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.