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Nitrox Rule of Thumb 32 vs 40

Discussion in 'SSI: Scuba Schools International' started by apivonka, Jul 29, 2018.

  1. apivonka

    apivonka Angel Fish

    # of Dives: 0 - 24
    Location: Indiana
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    From SSI
    My question : Why the higher EAN for shallower dives vs low EAN for deeper dives?


    Seeking to understand



    Capture.PNG

    Thank you
     
    Last edited: Jul 29, 2018
  2. DerekR

    DerekR Nassau Grouper

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    In order to have the highest possible O2 ratio, you have to decrease O2 enrichment as depth increases. I want the highest O2 content possible in my breathing air.
     
  3. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I have two reactions to this question.

    1. To answer your question, high percentages of oxygen in the mix become more dangerous at greater depths. Multiply the percentage by the depth (expressed in atmospheres of pressure) and you have the partial pressure of the gas at that depth. For most diving, a partial pressure of 1.4 is the most you should use, because spending much time at higher pressures can lead to convulsions. If you multiply the percentage of oxygen in air (0.21) by 7, you are over 1.4 and considered to be at risk, so you should not take air below 7 atmospheres (roughly 200 feet in sea water). The maximum depth at which you would be considered safe for a particular percentage is called its Maximum Operating Depth, or MOD.

    2. Given what I just wrote, I am surprised by the material in quotations. The normal MOD for EAN 40 is about 80 feet, so the quotation advocates violating the MOD for dives between 80-100 feet. The normal MOD for EAN 32 is about 110 feet, so the quotation violates that MOD as well. Those numbers do work if the safe partial pressure is 1.6 rather than 1.4. Most people consider 1.6 to be the "contingency" MOD only--you can go that deep for short periods of time if you need to. It is also used by technical divers for decompression stops., but not for normal diving.
     
    apivonka likes this.
  4. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    After my post, you added an image of the maximum operating depths from SDI material. You will see there what i was talking about with MODs.

    I want to get back to the quote in your post:
    "Many Nitrox divers make life even easier. They pick EAN40 for all dives shallower than 30 m/99 ft as this gives the most bottom time and pick EAN32 for dives right up to the maximum sport diving depth limit of 40 m/132 ft."​

    It says many diver do that to make their lives easier. That may be true. I have certainly not met all the nitrox divers in the world. I have not, however, met any that do this.
     
    Last edited: Jul 29, 2018
  5. Steve_C

    Steve_C Divemaster

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    This overlooks the possibility that a dive will get canceled or you get sick and now you have the wrong mix for a later dive. Since all my dives are 110 ft and above, in fact 105 and above, and since half my dives get canceled (I am in NC), and tanks are filled right after the previous dive, I just get 32 for every fill. As it is, I am often the last one back on the boat anyway, so more NDL with more hassle is not worth it to me on the shallower dives.
     
  6. tomfcrist

    tomfcrist NAUI Instructor

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    That do what? Dive Ean40, or run 1.6 like it’s no big deal. The former I find believable, the latter not so much.
     
  7. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I have never met anyone who routinely uses EAN 40 for dives down to 99 feet and EAN 32 for divs to 130 feet as standard practice.
     
  8. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I did not realize when I first posted that this was in the SSI forum. I am not an SSI diver. What is the source of the quote in the first post? Does SSI advocate using the 1.6 PPO2 standard for recreational dives?
     
  9. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
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    I see the latter regularly in the Keys...where banked nitrox is almost always 32...
     
  10. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Boulder, CO
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    All right, I will try to make this as clear as possible, since there seems to be some difficulty understanding what I wrote. Here is the original quote to which I referred. Please read ALL of it carefully.

    "Many Nitrox divers make life even easier. They pick EAN40 for all dives shallower than 30 m/99 ft as this gives the most bottom time and pick EAN32 for dives right up to the maximum sport diving depth limit of 40 m/132 ft."
    I said I had never met anyone who follows this practice.
     

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