Trimix gas switching

[saturated]

I was just curious. How many of you switch to air and how many of you remain on trimix through out deco and why. Also if you could I know many planners give reduced deco time if you switch to air please list your planning software. I currently use pro-planner and do not switch to air. As the amount of helium in the loop is reduced on ascent;however, my deco(according to pro-planner would be reduced if I perform a gas switch to air). While I do carry staged air, it does'nt make sense to switch. I have always believed that any deco mix should not contain more nitrogen than the bottom mix because of solubility/permeability problems between helium and nitrogen.

Sorry I just did a dive with 4 other turtle divers and we were split on "gas switches".

This has also been posted on the inspiration owners board;but, nobody seems to be able to substantiate the reasoning with physiological fact. Please no I thinks, I would ect. We are only interested in physiological facts.

 

[rbdave]

Hi Saturated,

I'm unsure if I should post or not. Initially you ask for opinions but conclude with " We are only interested in physiological facts." ???

Anyway here is what I do. Instead of flushing with Air I flush with my standard onboard dil 25/25 which gets you out of the water faster than staying on bottom mix. I do the flush at 42 metres which keeps my END at about the same as my bottom mix.

I use V-planner. Of course you would be able to get out of the water faster if you did an air flush.

My apologies if this is irrelevent to the thread.

Cheers

Dave

 

[fins wake]

... cross-post you question on the 'Dr Deco' section of this board, Saturated ... :doctor:

The likelihood of you getting 'physiological facts' there may be stronger than on the Inspiration owner's list (although there actually are some pretty decent 'scientific types' there too).

I have always believed that any deco mix should not contain more nitrogen than the bottom mix because of solubility/permeability problems between helium and nitrogen.

Not quite sure what you mean? Is this another classic 'Trey musing'? As has been mentioned on the Dr Deco subsection of this forum a few times, commercial outfits use this type of gas-switching a lot. There would certainly appear to be solid theoretical reasons for doing so, certainly from a neo-Haldanean perspective.

The disadvantage, as I see it, is that if you do the switch too deep, you will end up heavily narked, by anecdotal evidence more narked than you would have been if you used the high-nitrogen mix as bottom gas in the first place. (I haven't tried this myself at depth, so I speak from hearsay. I think Exley's early experiments pointed to this, as well.)

I flush with my standard onboard dil 25/25 which gets you out of the water faster than staying on bottom mix.

How is the ppO2 of your 'bottom mix' markedly lower than in your diluent? Are you running very low set points at depth?

BTW, I use V-planner (VPM-B) myself, mainly.

 

[saturated]

sorry guys, what I meant was I dont want to here some guy thats never used a ccr telling us how it should be done. I just want to here the reasoning behind both ways. I am currently researching this, in fact I got so interested I'm taking a couple days off. I know I really should get a life.

Finswake
No I dont lower my po2 at depth, what I mean is the nitrogen content of the dilutent. N2 is heavier than He and thus saturates/desaturates slower. When replacing he with n2 a period of supersaturation occurs due to the slower desaturation of the N2.

Most of my experience with mixed gas has been military and commercial and primarily heliox. And it has been awhile so please bear with me.

I have indeed done gas switching to air, but only when my bottom mix was not breathable on the surface. For example a 45 minute dive to 85msw at a USN approved po2 of 1.4 would give me a He mix of 14/86, so I would dive air to 40'. However, even with a dilutent mix of 14/86 my ccr would automatically add the o2 necessary to keep my po2 at optimum levels on the surface.
but also allow my dilutent to be safe at my intended target depth.Should I need to go into scc mode for an emergency, ect.

As I stated above most of my experience is with heliox. We dove heliox with o2 decompression with great success. And a ccr in effect does the same thing;however pro-planner gives a sig break in deco times if a gas switch to air is performed. I have been going through all my formulas and cannot figure out the reasoning behind the difference.

Thanks in advance for any help, I've been working abit too much and am all but braindead. I hope this makes sense.

 

[saturated]

hey guys, going to post tidbits of info as I get it. Most info is obtained from the Navy dive manual(by the way if anyone would like I have copies of all Navy dive, Salvage ect. manuals in pdf converted) Pm me and will send you your request.

Ok starting with straight Heliox21/79 Vs. Air profiles.

sample dive to 130' for 20min bottom time.

Heliox

130' 20 min descent rate 60 fpm-Ascent rate 30 Fpm ppo2 .7ata

time to first stop. 4:00min 9min deco at 10' total ascent=13:20min

Air

130' 20 min descent rate 60 fpm-Ascent rate 30 Fpm ppo2 .7ata

time to first stop. 4:00min 6min deco at 10' total ascent=10:20min

Of course this does'nt say much in itself;however, if we look at the times we see that for a very modest dive we are penalized 3 minutes for diving heliox. I would conclude from this that on a modest dive the solubility is a factor. As HE is more soluable than N2, it would make sense that more HE would enter the tissues in this time frame, even though HE also desaturates quicker than N2. Since the absorbtion rate is quicker more HE would be absorbed, Thus more He would have to be removed. Thus the additional alotted time for deco on He. This does make sense to me.

What I am trying to figure out is the effects of breathing air on the desaturation of the He. Would the intruduction of N2, a much heavier gas, reduce the tissues abbility to rid itself of the He?


As I recall, the addition of nitrogen results in a time of supersaturation. I believe that the addition of a "heavier" gas N2 would also inhibit the desaturation of the Lighter gas(He).

If not then maybe a gas switch does make sense. Since Nitrogen is absorbed slower, and helium desaturates quicker, then maybe the air switch allows the desaturation of the helium without a quick saturation of N2. But I dont think this is the case. When I get closer to some books will do some research and post more info.

Sorry for rambling, I'm feeling dumber by the moment.

 

[fins wake]

No I dont lower my po2 at depth, what I mean is the nitrogen content of the dilutent

I know. This wasn't directed at you, but a question aimed at rbdave: I found - and still find - the remark on not "staying on the bottom mix" somewhat confusing.

When replacing he with n2 a period of supersaturation occurs due to the slower desaturation of the N2.

This is an interesting point and makes sense if we are talking about only one 'generic inert gas'. We are however talking about two different gases, each with its own saturation and de-saturation half-times.

however pro-planner gives a sig break in deco times if a gas switch to air is performed. I have been going through all my formulas and cannot figure out the reasoning behind the difference.

I think this is for the reason you yourself outline below.

As HE is more soluable than N2, it would make sense that more HE would enter the tissues in this time frame, even though HE also desaturates quicker than N2. Since the absorbtion rate is quicker more HE would be absorbed, Thus more He would have to be removed. Thus the additional alotted time for deco on He. This does make sense to me.

Me, too.

Would the intruduction of N2, a much heavier atom, reduce the tissues abbility to rid itself of the He?

Why should it? It's not the same gas.

Since Nitrogen is absorbed slower, and helium desaturates quicker, then maybe the air switch allows the desaturation of the helium without a quick saturation of N2.

Precisely how I'm thinking. And because you specifically dive heliox (not trimix), the pressure differentials would really work in your favour with air or nitrox decompression, although it should work reasonably well with trimix/heliair as well. But of course, I'm merely a well-read diver, not a hyperbaric specialist.

This thread would ideally be answered by Dr Deco himself ... :doctor:

 

[saturated]

Thanks fins, like I said I am braindead. I do remember something on the gas models involving N2 and He;however, that may have been 20 years ago......This has been a good post for me though. I have been diving the same way for a longgggg time. Heliox and O2. And to be honest its gotten to the point where it has'nt required much thought. Just trying to make sure I'm not caveman diving.....:wacko:

thanks for your input

 

[fins wake]

I do remember something on the gas models involving N2 and He;however, that may have been 20 years ago

You're absolutely right! Some 27 years ago Bill Hamilton published a paper on the effects, specifically of switching from heliox at depth to air during decompression and found it seemed to allow accelerated decompression with no superficial counterdiffusion problems. It is a well-established practice for deep heliox bounce diving (Hamilton 1976).

Just trying to make sure I'm not caveman diving

I don't think you are. On the contrary, relying on the established practices of the US Navy Diving Manual and diving the rebreathers you do, I'd argue the reverse, you're at the forefront.

 

[saturated]

Ok guys heres the answer(kind of). I finally got access to all my materials, and doc deco confirms. Uptake of N2 and He have no bearing whatsoever on each other. He saturates and desaturates at a rate 2.645 times the rate of N2. So by doing a gas switch to air you eliminate uptake of He and allow the desaturation of He to begin. Since N2 saturates/desaturates at approx 1/3 the rate as He your uptake of N2 during deco is minimal. So by switching to air the rapid loss of He begins while the slower uptake on N2 also begins......This explains the big break you get on planners for doing the switch...

I still have alot of math to do to show specific comparisons, but will post when completed....

Hey fins thanks for unconfusing me

 

[madmole]

Interestingly I've been talking today to someone involved in producing deco plans for 250 - 300m dives and they are amasing a load of evidence that their divers are getting bent if they swap back to Nitrox or air at 40m or so, Staying on the He doesn't do this

Nitrogen and He do interact, its a thing called isobaric counterdiffusion and causes such nasties as Vestibular bends (a bend almost exclusive to folks who swap from He to Nitrogen mixes)

I think the current models are wrong on deep dives and the difference between He and nitrogen. The newer ideas that these guys are proposing mean that in the future we will have models that get you out faster on He than on Nitrox or air. Its to do with the larger nitrogen moleclules attaching to the He bubble seeds. A small He bubble is mobile and diffuses quickly, once the N attaches it alters the physical properties of the bubble. A sort of RGBM plus

Time for another batch of goats to go diving methinks