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how much does air weigh?

Discussion in 'Basic Scuba Discussions' started by H2Andy, Feb 26, 2007.

  1. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
    Location: NE Florida
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    ok ... i've tried a few searches and Google, but can't seem to find a quick answer


    we all know that a 80cf tank will hold 80cf of air (more or less) at its rated pressure

    so ... if you have an AL80 rated for 3,000 psi, it will hold 80cf of air at 3,000 psi

    now, we all know that a full cylinder weighs more than an empty one, right?

    so .... can anyone tell me what the weight of air per cf is?

    that way i will know how much a tank that weights 34 lbs empty will weigh when it is full to its capacity

    thanks!
     
  2. 3-Ring Octopus

    3-Ring Octopus Solo Diver

    # of Dives: 50 - 99
    Location: Eugene, OR
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    http://www.huronscuba.com/equipment/scubaCylinderSpecification.html

    Found that the other day when trying to calculate how much weight I need to take off going from my usual AL80's to LP95's for an upcoming trip to Seattle (I want more bottom time in good vis!! lol).

    Cheers,
    Austin

    Edit: Okay..maybe that doesn't help as much as I thought..it just gives bouyancy characteristics for each...hmmm...weight of compressed air. I guess we could do some fancy math. But "weight" would be dependent upon temperature. What temp will you be diving? lol http://www.engineeringtoolbox.com/air-desity-specific-weight-d_600.html I found that website with weights at given temps. We could work from there...
     
  3. Dan Gibson

    Dan Gibson Divemaster

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    http://www.engineeringtoolbox.com/air-desity-specific-weight-d_600.html

    I refer to the 70 degree F line.

     
  4. Lead_carrier

    Lead_carrier Instructor, Scuba

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  5. spectrum

    spectrum Dive Bum Wannabe ScubaBoard Supporter

    # of Dives: 500 - 999
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  6. OHGoDive

    OHGoDive Barracuda

    # of Dives: 50 - 99
    Location: OH, USA
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    Density of air is 0.0012 g/cm[SUP]3[/SUP] from here:

    http://www.iscpubs.com/articles/al/a0109bad.pdf

    I'll leave the conversion to cf and the calculation of how much a tank full of it weighs to you.
     
  7. H2Andy

    H2Andy Blue Whale

    # of Dives: 200 - 499
    Location: NE Florida
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    sweet, thank you all

    very nice
     
  8. Rick Murchison

    Rick Murchison Trusty Shellback Staff Member ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
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    About 40.4 pounds. But a more useful number is the difference in a full tank and the typical "end of dive" tank, which is about 5 pounds for an AL 80, for 'tis that number that tells you how much weight to carry.
    Just to shave a nit or two... a standard AL 80 holds 80 CF at 3100 psi; only 77.4 at 3000 psi.
    Both tank capacity (rated in CF) and the weight of air are dependent on what you accept as "STP" (Standard Temperature and Pressure) They are different for physicists (0C,760mm), meteorologists (59F,29.92 in) and marketing folks (whatever gives the "best" numbers for selling), and the answers vary accordingly.
    Rick
     
  9. fisherdvm

    fisherdvm Solo Diver

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    :wink: The weight of air is dependent on temperature, barometric pressure, humidity, and humility.
     
  10. Guba

    Guba Regular of the Pub

    # of Dives: 200 - 499
    Location: North Central Texas
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    Rick, I'm a science teacher, and your answer is about the most concise and straight foward I've seen. Good job!

    By the way, just today I'm showing my classes a powerpoint on "scuba physics". Part of the presentation is over Archimedes principle, the compression of gases at depth, and the weight of air. One set of pictures shows a Al 80 (full) on a set of bathroom scales, which reads 41 pounds. The next shows the same tank on the scales after a dive. The scales now read 36 pounds. Difference? (do the math). It seems to make the point well for my students.
    We also show Archimedes principle by weighing a decked out diver on scales on the dock. Next, the same diver is "standing" on the scales at 25 feet. Of course, he registers zero weight. Man, this sequence of shots was fun to shoot! My partner and I had a blast, and we got lots of quizzical looks from passing divers!
     

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