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Decanting - most efficient way?

Discussion in 'General Scuba Diving' started by Wibble, Jul 11, 2018.

  1. Wibble

    Wibble Fish don't talk
    UKD Supporter

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    Two questions about decanting. Want to end up with the most pressure in the target cylinder.

    1) Single to twinset.
    Should the manifold valve be opened and just decant?
    or
    Should the manifold be closed then decant into one side of the twinset, then disconnect and decant into the other side of the twinset, disconnect then open the manifold.

    2) Twinset to single
    Ditto?

    Further marks for workings!
     
  2. Vanny

    Vanny Active Member

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    1) shut manifold decant into both sides then open.

    2) shut the manifold. Use one side of the twin set for volume then side 2 to top up pressure. Leave the manifold shut and continue decanting 1 then 2.

    Poke doing maths.
     
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  3. Dave Whitlow

    Dave Whitlow Well-Known Member

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    @Vanny is correct.

    Do the maths with single 12 having 200 bar (2400 litres) and empty twin 12s.
    (sums ignore the 24 litres already in the twinset)

    Option a - manifold closed
    Step 1 fill side 1 giving 100 bar in each (1200 litres moved)
    Step 2 fill side 2 giving 50 bar in each (600 litres moves)
    Step 3 equalise to 75 bar

    Result: 1800 litres in twinset and 600 left in the single 12

    Option b - manifold open
    Equalise cylinders so 66.66 bar

    Result: 1600 litres in the twinset and 800 left in the single 12.
     
    Wibble likes this.
  4. jb2cool

    jb2cool Moderator
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    In my experience closing the manifold is never worse. It's sometimes barely noticeable and you might question whether the effort is worth it but you can sometimes magic up 10 more far or so making it worthwhile.
     
  5. nickb

    nickb Active Member

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    Pissing about with trying to make a cascade from a twinset is not really worth the bother, especially if you need to depressurise the whip in-between.

    The correct answer is:

    Get a booster
     
  6. jb2cool

    jb2cool Moderator
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    ....and a compressor to drive it.
     
  7. JohnL

    JohnL Well-Known Member

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    Difficult if you've got a garage without power. :whistling: I'll have to stick to dive centres, which is fine for my diving.
     
  8. snowman

    snowman Active Member

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    Don't have any experience personally OR the maths to back it up, but on a recent set of dives those 'in the know' filled the cylinders in their twinset separately.

    The logic above makes good sense to me, but I failed miserably at A-Level maths because things that were totally wrong made perfect sense to me! :D

    M
     
  9. Nick Ward

    Nick Ward Active Member

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    phantomlurker and Wibble like this.
  10. John F

    John F Member

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    What an interesting site, saved that one in favourites.
     

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