1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Dil & O2 cylinder sizes

Discussion in 'Rebreathers Closed and Semi Closed Circuit' started by Wibble, Oct 31, 2017.

  1. Wibble

    Wibble Fish don't talk
    UKD Supporter

    Joined:
    Apr 7, 2014
    Messages:
    4,714
    Likes Received:
    1,034
    Dumb non-box person question...

    Are 3 litre cylinders "better" than 2 litre cylinders?

    I appreciate that you actually only use the diluent for filling the loop on the bottom; once they're 'filled' then you don't need the diluent except for problems. Similarly the O2 is only required to replace the metabolised O2 (about 1 litre/min???). So therefore there's plenty of gas in 2 litres.

    However, if there's problems, wouldn't having additional gas be a benefit? For instance each time you flush the loop you'd need a fair amount? (Excuse my ignorance, not dived one nor taken any courses)

    Also if you're doing multiple dives, wouldn't 2 litre cylinders mean that you'd need to fill them in between dives?

    Are 2 litre tins that much lighter than 3 litre tins?


    This thought train was brought on by looking at the Red Bare which only has space for 2 litre tins unless modifications are made (the stand needs to be extended and there's only enough space for 'dumpy' 3 litre tins).
     
  2. Dave Whitlow

    Dave Whitlow Well-Known Member

    Joined:
    Oct 27, 2010
    Messages:
    847
    Likes Received:
    317
    Diluent is used is small amounts on the descent and also for filling the wing at the surface so, typically 15-30 bar (in a 3l) is used per dive.

    Diluent flushes can consume large amounts of gas, especially at depth, but a flush is an exception as a response to either high loop ppo2 or to check a cell anomaly.

    Oxygen use depends on the person, and work rate, but is typically 20 bar (in 3l) per hour, with additional oxygen being expended on the ascent, especially at 6m, as some oxygen is discarded with inerts during deco. I also flush the loop with oxygen to force the ppo2 to 1.6 to confirm my cells can read high.

    For most diving done by most people 2l bottles are enough. The need for refills after a few dives is generally resolved by owning multiple bottles (I have 4 sets) or by decanting from a 'bank'.
     
  3. nickb

    nickb Active Member

    Joined:
    Oct 13, 2009
    Messages:
    390
    Likes Received:
    152
    2l cylinders tend to be used on smaller rebreathers: AP Evolution, rEvo micro etc.

    I think that the Sentinel had 2l bottles as a design philosophy, although anything to reduce the burden of lugging one around would have been welcome and this appears to have been carried over to the vastly superior RedBare.

    A short stack Megalodon will often have 2l cylinders too but most CCRs will use 3l tins as these tend to offer the best trade-off between size and convenience.
     
  4. Vanny

    Vanny Active Member

    Joined:
    Apr 18, 2016
    Messages:
    398
    Likes Received:
    132
    Yes what Dave said. I've 3ltr cylinders. On my mix course at Chepstow the instructor used an inspo Evo with 2ltr cylinders and needed to change them/fill , certainly the O2 , between dives. Albeit deep dives. 3ltrs seem to give more flexibility and like Dave I have 2 sets, although 1 sets out of test because I'm tight and lazy.
     
  5. Wibble

    Wibble Fish don't talk
    UKD Supporter

    Joined:
    Apr 7, 2014
    Messages:
    4,714
    Likes Received:
    1,034
    Thanks for that.

    The Redbare did seem very nice (to a person with no experience of CCR) although it wasn't as clean around the back as the JJ and Inspiration (when mounted on a third-party frame). Whilst the idea of a JJ with twin 7's a-la GUE configuration seems a little, errm, esoteric, it's the fact that a JJ can be configured like this which is testament to how flexible it is.
     
  6. Vanny

    Vanny Active Member

    Joined:
    Apr 18, 2016
    Messages:
    398
    Likes Received:
    132
    I haven't seen the redbare in the flesh and know very little about it. The inspo in a frame can have any cylinder/valve config which I think is what your getting at with the GUE JJ. I haven't tried bigger than 3's on mine but the general vibe is that anything bigger e.g. 7's are a real heavy pig to dive. So whilst a nice to be able to do in reality fairy inpractical for normal diving. Ben in scapa uses 7's on his meg , my understanding, is due to the amount of wreck penetration he does the larger cylinders become his bail out, so good for a specific purpose.

    Recently I've set mine up to take 2 pairs of 3's , so 2x O2 , one for the unit , one for back up/bail out. 2xdil , usually mix driving the unit the other air for suit/bail. On shallow dives no need for separate bail out on deeper longer dives coupled with 2x7 bailout it improves my bailout choices.

    I think the units stock are all v good. All this dicking about might help or maybe just satisfying my need to tinker. Whilst the AP plastic box isn't v popular I still argue with myself if it was the best compromise, it's certainly a lot lighter than my current set up to shift around, so in time I may just go back to it, for the time being I'm very happy with how my set up dives and works.

    The JJ. Seems to get left pretty much stock , the AP seems to encourage butchering.
     
  7. nickb

    nickb Active Member

    Joined:
    Oct 13, 2009
    Messages:
    390
    Likes Received:
    152
    FTFY
     
  8. Vanny

    Vanny Active Member

    Joined:
    Apr 18, 2016
    Messages:
    398
    Likes Received:
    132
    I larfed so hard snot came out my nose. Thank you
     

Share This Page

  • About Us

    Our UKDivers community has been around for many years and prides itself on offering unbiased, helpful discussion among people of all disciplines and abilities. We are working every day to make sure our community is one of the best and friendliest around.
  • Support us!

    The management works very hard to make sure the community continues to run reliably. Care to support us? All donations go to the running costs of the forum: hosting charges, software maintenance, etc. We'd really appreciate it!

    Choose option:  

    UKD Username: