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Filling 8.5l twins by decanting from a 15l?

Discussion in 'Dive Equipment' started by Jimnibimnibob, Jun 29, 2020.

  1. Jimnibimnibob

    Jimnibimnibob New Member

    Jun 5, 2014
    Likes Received:

    Has anyone any experience of filling an 8.5 litres twin set for a second dive by decanting from a 15l cylinder? If so, what how much of fill do you get out of the 15l based on having run them down to about 70bar from you first dive?


  2. nickb

    nickb Well-Known Member

    Oct 13, 2009
    Likes Received:
    About 145 bar, assuming a few things.

    So, not really worth the shag
  3. Vanny

    Vanny Well-Known Member

    Apr 18, 2016
    Likes Received:
    Where as twin 12 based on the same gas use would leave you around 110/120 bar before topping. I feel like we’ve been here ?
  4. NickPicks

    NickPicks Active Member

    Jun 7, 2010
    Likes Received:
    you can calculate this.
    Assuming 230 bar in the single 15 and 70 bar in the twin 8.5s.

    Close the manifold, fill left cylinder - sum of cylinder contents: 15*230=3450 plus 8.5*70=595 makes total 4045: divide by total volume (8.5+15) to make equalising pressure of 172 bar.

    Fill right cylinder: total contents of 15 & R8.5 is 172*15 + 8.5*70 = 3177, divided by total volume makes 135 bar.

    Now open the manifold: Add the two 8.5 contents together: 172*8.5 + 135*8.5 = 2612, divide by total volume gives 154 bar.

    If you'd done it with the manifold open, you'd have got:
    15*230 + 17*70 = 4640, divide by total volume of 32 litres, gives you 145 bar.
    Tribal Chestnut and splinter like this.

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