Would heliox make as good a deco gas as nitrox?

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fnerg
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Would heliox make as good a deco gas as nitrox?

Post by fnerg »

I mean, besides the expense of it. Nitrogen dissolves directly into the blood, instead of binding to hemoglobin, right? So any gas that's not nitrogen, and has the right proportion of oxygen mixed in would work, you'd think.

Nobody uses heliox for decompression, so other than the expense are there other reasons?
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Joshua Smith
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Re: Would heliox make as good a deco gas as nitrox?

Post by Joshua Smith »

Some people use helium in their deco gas to avoid the largely misunderstood Isobaric Counter Diffusion. Other than that, what would be the point? Helium dissolves into tissues faster than nitrogen, and comes out of solution faster, too, which might not be a good thing for deco.
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fnerg
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Re: Would heliox make as good a deco gas as nitrox?

Post by fnerg »

The point would just be to lessen the risk of oxygen toxicity from grabbing the wrong bottle.
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Joshua Smith
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Re: Would heliox make as good a deco gas as nitrox?

Post by Joshua Smith »

fnerg wrote:The point would just be to lessen the risk of oxygen toxicity from grabbing the wrong bottle.
I dont understand how using heliox instead of nitrox could have anything to do with 02 tox.
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fnerg
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Re: Would heliox make as good a deco gas as nitrox?

Post by fnerg »

Nitrox 50 still has 50% nitrogen, so nitrogen in your blood diffuses into the breathing gas slower. 50% oxygen and 50% helium has no nitrogen, so maybe you'd offgas as fast as 100% with less risk of toxicity.
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Re: Would heliox make as good a deco gas as nitrox?

Post by fnerg »

Also, big disclaimer: I AM NOT EVER PLANNING ON DOING ANYTHING LIKE THIS EVER!

It just popped into mind, and I was curious.
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Joshua Smith
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Re: Would heliox make as good a deco gas as nitrox?

Post by Joshua Smith »

fnerg wrote:Nitrox 50 still has 50% nitrogen, so nitrogen in your blood diffuses into the breathing gas slower. 50% oxygen and 50% helium has no nitrogen, so maybe you'd offgas as fast as 100% with less risk of toxicity.
Current models suggest that this is not the case. I think I'll just stick with what I've been doing.
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Re: Would heliox make as good a deco gas as nitrox?

Post by fnerg »

I'll stick with what I've been taught too, but sometimes I start thinking of strange things, some involving pickled herring and bicycle tires.
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Jeff Pack
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Re: Would heliox make as good a deco gas as nitrox?

Post by Jeff Pack »

Mine usually involve a water buffalo :)
=============================================

- I got a good squirt in my mouth
- I would imagine that there would be a large amount of involuntary gagging
- I don't know about you but I'm not into swallowing it

CCR discussion on Caustic Cocktails.
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Re: Would heliox make as good a deco gas as nitrox?

Post by Joshua Smith »

fnerg wrote:I'll stick with what I've been taught too, but sometimes I start thinking of strange things, some involving pickled herring and bicycle tires.
That field remains strangely unexplored. Nat Geo turned down my request for grant money again this year.
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Re: Would heliox make as good a deco gas as nitrox?

Post by Grateful Diver »

Jeff Pack wrote:Mine usually involve a water buffalo :)
I'd be a little cautious about that one ...

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Re: Would heliox make as good a deco gas as nitrox?

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fnerg wrote:Nitrox 50 still has 50% nitrogen, so nitrogen in your blood diffuses into the breathing gas slower. 50% oxygen and 50% helium has no nitrogen, so maybe you'd offgas as fast as 100% with less risk of toxicity.
You'd offgas your N2 just as fast since the diffusion gradient would be as high as it could be. Trimix is a good intermediate gas choice (e.g. 50% O2, 25 or 50% helium, balance N2) for long decos like 4 hrs since you can then eliminate the helium quickly at your shallow 100% stop. For ocean diving its not typically used since we don't have long bottom times creating 70ft stops that are long enough to accumulate large amounts of N2 (from your deco gas) which you then have to eliminate still shallower in your deco.
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Re: Would heliox make as good a deco gas as nitrox?

Post by fnerg »

So then it wouldn't help enough for the added expense, and you still need to eliminate the dissolved helium with 100% o2 anyway. Got it.
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Re: Would heliox make as good a deco gas as nitrox?

Post by CaptnJack »

fnerg wrote:So then it wouldn't help enough for the added expense, and you still need to eliminate the dissolved helium with 100% o2 anyway. Got it.
The cost is slight. Its only advantageous when 1) the deco stops are >100ft END and you're trying to avoid deep deco stops with 'excess' END or 2) the intermediate deco stops are so long you are still on-gassing N2 and to shorten your overall deco you switch to trimix since on 100% you can get rid of helium faster than you can off-gas N2.
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Jeff Pack
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Re: Would heliox make as good a deco gas as nitrox?

Post by Jeff Pack »

Grateful Diver wrote:
Jeff Pack wrote:Mine usually involve a water buffalo :)
I'd be a little cautious about that one ...

Image

... Bob (Grateful Diver)
Sometimes the water buffalo gets pissed off though
=============================================

- I got a good squirt in my mouth
- I would imagine that there would be a large amount of involuntary gagging
- I don't know about you but I'm not into swallowing it

CCR discussion on Caustic Cocktails.
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kdupreez
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Re: Would heliox make as good a deco gas as nitrox?

Post by kdupreez »

To my understanding, theoretically, yes, you could use pure Oxygen/Helium mixes (Heliox) in leu of pure oxygen for decompression, especially if you on gassed non-helium mixes (air), because all that accelerated deco is doing, is creating an artificial gradient of the inert gas partial pressure to increase the rate at which it diffuses out of tissues. in this case, a heliox mix creates the same PPN2 gradient than 100% O2 would.. (except O2 is probably better for your body than He)

The problem is that because He is also an inert gas, you are now creating the opposite PPHe gradient, you will counter diffuse helium into your tissues (on gassing helium) so its not really what you want and you are trading one for the other.. especially bad idea since He diffuses at a much higher rate than N2 and could theoretically produce bubbles easier. So.. switching from a low to high He mix during deco is bad idea in my opinion.

As I understand it, you can and should use helium mixes as very effective deco mixes deeper where high oxygen percentages are not viable strategy (i.e oxygen toxicity) for raising PPN2 differentials and so by using helium you can do the same for efficient and early accelerated N2 off gassing and because helium diffusion rates are high, they will theoretically catch up when you eventually switch to Nitrox and maximize the He gradient again.

This only applies of course if you were diving Trimix as back gas to begin with and you want to employ a deco strategy to keep PPN2 gradients high to offgass that slower diffusing bad nitrogen boys first and effectively and keep the more reactive faster diffusing PPHe manageable till you hit 50% nitrox and accelerate He off gassing and when you hit 100% oxygen, accelerate both He and N2 offgassing.
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Re: Would heliox make as good a deco gas as nitrox?

Post by Argon »

In deep trimix diving, with bottom mixes of say 14/60, the majority of tissue loading at the beginning of the ascent will be with helium. So in pracitce, deco is usually accellerated by weaning off helium as soon as IBCD permits for the same reasons you proposed about heliox as a deco gas for offgassing nitrogen. In deep trimix diving, nitrox is the best deco gas.

I suppose if one were to incurr heavy deco from deep or very long air dives, where all the loading was from nitrogen, the situation might be differet. I know very little about cave diving elswhere, but here in the pacific nw, most tech diving that results in dives with so much deco that the CNS clock becomes an issue (when bottom gas/deco gas ppo2 of 1.4/1.6 rules are applied) are going to be deep trimix activities.

I have friends in the tropics that do 200ft air dives routinely that result in big deco. Here too though, practicality precludes the use of heliox for deco. they are doing deep air dives because helium is not available in their locales :)
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