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Short Air Fills

Discussion in 'General Scuba Equipment Discussions' started by bug catcher, Jun 26, 2012.

  1. bug catcher

    bug catcher Solo Diver

    # of Dives: 200 - 499
    Location: Wisconsin
    46
    10
    0
    Does anyone know approx. how much cubic feet of air is lost on a short fill of say 2700lbs. on a Alu. 80cf tank? I have a HP 130cu and read somewhere a fill of 3000lb being 440lb light would only give me approx. 90cf so I wondered if the reduction on a Alu. 80 would be proportionately the same? On a shallow dive I don't mind the short fill but I hate to loose bottom time on a paid charter. I know to tell the shop to fill slower (and I have) but it don't sink in and they won't put the tanks in cooling water. Thanks in advance!
     
  2. nimoh

    nimoh Public Safety Diver

    # of Dives: 500 - 999
    Location: Rochester, MN
    3,455
    1,690
    113
    Al 80:
    80cuft/3000psi = 0.02667 cuft/psi

    0.02667 cuft/psi * 2700 psi = 72 cuft (i.e you are losing 8 cuft of gas with the short fill)

    HP 130:
    130cuft/3442psi = 0.03777 cuft/psi

    0.03777 cuft/psi * 3000 psi = 113 cuft (i.e. a little better than 90 cuft)
     
  3. mala

    mala DIR Practitioner

    # of Dives:
    Location: old hampshire
    597
    198
    43
    i can do the metric maths .

    just multiply the size of tank by the pressure.

    so a 12ltr at 200 bar has 2400lts of gas.

    at 190 bar it would have 10 x 12lts less.(2280lts).
     
  4. nimoh

    nimoh Public Safety Diver

    # of Dives: 500 - 999
    Location: Rochester, MN
    3,455
    1,690
    113

    the math is so much easier in metric (I had to use a calculator for my answer)
     
  5. mala

    mala DIR Practitioner

    # of Dives:
    Location: old hampshire
    597
    198
    43

    you cant take a calculater underwater.
     
  6. Peter_C

    Peter_C Solo Diver

    # of Dives: I just don't log dives
    Location: Santa Rosa, CA
    5,889
    741
    113
    Most of it is simple math.
    130/3442=.03776x3000=113
    An AL80 is actually a 77cf tank.
    77/3000=.025666x2700=69

    Leave the tanks for a longer period of time at the dive shop and do not wait on them. Then when you go to pick them up they can top them off. No need for cooling water, and many will argue the cooling water can get into the tanks.
     
    Bigd2722 likes this.
  7. nimoh

    nimoh Public Safety Diver

    # of Dives: 500 - 999
    Location: Rochester, MN
    3,455
    1,690
    113
    I agree, but you should probably know how much air you have before going underwater :)
     
  8. mala

    mala DIR Practitioner

    # of Dives:
    Location: old hampshire
    597
    198
    43
    i do.
    its the op that was asking the question.

    you just never know what might happen underwater and when a quick calculation might just safe your life.


    if i had to work out cu/ft ,psi,water depth in feet and sac rate in whatever you use i would be lost.
     
  9. nimoh

    nimoh Public Safety Diver

    # of Dives: 500 - 999
    Location: Rochester, MN
    3,455
    1,690
    113
    Well, isn't this thread just getting a bit hijacked, so I only have one more comment.

    I don't see anywhere in the original post asking how to do this calculation underwater, but in any case, I wouldn't bother with either the metric or imperial calculation to save my life on a dive, I would be heading for the surface hoping I (and my buddy) have enough air for the journey.
     
    Bigd2722 likes this.
  10. dfx

    dfx Regular of the Pub

    # of Dives: 200 - 499
    Location: Binbrook, ON
    1,834
    647
    113
    It's quite easy to answer the OP's question without calculator. When 3000 psi give you 80 cu ft, then 300 psi give you 8 cu ft.

    80 - 8 = 72.
     

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