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Decanting result?

Discussion in 'General Scuba Diving' started by Zubar, May 17, 2017.

  1. Zubar

    Zubar Active Member
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    I need some mafs help. If I had a twin 7s set (14l) down at say 50 bar, and topped it back up from a set of twin 12s (24l) at say 220bar, what would I get in the 7s? This should be easy but I'm a mathstard.

    Trying to decide if i should ditch the 7s for a pair of 12s as a single set, just don't want to admit I need to and have the expense.

    Thanks buddys

    C
     
  2. becky9

    becky9 Diving bore!

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    157 bar x
     
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  3. Zubar

    Zubar Active Member
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    Hmmmmm. 50 bar was just a lowest pressure example. 157 wouldn't give much of a second dive, even if it is shallow rib dives.

    Thanks Beks
     
  4. Stretch_y2k

    Stretch_y2k New Member

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    170 bar if you cascade from the 12s separately.

    Sent from my SM-G900F using Tapatalk
     
  5. becky9

    becky9 Diving bore!

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    Its the equiv of around 180odd bar in a single 12, so not that bad really. Q is I guess are you going to get down to 50 bar, you'd prolly have more depending on your buddy x
     
  6. Zubar

    Zubar Active Member
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    yep, on a recreational dive, if I started with 14l at 220 bar i'd likely be finishing with 80 bar which is more sensible and realistic.

    What is the formula for working this out?
     
  7. becky9

    becky9 Diving bore!

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    Yep thats what I thought. You can cascade as stretch says closing the manifold, but its not a huge difference.

    v1 = Cyl 1 size in litres
    p1 = Cyl 1 pressure
    v2 = Cyl 2 size in litres
    p2 = Cyl 2 pressure
    p3 = Settled pressure

    p3 = (v1xp1)+(v2xp2)

    I have an excel sheet with this on and some other stuff, I'll see if I can find and send it over

    x
     
  8. becky9

    becky9 Diving bore!

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    Ooops missed the divide by total volume - p3 = (v1xp1)+(v2xp2) /( p1+p2)

    x
     
  9. barrygoss

    barrygoss Active Member

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    This is a bit of a simplification

    It's. P1st x Volfirst + Psecond x Volsecond = Vol total divided by (volfirst add volsecond)

    So for twin twelves for the first dive and a 15 for the top up.

    Finish first dive on 100bar on the twins.

    24x100 + 232x15 = 5880/39 = 150 bar.

    B
     
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  10. becky9

    becky9 Diving bore!

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    Yep I missed the divide as I said above x
     
  11. nickb

    nickb Active Member

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    (Pressure * volume of vessel 1 plus pressure * volume of vessel 2) divided by the total volume of vessels 1 and 2
     

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