Editing Trimix (breathing gas) (section)

==="Standard" mixes===
Although theoretically trimix can be blended with almost any combination of helium and oxygen, a number of "standard" mixes have evolved (such as 21/35, 18/45 and 15/55—see [[#Naming conventions|''Naming conventions'']]). Most of these mixes originated from starting by decanting a given pressure of helium into an empty cylinder, and then topping up the mix with 32% nitrox. The "standard" mixes evolved because of three coinciding factors — the desire to keep the [[equivalent narcotic depth]] (END) of the mix at approximately {{convert|34|m|ft}}, the requirement to keep the partial pressure of oxygen at 1.4 ATA or below at the deepest point of the dive, and the fact that many dive shops stored standard 32% nitrox in banks, which simplifies mixing.<ref name="TDISDI manual" /> The use of standard mixes makes it relatively easy to top up diving cylinders after a dive using residual mix — only helium and banked nitrox are needed to top up the residual gas from the last fill. This can be also be applied by using a compressor with a [[Nitrox#production|membrane separation]] system or [[Nitrox#Production|continuous blending]] set for a Nitrox32 output.<ref name="Coenen and Zivkovic 2015" />

The method of mixing a known nitrox mix with helium allows analysis of the fractions of each gas using only an oxygen analyser, since the ratio of the oxygen fraction in the final mix to the oxygen fraction in the initial nitrox gives the fraction of nitrox in the final mix, hence the fractions of the three components are easily calculated. It is demonstrably true that the END of a nitrox-helium mixture at its [[maximum operating depth]] (MOD) is equal to the MOD of the nitrox alone.<ref name="Coenen and Zivkovic 2015" />

====Heliair====
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'''Heliair''' is a [[breathing gas]] consisting of mixture of [[oxygen]], [[nitrogen]] and [[helium]] and is often used during the deep phase of dives carried out using [[technical diving]] techniques. This term, first used by [[Sheck Exley]],<ref name="bowen" /> is mostly used by [[Technical Diving International]] (TDI).

It is easily [[gas blending|blended]] from helium and [[air]] and so has a fixed 21:79 ratio of oxygen to nitrogen with the balance consisting of a variable amount of helium. It is sometimes referred to as "poor man's trimix",<ref name="bowen" /><ref name="gentile" /> because it is much easier to blend than trimix blends with variable oxygen content, since all that is required is to insert the requisite [[partial pressure]] of helium, and then top up with air from a conventional compressor. The more complicated (and dangerous) step of adding pure oxygen at pressure required to blend trimix is absent when blending heliair.

Heliair blends are similar to the standard Trimix blends made with helium and Nitrox 32, but with a deeper END at MOD.
Heliair will always have less than 21% oxygen, and will be hypoxic (less than 17% oxygen) for mixes with more than 20% helium.