Decompressing under GF ceiling rather than set depth?

A recent (very interesting) SB thread about oxtox, CNS clock and OTUs made me think about how I deco on O2. Usually, once I switch to O2, I finish my 20 ft stop and many times even stay there for my 10 ft stop then go up. Occasionally I get to 90% CNS clock and even higher. I always take backgas (usually 32%) breaks if more than 15 minutes continuously on O2. I never had any issues but that doesn't mean much. I use Shearwater computers and they do give me the ceiling for my chosen GF. I am considering following the GF ceiling rather than the depth stop, which would move me out of the 1.6 PPO2 pretty quickly and reduce the O2 exposure (thus CNS clock, OTUs and the risk for oxtox). Shearwater allows you to do so once acknowledging the blown stop depth (don't know about other computers).
Has anybody done so routinely in order to decrease their O2 exposure?

Old article,but some relevance http://www.wkpp.org/?page_id=2594
The Wkpp regularly pushed their CNS clock well beyond 100%, and never did 10ft stops, choosing the partial pressure benefit of doing it all at 20ft. If you dig up some of their old dive reports, rationale for what they did would be imbedded in there.

I think there’s merit to your proposed strategy.

However...

The strategy of low bottom po2s and frequent gas breaks for sure works. Like Kelly said, guys have been racking up downright preposterous values on the “CNS Clock” for years without incident. I did a dive that ended with a CNS Percentage of 1,036 back in July. That’s just one anecdote but there are many similar examples.

I try to replicate what other successful folks have been doing. This seems to work well.

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Originally Posted by PfcAJ

I did a dive that ended with a CNS Percentage of 1,036 back in July. That’s just one anecdote but there are many similar examples.

I try to replicate what other successful folks have been doing. This seems to work well.

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LOL AJ, don't think I've ever done a 10 ft stop and I have a few deco dives... ;)

Safe diving,

Rich

I've experimented with 6m (20ft) last stop and following the ceiling up, but it's the constant depth alarm is rather distracting and I don't really like the idea of a constant alarm being part of planned procedure.

You mentioned both staying at 20ft for your 10ft stop and following the ceiling up - they're kind of incompatible. Have the last stop set to 10ft and you can still stay at 20ft if you feel it's necessary. Did you plan on following the ceiling for the last 10ft?

Kelly and AJ, that?s about what I?ve been doing and thinking, good to know others are doing the same and have done so many many times with no repercussions. Thanks!

mrki, what I was asking was if anybody does their deco following the GF ceiling rather than the customary 20 ft and 10 ft stops, in order to minimize the 1.6 ppo2 at 20 ft. I personally don?t, and many times finish all my deco at 20 ft. However, in places where is easy to move up and keep a shallower depth (like JB for example), I do move up to finish deco there. Sorry my post wasn?t clear but I wasn?t thinking following the GF ceiling literally every foot to surface, just getting out of the 1.6 ppo2.

ONCE and only once, I followed the GF ceiling exactly from 50' up. This was only an attempt to keep my deco pressure differential higher and get my arse out of the water. The dive was planned to be 5 hour and ended up being 9 hour. I did the entire 10' stop at 10' until clear.

Full disclosre - I did end up with some pretty bad shoulder pain afterwards.

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I have heard of people riding the GF to the surface, but not for the purpose of minimizing O2 exposure, but rather, to minimize total deco time. The argument given was that 10 foot increments for stops is arbitrary. If you accept that your GF ceiling is your accepted deco stress risk limit, you are OK so long as you stay below it. Of course, it's hard to discuss the safety of this approach without having a discussion regarding your chosen GF settings...and what is "safe" here will always be variable from person to person.



On deep dives with very long deco runs, this can produce some VERY impressive reductions of total deco time. I haven't heard of anyone doing this having any issues as compared to their usual deco routines, other than what Dsix mentioned above. I personally haven't tried this - I can't mentally get around clearing the missed deco stop alerts.


I imagine that people who are employing this strategy are well accustomed to seeing CNS numbers several times higher than 100% on dives. I am not doing near the dives that these folks are doing, and I routinely crush CNS on my rebreather. If I end up doing a 10 foot stop, it's usually because I am looking to get out of the water sooner, with the lower pO2 for deco as an added bonus. I find that the location of the dive also greatly impacts if I am doing a 10 foot stop. At ginnie, I do most of my 10 foot stop at 20 feet, because I like that little alcove I can park myself in at 20 feet. At eagles nest, though, you can bet I'll be lying on the log at 10 feet, all while hoping to not see any gators.

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Originally Posted by TotDoc

. If I end up doing a 10 foot stop, it's usually because I am looking to get out of the water sooner, with the lower pO2 for deco as an added bonus

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I dont buy into the concept that by staying at 10' and thus shortening the overall Deco time one is really better off than with staying they same shortened time at 20'
This is die to the fact that I beoieve that the steuer gradient is in fact negated by the lower PO2 and reduction of Oxygen Windows.
Of course staying within the model it shows different results but then always keep in mind that it is exactly that.. only a model and not real physioligy. ;)

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Originally Posted by Nitrogenius

only a model and not real physioligy. ;)

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All I know is that doing the 10 foot stop gets me out of the water faster, without needing to click off that I didn't complete a stop that my computer wanted me to do. I am sure I will never know enough details about the models to know if it's OK to short change my 20 foot stop or not, but I presume there is a reason it is calculated the way that it is.

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Originally Posted by LMN

mrki, what I was asking was if anybody does their deco following the GF ceiling rather than the customary 20 ft and 10 ft stops, in order to minimize the 1.6 ppo2 at 20 ft. I personally don?t, and many times finish all my deco at 20 ft. However, in places where is easy to move up and keep a shallower depth (like JB for example), I do move up to finish deco there. Sorry my post wasn?t clear but I wasn?t thinking following the GF ceiling literally every foot to surface, just getting out of the 1.6 ppo2.

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I was suggesting that setting last stop to 10ft and then having the choice to finish deco at either 10 or 20ft was (at least in my case) a beter alternative to having it set to 20ft and then watching those blinking red digits for the last stop.

After a few runs involving hours of 2.8Bar O2 with 5 minute air breaks after every 20 minutes of O2, I've kind of lost any fear of doing Deco at 1.6 O2.
Sure I would not do a long dive at Ginnie on a 36% because others have tried it and died, but doing deco on 1.6 or for short periods higher is not something that I feel uncomfortable with.
What I don't like are shoulder, elbow and hip pains along with skin bends or exaustion, so if I have to run the O2 a little bit higher in order to feel great after the dive that's ok.

Michael

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Originally Posted by TotDoc

All I know is that doing the 10 foot stop gets me out of the water faster, without needing to click off that I didn't complete a stop that my computer wanted me to do. I am sure I will never know enough details about the models to know if it's OK to short change my 20 foot stop or not, but I presume there is a reason it is calculated the way that it is.

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That's why I meant within the model.. That is what your computer is showing. The question I have is really if especially to that question hi PO2 + lower gradient or lower PO2 and steeper gradient is the most efficient way to spend a given time for deco.


Your computer would also let you out of the water earlier if you would instead of going to 10' at that moment e.g. change your GF hi..


The real question is: If you run into constraints that you can only spend a set amount of time between 20' and surface on oxygen where and how is that exact same time spend best from a decompression perspective. The models answer is easy that would be as soon as possible get to 10' but is that true in reality given you loose PO2 ? I doubt it.
On the other hand the question would be, what is your constraint: it has to be something like thermal stress or exhaustion etc., cause if it is gas on OC there would be additional incentive to get to 10' as you will be able to increase overall deco time that way..


Intersting one to look at from comparing it by exact same time.. Maybe some of those heat maps from the infamous deep stop discussion several years ago on RBW could give some insight. It would still stay "within" the model, but it would at least show the stress on ALL compartments, while the computer will only always have assessed on the leading compartment..

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Originally Posted by Nitrogenius

Maybe some of those heat maps from the infamous deep stop discussion several years ago on RBW could give some insight. It would still stay "within" the model, but it would at least show the stress on ALL compartments, while the computer will only always have assessed on the leading compartment..

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Maybe...but you can also see the saturation of the compartments used in the computer calculation on the shearwater...it's able to be selected during the dive, and is a cool thing to watch during deco, especially after a deep dive like eagles nest, where you really get some loading in the slower tissue compartments.



This obviously provides no clear answer to the question at hand, but I didn't know if you were aware the computer had that feature or not


Here's the Shearwater video of the graph:

https://www.youtube.com/watch?v=O81vX79X_mA

Food for thought:

I understand that there are several documented cases of O2 seizures at 20'. Are there any documented cases at 10'? Is there any chance that the lessened depth (read as water pressure) would have any significant impact of it. I know that on the surface one can get away with a lot more, but is this only because of immersion and just having your face wet would have the same effect? The PO2 would be lower but it would still be O2.

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Originally Posted by TotDoc

This obviously provides no clear answer to the question at hand, but I didn't know if you were aware the computer had that feature or not

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I am aware. Thanks. While this provides you some info in that regards it does not really Provider the same insight as those heatmaps in the sense of "overall" stress..

All, I routinely race tractors with my neighbor and good friend of many years, Erik Baker since neither of us can cave dive anymore. He has forgotten more about deco than most people know. (not really, he forgets nothing). He was also my go to deco planner for many years as I was his worst case scenario tester for his models. All that being said, no matter if your last stop is 20' or 10' make sure those 20' of ascension is done in a very slow manner. In other words, Erik's advice was to move up a foot at a time by the minute. He explained at the time that the pressure gradients at these depths are the most influential on your lively hood than anywhere else in the water table. To further the point, his advice about tables was to follow whatever table or computer you want as long as you pay attention to the 20' and above stops and keep them slow, slow, slow.

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Originally Posted by michael-fisch

Sure I would not do a long dive at Ginnie on a 36% because others have tried it and died, but doing deco on 1.6 or for short periods higher is not something that I feel uncomfortable with.

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Funny, 36% at Ginnie used to be standard, and that is when Ginnie really pumped. We since have gotten smarter,but remember lots of dives there, and many other people, without symptoms.

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What I don't like are shoulder, elbow and hip pains along with skin bends or exaustion, so if I have to run the O2 a little bit higher in order to feel great after the dive that's ok.

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If someone is having the above they are bent. Skin bends which we have blown off as nothing is actually a type 2 hit.

I set my computer (shearwater) for last stop at 10 feet, regardless of whether I'm in the ocean or in a cave. In the cave I will do my 10 foot stop at 10 foot, in the ocean I go to around 12 to 15 feet depending on how much "texture" there is on the surface. I used to have a VR3 that prominently displayed %CNS on the home screen. At 1.6 PPO2 the clock goes 2.5% per minute. It is like a freight train, 1% every 24 seconds. People have died from O2 tox/seizure, it's something I have strived to avoid. Even in Ginnie once I clear the 20 foot stop I may go to 18 feet, that reduction in depth is enough to slow down the CNS clock, the goal is to clear the 20 foot stop and get up off of the 20 foot depth. There are few absolutes in the world - My personal tolerance to oxygen seizures is unknown, but there is a limit to it. I don't want to find it.