heliox

[Iguana Don]

i hope this doesn't sound like a stupid question, but here it goes anyway- instead of filling your tank with "air" or a nitrox blend, why couldn't you fill with o2 & helium, therefore emiminating the nitrogen problem? unless it is cost preventative. i'm not a new diver but this has been on my mind & none of the people in my area can answe this question.

 

[DVG]

That is a fascinating question to which I hope you get an answer cause I'd like to know the answer also. There must be some disadvantage to having no N2?

 

[Sharkbait]

Mixing oxygen and helium as a breathing gas is called Heliox and is commonally used for deep commerical diving.

Tech divers often do a three way mix with oxygen, helium and nitrogen to keep a equivlant nitrogen depth (narcosis level) of 100 FSW. This mix is called Trimix and is very common in deep recreational diving.

Yes, they are expensive and require specialized training and equipment.

 

[Iguana Don]

Thanks for the reply Sharkbait.
Yes, I've read about tri-mix, but nobody has explained why you still add "air" with its nitrogen. couldn't you just add o2 & He & do away with all the problems associated with nitrogen. i'm not a tech diver, but would like an answer from one of you diving gurus.

[Edited by don on 09-06-2000 at 12:38 PM]

 

[Norm]

Hi Don. Two things to remember during this short note: 1. We are still learning about physics in diving and 2. No one knows everything about those physics. Now, trimix contains oxygen, nitrogen and helium. Heliox contains no nitrogen. Nitrogen is the dangerous mistress in our tanks when we dive air deep causing nitrogen narcosis. Nitrogen is also the culprit when get bent because our bodies do not metabolize nitrogen, only oxygen. We always 'thought' that we did not need the nitrogen in our breathing atmosphere, since we do not process it. (More on that later!) Helium comes out of solution very rapidly. The molecules in He are so small that helium can evern penetrate glass! It comes our of solution in our bodies the same way - easy and fast, thus the necessitated SLOW ascent rates when on mix. We vary the amount of helium and oxygen that we add and the amount of nitrogen that we remove from our mixes according to the depth to which we are going. The deeper we go the less oxygen we need, just as the nitrogen. Deeper dives, (some commercial diving), can use Heliox as no nitrogen is needed and the cooling effect of He can be offset by hotwater suits. Deco is also (often) under much more controlled conditions. Also, the huge amount of VERY expensive He needed is easier to offset. Now for the kicker: Only in the last few years have we discovered that DEEP divers do better if a tiny amount of nitrogen is added back to the mix. Why? They're still working on that one! I hope this helps. Norm

 

[Iguana Don]

Thanks guys:
I knew I could get an answer here, I Have some more questions, but let me absorb, (no pun intended) this first. Again thanks.

 

[Drew Sailbum]

Generally, heliox is not used other than in technical applications. Reasons are cost as well as the tendency for heilum to come out of solution faster than nitrogen. This requires the slower ascent rates which have already been mentioned. Helium also has a different thermal conductivity, and will cool the body more than a nitrox mix.

Other lesser concerns when using helium mixtures (particularly trimix) have to do with the ability to determine the exact proportion of helium in a mixture. It's easy to do if you fill the tank starting from empty, but more difficult if you want to top up a partial tank.

In VERY deep (600+ fsw) dives, a diver using heliox may experience HPNS - High Pressure Nervous Syndrome. Think - helium narcosis, but the exact mechanism of physiological action is not understood. Adding some nitrogen back to the mix seems to help remedy this problem

 

[blacknet]

Hello,

Isn't this the wrong forum for this type of thread?

Ed

 

[Rick Murchison]

Allow me to sum up...
The simple answer to your question has two parts.
1. It's expensive.
2. You have an *increased* risk of DCS when using He.
There are other more complicated things to consider... thermal properties, gas containment, even long-term metal embrittlement among others. But those two are the main reasons Helium just doesn't make sense for dives where narcosis isn't a factor - and why just adding enough Helium to keep Nitrogen exposure at a "no significant narcosis" level makes sense at moderately deep depths.
For example, if you want to keep your oxygen exposure at 1.4 ATM or below, and your equivalent Nitrogen exposure to 80 FSW or less, no Helium is called for for dives of 102 feet or less. (Use EAN34 at 100')

Rick

 

[Iguana Don]

Hello,

Ed,

This thread was started in September of last year, long before the "Technical Forum" inception.

But as you stated, it needs to be moved to there.

Thanks for pointing this out.

ID