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12ltr or 15ltr tanks

Discussion in 'General Scuba Diving' started by kevin b, Jan 16, 2019.

  1. kevin b

    kevin b Member

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    Currently dive in wetsuit & bcd with 12 ltr single tank to about 20 metres but moving over to drysuit & wing. I want to dive deeper so was wondering would I be better using single 15 ltr tank to compensate going deeper & using drysuit with wing.
     
  2. Vanny

    Vanny Active Member

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    What sort of diving are thinking of doing ? I’m guessing uk stuff.
     
  3. Tribal Chestnut

    Tribal Chestnut Well-Known Member
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    I went for a 15 when buying my first cylinder and, knowing what I know now, would also add a pony of a decent size (eg an ali40 which is 5.5 litres or thereabouts).
     
  4. Dave Whitlow

    Dave Whitlow Super Moderator
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    Yes. Assuming the typical 2 dive day a 15l for first dive and 12l for the second dive is a good combination. A pony can be added later if you feel the need.
     
  5. jb2cool

    jb2cool Moderator
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    A 15 will last longer than a 12 so if diving to 25m for example you have to decide how long you want to stay and whether that is worth the cost/weight difference of a larger cylinder. If you were buying from scratch then a 15 would make sense but if you already have a 12 then you might decide it's not worth the cost or swapping. I did start off with a 12 but then changed to a 15 and then changed to a twinset as I was diving deeper regularly and wanted to extend times.
     
  6. kevin b

    kevin b Member

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    UK diving, hoping to do some wreck diving.
     
  7. kevin b

    kevin b Member

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    Thanks
     
  8. kevin b

    kevin b Member

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    I
    I have 12 ltr tanks but not interested in twinset diving.
     
  9. kevin b

    kevin b Member

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    That's a possibility
     
  10. Vanny

    Vanny Active Member

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    Ok. What sort of depths interest you ?
     
  11. kevin b

    kevin b Member

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    20 to 40 metres
     
  12. Tel

    Tel Super Moderator
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    In that range I'm a great believer in indy twin 12's and have dived them for about 20 years now :)

    I get two cylinders so 2x shallow(ish) rib or hardboat dives rigged as singles a day not a problem
    and if I do want anything deeper the same rig can be twinned up fast. This has meant I've got loads
    of gas for deeper stuff, but if blown out and having to dive somewhere shallow, still have the option
    to go single in about 3 mins :)
    .
    I'm lucky in that I have a stupid number of cylinders to play with and do use a 15 and pony every
    now and again, but for trim, ease of use and better redundancy, the twin 12's are better no question.

    It's also tends to be way cheaper as well for most common stuff. Two dives a day = 2x cylinders,
    add redundancy 3x cylinders - Twin 12's just 2 cylinders :)
     
  13. Nick Ward

    Nick Ward Active Member

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    Why not twin set diving?

    25-30m + is definitely edging into two-cylinder territory - bottom time aside, redundancy for peace of mind is a boggy

    Doesn’t have to be twin 12s - a 15 plus a small stage (5-7l) would work well if you’re dead against back mounted twins and would see you ok down to 40

    I wouldn’t go past 25 without redundancy ymmv...
     
  14. Vanny

    Vanny Active Member

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    This is what I was getting at. Depth and some bottom time will dictate what kit you’ll need. 30-35 mtrs nitrox minimum 15 ltr but for any meaningful dive time a deco gas as well.

    Your interested in hitting wrecks to 40 mtrs, loads of great stuff round the uk, but you say your not interested in twin 12’s. To be honest wrecks in that range will probably be 2-3 hour round trip , 50 quid or more boat fees for maybe 10-15 mins bottom time on a single/single and deco, ignoring limited gas supply/reserves. I reckon you’ll find your wasteing your time.

    Consider a twinset & training then deco training. Or wait until your prepared to.
     
  15. kevin b

    kevin b Member

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    Thanks for replys it gives me something to mull over
     
  16. Wibble

    Wibble Fish don't talk
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    The ali40 is a great cylinder. Light and small and very easily clipped on. It also gives you some experience with stages and can easily be used for a deco gas when you're ready. A stage also means you have full control of it as it's in front of you and can be given to someone else who needs it.


    @Vanny's comment of the effort you put into a trip is a very good point. As you go beyond 30m, your need for redundancy becomes more critical and your bottom time's shrink.

    For example, 35 metres on 30% Nitrox with a single 12. Using Multideco planning software, for no deco you've about 14 mins on the bottom. That bottom time could be extended with deco, assuming using the back-gas to exit with 50 bar, this would give you 25 mins on the bottom with 11 mins of deco (on backgas), surface to surface of 42 mins.

    For comparison, pushing that to the limit with twin 12s and two 7 litre stages using 50% and 80% deco gas would give you 65 mins on the bottom, surface to surface of 103 mins (accounting for a single gas failure too).

    Basically, a 12 leaves plenty of gas for 35 metres and no deco, with a pony/stage used for redundant gas emergencies. Extending the bottom time needs deco and will require more gas than a 12 allows.

    So, recalculating for a 15... Max time would be 30 mins bottom time with 18 mins deco; 54 mins surface to surface.
     
  17. Dave Whitlow

    Dave Whitlow Super Moderator
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    :eek: Yes, but a twinset and two stages is a lot more complicated for a 35m dive, and yet so limiting! ;)

    If you ware looking to maximise bottom time you should at least plan with the best equipment for the job (rebreather)!
    Observation tells me over 90% of divers are content with their single cylinder configuration and the dive time that allows. It is compact, relatively inexpensive to run, and doesn't require an understating wife and a whole garage dedicated to diving!
     
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  18. nickb

    nickb Well-Known Member

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    Who the hell, in their right mind, is taking two 7l deco bottles in for a 35m dive when one AL40 of O2 (the correct deco gas for this dive if your rebreather is malfunctioning/unavailable) will suffice?
     
    Dave Whitlow likes this.
  19. nickb

    nickb Well-Known Member

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    Actually, this is relevant for you @Wibble as this is a typical Scapa dive (albeit the average depth will probably be a little less, on the cruisers at least).

    By all means fill your boots and insist on diving with two deco gases but be prepared for some sideways glances.

    Put 32% in your twins and carry a 7l or AL40 of O2 and you'll be sorted for everything except the Markgraf for which you'll want 28% (and a squirt of He if you fancy it) and O2 or 50%&O2 if you must.
     
  20. Wibble

    Wibble Fish don't talk
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    The 50 + 80 was just to illustrate standard gasses and planning to show the progression with more kit and caboodle.

    I'd normally do that sort of dive with a couple of deco tins with 80%; either 7 litre or ali40. Only need one tin, the other is only for a backup to guarantee deco gas as there's not enough backgas without considering teammates.

    Am up in Scapa twice this year and will be using that config, probably with some He for the deeper dives. Really looking forwards to it as it'll be my first time. Hoping to dive in Loch Ness on the way back.
     

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