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Thread: Comparative sizes of parts in rebreathers

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    RBW Member Anthony Appleyard is an unknown quantity at this point Anthony Appleyard's Avatar
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    Comparative sizes of parts in rebreathers

    What is the maximum safe pressure of rebreather gas cylinders, or any gas cyinders? Has that pressure advanced much beyond 200 bars in recent years?

    Some rebreathers that have everything in a big backpack box, have the gases in spheres instead of cylinders. Can the walls of a gas sphere be thinner than the walls of a gas cylinder for the same diameter for the same maximum safe pressure?

    Is there a rule for the ratio between the amount of absorbent to be carried and the amount of oxygen to be carried, both for the same dive period?

    The ratio between the amount of diluent to be carried and the amount of oxygen to be carried will likely vary according to the diving to be done; but is there a rule-of-thumb rule for this ratio?

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    RBW Member tbone1004 is an unknown quantity at this point tbone1004's Avatar
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    Re: Comparative sizes of parts in rebreathers

    300 bar is normal in Europe, 250bar is about the most you see in the US

    the biggest restriction is going to be the ability and cost of compressors to go much higher than that. 300 bar compressors are the "norm", but it costs a lot of money to go up to the 400bar compressors that many fire departments here are using with their composite cylinders. Storage banks also get extraordinarily expensive. To go from 300bar to 400bar is about a 2x increase in cost.
    The other issue that is CCR centric is the safety of high pressure oxygen fills. I'm still not 100% comfortable with 200bar O2 fills from a safety standpoind.

    regarding the spheres, yes they can be made with thinner walls than a cylinder, but getting them made is prohibitively expensive which is why no one is currently producing them. The smCCR guys would love for them to come out, but no one is making them.

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    Re: Comparative sizes of parts in rebreathers

    I think you can get them from Acme's representative, Wile E. Coyote.
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    RBW Member Anthony Appleyard is an unknown quantity at this point Anthony Appleyard's Avatar
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    Re: Comparative sizes of parts in rebreathers

    Around 1960 here in Britain the maximum pressure of Siebe Gorman aqualung cylinders was 120 bars underwater, 150 ats on the surface.

    Quote Originally Posted by tbone1004  View Original Post
    300 bar is normal in Europe, 250bar is about the most you see in the US
    The image at this link shows 3 USA naval rebreathers with spheres instead of cylinders.

    https://commons.wikimedia.org/wiki/F...abian_Gulf.jpg

    Quote Originally Posted by tbone1004  View Original Post
    regarding the spheres, yes they can be made with thinner walls than a cylinder, but getting them made is prohibitively expensive which is why no one is currently producing them. The smCCR guys would love for them to come out, but no one is making them.

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    RBW Member tbone1004 is an unknown quantity at this point tbone1004's Avatar
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    Re: Comparative sizes of parts in rebreathers

    Quote Originally Posted by Anthony Appleyard  View Original Post
    Around 1960 here in Britain the maximum pressure of Siebe Gorman aqualung cylinders was 120 bars underwater, 150 ats on the surface.



    The image at this link shows 3 USA naval rebreathers with spheres instead of cylinders.

    https://commons.wikimedia.org/wiki/F...abian_Gulf.jpg

    modern cylinders are filled to up to 300bar in europe, and 250bar in the US. That's the reality now

    regarding the USN rebreathers, yes those use spheres, they are inconel spheres and are 3l cylinders rated to 200 bar. They are about $2000 USD used, if you can find them. There are no new sources without purchasing a MoQ of about 50 from the OEM in California

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    RBW Member Doomanic is an unknown quantity at this point Doomanic's Avatar
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    Re: Comparative sizes of parts in rebreathers

    In the U.K. the norm is 232bar. 300bar cylinders are available but fill stations capable of 300bar fills are few and far between. Amongst the people I know with 300's a "good" fill is anything over 270bar. As a "good" fill for 232's is 220 and my scrubber will run out long before my O2 are 300's worth the weight and hassle?

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    Re: Comparative sizes of parts in rebreathers

    Does anyone boost O2 beyond 200 bar?

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    Re: Comparative sizes of parts in rebreathers

    Since my gas supplier supplies 300Bar O2 and I run a 5 tank O2 cascade, I rarely finish an O2 fill below 270Bar. It does mean though that I'm returning O2 tanks still with 80-100Bar in them. At the prices I pay for 300 Bar O2 I would have to not return 300+ O2 tanks at 100Bar in order to pay for a used booster. It just isn't worth it.
    Michael

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    Re: Comparative sizes of parts in rebreathers

    Real gas laws lead to diminishing returns on higher pressure cylinders.

    A cylinder pumped to 300 bar does not contain 50% more than the same cylinder pumped to 200 bar.

    Up to 200 bar compression is pretty much linear (ideal gas laws). I seem to remember it being about 10% less than linear at 300 bar, so the 300 bar cylinder is only as good as 270 bar if everything were linear (my memory for numbers could easily be wrong).

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    Re: Comparative sizes of parts in rebreathers

    Quote Originally Posted by Anthony Appleyard  View Original Post
    Around 1960 here in Britain the maximum pressure of Siebe Gorman aqualung cylinders was 120 bars underwater, 150 ats on the surface.



    The image at this link shows 3 USA naval rebreathers with spheres instead of cylinders.

    https://commons.wikimedia.org/wiki/F...abian_Gulf.jpg

    Anthony,

    Those divers are diving Mark 16 Mod 0 rebreathers (there is no switch on the top that would designate them as Mod 1, and they don't have our PEA in it which would designate them as Mod 2 and 3). The spheres are made of Inconel, and are still produced by Cobham. Tavco in Chatsworth, CA used to make them. They are 3,000 psi working pressure, with 4500 psi test pressure.

    There have been "special" spheres made which can handle much higher pressures.

    None of these are CE or DOT approved, btw. The Navy spheres carry the "DOT-E" exemption certificate.

    The only "civilian" unit I know of that used Spheres was Dan Wible's CCR-2000 that was eventually built by Olympic Submarine - those were the last spheres that Tavco made. Olympic stopped making them quite a while ago, and though Dan sold a number of units (I added a Hammerhead Controller to Tom Mount's personal unit) the CCR-2000 never quite caught on.

    Kevin Juergensen
    Juergensen Marine, Inc.

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