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Independent doubles

Discussion in 'Advanced Scuba Discussions' started by paddydiver, Oct 20, 2010.

  1. paddydiver

    paddydiver Angel Fish

    # of Dives: I'm a Fish!
    Location: Ireland
    I want to make an Independent doubles rig, one tank air for the deeper part of the dive and switching to a nitrox mix for the shallow part to extend dive time.
    would appreciate some feedback from anyone using a setup like this.

    Using twim alu 11.5l if I drain one side to about 1/3 of air capacity and switch to the other at about 20 - 30m depth would you have serious trim issues?

    If this is the case would steel 10l's be significantly better?

    What is a good and portable band system to use with this and any opinions on using one of the smaller oms wings, say single bladder 27kg?

  2. waterpirate

    waterpirate Solo Diver

    # of Dives:
    Location: Delmarva peninsula
    There are a lot of things to consider when planning an indy set up over manifolded doubles. adding two different mixes is adding another layer of complexity. Why not dive small indies for back gas and sling a hot mix for the shallow part of the dive?
  3. stairman

    stairman Manta Ray

    # of Dives: 500 - 999
    Location: florida
    Youll be swimming sideways as one tank is depleted while the other stays full. Waterpirates suggestion sounds like a better idea.
  4. ianr33

    ianr33 Solo Diver

    # of Dives: 200 - 499
    Location: Wah Wah Land
    Trim will not be a problem.

    What happens if you lose all the gas in the air tank at depth though? You will probably be below the MOD of the nitrox tank.

    Much better,IMHO to get deco training, dive double tanks (ether independent or manifolded) and sling some O2 for deco.
  5. Hickdive

    Hickdive Solo Diver

    # of Dives: 1,000 - 2,499
    Location: Glasgow, UK
    Trim won't be a problem.

    If you're using a conventional BCD or have a BP/W with cam band slots then buddy bands are a lightweight, portable option;


    There are a couple of points to bear in mind though.

    If you will be deeper than the MOD of the nitrox at any point in the dive then you need to plan/equip on the basis of having a single cylinder of air. The nitrox is useless as a bailout below MOD.

    Also, you need to be certain there is no possibility of accidentally breathing the nitrox below MOD by inadvertently mixing up the 2nd stages.
  6. rongoodman

    rongoodman ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Albany, NY
    Having two different mixes in tanks you can't see strikes me as a really bad idea. If you're going to go into deco, get the training and sling your deco gas so you and your buddy can verify the mix and depth before you breath it.
  7. DA Aquamaster

    DA Aquamaster Directional Toast ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: NC
    As noted above, trim will not be an issue.

    Again as noted above you will however have different gasses with different MODs in each tank, and will be doing so in a manner not 100% consistent with safely managaing gasses with different MODs. People die doing that and switching to the wrong gas is still the number one killer of open circuit technical divers.

    So, there are two major problems:

    1) You are adding significant additional risk in terms of gas switching issues and it could easily get you or someone else killed.

    What happens for example if you are at 150 ft on 24% nitrox and the reg fails? If you have 40% if the other tank, bailing out to 40% is not a real good option.

    Similarly, what if an OOA diver comes calling for gas? One of you is going to get the short straw and end up on a gas with a very high PPO2 and it may take a minute or two to get sorted out before you can even start an ascent. Toxing on the hot mix becomes a possibility in this case.

    The only way around it is to add another seocnd stage to at least the bottom mix reg, and that still won't eliminate the risk of someone grabbing the wrong on in an emergency.

    2) The idea behind indpendent doubles is to provide 100% redundancy in the event of a failure as when properly managed, the gas in either tank by itself will be enough to get you back where you started and/or to your first deco gas. Doing it your way does not achieve that.

    You would be far better off with a single tank on your back for the deep portion and a single tank carried as a stage for the shallow portion of the dive. At least then you will be abel to follow a reasonable gas switching protocol and any OOA divers will be using a back gas reg, and not the hotter/shallower MOD nitrox mix.
  8. waterpirate

    waterpirate Solo Diver

    # of Dives:
    Location: Delmarva peninsula
    All of you said much better, what I was thinking.
    However the question was to extend time, not do deco?
    i.e. I dive to x untill ndl is reached then ascend to a shallower depth where a hot nitrox mix say 36is the appropriate mix. At a minimum a 2 gas computer would be required and all the problems still remain, as well as a lack of understanding of multi-level profiles with an imperfect mix for one of them on open circuit.
    Maybe a ccr is in order? lol
  9. AfterDark

    AfterDark Solo Diver

    # of Dives: 1,000 - 2,499
    Location: Rhode Island, USA
    I use ID's to extended my dive time and a redundant system for solo diving. Trim is not an issue.

    I use 2 LP 72's or 2 LP 90's. The best bands are the ones that cost the least.

    Using a single wing with doubles will not work. Also using a double wing with a single tank doesn't work well either.

    I only use air. I am a stranger to Nitrox and always will be. I don't like the fact I'm limited to a particular depth on any given dive. So I wouldn't comment on the Nitrox
  10. AfterDark

    AfterDark Solo Diver

    # of Dives: 1,000 - 2,499
    Location: Rhode Island, USA
    oopps duplicate
    Last edited: Oct 20, 2010

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