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What % Nitrox settting on dive computer?

Discussion in 'Basic Scuba Discussions' started by chipster, Sep 30, 2003.

  1. chipster

    chipster Angel Fish

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    Ok hope this is not to silly of a question.....

    I have a Uwatec Air X Nitrox, and the Nitrox settings have increments of 2. If I have calculated or measured my O2 to be say 27%. Do I set the dive computer at 28% to err on the side of depth?

    Thanks

    Brian
     
  2. chrispete

    chrispete Divemaster

    # of Dives: 1,000 - 2,499
    Location: Seabrook, TX
    949
    27
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    Yep, I'd say that would be a safe bet. Another thing, you should never dive any nitrox mix based on calculations alone, always be 100% sure by testing the mix personally before diving.
     
  3. trewbs

    trewbs Guest

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    you are always meant to round the O2 % down rather than up. This means that your computer's decompression calculations are more conservative.

    I you round O2% up, then your computer thinks that you are absorbing less N2 than you actually are so the data it gives you is wrong.

    However, you must calculate your MOD for the exact mix.

    Hope this helps

    Tom

    p.s. I just finished my nitrox course so I'm feeling pretty knowlegable!
     
  4. japan-diver

    japan-diver Dive Charter

    # of Dives: I'm a Fish!
    Location: Okinawa, Japan
    1,488
    196
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    As a nitrox instructor I would say round down as stated in the previous post.

    Although 1-2% would realy not make much of a difference.
     
  5. chrispete

    chrispete Divemaster

    # of Dives: 1,000 - 2,499
    Location: Seabrook, TX
    949
    27
    28
    Duh. :loopy: Thought about it, and there would be potential problems with rounding either way. If you round up to 28% you will lose accuracy on nitrogen tracking - the computer will think that you have taken on less N than you really have. If you round down to 26% you will have incorrect tracking of your CNS% and the computer will display an MOD that is too deep and a PO2 that is inaccurate...
     
  6. Big-t-2538

    Big-t-2538 Divemaster

    # of Dives: 100 - 199
    Location: Dayton, OH
    3,497
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    HEre is what I would do...

    1 - get the mix analyzed to find out what's in it.

    2 - Set the % on your computer by rounding down...(i.e. if you analyze at 27.3%...use 26 on the computer)

    3 - Dive the MOD for EANx 28...
     
  7. chipster

    chipster Angel Fish

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    Wow talk about conflicting responses...

    with regards to having in analyzed, not always possible if you have a nitrox fill from the LDS than go somewhere where they do not have nitrox. Must to a calculation derived from the amount of top up of air in the tank for dive #2. I would tend to agree with mine and others here that rounding up would be more cautious then rounding down. Would think that if I rounded down might accidently exceed MOD. Also I have some wiggle room for % of obsorbing N2 as I have my computer set at 1.5ATA not 1.6. Needless to say it sounds here, that one should be cautious with there MOD If rounding up, and on the flip side dont push the max BT if rounding down.
    Sound about right???????:doctor:
     
  8. raviepoo

    raviepoo Manta Ray

    # of Dives: 500 - 999
    Location: in exile in the Pennsyltucky Archipellago
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    It is if you have own analyzer. You can build an el cheapo O2 analyzer for about $100. Is your safety worth $100?
     
  9. DeepScuba

    DeepScuba Manta Ray

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    Here's my opinion/thoughts.

    You have a 27% MIX.

    Set your computer at 26%. This will be (slightly) conservative for the Nitrogen loading.

    I'm assuming you're diving 1.4ATA for the PO2, as in, you have wiggle room, so it's more of a mental note of your MOD.

    Assuming 1.4 PO2 MAX, you won't realistically ever have a PO2 problem, so PO2 is a mute point.

    Your Nitrogen loading is the controlling factor, so set that for "conservative" and round down.

    NOW, if you're crazy, sloppy, whatever and dive at 1.6 PO2, you're in trouble (potentially), which is why you don't ever dive your dive at 1.6 PO2!

    Besides, O2 analyzers have an error factor already, so don't push it for 1%

    Regards.
     
  10. Big-t-2538

    Big-t-2538 Divemaster

    # of Dives: 100 - 199
    Location: Dayton, OH
    3,497
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    O.K...first off...I'd look at why you have your computer set to 1.5. Are you doing something that requires you to be pushing your limits? Why push them if it isn't necessary?

    As for not having an analyzer...well, that happens. In that case, I wouldn't be diving with my computer, b/c it isn't going to be able to help you at all. You have to be able to reason out for yourself what is the intelligent thing to do. You might consider staying within the MOD for a very conservative mix % (I would say 2% higher than what you think you've calculated) and not exceeding the max BT for a lower conservative mix % (like 2% lower than what you think). This method gives you the shallowest MOD, and the least amount of Bottom time for the mysetery gas. If you want to be ultra conservative....use the MOD for what was initally in the tank, and then the max BT for switching to air. Do you see where the conservatism is coming into play? you have to think a little.

    All this time, your computer isn't going to be helpful b/c you don't know what is in the mix. Unless you have a really accurate pressure gauge on your tank, the number is only going to be ballpark at absolute best. And the computer is already ball-parking what your N2 loading actually is. At that point, it becomes a bottom timer/depth gauge.
     

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