So kind of like switching to an intermediate decompression trimix on OC ? The difficulty for me would be this : from the time you start to add air to the loop until the time you flush with 02 at 6m, you have no way of knowing what fraction of helium and nitrogen is in the loop. To allow for this accurately in your decompression planning is impossible. IMHO if you are going to change the makeup of your diluent it should be a full flush and proper planning.
Gas switch using air
- Thread starter JukkaO
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matt
Well-Known Member
matt
Well-Known Member
I'm still unclear on the benefit, BTW. Perhaps there isn't one and it's just a discussion - which is also fine!
Matt.
Less helium on the ascend would be better. How much less and how to manage it is the challenge. The portion is getting lower on ascend always but not enough without any flush.
Another question: What helium% the planners are using in calculations? It has been said here many times that it is not possible to know what it really is on ascend. But still I guess less is better.
Jukka
matt
Well-Known Member
Not sure that's either true or provable.
I'm now of the opinion that helium is my friend and therefore I keep on the mix all the way up for the reasons stated.
Matt.
matt
Well-Known Member
Not sure it applies physiologically regardless of the algorithm but absolutely no way to tell for sure!
As an OT, could someone with VPM desktop run the profile side-by-side with Buhlmann with that switch (full flush 60m I guess is the only practical way unless we work out a dozen gas switches breath-by-breath
) so we can see that difference?Matt.
It is quite impossible that the optimal mix is the one we get just by doing nothing to have it. If less is not good, should we add some He then? If we one day know, we can think how to use that info. Or as many of us have done we just try something and see how it goes. It just does not proof much as the whole deco theory is so inaccurate. I like to discuss first if there is someone already who knows. New ideas might also rise up when discussing. All the comments are very wellcome.
Jukka
matt
Well-Known Member
Approach from a different angle.
When you ascend using OC would you switch to gas with proper END for the depth?
Or would you like to keep He high during the whole ascend?
Here we have two OC calculations from V-Planner. 20min at 120m.
One represents how it goes with CCR without any flush. He is decreasing slowly.
Another is with diluents with optimal END for the depths.
Calculations are not exact but you get the idea about the difference. Both are OC to keep it simple and comparable.
One could cut 1,5h from 5h dive.
Off gassing of He is not included.
It is not only about comfort but shorter deco is more safe in many ways if you can do it with the same DCS risk.
Jukka
When you ascend using OC would you switch to gas with proper END for the depth?
Or would you like to keep He high during the whole ascend?
Here we have two OC calculations from V-Planner. 20min at 120m.
One represents how it goes with CCR without any flush. He is decreasing slowly.
Another is with diluents with optimal END for the depths.
Calculations are not exact but you get the idea about the difference. Both are OC to keep it simple and comparable.
One could cut 1,5h from 5h dive.
Off gassing of He is not included.
It is not only about comfort but shorter deco is more safe in many ways if you can do it with the same DCS risk.
Jukka
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matt
Well-Known Member
Yes - but that's not due to choice it is due to necessity & practicality. My carried choices OC are limited.
Dil=6/72, SP=1.3, GF=15/85, 120m, 20 mins:
Ascent
#1 SP=1.3, Dil=6/72, Total Time=210
#2 SP=1.5, Dil=6/72, Total Time=178
#3 SP=1.3, Dil=Air@63m, Total Time=153
#4 SP=1.5, Dil=Air@63m, Total Time=141
So we could save 69 mins by flush and higher SP. But I personally don't feel good after that, so I do not take those flushes anymore and settle for 32 minutes saving.
If I wanted more then I would shave without flush, perhaps as much as half the 37 minutes extra suggested saving.
I'm not sure you can, I feel quite bent on #3 & #4. YMMV.
Matt.
matt
Well-Known Member
Personally I have not dived OC to 120m, my deepest OC is around 80m due to the gas carried.
Do you consider 1.3 and 1.5 high ppO2? I use those all the time.
You say the move to air is dramatic; what would you use - we used to move first to 50% at 21m, is that more or less dramatic?
My only point is that I don't think moving off the gas onto N2 in CCR diving gives a decent result when diving deep.
Matt.
I try to find out the difference with different He contents in the mix. 1.1 and 1.3 that I used in calculations are conservative for most divers and good when comparing different He%. I also took off air brakes. O2 dosage is quite high already with 1.1 and 1.3. Air brakes had just made the calculations more complex to compare.
In the picture attached below I describe He% changes starting from 100m and the difference what happens in CCR just by tuning PPO2 and what could be done within the limits of reasonable END. Switch to air at 63m is out of space compared to what I am writing.
I am not trying to discuss what divers did on dives in the past but what might be the future.
Jukka
In the picture attached below I describe He% changes starting from 100m and the difference what happens in CCR just by tuning PPO2 and what could be done within the limits of reasonable END. Switch to air at 63m is out of space compared to what I am writing.
I am not trying to discuss what divers did on dives in the past but what might be the future.
Jukka
matt
Well-Known Member
I admit I did not fully follow your chart...what's the gas list?
I've also tried 20/30 flush at 63m, with the same result (I also tried air at 42m).
Matt.
Clare
Retired - going diving
Talked to some old wkpp guys about this. Jarrod spoke of switching to air deep as being like being hit between the eyes with a sledgehammer.
I don't really care that much about what I can get a theoretical model to do if I twist it. One benefit of CCR is the gradual changes it makes on ascent. Same reason my deep deco gases have helium in.
Thanks for all the comments received through different channels.
Comments about switching to air are misleading the discussion in this forum. There has been no suggestions or support to do that.
The drawings earlier are describing gradual change in He content. The idea was just to add small amounts or air to the loop but the whole method of changing the mix is not important in this phase. The big question is is it the best method to have more than 50% He in the loop still at 20m. I do not mean that the blue line is the way to go but it just gives the idea that it is a question about a big difference in the calculations.
I recognize two different views. Some think that planners are right and having less He in the loop during ascend allows less deco. Some others think that planners do not understand how He is leaving the body and it does not make the deco shorter if the He starts to off gas deeper. Some divers just cut the shallow deco alhtough planners does not.
Jukka
Comments about switching to air are misleading the discussion in this forum. There has been no suggestions or support to do that.
The drawings earlier are describing gradual change in He content. The idea was just to add small amounts or air to the loop but the whole method of changing the mix is not important in this phase. The big question is is it the best method to have more than 50% He in the loop still at 20m. I do not mean that the blue line is the way to go but it just gives the idea that it is a question about a big difference in the calculations.
I recognize two different views. Some think that planners are right and having less He in the loop during ascend allows less deco. Some others think that planners do not understand how He is leaving the body and it does not make the deco shorter if the He starts to off gas deeper. Some divers just cut the shallow deco alhtough planners does not.
Jukka
divetheworld
New Member
Matthew mentioned twice the risk of gas shifts on the compartments, ok it was subtle but no one seemed to pick up on it.
Making significant gas shifts close to your compartment limit runs a high risk of ICD. The anecdotal posts above give credence.
As a side note;
What can be surprising is the effect on computed deco when working with gases in some older computers. If you were running a 10/52 for example, at the last stop at 3m (sp 1.3) - some older computers would show different TTS if you changed to OC 100%.
Interesting how they handle inerts in the calcs. I bring this up in light of Matts post above with the loop mixes. Ponder away.
Making significant gas shifts close to your compartment limit runs a high risk of ICD. The anecdotal posts above give credence.
As a side note;
What can be surprising is the effect on computed deco when working with gases in some older computers. If you were running a 10/52 for example, at the last stop at 3m (sp 1.3) - some older computers would show different TTS if you changed to OC 100%.
Interesting how they handle inerts in the calcs. I bring this up in light of Matts post above with the loop mixes. Ponder away.
What do you mean with "significant gas shifts"? The blue line indicates such as from 70% to 60% at 100m and I think even smaller step is still very usefull. The curves are made with current V-Planner.
Jukka
divetheworld
New Member
Now we are talking inner ear ICD. Significant is not defined, its more of a sliding scale of risk.
However, a change of gas at a compartment limit will result in transient increases in total inert gas pressure. How much increase? Dunno, I've seem little empirical data or studies to comment.