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Capernwray Nitrox MOD

Discussion in 'General Scuba Diving' started by JohnL, Jul 6, 2018.

  1. JohnL

    JohnL Well-Known Member

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    Had a Nitrox fill in Capers yesterday. When we measured the mix, the guy referred to a table and, for 32%, told me that the MOD was 40m. I always dive Nitrox, so have a good idea on MOD and know that 32% = 34m give or take, so queried it. The boss turned up and told me that the table was fine and, on enquiry, found that they were using 1.6 rather than 1.4. Whilst I recognise that 1.6 is for use when helping your buddy when he is in trouble, I've never seen it used at a fill station - I guess there could be an argument that you need to know how far down you can go to save your buddy before you kill yourself!
    All this is slightly irrelevant for Capers that has a maximum depth of 20m, but a tank filled there can be used elsewhere - what do you think?
     
  2. Vanny

    Vanny Active Member

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    I guess you could argue that’s the maximum you could use it at , being at 1.6 ppo2. For back gas most agencies , probably all would shrink at the suggestion nowadays.

    Your BSAC sports ? So a ppo2 of max 1.4 upto max 36%. For simplicity BSAC say on no stop dives you can use 36% @ 30 mtrs and 32%@35mtrs (1.44ppo2).

    Capers are being a bit silly.
     
  3. nickb

    nickb Active Member

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    It’s common practice to calculate MOD using a PO2 of 1.6

    1.6 divided by 0.32 is 5. Therefore the MOD of 32% nitrox is 40m

    They are correct. If you have a nitrox ticket you should know this and querying it makes you look a bit daft TBH
     
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  4. Dave Whitlow

    Dave Whitlow Well-Known Member

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    Yep, as @nickb said, they are correct in their marking.

    My 32% bailout is marked MOD40 and my oxygen has MOD of 6.

    For the active phase of a dive, planning to use a maximum ppo2 of 1.4 is the common convention.
     
  5. Wibble

    Wibble Fish don't talk
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    MOD = (dose / mix) - 1
    For backgas the dose will be 1.4
    = (1.4 / 0.32) - 1
    = 4.375 - 1
    = 33.75m

    For DECO gas, the dose will be 1.6
    = (1.6 / 0.32) - 1
    = 40m
     
  6. JohnL

    JohnL Well-Known Member

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    Maybe for centres supplying tech divers; of the ten places that I've got fills, only one used 1.6 - so not that common practice. And, while I have a ticket and did know about the two PO2 levels, I have never heard of using the higher level as deco gas. Not included in either PADI nor BSAC basic training, so don't feel daft at all. But a little more knowledge gained, even if of little use as I have no ambition to go beyond no-deco diving.
     
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  7. Wibble

    Wibble Fish don't talk
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    Just a thought - rec v tec...

    The 1.6 vs 1.4 numbers are there to control the two oxygen 'clocks' (CNS & pulmonary). A PPO2 of 1.4 gives about 150 minutes max, whereas 1.6 gives 45 mins (reference). Recreational dives are unlikely to have 45 mins bottom time at depth due to the restriction of a single cylinder and 40% O2 (which gives a MOD of 30m at 1.6).

    So not (too) dangerous.
     
  8. Vanny

    Vanny Active Member

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    You don’t look daft it’s a genuine question , one I’d rather someone asked. Whilst 1.6 is the max all us recreational divers will use handing a cylinder to relatively new Divers , whether qualified or not with use this to 40 mtrs. Is a bit short sighted as no teaches 1.6 for bottom gas.

    BSAC do teach 1.6 for deco , but in the ADP course.

    The ideology is that your more susceptible to CNS over 1.4 when working at depth and less susceptible when relaxed off gassing on deco shallow.
     
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  9. Iain Denham

    Iain Denham Active Member

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    My local gas supplier always marks it up as 32% 1.6 PP02 at 40m I thought it was quite common, Nutties in Essex?
     
  10. Tribal Chestnut

    Tribal Chestnut Well-Known Member
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    Let’s not create a climate in which people are afraid to ask questions.
     
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  11. Dave Whitlow

    Dave Whitlow Well-Known Member

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    It is the correct marking for the MOD (maximum operating depth) of the gas, this being the maximum depth at which it is deemed safe to breathe the gas, at a PPO2 of 1.6 bar.

    However, and quite separate, for dive planning a lower PPO2 is used to reduce the risks of extended oxygen exposure issues (CNS and whole body).

    The confusion arose from not separating the hard gas limit from sensible dive planning.
     
  12. Vanny

    Vanny Active Member

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    I guess the main thing here is to analyse and mark your own MOD. No confusion then. Can’t remember any shop marking my cylinders.
     
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  13. Tel

    Tel Super Moderator
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    l think they've opened up a can of worms.

    This is a pumping station not a diving adviser and while 1.6 might be ok for some dives, that's not what your
    cert says.

    IF they had two tables and had asked you what your max for cert was (did they check your Nitrox card? )
    that would have been ok, IF they just displayed two tables (1.4/1.6) and let you decide yourself again that's ok,
    but to tell you that it's 40m when your max for 1.4 was 34m would (as you say) if you took the gas elsewhere
    and as Capers have no idea as to what the variables are, could have put you at risk.
     
  14. splinter

    splinter Active Member

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    This. Whenever I get nitrox fills, at most they put a piece of masking tape with the % on. No comment on MOD. I then analyse at home and mark it up fully myself, then analyse again the day I use it.

    Sent from my SM-G900F using Tapatalk
     

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