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O2 better than 50% for single deco gas dives?

Discussion in 'Advanced Scuba Discussions' started by Pao, Oct 16, 2019.

  1. Pao

    Pao Manta Ray

    # of Dives: 200 - 499
    Location: Manila, Philippines
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    Scubatech Philippines (run by the perma-banned Devon Diver) recently posted a link to this paper. Although the sample size is small, it makes for a very interesting read. Does it mean that if you are limited to a single gas for deco, 100% would be much better? It it is right smack at the allowed dives for entry level tech diving (45 m limit, single deco gas). Should agencies reconsider their entry level tech courses? TDI AN/DP seems to have an advantage here since you are allowed up to 100% O2 for deco.

    Influence of oxygen enriched gases during decompression on bubble formation and endothelial function in self-contained underwater breathing apparatus diving: a randomized controlled study

    TLDR: It compares air, 50% and 100% for deco on a 45 m 20 min dive using the VPM model. O2 comes out on top with significantly less bubbles and no significant detriment to endothelial function compared to air. Caveat: n = 10.

    Here is the profile:
    Profile.jpg
     
  2. PfcAJ

    PfcAJ Orca

    # of Dives: 5,000 - ∞
    Location: St Petersburg, Fl
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    “Better” needs to cover more than just deco outcomes.

    Youre correct of course that oxygen is the “best” deco gas, as it has no inerts.

    However, 50% is often a better logistical choice because you can get on it much sooner than oxygen. This dramatically reduces your minimum gas requirements on a 45m dive.

    Because of the increased min gas volume, I don’t think oxygen is a “better” gas for this scenario.

    Also VPM sucks.
     
    kensuf, RainPilot, shoredivr and 15 others like this.
  3. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
    Location: Virginia, USA
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    I’ll take the extra few minutes of deco and be able to get off backgas quicker if nessecary.

    also, 100% fills aren’t always available at high pressure...lack of booster...etc.

    A monkey can blend 50%...1101 and top it.
     
  4. broncobowsher

    broncobowsher Solo Diver

    1,101
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    Depends on the profile.

    Pick your favorite dive calculator program and plug in your dive profile. Try different gasses. 50%, 70%, 80%, 100%. Also check gas consumption. How much will you carry with you? How bad do you really need to get out of the water 2 minutes sooner?

    Oh, there are more variables. Start changing the GF or conservatism factors in you software and see where that gets you?

    Want even more confusion? Probably not but this is scubaboard so it will happen anyway. Do the above profile but for deco switch to a 50/50 mix. Now you jump on the deco mix much deeper instead of 20 feet with pure O2. But you decompress as if you were on 100%. Now this does come at added expense. And it would be better to run that Helium at the bottom but that will screw with the advantage of using it as a deco gas.
     
  5. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
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    Those results would probably change if you weren't narced on the bottom...
    ie trimix changes all the inert loads, the time it takes you to arrive at the switch depth, and the efficacy of both gases
     
  6. FreeFlyFreak

    FreeFlyFreak Solo Diver

    # of Dives: 25 - 49
    Location: California
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    This is comedy gold right here:

    "The divers restrained from alcohol and tobacco usage for a minimum of 6 hours before the dive"

    But then again, it is Croatia. They might get no volunteers if they had to go longer than that between cigarettes and vodka shots.
     
  7. happy-diver

    happy-diver Skindiver Just feelin it ScubaBoard Sponsor

    # of Dives: 2,500 - 4,999
    Location: same ocean as you
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    Imagine if you went in to get some fills one day and the dudes were in there on all fours hopping around making noise
    and throwing faeces at the customers
    I would love to see that
    on film
     
    Bob DBF likes this.
  8. lermontov

    lermontov Manta Ray

    # of Dives: 500 - 999
    Location: christchurch
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    looking a the graph where talking about a few minutes and as already mentioned id sooner take 50% so i can get on it earlier - its a greater distance from say 45m to 6m if it all goes wrong
    the few minutes difference is neither here nor there when theres such a wide variation of physiology, conditions and GF choices amongst divers, - why hand ring over a few minutes deco on what is for tech diving a relatively shallow dive

    it may be better but what is the risk % increase over other gas mixes -not sure if this is simply a theoretical discussion and we are still inside an accceptable margin
     
    FreeFlyFreak likes this.
  9. Pao

    Pao Manta Ray

    # of Dives: 200 - 499
    Location: Manila, Philippines
    997
    210
    43
    I found this interesting since I am currently certified at this level (AN/DP). The argument that minimum gas requirements will be increased is a factor, but running ZHL-16C GF 50/75 yields only a 300 liter difference in the projected back gas requirement (14 bar for 2 x 11 liter tanks). But putting the next gas available at 6 meters is a little uncomfortable. O2 is readily available and cheap here (12-15 USD for a K or T tank) and you do not need a prescription, so logistics won't be a problem unless you are in a very remote place. In Coron, Palawan O2 is actually cheaper since there is no blending involved.
     
  10. Centrals

    Centrals Barangay Pasaway

    # of Dives:
    Location: Hong Kong
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    If I can get hold of 50%, I can get other mixes as well.
    I would opt for 50 and 70 and get off the water even faster than a single 100%(based on GF20/80).
     

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