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Nitrox Rule of Thumb 32 vs 40

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

  1. tomfcrist

    tomfcrist NAUI Instructor

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    Yeah, ok John...so you are saying you’ve never met anyone who does both of those things. I get it.
    I’ve never seen anyone use EAN40 for any dive...so I guess my answer would be the same.

    Separate question:
    Have you ever met anyone who regularly dives nitrox to a PpO2 of 1.6 on recreational dives as a standard practice?

    Ie: filling with EAN32 and not really giving a rip about the PpO2 as long as they are above 132fsw.

    For me, yes I have met those people...and there are a lot of them. Is it best practice? No. Is it common? Absolutely.
     
    KWS likes this.
  2. rick00001967

    rick00001967 Tech Instructor

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    NO..... they do not. below is a quote (in part) from the ssi nitrox instr manual

    "This procedure is to be taught in a manner that emphasizes the
    importance of remaining within a limiting ppO2 of 1.4 atmospheres"
     
  3. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    Im going to give you an answer that is not number sup-ported, but is a process explanation. O2 is a poison. O2% times depth == poison affect on diver. Much like drinking affects your driving. you can drink many low concentration drinks and have the same effect as fewer higher concentration drinks. So equate depth to number of drinks and alcohol content as O2 content. So to maintain a functional alcohol level in your body the more drinks you plan to have the lower the alcohol content in them has to be,,,,,,, to still be able to walk. You can get just as plowed on 3% beer as you can on 151 rum. beer just takes more of it. ............ Although O2 like N2 is toxic on the surface the body can deal with it,,,,, but take that O2 concentration down to deep water where the depth multiplies the affect of the O2 and you will have problems. The lower the O2% the deeper you can go. It is a trade off.

    After a prescribed safety margin it is accepted to use an O2 level of 1.6 or less as a safe level for the body to work with. Rec divers have added a bit more safety margin and use 1.4 as the continuous limit. The body reactions are pretty linear up to about 1.65 where the effects become more exponential rather than linear. General thoughts is that at a level of 2 in most divers will start to feel the effects of the O2 poison. by a level of 3 things are not good at all loss of body conro and by 4 you are dead. Not precise numbers but good enough for explaination purposes. So you take the O2 content of the gas you are breathing (air is 20%) times the depth in Atmospheres. 100 ft is 4 ATM so the level our body is at is 4 times .2 or .8 VERY MUCH WITH IN THE MAX LEVEL OF 1.4. now go to 32% and at one hundred feet you now have a little less than 1.3 .32 x 4 ATM or 1.28. If you were breathing 40% at 100 ft you would be at 1.6. Still technically safe and below the exponential increase amount of about 1.65 for the NORMAL person but it is accepted practice that you do not tempt fate by doing so. Think of the avg person as the football jock warm water on no meds and no cold at the time , not stressed etc.

    Your class will or use to teach you to calculate MOD (max operating depth, for a given O2 content) at depth in ATM divided into your limit of 1.4 So that if you are going to 132 feet that is 5 ATM divided into 1.4 and that gives you .28 which means if you go to 132' to do your dive you should have no more than 28% O2 to not exceed the 1.4 limit. That percentage allows you to drop a tool to say 140 ft allow you to go after it and not exceed the 1.6 limit. These limits are not a monster that will kill you at the moment you hit the level. There is a time factor involved, so hypothetically you can go to say 2.0 for a couple of minutes because it takes time for the body to catch up to the new level.

    I one time on the Oriskani found my self at 1.85 with 32% because iw as not watching my depth while looking for a land mark on the ship. When i figured out that i was seeing the deck at 150+ feet or so i beat feet to about 70 ft to not just stop the increasing level but to reduce the level on my body to a depth of 70-100 ft quickly. That incident was not pushing my luck it was a result of not paying attention and implementing an immediate response to the problem. since I could not change gas to control the level i had to change the depth. The rest of my dive remained well less than 100 ft.
     
    Last edited: Jul 29, 2018
  4. The Chairman

    The Chairman Chairman of the Board

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    I've never met a diver on 40% either. I dive 32% almost exclusively on OC. When I read the title I thought you were referring to the "Rule of 140" used for planning only. Subtract the depth from 140 to get a rough idea of how much time you can spend at that depth.
     
  5. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    I have never met anyone who openly advocated diving to 1.6 as a standard practice. I personally would have no qualms about going there for a reasonably short period of time, and I always go to 1.6 as a limit for decompression stops.

    There used to be a dive shop in Pompano Beach called Fill Express, and providing fills was their primary business. They had a 15 tank cascade of EAN 40, so somebody had to be using it.
     
  6. CuzzA

    CuzzA Solo Diver

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    I don't advocate or try to convince anyone to dive a certain way, but my computer is set at 1.6. I'd be happy to see some real evidence this is taking a risk. My guess is the 1.4 recommendation is just over the top conservatism from training agencies who are at the top of the list on lawsuits.
     
  7. The Chairman

    The Chairman Chairman of the Board

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    Actually, they had it figured out how to blend that with air to provide any mix you asked for. I loved those guys and was sad to see them go.

    Advocate? No, not really. However, I always set my alarms to 1.6.
     
  8. tomfcrist

    tomfcrist NAUI Instructor

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    I have seen a lot of instructors and just plain Jane old divers treat the recommendation of 1.4 with a max of 1.6 the same way they treat the recommendation of 100’ max depth for recreational divers...the tables go to 130.
     
  9. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

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    Believe me, that had the cascade, too. I got my gas blending certification there.
     
  10. giffenk

    giffenk Great White

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    Who recommends 100 for recreational divers?
     

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