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Using a nitrox 2x gas switching computer as backup on trimix

Discussion in 'Technical Diving' started by Alex Denny, Oct 27, 2018.

  1. Alex Denny

    Alex Denny Active Member

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    Hi all,

    A quick question - probably a silly one. I have a trimix gas switching computer (a Helo2, but not sure it matters for this question) and I also have a suunto vyper which I use as a backup.

    I always plan trimix dives and take a slate - so if my main computer were to snuff it, I can change to the slate and use the viper to get me out of the doo-doo to make all my stops etc.

    My question is this - do I have to have the computer in gauge mode? Or can I actually still fudge using the computer as a computer (along the lines given below)? I have heard various references to whether helium penalties were real and that the actual decompression profile isn't all that different.

    So - if I were to set nitrox gas mix 1 to say 20% (i.e. 80% nitrogen) when diving 20/30, or 18% if diving 18/35 and then gas mix 2 to my travel gas (likely 32%) and do any deco above 9m on the rich gas (50% or 80%) but using the deco profile the computer gives me for the lean gas throughout because it only does two gasses, would this be sufficiently conservative? Would I likely get bent, or would the computer lock me out for some other reason earlier?

    Thanks...

    Alex
     
  2. jb2cool

    jb2cool Moderator
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    What software do you use to plan your dives? You could probably answer this yourself by planning the same dive with and without helium to see the difference.

    I myself wouldn’t bother, my buddy’s computer is my backup computer. Just stick to using the vyper in gauge mode and stick to your plan (keeps you thinking that way rather than simply following the computer)
     
  3. Wibble

    Wibble Fish don't talk
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    Playing with some numbers: 45 metres for 40 mins using 50% deco gas running Buhlmann 50:80 — not the suunto algorithm(s)...


    Depth Stop Run Mix pO2 EAD

    Des 45 - 2 20/30 - -
    Lvl 45 37 40 20/30 1.09 25
    Asc 21 - 42 20/30 - -
    Stp 21 2:20 45 50 1.54 10
    Stp 18 1:00 46 50 1.39 8
    Stp 15 1:00 47 50 1.24 6
    Stp 12 4:00 51 50 1.10 4
    Stp 9 7:00 58 50 0.95 2
    Stp 6 34 92 50 0.80 0
    Sfc - - 94 50 - -



    Depth Stop Run Mix pO2 EAD

    Des 45 - 2 20 - -
    Lvl 45 37 40 20 1.09 46
    Asc 21 - 42 20 - -
    Stp 21 2:20 45 50 1.54 10
    Stp 18 1:00 46 50 1.39 8
    Stp 15 3:00 49 50 1.24 6
    Stp 12 5:00 54 50 1.10 4
    Stp 9 7:00 61 50 0.95 2
    Stp 6 38 99 50 0.80 0
    Sfc - - 101 50 - -


    This shows that the same dive on 20/0 is longer than the same dive on 20/30, therefore is more conservative.

    BUT... I just wouldn’t trust the suunto to not screw it up especially if they’re running different suunto algorithms (RGBM vs tek woteveritscalled)

    You need to run it through the suunto proprietary planners and see.


    The other thing is all dive computers, even “bent” ones, will show the depth and dive time, so run it off your slate, or at least check it with your slate.


    Have found massive discrepancies when working with Buhlmann on one computer and the ridiculously over-cautious suuntos. @jps and I fount up to 20 mins extra on suunto after the Shearwater’s cleared.
     
  4. Alex Denny

    Alex Denny Active Member

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    I gave it a go today as a test. Used V-planner to write the slate out for a 50m 20/30 mix dive. Used a Helo2 on one wrist (which was more conservative than the slate plan, even though it was a ‘real dive’ not a square profile), and had the vyper on my other wrist. I decided to use air as gas 1 on the vyper and the travel gas (32%) as gas 2 so I could track the profiles throughout. Pretty similar on the two suuntos (both more conservative than the slate). The problem came, as expected, when I switched to the deco gas (80%) because the vyper was obviously convinced I was still on 32%.

    I stayed at 6m for as long as I could be bothered but got out with another 6 or so minutes of deco to go on the vyper and set it beeping at me wildly - though it is only a ‘bit’ bent and hasn’t locked me out for two days in a fit...

    The moral is, I guess, that you could use a vyper as a backup in nitrox dive mode if you are only doing one dive and don’t mind bending it by following your main computer or slate.

    But why bother if you can run it in gauge mode as said above...

    (Interestingly, the Vyper Novo has 3x gas mixes so could have taken the 80% mix - in which case I think the profile would have been very similar indeed!!)
     
  5. nickb

    nickb Active Member

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    Why are you using a travel gas for a normoxic dive?
     
    jb2cool likes this.
  6. Alex Denny

    Alex Denny Active Member

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    Because it’s part of development/training before moving onto hypoxic trimix. Also 32% above about 30m is quite a lot cheaper than burning through helium.
     
  7. Wibble

    Wibble Fish don't talk
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    Which algorithm are you using in V-Planner? VPM or Buhlmann+GF? (Or is that just Multideco?)

    For hypoxic dives you will need / should have two computers running the same algorithm, preferably a common one used for deep dives. i.e. ditch the Suuntos.

    If for no other reason than it'll start making big differences for hypoxic dives, say 80 metres. At that depth you can't mess around and you need reliable kit that matches your plans -- and that of any team mates you're diving with.

    I'm sure that even team-oriented diving would prefer a backup computer on your wrist. Relying on a team mate is fine for shallower, normoxic dives but isn't it too risky for hypoxic dives: you could loose each other on the ascent, at the bottom... I prefer the old adage: if it's important, carry two. And computers definitely sit in that category.

    Isn't a travel gas normally the lowest O2 deco gas, e.g. 50% or possibly a third deco cylinder with a rich trimix.
     
  8. Alex Denny

    Alex Denny Active Member

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    I will almost certainly end up using different computers in the future. At the moment, I’m not qualified to do any hypoxic dives / beyond 60m. Hence why I’m thinking about these questions.

    It’s a little like the ‘buy a rebreather’ advice. Absolutely worth it if you’re doing multiple deep dives in a year. Expensive if it is a relative rarity. I’m not yet sure which camp I’ll be in (but my wife is my main buddy and isn’t qualified to depth beyond 40m...)
     
  9. jb2cool

    jb2cool Moderator
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    Whatever you do, upgrade from v-planner to multi-deco. It’s a free upgrade and you then have the choice of buhlmann or VPM rather than just VPM (which is less popular these days)
     
    Alex Denny likes this.

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