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BSAC published new gas density recommendations

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

  1. NothingClever

    NothingClever Manta Ray

    # of Dives: 200 - 499
    Location: Red Sea and Atlantic Ocean
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    In my neck of the Red Sea woods, people will relate to a BSAC article.

    If I show them some DIR article from that long ago, they’ll just disregard me as some street corner preacher. Unless, of course, they’re some Dutch cats from that bizarre George Irvine hero worship club and then they’ll think I’m some kind of theologian steeped in the ancient texts.
     
  2. Edward3c

    Edward3c Instructor, Scuba

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    So the cost of your diving is more important that having a safe mix.
     
  3. atdotde

    atdotde Solo Diver

    # of Dives: 200 - 499
    Location: Munich, Germany
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    Maybe not the tables, but Subsurface has been computing gas densities for a while (implemented after watching on of Mitchell's videos on youtube). You can see them displayed in the info box. Screenshot 2020-09-17 at 10.18.06.png
     
    wnissen, taimen and NothingClever like this.
  4. KenGordon

    KenGordon Rebreather Pilot

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    The various hyperbaric academic societies charge a subscription to get their publications. There is a cost in providing a service even if the origin of the paper is funded already. BSAC has costs and Is funded by subscription. If it gave away the product would not be offering value to those who fund it.

    You can read the papers and do the maths for yourself, or you can spend a few quid (about 75USD) to have access to the various BSAC resources.

    BSAC have been instrumental in raising the profile of this issue, they certainly put 600 divers in a room with Simon Mitchell to hear about it.

    As to OC vs CC, I will guess they said “Gavin, what about OC?” and did something based on the answer.
     
  5. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Europe
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    Gas blender toolkit also available as a standalone android app can be used for calculating gas densities and ENDs.
     
  6. kensuf

    kensuf Cave Instructor

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    ICD isn't a problem for diving within normoxic dive depths. Counterdiffusion may be an issue diving at hypoxic ranges, but generally isn't an issue diving at depths shallower than 80m.

    Staying on helium mixes until you're at about 70' will help minimize the risk of ICD.

    Candidly, a bigger concern beyond ICD is narcosis when making gas switches to non-helium mixtures. Back when I did my very first hypoxic dive, we were told "you need to get off the helium as quick as possible" and our protocol involved switching back to air at 190-200'. The problem is this abrupt switch is extremely more powerful of a narcotic hit than if you were to descend from the surface to 200' on air.

    While I can assure that switching from trimix to air at 200' was like being kicked in the head by a mule, I can also tell you that the switch to nitrox or air even as shallow as 110' can produce extreme narcosis as well.

    My recommendation is stay on helium mixes until you're at 70'/21m before switching to nitrox. If you're on a CCR, just stay on helium DIL.
     
  7. Deep_Down

    Deep_Down Angel Fish

    # of Dives: 2,500 - 4,999
    Location: Los Angeles
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    You are totally right, but look at the tables.
    They recommend an He mix of 54% for a dive to 60msw. If done as it is usually done, with one switch to EAN50, the PN2 differential is much bigger than with a 18/35, and there is where I see a problem. Another gas is then needed or just to use the back gas until reaching 02. I personally prefer for such a shallow dives to Deco on a EAN50, and therefore will keep using my loved 18/35.
     
  8. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
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    I can't see the tables, but I suspect the differences will be in standard gases. The "next" WKPP gas is 15/55 and for that you'd have 35/25 as a deco gas so it wouldn't be a switch straight to 50%.
     
  9. Deep_Down

    Deep_Down Angel Fish

    # of Dives: 2,500 - 4,999
    Location: Los Angeles
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    Exactly. And here is where I dont really agree with the tables. They recommend too much He in the normoxic range, where usually only one switch is done and therefore the pn2 differential is too big, in my opinion.
    I totally agree with your point of view.
     
  10. Deep_Down

    Deep_Down Angel Fish

    # of Dives: 2,500 - 4,999
    Location: Los Angeles
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    They recommend to 64msw 18/54, instead of the 18/35. If you then switch to EAN50... the pn2 differential is big enough to not like it (at least myself).
     

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