 0.16 bar we will suffer significant hypoxia, at greater than 1.6 bar we risk the effects of oxygen toxicity. At worst, this may manifest as fitting, unconsciousness and subsequent drowning. Hence the safe range for PO2 is not particularly wide. Using air, we can reach a PO2 of 1.6 at a depth of 66m (7.6 ATA). Breathing 100% O2 this occurs at only 6m. So in planning our maximum depth we must be mindful of these limits. The duration spent at high PO2 levels is also important. In addition, O2 may be equally narcotic to nitrogen so replacing nitrogen with oxygen will not relieve the narcosis associated with depth.
Nitrogen. An inert gas (not metabolically important) which is the primary gas implicated in DCI and "Nitrogen" narcosis. Martinis Law so often quoted (and so useless!) states that each 10m descended feels like drinking one martini. At extreme depth total loss of ordered thought and co-ordination ensues and the diver becomes unconscious or runs out of air and drowns. This narcotic effect is most likely related to an effect of some gases on cell membranes and correlates well with their lipid solubility (the values for O2 and N2 are very close). Gases with a lower lipid solubility have less narcotic effect (He, Neon, hydrogen), conversely a gas like Xe is 4 times worse (it is a good anaesthetic!).
When it's time to ascend we know we must allow time for nitrogen to "off gas" at an appropriately slow rate to avoid bubble formation and clinical DCS. The amount which has been taken up by the tissues depends on the PN2 breathed (percentage breathed ´ ambient pressure), the time at depth, the blood flow to the tissues (high for muscle, low for fat), and the lipid content of the tissue (low for muscle, high for fat). So diving with a gas mix containing less N2 will allow greater time at depth without decompression penalty.
Helium. Substituting N2 for He may offer significant advantages but also generates a new set of problems! Helium is expensive to buy and it is a finite resource. It is less lipid soluble than N2 and so its narcotic properties are effectively zero at even great depth (>300m). Its uptake into tissues is still significant however because of its small molecular weight (4 cf N2 which is 28) the final result of which is a greater decompression penalty at most depths compared with N2. The low density of helium decreases the work of breathing at depth which may help reduce the risk of CO2 retention, a danger which should not be underestimated in deep diving. The low density may also increase gas consumption however. Communication whilst breathing helium is difficult for those with full face masks due to its unique sound conduction properties (Donald Duck voice). Also helium's high thermal conductivity will allow significant respiratory heat loss in technical divers. At great depth it may cause a neurological excitatory phenomenon known as HPNS (High Pressure Nervous Syndrome).
 The golden rules of gas blending: check what you have got, trust no-one else!
This I hope provides some background into the properties of the various gases used. Next month I'll look at how and when divers use the different mixes. |
