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BSAC published new gas density recommendations

Discussion in 'Technical Diving Specialties' started by Deep_Down, Sep 15, 2020.

  1. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

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    I agree, that's a big swing. I'd want 35/25 or 30/30 or something in the middle
     
  2. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

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    People have been switching from 15/55 to EAN50 without an intermediate gas for 20years.

    Way back in the TDS forum days there was ample evidence that 70ft trimix to nitrogen mix switches were no big deal. Seems like the stupid IBCD monster has again reared its head and gets hundreds of posts and thousands of words but nobody has ever seen it without being narced out of their gourd. Keep your ENDs <100ft and it is a non-problem.

    ppO2 of 1.4 is too high for me, i might dive that gas as a contingency but I plan for 1.2 to give me some wiggle room
     
    Matan, laikabear, PfcAJ and 2 others like this.
  3. elmo

    elmo DIR Practitioner

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    I couldn't agree more. It's interesting that BSAC is taking gas density seriously on the basis of Simin Mitchell's work, and so we all should, but don't seem to pay attention to his work on ICD. I hate to paraphrase but his work (and others) pretty much concluded that it isn't an issue at normoxic depths, which is backed up by successful use of standard gasses for decades.
     
  4. Dr Simon Mitchell

    Dr Simon Mitchell ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Hello,

    What Elmo says is pretty much on the money. One correction though. The data that defined a gas density threshold beyond which there is a substantially greater chance of CO2 retention were collected by Gavin Anthony, recently retired from QinetiQ. He deserves the credit for that. I was the catalyst for getting the data published. I wrote the paper and Gavin and I published it together [1].

    I agree that the risk of inner ear DCS following helium to nitrogen switches is being over-called on this thread. Our previous work [2] suggested that the increase in supersaturation in the inner ear provoked by gas counter diffusion after a helium to nitrogen switch was relatively small. It would only be likely to cause harm if the inner ear was already substantially supersaturated as a result of decompression in the period leading up to the gas switch. This in turn would be most likely during the early decompression phase of a very deep dive.

    It may interest you to know that David Doolette published an NEDU study [3] in which divers performed switches from pure heliox to air (massive violation of related technical diving ‘rules’) in decompression from dives to ‘intermediate’ depths between 45 and 67m (150 – 220ft). In 104 dives they never saw any inner ear DCS, and just to be clear, the inner ear is the only organ that could plausibly be adversely affected by gas counter diffusion.

    I think that in an intermediate depth dive (eg, the range of the NEDU study) on open circuit where gas switches are obviously necessary, I would be much more concerned about gas density and the potential for CO2 retention than I would be about a risk of gas counter diffusion and DCS implied by having to use a higher helium trimix that kept me within the density recommendations. I doubt that many dives deeper than 90m / 300ft get done on open circuit these days and when using rebreathers there is much less reason to do gas switches (dil flushes). We used to do that quite a lot in the 2000s but most people are just staying on the bottom dil all the way these days.

    References

    1. ANTHONY TG, MITCHELL SJ. Respiratory physiology of rebreather diving. In: Pollock NW, Sellers SH, Godfrey JM (Editors). Rebreathers and Scientific Diving. Proceedings of NPS/NOAA/DAN/AAUS June 16-19, 2015 Workshop. Wrigley Marine Science Center, Catalina Island, CA, 66-79, 2016. Available from: Rebreathers and Scientific Diving Workshop Proceedings

    2. DOOLETTE DJ, MITCHELL SJ. A biophysical basis for inner ear decompression sickness. J Applied Physiol 94, 2145-2150, 2003. Available from: https://journals.physiology.org/doi...l_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org

    3. DOOLETTE DJ, GERTH WA. Safe inner ear gas tensions for switch from helium to air breathing during decompression. NEDU TR 12-04. Panama City (FL): Naval Experimental Diving Unit; 2013.

    Simon M
     
  5. 100days-a-year

    100days-a-year Solo Diver

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    Thanks Doctor Mitchell for helping keep the air clear on inner ear DCS and gas switches.
    I think sometimes the point is missed that narcosis and it's potential dangers far outweigh the slim possibility of inner ear DCS caused by gas switches.

    I do however doubt that even with BSAC recommending lower gas densities that expense and availability will prevent more widespread use of HE even if it has widespread acceptance in OC. I'd love to have had the option to dive more HE but most years it would have added $10 to $20K per year to expenses. I made the decision to limit depths and conditions to manage narcosis.
     
    NothingClever and rjack321 like this.
  6. Dr Simon Mitchell

    Dr Simon Mitchell ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Hello,

    Great point to raise the issue of narcosis - the discussion was on gas density so I did not think to include that, but it is definitely part of the comprehensive answer.

    I expect you are right about cost of helium being a barrier to the density recommendations being followed in deeper open circuit diving. That is partly the basis for my previous comment that "I doubt that many dives deeper than 90m / 300ft get done on open circuit these days". Helium is just too expensive. Pragmatically speaking, and even as an author on the paper that promulgated the guideline, I have to acknowledge that failure to comply with density recommendations does not mean that a disaster is going to occur. It just means the risk is higher. Technical diving involves lots of decisions about risk and balancing one against the other, so this is no different really. Having said that, where an avoidable risk factor has been identified then we should try to avoid it, and for rebreather divers there is no cost reason to avoid following the density recommendations.

    Simon M
     
  7. njmorrell

    njmorrell Captain

    # of Dives: 1,000 - 2,499
    Location: Miami, FL.
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    Way back in May 2017 the BSAC recommended limiting gas density to 6.2gm/l and PN2 switches to 0.5-0.7 bar.

    Having looked at the recently published gas density tables it doesn't seem much has changed since May 2017.
     
  8. Matan

    Matan DIR Practitioner

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    There are probably hundreds of documented dives where GUE divers are switching from 15/55 to Nx 50% (it's what Tech2 certifies you to do).

    12/65 I'd switch to 21/35 as this is a 3 deco gas dive - not because of counter diffusion (common aren't we over this in 2020 already?) but because a dive to 88m (I think it's 290ft?) requires another deco gas to have normal minimum gas on the bottom + more efficient and more options during deco.

    but - if the diaper hits the fan I'd do my best on backgas + buddie's backgas to get to 21m and switch from 12/65 to 50% w/o thinking too much on counter diffusion - I'd be more worried about my life in general during that scenario :) (then again - buddy will have 21/35 we can share so not going to happen).


    Matan.
     
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