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12ltr or 15ltr tanks

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

  1. Dave Whitlow

    Dave Whitlow Super Moderator
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    Yes, that is all very interesting except the OP asked for guidance on whether a 15l cylinder is a worthy contender for his next acquisition. It is.

    Perhaps you can explain how a rubbish mix like 80% can be considered a 'standard gas'!
    Scapa is nice easy diving which is rarely blown out. You'll enjoy it, although two stages seems a lot of work and quite costly on gas! For maximum crew appreciation you should side-mount all your bottles :D

    For what it is worth gas costs for a week of CCR diving in Scapa, doing to 2 x 90 minute dives per day, is likely to be around £30 (assuming no bailout is required).
     
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  2. Wibble

    Wibble Fish don't talk
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    Prefer to mount two stages left. With shorter clips they don't excessively dangle and 7 litre/ali40 are light and easy enough to handle on the boat -- certainly when compared with Ali80's.

    As for 80%... it's efficient for deco as you can get on it at 9m; doesn't rack up the O2 exposure like 100% does; and can be pumped to 232 bar without boosting (O2 fill to 185 bar and air top). So for the Scapa depth dives (35 to 45 metres) would easily give two or three dives from a 7 litre tin. Will do more planning of course.

    Agreed that a box does offer gas efficiency, but I don't have one (yet) and it does need a lot of training and practice. Can't overlook the additional faff they bring too. But yes, of course I want one.
     
  3. jb2cool

    jb2cool Moderator
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    You can get on it earlier but as soon as you move away from the 9m stop it loses its efficiency. O2 is efficient at off gassing inert gas since it has no inert gas in it maximising the gradient.

    Of course 80% works and it is the choice of some but it’s not my choice and although a popular gas for some.
     
  4. Leeb2018

    Leeb2018 Active Member

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    I use a 80%. I dive rebreather so helps keep your cns down and can be cross patched in if needed. For shallower stuff I carry a bottom gas in case I need to bail out from depth but also is available for cross patching. Deeper stuff is bottom mix and 80%. Really deep stuff we all carry a bottom gas but mix up the other gas between us for team bail out
     
  5. Wibble

    Wibble Fish don't talk
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    To which I'll apologise for calling 80% a 'standard' gas! I'd happily change 'standard' for 'popular' gas.

    Last year I did a dive out of Portland -- the excellent Frognor -- which was a minor miracle to do the dive as I'd been blown out of two other boats that day (they forgot to tell me they'd cancelled the night before, so I drove the 3+ hours arriving at 0700 and was definitely not amused). Anyway, did a lovely dive off of Scimitar and had a limit of 90 mins for the dive. I had a twinset of 32% and an ali40 of 100%. Was 60 mins on the bottom and 30 mins deco. I was surprised to see I'd pinged a CNS warning on my computer as I'd hit the limit. Wouldn't have seen that on 80%.
     
  6. Nick Ward

    Nick Ward Active Member

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    Stick by your guns - 80% is both standard and popular

    For BSAC divers it’s the richest mix you can use until you’ve done the second level Trimix course...
     
  7. timmyg

    timmyg Super Moderator
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    Most people I know use 80% as their richest gas, regardless of agency as the deco difference is only a couple of mins but the CNS can be up to half, but that’s a different topic.

    For Scapa, a few months ago anything more than 1 stage & you’ll have questions asked.
    AM. I was doing 60-80 min RT’s on the deeper wrecks on Nitrox (air top from day before) with a 50% Ali 80. That 80 would last 2 dives. This was also using DPVs so breathing rate was less.
    PM. 60 min RT on Nitrox 32/36 depending on the depth.
    I took a second stage on the boat in case of equipment failure but it was not used.


    TG

    Sent from my iPhone using timmytalk
     
  8. Wibble

    Wibble Fish don't talk
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    Glad I'm not the only one who likes 80%.

    Regarding the second stage, I've kind of got used to having a backup tin just for self-sufficiency as I'd be in a proper mess if the deco stage failed and I've pushed the backgas down to 50 bar. I find the ali7 works well with a small ali40 sitting next to it.

    As for Scapa, going to have to think carefully about the kit I'll be taking as there's two of us in the car. Was thinking of taking a second twinset with a relatively strong helium mix which I'd decant into the main twinset, i.e. two helium dives. Thinking that for 45m dives we'd only need 10 or 20% helium to lift the narc.

    Hmm, thinking about it, could consider taking an ali80 with a very strong helium mix -- 20/80 -- and decanting that into one twinset a couple of times, then topping off the ali80 as a bottom stage, e.g. 3 deeper dives. I guess it depends on the cost of a helium mix I can get down here (I think I can get it for half the price of a mix up there).
     
  9. timmyg

    timmyg Super Moderator
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    VW Passatt estate.
    1x X-CCR
    2x twin 12's (one for a friend flying up)
    3x ali 80's (one for a friend flying up)
    1x ali 7
    2x DPVs
    2x camera rigs, arms etc...
    2x kit bags with fins, DSMBs etc...
    Analysers etc...
    Enough spares for an imaginary diver inc primary lights, masks, spools etc...
    3x drysuits
    2x clothes bags
    2x laptops etc...
    2x passengers

    Car was on its axels but still did 52mph on cruise control.
     
  10. Wibble

    Wibble Fish don't talk
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    Definitely taking a spare drysuit after the shenanigans in Oban last year!

    Thinking about the helium, that ali80 idea would work well for two of us bubble blowers as we'd only need one twinset and one ali80 each (plus a couple of 7's and an ali40).
     
  11. timmyg

    timmyg Super Moderator
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    I didn't bother with He. There was only 1 dive below 40.

    TG
     
  12. korsakof

    korsakof Member

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    Have you tried sidemount?
     
  13. Wibble

    Wibble Fish don't talk
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    That's very useful to know, thanks. I've found that although I can dive below 40m on Nitrox, the narcosis is a bit too strong, so a squirt of helium helps -- even 10%.

    The problem with Sidemount is they are hated on hard boats!

    I dive sidemount in freshwater, particularly overhead dives. Much easier to kit up and carry the cylinders separately. However, on a moving boat it can be difficult to get the cylinders on and it really annoys the other divers as you're a lot wider!
     
  14. korsakof

    korsakof Member

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    Seriously? Why are they hated? It’s comfortable and easy. Don’t the go to the water one by one?
     
  15. Wibble

    Wibble Fish don't talk
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    On our UK hard-hulled boats the sidemounters are generally the target for lots of piss-taking. Mainly because the back-mounters are jealous :)

    Most of the diving I do in the UK is on hard boats. These generally take about 12 divers and have specially designed benches to tie your twinsets and rebreathers on to -- even single cylinders! When you've sidemount kit, your cylinders will roll around a lot and it annoys the other divers as you're also taking up more space.

    However, RIB diving (inflatable boats) is another thing though as cylinders are much easier to put on when sitting on the sponson (inflated bit). It's also easier to pass up the cylinders up and climb back on.
     
  16. korsakof

    korsakof Member

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    WTH!!! Sorry if I understand the get annoyed because the tanks make noise when the are rolling up?also if there are other kind of tanks they will touch and make also noise, the gap is the gap. I think that you can attach somewhere to the boat.

    You need more space is obviously but you can do the first, the last or in the water. I don’t see the problem.
    If I go I will keep my sidemount set at home and take mi old bcd.hehehe.
     
  17. Dave Whitlow

    Dave Whitlow Super Moderator
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    :eek: Not for me. That very much depends on your tolerance for narcosis and acceptance of risk. If it is an option then I would always advise use of helium for that dive and some of the others are close to 40m.

    The first time I dived the Markgraf (in 2009), I was an ART diver diving a twinset of 25/25 with an AL40 of 50%. It was quite dark and foreboding in the shadows of the hull, down at the seabed, and I was grateful for the clarity of thought that helium gave me.
     
  18. Dave Whitlow

    Dave Whitlow Super Moderator
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    Looking back at my records the depths I've seen in Scapa are:

    Bayern Turrets 39-42m
    Brummer 35-36m
    Coln 32-36m
    Dresden 32-38m
    F2 15-18m
    Karsruhe 25-28m
    Konig 39-42m
    Kronprinz 38-41m
    Markgraf 42-47m
    Marra 17m
    Sydlitz Debris 20-22m
    Tabarka 16m
    UB116 30m
    There is a place for sidemount diving and the rolling deck of a boat is not one of them, especially the sloping decks found of a converted fishing boat.
     
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  19. Wibble

    Wibble Fish don't talk
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    Thanks for those depths - very useful.

    Thinking the Ali80 with 20/80 might work quite well with some judicious decanting -- should get enough out for all but the Markgraf which deserves a richer mix.
     
  20. Leeb2018

    Leeb2018 Active Member

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    Get a rebreather gas choice is a lot easier and cheaper. 16/44 covers you to 60m. It costs £15 for a fill and you get hours of dive. Plus more bottom time

    Sent from my SM-G960F using Tapatalk
     

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