The composition of regular heliox gas used in scuba diving?
I would like to know the general composition of the heliox gas mixture used in scuba diving
any help would be appreciated
thanks
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Tags: composition, diving, heliox, regular, scuba, used
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Heliox is an artificial mixture of helium (He) and oxygen (O2), generally used as a breathing gas only for extreme diving depths. The exact fractions of He and O2 in the mixture are chosen according to the intended depth of use, to minimise the risk of O2 toxicity at high partial pressures (pO2 > 1.6 bar). e.g. for a dive to 100 m depth (330 ft, 11 bar ambient pressure), the O2 fraction would have to be ≤14.5% (pO2 = 0.145 x 11 = 1.595 bar), with He making up the other ~86%.
He is used because it is biologically inert, and unlike the other readily available inert gas (nitrogen, N2), does not have a severe narcotic penalty at high partial pressures. (N2 starts having a noticeable effect on performance at pN2 ≥ 2.8 bar, i.e. at ≥25 m/80 ft when diving on air). For deeper diving, He is simply used as a ‘filler’ gas, to reduce/replace the fraction of nitrogen (N2) in a gas mix, while keeping the O2 content below toxic levels.
Heliox is actually NOT generally used in SCUBA (self-contained underwater breathing apparatus) diving, since He is very expensive, and open-circuit SCUBA requires very large quantities of gas (relative to the small amount of O2 actually used by the diver). Use of heliox is thus generally restricted to commercial/saturation diving applications (e.g. oil rig divers), for which breathing gas is supplied through umbilical hoses from the surface or a diving bell, rather than being ‘self-contained’. Large petrochemical companies can easily afford to buy the large quantities of He required (indeed, much He is actually extracted from natural gas in the first place).
To put things in perspective, sport divers (up to about 40 m depth, 130 ft) generally use compressed air (79% N2, 21% O2), or nitrox/EANx (less N2, more O2, up to 40%) as breathing gas; due to O2 toxicity/N2 narcosis considerations, depths are limited accordingly.
To reduce the risks of narcosis/toxicity at depths >40 m or so, technical (decompression) divers replace some of the N2/O2 in their bottom gas with He, forming ‘trimix’ (or ‘heliair’, if the He has simply been used to dilute normal air). Some tech divers even advocate trimix for any dives >30 m (100 ft). Decompression stops will be done with high-O2 (≥50%) nitrox mixes (or 100% O2 for the shallow stops), to flush out the He and N2 absorbed during the bottom phase.
Although heliox would in theory be ‘better’ for tech-diving, the expense of such gas-fills puts it out of reach of most private individuals, at least for open-circuit divers. Closed-circuit (rebreather) divers do not need such large quantities of gas, and so may well choose to use heliox rather than trimix.
For the most part I agree with tjs282.
Heliox is usually used in close circuit re-breathers for depths deeper than 100 feet (usually limited to 600 feet) with limited or no decompression time . The down side of heliox is that it steal the heat from your body.