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Thread: formula to calculate volume of air in a cylinder

  1. #1
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    Default formula to calculate volume of air in a cylinder

    I tried to search the forum without success.

    How do I calculate the storage volume of air in a cylinder given the pressure range and the size.

    Example if i want to work out which will give more charges between 2 cylinders of different size and working pressures:

    1) A 6L cylinder going from 300 bar to 150 bar (150 bar working range) and
    2) A 10L cylinder going from 200 bar to 150 bar (50 bar working range)

    I would appreciate a formula which i could work these out and extrapolating it to 232 bar tanks with a different size etc.
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  2. #2
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  3. #3
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    ahhhh i see it, thank you Riaan
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  4. #4
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  5. #5
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    No formula needed actually. 1 bar = 1 atmosphere, therfore a 300bar 10L cylinder holds 3000L when full(at sea level), and 1500L when at 150bar. This can be used to calculate how many fills for your rifle if you know its cylinder volume. Just convert the leftover pressure to the amount of litres left in the rifle and subtract what is needed to fill the rifle from the cylinder and note what will be left in the cylinder after the first fill. Do this calculation over and over till you get to the least amount of "shootable" air left in the cylinder. Then you have a good "more or less" idea of fills from your cylinder.


    I say "more or less" because you'll always waist a bit of air by bleeding the excess after each fill.

    Hope it helps
    Last edited by madness; 07-09-18 at 00:51.
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  6. #6
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    [QUOTE=madness;637285]No formula needed actually. 1 bar = 1 atmosphere, therfore a 300bar 10L cylinder holds 3000L when full(at sea level), and 1500L when at 150bar. This can be used to calculate how many fills for your rifle if you know its cylinder volume. Just convert the leftover pressure to the amount of litres left in the rifle and subtract what is needed to fill the rifle from the cylinder and note what will be left in the cylinder after the first fill. Do this calculation over and over till you get to the least amount of "shootable" air left in the cylinder. Then you have a good "more or less" idea of fills from your cylinder.

    Using this example:

    1) Dive cylinder: 300 bar to 250 bar = 3000 L minus 2500 L= 500 L (of usable air)
    2) Airgun cylinder uses (250 bar to 150 bar of charge every fill which is 100 bar x 500ml) = 50 L of air

    500 L of usable air divided by 50 L per fill gives approx 10 fills

    This approximates with the 9 fills generated by the forum formula.

    Its convenient using the formula, but a much happier experience knowing how the calculation is arrived at

    Got it now....thank you all
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