Gas switch using air

Simon Mitchell said:
Hi John (I'm guessing),

See...

DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003

We made some estimates in relation to the sort of dive being discussed here.

You are absolutely on the money with your first sentence, and it is part of the issue that this discussion has ignored. It is He to N2 switches when a vulnerable tissue (specifically the vascular labyrinthe) is already supersaturated that maximises the risk, rather than the gas switches per se. That's why the term "isobaric" is not really correct in this setting. There is nothing isobaric about the circumstances underwhich these switches are made. We are in the process of ascending (which is not isobaric) and because of that there may be pre-existing supersaturation. A small transient increase in tissue supersaturation arising from a gas switch can be the "straw that breaks the camels back" under these circumstances.



In theory no, not really. Except if I really was on O/C I would bump the oxygen fractions to produce a PO2 as high as you would be happy to accept as a set point on a CCR. A CCR would take care of that. This is all fairly theoretical though. The plan seems somewhat impractical as an OC dive, and difficult to "regulate" on a CCR dive.

If i don't respond to any replies it is because I am on my way to Bikini!!! and may not have internet access for the next 12 days.

Simon M
Click to expand...

From the article I got some thoughts for better profile & mixes for a dive like:
Time spent around 100…120m 28min.
Bottom mix He 65% could be cut slowly to 55% During ascend from 120m to 100m. Extra 5min stop at 78m.
This would cut deco oblication and inner ear gas tension might be less supersaturated.
The article is just one study… but a great one I guess. Is it all we know about this issue?

I see no connection between step size issue and the article but just the idea to controll the rate of change from He to N2. Is there any deeper theory available behind the 0,5 bar rule? (It is not a rule anymore if you make the steps small enough).

Is there someone who knows some other ideas how to plan such a dive if mix management would not be any issue? Or what did I undestood wrong? CCR can not be automatially right.

I dive between the danger of long dives and IEDCS. I suppose it is possible to do it one hour more safe if I just would know how.

I really do not understand after all this how the “part of the question” mix list is of no big risk as it is so much less concervative than what CCR gives without any flushes?

Jukka
 
I don't think it is a rule, it is only a suggestion. I don't think much in the way of real fact is actually known.

I don't know why you would try stepping the gas as you suggest, it seems a little odd to me as the off-gassed helium is going into the loop and not into the water so comparison with OC seems flawed.

I don't have the analysis equipment to check what's going on...and for me at least no need to take an extra risk for a small extra saving.

I might explore what "at or near a ceiling" means if I get time.

Matt.
 
I'm another one who's anti-flush. I just do the deco. As they say: What are you going to do with the hour you save? Invent a cure for cancer?

Janos
 
Janos said:
I'm another one who's anti-flush. I just do the deco. As they say: What are you going to do with the hour you save? Invent a cure for cancer?

Janos
Click to expand...

If I rip my suit I might still survive if my deco oblication allows early surfacing.
Jukka
 
JukkaO said:
If I rip my suit I might still survive if my deco oblication allows early surfacing.
Jukka
Click to expand...

Anyone can cut tables for GF 45/140 for when your backside is in a sling and breathing any gas is better than water but you don't come across as talking about doing that so dare I say either do the deco or accept that Haldine et al did not get around to bending enough goats to answer your questions with scientific facts...

Deco is still more art than science....
 
JukkaO said:
I am just a clown trying to dig some art out of scientists.
Click to expand...

"Let me first say I am not going to entertain any discussion or argument on any of this with anyone who wants to read me ancient history. My stuff is based on what works, and I have zero interest in what anyone else things..."
 
matthewoutram said:
I don't think it is a rule, it is only a suggestion. I don't think much in the way of real fact is actually known.

I don't know why you would try stepping the gas as you suggest, it seems a little odd to me as the off-gassed helium is going into the loop and not into the water so comparison with OC seems flawed.

I don't have the analysis equipment to check what's going on...and for me at least no need to take an extra risk for a small extra saving.

I might explore what "at or near a ceiling" means if I get time.

Matt.
Click to expand...

It is not about the rule or suggestion... there is of course some thinking behind that I am interested about.

I am interested about the theory. Practice is a step in the future perhaps.

There is nobody doing it today. I am just interested to know how it is. Before I plan to make it real.

“I might explore what "at or near a ceiling" means if I get time.” I guess in writing it is just what current planners suggest. DCS is around the corner always. We just do not know too exactly how far IEDCS is during ascend.

Please try not to put my thinking to reality for the next dive.
I hope there could be some space to think what might be the theoretical optimal situation.
If the difference would be minimal in practice, then it is easy to make a practical conclusion that there is nothing to achieve. Now there are only “I do not support the idea” comments. You do not need to support it. Just point my logic´s wrong and I stop disturbing readers. There is no info of my own. I just collect together what divers are doing and what planners etc. suggests.

Jukka
 
Jukka,

I have a genuine question / scenario for you to consider and I would like you to tell me what the dive computer should do / will think it's doing v's what will be in the loop therefore what your deco calculations are based on as I think this is one area even I need some help with.

Scenario: I descend to 100M on something suitable... say 10/70, I then do some bottom time, say 30 minutes. I then ascend to stop depths and work up to 30M where I decide to change the DIL and the dive comp to 21/30 and then work up to surface.

1 - If I don't loop flush what should the computer be using as the inert values in the loop?

2 - If I loop flush what should the computer be using as the inert values in the loop?

3 - How will it know the difference in either scenario?

4 - What would be the deco profiles in each case?

I don't want you trying the hard maths but I am keen to know how you think it should respond as that is both your logic I am interested in and also wanting to know how the dive comp will actually respond?
 
There are some curves in post 32 on page 4.

1. Upper curve describes the He content when only raising portion of the O2 takes some space from inerts. Off gassing would change the curve a but it does not change the deco profile much in that case as inerts are so high anyway.

2. The mix in the computer should be changed according the flush result. Full flush needs lots of gas and off gassing will change the situation soon. Not too accurate for calculation but some estimate could be used as the whole calculation is quite inaccurate. I am not thinking of such a flush as the IEDCS risk seems to be remarkable then. More effect and less risk between 120...100m.

3. I do not understand the question. If it is about off gassing we should find out more about off gassing or how to measure the mix.

4. Longer deco profile in post 32 describes CCR. Another one is telling how much shorter is the V-planner result with mixes changed along a stable END values. It is not realistic exactly... just to give the idea about the difference.

Jukka
 
JukkaO said:
There are some curves in post 32 on page 4.

1. Upper curve describes the He content when only raising portion of the O2 takes some space from inerts. Off gassing would change the curve a but it does not change the deco profile much in that case as inerts are so high anyway.

2. The mix in the computer should be changed according the flush result. Full flush needs lots of gas and off gassing will change the situation soon. Not too accurate for calculation but some estimate could be used as the whole calculation is quite inaccurate. I am not thinking of such a flush as the IEDCS risk seems to be remarkable then. More effect and less risk between 120...100m.

3. I do not understand the question. If it is about off gassing we should find out more about off gassing or how to measure the mix.

4. Longer deco profile in post 32 describes CCR. Another one is telling how much shorter is the V-planner result with mixes changed along a stable END values. It is not realistic exactly... just to give the idea about the difference.

Jukka
Click to expand...

My questions were a bit of a trick question as the reality of a DIL switch without a flush is that on an ideal ascent there is no new DIL used. More so just because you have told your computer that it's got a new DIL does not mean that the loop ratio matches that DIL. Your right the degrading DIL level drops as the O2 goes up but the problem you have is that HE and N2 behave differently and that the HE behaviour is far from known today hence why I keep rolling out the goats.

This variance in behaviour is significant unlike your response 1 as tissue tensions will change as a result of these 2 being swapped and although V planner may say you get out of the water quicker with less HE it won't tell you if your about to bend your middle ear doing so.

I guess the point I keep making is that you are surfing the gradient maths rather than tissue science and I am genuinely unsure / curious to know how something like a Shearwater can track In-Loop HE which is the one that will bite you if you go changing mixes to what is a 2 part unknown ratio on those 2 variables at very steep gradient ratios between the in loop mix and what ever else your going to throw on it.

I suspect now would be the time for a polite scenario based email to Bruce to see if he can comment on how the computer would treat it to know better?
 
Mostly I am interested what are the best mixes during ascend. How to get those in CCR is a possible step to take later if we first know what to aim.

Question 1 with high inerts during ascend.... I suppose He is so high anyway that the deco result is about the same as I (V-Planner) calculated. Relatively big errors at shallow do not change the calculation much.

Jukka
 
I'm not sure even how "good" the base VPM/Buhlmann algorithms are when we consider this type of optimisation.

Certainly they are proven for some value of proven, but I'd say for the dives we mention here (over 80m, long duration) gas optimisation based on those algorithms may not be practical.

Matt.
 
matthewoutram said:
I'm not sure even how "good" the base VPM/Buhlmann algorithms are when we consider this type of optimisation.

Certainly they are proven for some value of proven, but I'd say for the dives we mention here (over 80m, long duration) gas optimisation based on those algorithms may not be practical.

Matt.
Click to expand...

Not enough goats were bent to know how much you can push HE through it's own gradients reliably... Some people think that 25/125 is a safe HE gradient but no one has played with that and deliberate counter diffusion to find out how to end this discussion..

If only Ken Lorp was on this board..... :starwars:
 
Peter L said:
Not enough goats were bent to know how much you can push HE through it's own gradients reliably... Some people think that 25/125 is a safe HE gradient but no one has played with that and deliberate counter diffusion to find out how to end this discussion..

If only Ken Lorp was on this board..... :starwars:
Click to expand...

A friend of mine believes that the difference between He and N2 half-times is almost insignificant and therefore routinely changes gas to Air @ 42m without the flush and with no ill-effect.

If I must get out early then along with the high PO2 I'd ditch the 3m stop, but I never tried it, and I hope never to.

I'd prefer that to gas-switching.

Matt.
 
matthewoutram said:
A friend of mine believes that the difference between He and N2 half-times is almost insignificant and therefore routinely changes gas to Air @ 42m without the flush and with no ill-effect.

If I must get out early then along with the high PO2 I'd ditch the 3m stop, but I never tried it, and I hope never to.

I'd prefer that to gas-switching.

Matt.
Click to expand...

But a DIL swap without a flush introduces no net new N2 to the loop? He's skipping deco for the fun of it.

I so wish Ken was on here to talk with Jukka...... :)
 
Peter L said:
But a DIL swap without a flush introduces no net new N2 to the loop? He's skipping deco for the fun of it.

I so wish Ken was on here to talk with Jukka...... :)
Click to expand...

Yes. And I'd say this is better than doing the flush. Flush is evil. Deco is what you can get away with on the day.

Oh, and he doesn't swap to air, he doesn't even carry air. It's just the profile he trims.

Matt.
 
matthewoutram said:
Yes. And I'd say this is better than doing the flush. Flush is evil. Deco is what you can get away with on the day.

Oh, and he doesn't swap to air, he doesn't even carry air. It's just the profile he trims.

Matt.
Click to expand...

I could plug 100% O2 in on my DIL on that program, it makes as much sense and impact.

We all know HE is a fast gas and it's suspected that it's being handled conservatively even on a 100/100 GF v's N2 but the problem with a fast gas and high tissue tension is that you can get bendy just as quick if the ICD gradients are against you which is where dil flushes come to play.

I am much the same on your idea, surf the loop mix up, make it gradual by varying the one substance you have control on rather than taking pot luck on your actual deco mix. If you have big balls or a reason get to surface fast move the High GF above 100 ala what Shearwater will let you do rather than inventing some random way of fudging the deco obligation.
 
You must log in or register to reply here.
Back
Top