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Thread: Air & Nitrox, which first?

  1. #1
    Grouper
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    Air & Nitrox, which first?

    Just finished the nitrox course and of course thought of a question the next day. If for some reason you were doing multiple dives and couldn't do them all on nitrox what do you dive first, the air or the nitrox? I came up with reasons for both but didn't know if they were valid.

    Dive deep on nitrox first to lesson nitrogen loading.
    Dive nitrox on the later shallow dive to help with off gassing.

    Anyone know the real answer?

  2. #2
    I would think this is similar to the reasoning of doing your deepest dive first. When I first learned to dive I was taught you should do the deepest dive first. Recent teachings are suggesting the is no basis for this rule. Previously, all data was on deepest dive first. So the rule of deepest dive first came from the fact there was little data on shallow dives first. Now there is data to show that diving the shallow dive first adds to increased risk of DCS (Learn To Scuba Dive | Scuba Diving Magazine).

    The idea behind diving nitrox is similar to reducing the depth of the dive. You will note that the nitrox tables are really the RDP tables with the depths changed. Or in other words, a dive on nitrox is equivalent to a deeper dive on air, as far as nitrogen loading goes. So if I wanted to follow the old rule of deepest dive first then I would have to figure out which combination gives me the deepest dive first. The combinations would be:

    dive A, nitrox: equivalent air depth X1
    dive B, air: depth Y2
    dive A, air: depth Y1
    dive B, nitrox: equivalent air depth X2

    These are the two combinations you are looking at. There are four combinations:

    X1 > Y2 then dive nitrox first (dive A then dive B)
    X1 <= Y2 then dive nitrox second (dive B then dive A)
    Y1 > X2 then dive air first (dive A then dive B)
    Y1 <= X2 then dive nitrox first (dive B then dive A)

    There is the order of nitrox and air plus there is the order of dive A or dive B (assuming you can change the order of the dives). However, with the recent studies on deepest dive second it really should not matter.

    Now this is all with the understanding that these rules are all statistical guesses and any combination could result in DCS. The only true way to avoid DCS is to stop diving.
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  3. #3
    Grouper
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    OK ,now my head hurts.

    I follow what you're saying though with one exception which may just be a slip on your part. You stated about half way in that a dive on nitrox was equivalant to a deeper dive on air; I thought it was equal to a shallower air dive?

  4. #4
    Doh. You are right, I stated it backwards. A dive on Nitrox is equivalent to a shallower dive on air.
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  5. #5
    Barracuda
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    nitrox first. and it doesn't necessarily mean deeper.
    i'm not as think as you drunk i am ...

  6. #6
    Grouper
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    Nitrtox first gives you shorter obligate surface intervals and longer depth times later. I really don't think it matters which is first, though I would tend to opt for early Nitrox. Personally, I plan using both dive tables (regular and Nitrox) and see which fits my diving day better. Usually it doesn't matter much (theoretically) so I usually choose to N load less early on. My Icon keeps track and lets me know when to stop. lol

  7. #7
    Megalops Atlanticus Diver Kat's Avatar
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    We do Nitrox first, basically for the reason Charon listed.

  8. #8
    Grouper
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    Quote Originally Posted by Charon View Post
    Nitrtox first gives you shorter obligate surface intervals and longer depth times later. I really don't think it matters which is first, though I would tend to opt for early Nitrox. Personally, I plan using both dive tables (regular and Nitrox) and see which fits my diving day better. Usually it doesn't matter much (theoretically) so I usually choose to N load less early on. My Icon keeps track and lets me know when to stop. lol
    Talking about using different tables, nitrox class was the first I remember hearing about the NAUI RGBM tables. They look super simple to use AND give you longer dive times than the usual tables, whether diving air or nitrox. Since I'm a believer in if it looks too good to be true, what am I missing about using the RGBM tables?

  9. #9
    Shark
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    hmmmm we have always used nitrox for the second dive. but our first dive is often below 111 ft which is past the nitrox safe zone
    God is good, no matter what!!

  10. #10
    Quote Originally Posted by scubajane View Post
    hmmmm we have always used nitrox for the second dive. but our first dive is often below 111 ft which is past the nitrox safe zone
    Around here we fill for the required MOD rather than just EAN36 and EAN32. So I think you mean 111ft is the Maximum Operating Depth (MOD) for EAN32 if you are using the ppo2 of 1.4. I often dive EAN28 to a MOD of 132 ft and I'd never go passed 82 ft. if I was diving EAN40.
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