Editing Heliox (section)

== Usage in diving ==
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{{Listen|right|filename=Helium article read with helium.ogg|title=Effect of helium on a human voice|description=The effect of helium on a human voice|format=[[Ogg]]}}

Helium diluted breathing gases are used to eliminate or reduce the effects of [[inert gas narcosis]], and to reduce [[work of breathing]] due to increased gas density at depth. From the 1960s saturation diving physiology studies were conducted with helium from {{convert|45|to|610|m|ft|abbr=on}} over several decades by a Hyperbaric Experimental Centre operated by the French company [[Compagnie maritime d'expertises|COMEX]] specializing in engineering and deep diving operations.<ref name="Hydra 8">{{cite web |url=http://www.comex.fr/suite/ceh/histo/histo%20anglais.html |title=Extreme Environment Engineering Departement Hyperbaric Experimental Centre - History |access-date=2009-02-22 |url-status=dead |archive-url=https://web.archive.org/web/20081005073910/http://www.comex.fr/suite/ceh/histo/histo%20anglais.html |archive-date=October 5, 2008 }}</ref> Owing to the expense of helium,<ref name="Fills">{{cite web |url=http://www.fillexpress.com/fills.shtml#prices |title=Example pricing for filling cylinders |access-date=2008-01-10 |archive-url=https://web.archive.org/web/20080116133910/http://fillexpress.com/fills.shtml#prices |archive-date=2008-01-16 |url-status=dead }}</ref> heliox is most likely to be used in deep [[saturation diving]]. It is also sometimes used by [[Technical diving|technical divers]], particularly those using [[Diving rebreather|rebreathers]], which conserve the breathing gas at depth much better than [[open circuit scuba]]. Use of heliox mixtures is known as heliox diving, a subs-category of mixed gas diving, also known simply as gas diving.<ref name="Lee et al 2020" >{{cite journal | vauthors = Lee DW, Jung SJ, Ju JS | title = The effects of heliox non-saturation diving on the cardiovascular system and cognitive functions | journal = Undersea & Hyperbaric Medicine | volume = 47 | issue = 1 | pages = 93–100 | date = First Quarter 2020 | pmid = 32176950 | doi = 10.22462/01.03.2020.10 }}</ref>

[[File:IMCA Heliox shoulder quartered.svg|thumb|[[Diving_cylinder#Surface_finish,_colour-coding_and_labeling|Heliox  diving cylinder color coding]]: Illustration of cylinder shoulder painted in brown and white quarters]]
[[File:IMCA Heliox shoulder.svg|thumb|Illustration of cylinder shoulder painted in brown (lower) and white (upper) bands]]

The proportion of oxygen in a diving mix depends on the maximum depth of the dive plan, but it is often [[wiktionary:hypoxic|hypoxic]] and may be less than 10%. Each mix is custom made using [[gas blending]] techniques, which often involve the use of [[booster pump]]s to achieve typical [[diving cylinder]] pressures of {{convert|200 to 300|bar|psi|abbr=on|lk=on}} from lower pressure banks of oxygen and helium cylinders.

Because [[sound]] travels faster in heliox than in air, [[human voice|voice]] [[formant]]s are raised, making divers' speech very high-pitched and hard to understand to people not used to it.<ref>{{cite journal | vauthors = Ackerman MJ, Maitland G | title = Calculation of the relative speed of sound in a gas mixture | journal = Undersea Biomedical Research | volume = 2 | issue = 4 | pages = 305–310 | date = December 1975 | pmid = 1226588 | url = http://archive.rubicon-foundation.org/2738 | access-date = 2008-07-08 | url-status = usurped | archive-url = https://web.archive.org/web/20110127113335/http://archive.rubicon-foundation.org/2738 | archive-date = 2011-01-27 }}</ref> Surface personnel often employ a piece of communications equipment called a "helium de-scrambler", which electronically lowers the pitch of the diver's voice as it is relayed through the communications gear, making it easier to understand.

[[Trimix (breathing gas)|Trimix]] is a less expensive alternative to heliox for deep diving, which uses only enough helium to limit narcosis and gas density to tolerable levels for the planned depth.<ref name=stone>{{cite journal | vauthors = Stone WC |year=1992 |title=The case for heliox: a matter of narcosis and economics. |journal=AquaCorps |volume=3 |issue=1 |pages=11–16 }}</ref> Trimix is often used in [[technical diving]], and is also sometimes used in [[professional diving]].

In 2015, the [[United States Navy Experimental Diving Unit]] showed that decompression from bounce dives using trimix is not more efficient  than dives on heliox.<ref name=NEDU2015-4>{{cite journal |vauthors=Doolette DJ, Gault KA, Gerth WA |year=2015 |title=Decompression from He-N2-O2 (trimix) bounce dives is not more efficient than from He-O2 (heliox) bounce dives. |journal=US Navy Experimental Diving Unit Technical Report 15-4 |url=http://archive.rubicon-foundation.org/10576 |access-date=2015-12-30 |archive-date=2017-07-07 |archive-url=https://web.archive.org/web/20170707084659/http://archive.rubicon-foundation.org/xmlui/handle/123456789/10576 |url-status=usurped }}</ref>