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Weighed some tanks at various PSIs and got unexpected results.

Discussion in 'Basic Scuba Discussions' started by CaveSloth, Oct 16, 2019.

  1. CaveSloth

    CaveSloth Barracuda

    # of Dives: 50 - 99
    Location: The Deep South
    242
    43
    28
    On a pair of doubled LP85s:

    92.4 lbs at 3750 psi
    86 lbs at 2350 psi
    82 lbs at 1450 psi
    75.3 lbs empty (15 psi)

    On one AL80:

    39.2 lbs at 3100 psi
    35 lbs at 900 psi

    On another AL80:

    39.2 lbs at 3100 psi
    36 lbs at 1375 psi

    I calculate that the LP85s contain around 75.8 cf of air at their service pressure of 2640, and the AL80s contain around 70.5 cf at their service pressure of 3000 psi.
     
  2. OceanEyes

    OceanEyes PADI Pro

    # of Dives: 5,000 - ∞
    Location: Hollywood, Florida
    279
    236
    43
    You might want to rethink some of your math.

    According to the Huron Scuba chart the actual volume of the cylinders you mention when filled to their rated capacity are as follows:
    Luxfer 80- 77.4 cubic feet
    Luxfer S80- 78.2 cubic feet
    Catalina S80- 77.4 cubic feet
    OMS 85- 85 cubic feet
    As you did not specify which aluminum 80 you weighed, I've listed the three that are currently available in the USA that have a working pressure of 3,000 psi.
     
  3. CaveSloth

    CaveSloth Barracuda

    # of Dives: 50 - 99
    Location: The Deep South
    242
    43
    28
    Those may be the manufacturer published capacities, but I am showing the actual recorded weights.

    Can you show me where my math is off? Did you calculate the capacities yourself based on the psi and lbs I recorded and come up with different numbers?
     
  4. FreeFlyFreak

    FreeFlyFreak Solo Diver

    # of Dives: 25 - 49
    Location: California
    201
    142
    43
    What was the temperature of the tank when weighed?

    Was it the same temperature when weighed full as when weighed empty or low?
     
  5. rhwestfall

    rhwestfall Solo Diver

    # of Dives: 200 - 499
    Location: "La Grande Ile"
    12,334
    11,046
    113
    Need to show your math - As you said "75.3 pounds empty (15 psi)", is the 92.4 pounds at 3750 the gauge reading or gauge +15?
     
  6. CaveSloth

    CaveSloth Barracuda

    # of Dives: 50 - 99
    Location: The Deep South
    242
    43
    28
    Temperature was before and after diving, maybe 20 degrees or so different. Could have an effect possibly.

    Before diving would have been a higher temperature. The before figure would have been the one at highest pressure which was taken just before a dive. The fact that I get very similar cf per psi at all measurements though tells me that temperature had little effect.
     
  7. CaveSloth

    CaveSloth Barracuda

    # of Dives: 50 - 99
    Location: The Deep South
    242
    43
    28
    I would love to see how you got 84-85.

    Here is my math:

    92.4 - 75.2 = 17.2 lbs of air at 3750 psi
    86 - 75.2 = 10.8 lbs of air at 2350 psi
    82 - 75.2 = 6.8 lbs of air at 1450 psi

    17.2 / .08 = 215 cf @ 3750
    10.8 / .08 = 135 cf @ 2350
    6.8 / .08 = 85 cf @ 1450

    based on the numbers above we can calculate that the tanks contain:

    17.4 psi / cf
    17.4 psi / cf
    17.0 psi / cf

    1 / 17.4 = .05747 cf per psi

    2640 x .05747 = 151.7

    divide by 2 since they are doubled:

    Each tank holds 75.86 cf at 2640 psi.
     
  8. FreeFlyFreak

    FreeFlyFreak Solo Diver

    # of Dives: 25 - 49
    Location: California
    201
    142
    43

    Without a doubt it would.

    Try this thread I haven't dug into it yet.

    Tank volume calculations..what was the temp used?

    Pressure in a tank varies quite a bit with temperature.
    Same volume of gas but higher temp gives higher pressure inside tank.
     
  9. CaveSloth

    CaveSloth Barracuda

    # of Dives: 50 - 99
    Location: The Deep South
    242
    43
    28
    Weird though that I got a consistent ~17 psi per cf or air in the tanks at all measurements, regardless of the temperature variation.
     
  10. RayfromTX

    RayfromTX Student Of Gas Mixology Staff Member ScubaBoard Sponsor

    # of Dives: 100 - 199
    Location: Hill Country of Central TX
    5,879
    5,386
    113
    I posted based on my weights on my lp85s. Running the numbers on yours I get 77. I use .08 lbs per cu ft for the weight of air. I ignore real gas laws.
     

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