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  1. #21
    TDF Member Scuba steve's Avatar
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    Ok so bear with me on this 1 and paint a picture . if you imagine looking at a bar door . and nitrogen molicules as big burly rude rugby players and helium as little danty girls in their high hell shoes . The girls want to leave the bar and the rugby players want to enter . There is only a limited space they can travel trough so the very rude and bigger rugby players burst there way into the bar . While the girls wait for the rugby lads to come in so they can leave . Whilst this happens and the bigger bullies there way in the smaller get frustrated and blow their tops (reduction in ambient pressure helium comes out of solution and form a bubble)

    The .5bar comes from the use of 50% as a deco gas . The real problem is when using high helium at depth and the switching to an air mix as the "first deco gas" , this has been linked to ICD or inner ear
    Bends and the use of air on these type of dive has been disbariged for year
    You dont have to be able to spell to be a diver

  2. #22
    Established TDF Member matt's Avatar
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    Quote Originally Posted by Scuba steve View Post
    The .5bar comes from the use of 50% as a deco gas
    Nope, that's not related to the 0.5bar "limit" for the ICD calculation...

  3. #23
    TDF Member Scuba steve's Avatar
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    That's where the suggestions starts . It's also reducing the nitrogen content by 30% but increases or opens the oxygen window aswell
    You dont have to be able to spell to be a diver

  4. #24
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    Quote Originally Posted by BTS View Post
    Pesonally I don't care how, all I need to know is what it is and how to avoid any associated issues, no need to make it all scientificky...
    Dive closed circuit & don't switch your diluent, the unit will smoothly reduce partial pressures of He/N2...... simples!!

  5. #25
    Old but keen Mark Chase's Avatar
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    Quote Originally Posted by Scuba steve View Post
    That's where the suggestions starts . It's also reducing the nitrogen content by 30% but increases or opens the oxygen window aswell


    Ka Ching

    ICD and the 02 Window

    Two myths for the price of one thread


    As I said before ICD dosent affect normal divers doing normal dives. In fact I have never herd of any one getting ICD on any dive shalower than 150m. There are incidents on world record attempt dives with the typical nausia vomiting and vertigo symptoms, but for dives in the 0-100m range its not rely an issue.

    Theres the thing about switching to air deep from trimix, but in truth 10,s of 1000s of divers used to do this regular as clockwork without ill effect. Have a chat to Kevin Gurr next time you see him at a dive show. He was running Air as inboards diluient on his Sentianl and Borris and flying the unit on off board gas but flushing with Air on the ascent to reduce deco. Very old school diving I admit, but it seems some people still like the idea of getting off the HE as soon as possable.

    50% is a comon gas choice for deco but no more so than 32%. In my early years on OC by far the most comon combination of deco mix was 32% and 80% and we would switch to 32% at 40m streight from back gas. QED I could have been doing a 80m dive on 14/65 and then gas switch directly on to 32% @ 40m for deco.

    We all did this. This was the norm. No one got ICD

    On deep dives down arround 100m we would add 21/35 as a travel gas and deco mix. Wed decend on 21/35 to 60m then switch to back gas (say 10/70) and on ascent wed switch back to 21/35 for deco at 66m. The 35% He wasn't to avoid ICD, it was to avoid the monster narcosis hit of switching to air.


    As for the 02 Window?

    That was investigated and found to be total myth with zero suporting evidance.

    There are however two points often asociated with the myth that i do agree with. 1: it takes two mins for any gas switch to take full effect. So staying an extra 2mins at 21m on 50% does help get the gass fulley into the system. 2: Replacing as much of the nitrogen with 02 as possable, will speed up deco. Prety obvious when you think about it.

    ATB

    Mark

  6. #26
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    Quote Originally Posted by Mark Chase View Post
    Ka Ching

    ICD and the 02 Window

    Two myths for the price of one thread

    ...
    Mark
    Ha ha... Think of it this way then, I'm after the "maths behind the myths".

    (Oh, and in the context it's used, there's nothing mythical about Scuba Steve's comment on the O2 window. The type he's referring to is simply increasing the off-gassing gradient.)

    DLK
    Last edited by dlk; 04-01-2013 at 09:45 AM. Reason: added o2 window stuff

  7. #27
    "Three Sheds" Janos's Avatar
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    Quote Originally Posted by Mark Chase View Post
    As I said before ICD dosent affect normal divers doing normal dives. In fact I have never herd of any one getting ICD on any dive shalower than 150m.
    Not even switching diluent to air? Quite a few examples of that.

    Janos
    You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves
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  8. #28
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    Quote Originally Posted by Scuba steve View Post
    Ok so bear with me on this 1 and paint a picture . if you imagine looking at a bar door . and nitrogen molicules as big burly rude rugby players and helium as little danty girls in their high hell shoes . The girls want to leave the bar and the rugby players want to enter . There is only a limited space they can travel trough so the very rude and bigger rugby players burst there way into the bar . While the girls wait for the rugby lads to come in so they can leave . Whilst this happens and the bigger bullies there way in the smaller get frustrated and blow their tops (reduction in ambient pressure helium comes out of solution and form a bubble)
    ...
    ICD is a diffusion process. Diffusion is governed by molecular weight (rate) and pressure gradient (amount). See Dalton's law & Graham's law.

    The girls daintily skip past the rugby players without even noticing them.

    DLK

  9. #29
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    Quote Originally Posted by Janos View Post

    1) You dive a very rich trimix (in fact, let's make it Heliox - say 10/90). You spend a while on the bottom. Because you're diving Heliox, you won't on-gas any Nitrogen, but you will on-gas Helium. You do your stops on 10/90 until you hit (say) 21m. At this point (because you've got to stop) you're right on the saturation limit. If you switch to Nitrox 50 then the Nitrogen will rush in very quickly, far quicker than the Helium will leave, giving you a ICDB bend.
    ...

    Janos
    Nice, concise explanation.
    Now the $64,000 question: Why will the nitrogen rush in much quicker than the helium leaves? Dalton and Graham seem to think otherwise.

    When I looked at the numbers, I couldn't see how to make this happen. I either made an error or I'm missing something.
    Steve Burton gives a rationalization (in the link I posted earlier) but I think he's confusing quantity with partial pressure.

    DLK

  10. #30
    "Three Sheds" Janos's Avatar
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    Quote Originally Posted by dlk View Post
    Nice, concise explanation.
    Now the $64,000 question: Why will the nitrogen rush in much quicker than the helium leaves? Dalton and Graham seem to think otherwise.

    When I looked at the numbers, I couldn't see how to make this happen. I either made an error or I'm missing something.
    Steve Burton gives a rationalization (in the link I posted earlier) but I think he's confusing quantity with partial pressure.

    DLK
    Because the Nitrogen pp gradient is huge - 1.6 bar (ambient) to zero (tissues). The Helium coming out isn't a straightforward simple diffusion across a membrane - but a complex thing involving perfusion from slow tissues to blood to lungs etc...
    You can lead a horse to water but you can't climb a ladder with a large bell in both hands - Vic Reeves
    Hellfins - a friendly London dive club
    My music video: Dive the UK, cos that's the way it is. Huh!


 
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