Heliox Vs. Trimix

[w ripley]

I am curious why anyone would u$e a Trimix above 21% rather than EAN. Is it an issue with available tables/decompression algorithms?

There are some who will blend HE in their deco mixes to address isobaric counter diffusion. I know divers who are greatly concerned with ICD when diving very deep with extended bottom times. If I'm diving deep I will add HE to my 70 foot bailout, for example, as everything up to that point is usually 21% O2 or less. I've never had ICD issues, and know there is always a ICD debate going on in some diving forum about it, but it's they way I've been diving for years and figure a little HE buffer at the 70-foot stop isn't necessarily a bad idea.

 

[Ozgarr]

Well I thought the US Navy considered heliox any mix of helium and O2 and the term is used that way in commercial diving. But recreationally NAUI and others here in the States consider heliox any trimix with >normoxic levels of O2. Things like 26/17, 25/25, and 30/30 are all considered "heliox" here (althought the term is not common).

Trimix being the more generic term for any combo of O2/He/N2

Here's a tidbit from wiki:
The National Association of Underwater Instructors (NAUI) uses the term "helitrox" for hyperoxic 26/17 Trimix, i.e. 26% oxygen, 17% helium, 57% nitrogen. Helitrox requires decompression stops similar to Nitrox-I (EAN28) and has a maximum operating depth of 44 metres (144 ft), where it has an equivalent narcotic depth of 35 metres (115 ft). This allows diving throughout the usual recreational range, while decreasing decompression obligation and narcotic effects compared to air.[16]

Is that what you're talking about. Helitrox, not Heliox?

 

[Ozgarr]

Ahh... My apologies. Read the first page, and missed the answer in the middle. >.<

Ignore the last response.

 

[rjack321]

Ahh... My apologies. Read the first page, and missed the answer in the middle. >.<

Ignore the last response.

NP that stupid little "TR" mixing us all up lol.

 

[Blackwood]

It would be interesting to understand how that convention evolved.

I suspect it's a way to identify gases as being either hypoxic or not.

Some agencies use the term helitrox, others heliair, and still others triox to denote normoxic or hyperoxic trimixes.

Personally, if a gas is comprising of mostly of oxygen, helium and nitrogen, it's "trimix" to me. I see no value added by the norm/hyper specific terms. I just call out the mix (e.g. 18/45).

 

[ajduplessis]

You like the 220 range!!!

 

[lssebek1]

This is not from a Tech diver, just a curious guy who reads a lot... but you asked disadvantages of Heliox and Trimix...


As I understand it, the very small size of the helium molecule relative to the Nitrogen and Oxygen, which are both much further up the periodic table, provides it's own challenges regarding deco obligations.
Being so small it diffuses through the tissues much more rapidly than nitrogen does, and als comes out of saturation and bubbles more readily. So Ascents are much more a matter of controlling speeds and keeping within the ascent speeds plotted for the dive, and in an emergency there is a much shorter period on a crash surface to chamber ride in which the gasses bubble out and serious/irreversable damage can occur.

On the other hand, it is far less narcotic (not non-narcotic) than Nitrogen. Trimix is to Heilox as Nitrox is to Air. The percentage of Nitrogen means there is a lesser percentage of Heilum, so offgassing through the breathing is faster, same as breathing 100% O2 is sometimes used to offgas nitrogen loadings.




I'll be glad to be corrected by true Tech and Tri/Heilox divers... Always glad to learn

I am also not a tech diver, just an avid tech reader.

The understanding of decompression science underwent a dramatic revision a number of years ago thanks to decompression physiologist David Doolette, Ph.D. and the team at the Navy Experimental Diving Unit (NEDU). Their report, “Decompression from He-N2-O2 (TRIMIX) Bounce Dives Is Not More Efficient Than He-O2 (HELIOX) Bounce Dives,” published May, 2015, dispels a belief about mixed gas diving that has persisted since the NEDU developed the first working heliox tables nearly 80 years ago.

Their findings: A bounce diver’s decompression requirements depend solely upon the time, depth and level of oxygen (PO2) over the course of the dive regardless of the fraction of helium and or nitrogen used in the breathing mix. In other words the so-called “helium penalty,” i.e. the extra stops and decompression time required when breathing helium mixes on a surface-to-surface bounce dive, does not exist.

The corollary? Virtually all of the existing tables, dive computers and deco software, used by technical divers today, particularly the Buhlmann ZH-L16 algorithm which fixes nitrogen halftimes as 2.65 times longer than those of helium, have a legacy bias that adds increasing decompression time the higher the fraction of helium in the mix.

Unfortunately the existing decompression algorithm dogma is a huge ship to try to turn around, so it will probably take decades to update the existing algorithms based on this (relatively) new finding