Heliox Vs. Trimix

[Rainer]

Richard, are you sure you aren't confusing the NAUI term helitrox (note the "tr") and heliox. Everyone I know who uses the latter is referring to a mix of only helium and oxygen (i.e. what most commercial and military divers use on deep dives).

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

 

[rjack321]

Richard, are you sure you aren't confusing the NAUI term helitrox (note the "tr") and heliox. Everyone I know who uses the latter is referring to a mix of only helium and oxygen (i.e. what most commercial and military divers use on deep dives).

You're right, my bad. I completely spaced the special NAUI "tr"

Its all trimix to me, they breathe 20/80 heliox in the hospital and stuff but never in real life diving around here.

 

[rjack321]

I'm confused by your post. Whenever I see two percentages together, and they don't add up to 100%, I figure they are trimix. That's one of the reasons it's a bad habit, IMO, to call a 50% nitrox mix 50/50, as I assume it is a Heliox mix. Could you elaborate?

Around here the convention is O2%/He%/balance N2
26/17 = 26% O2, 17% He, 57% N2

50/50 would be 50%O2 and 50%He, which is used as a deco gas by some.

On SB I always refer to nitrox as EAN32, EAN50 etc to help avoid these vagueries

 

[Akimbo]

... 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)...

It would be interesting to understand how that convention evolved. What do recreational divers in your circle use a Trimix above 21% O2 for? Why not use EAN to save money and simplify logistics?

Occasionally I would hear the terms "lean" and "rich" used to indicate above or below 21% O2 mixes, which usually implied using a different compressor, but I never ran across this use for Heliox. Because we only used HeO2, the verbal shorthand was just the Oxygen percentage, like: 1½% for a 1½% Oxygen 98½% Helium mix. Written labels always spelled it out though. "Mix" or "Mixed Gas" always meant HeO2 to us.

There was talk of using He/N2/O2 Trimix offshore after a Helium price spike in the late 1970s but everybody just installed Helium reclaim systems, push-pull (surface-based eCCR) systems, and automatic mix-makers instead. Nobody had the tables and Trimix was too depth limited anyway. Comex in France has experimented with Hydrogen/Oxygen for really deep work, but I haven't heard of it making it into the field.

Edit:

You're right, my bad. I completely spaced the special NAUI "tr"...

I think I was writing this replay while you posted this — the media blames it on satellite delay, I'm just a slow typist.

 

[rjack321]

Sorry Akimbo NAUI's term is actually "helitrox" a silly bastardization that tripped me up.

"heliox" is still as you describe I just glossed over the "tr"

 

[Akimbo]

Sorry Akimbo NAUI's term is actually "helitrox" a silly bastardization that tripped me up.

"heliox" is still as you describe I just glossed over the "tr"

Completely understandable, I probably would have done the same. I am curious why anyone would u$e a Trimix above 21% rather than EAN. Is it an issue with available tables/decompression algorithms?

 

[Rainer]

It's simply an issue of wanting to reduce narcossis in the 90-130' range.

 

[Akimbo]

Heliox is an easier gas to manage as there is no nitrogen to balance with the helium and oxygen….

The way I understand it, the US Navy started Helium Oxygen research in the late 1920s/early 1930s. Helium was cheap and government owned, gas analysis required a chemistry lab, and nobody had a clue what tissue permeability rates were. EDU (Experimental Diving Unit) had to install pens in the Washington Navy Yard for all the goats who contributed dearly to the tables in our dive computers. I think it was motivated more by reluctance to add a third gas variable to the experiments than anything else.

By the time I came along, the show-stopper we always considered first was if we had proven tables for the mix. Logistical issues were easy to overcome. Very few military or commercial diving operations used manual partial pressure mixing for bulk gas by the late 1960s — it was delivered to the pier in quads or tube trailers by industrial gas suppliers.

The general practice I observed during my career for surface supplied operations was Oxygen was only verified and tagged when gas was delivered. After that, the gas rack operator only monitored pressures and sources. Oxygen monitors are all over the place on Sat systems, but tenders rarely got to touch them on surface supplied jobs.

 

[Akimbo]

It's simply an issue of wanting to reduce narcossis in the 90-130' range.

Really??? Seems like a lot of cost and trouble for a really small gain.

Granted that subjects were experienced Navy divers doing familiar tasks, but all the test results I have seen show narcosis to be so negligible above 130' that it was a non-issue. Performance changes didn't really begin to show up until deeper than 150'. Even then, all the pipe puzzles and knots were correctly completed; just more slowly, until around 200-220'. Knots almost always held, but parts to the puzzles began to disappear. I know tying a bowline requires my full concentration and at least twice as long at 250' on air.

I suppose tasks like mental math might be more telling. I have never seen visual acuity tests at depth in the water. I have seen 40+ year old dry chamber math tests that didn't show much effect above 130', but that is much easier than in the water. I wouldn't be surprised if these military tests were the basis for the recreational depth limit today.

 

[Rainer]

A stage of 25/25 or 30/30 is actually pretty cheap (especially relative to the travel, charter, and ancillary costs of diving).

If you're doing complex dives (overhead penetrations, long distance scooter runs, fighting currents or flow, calculating and tracking average depth and decompression requirements on the fly, etc) a clear head is nice. That you might decide to do the same dives on EAN is nothing more than a personal and team risk assessment decision.