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Metric measurements?

Discussion in 'Basic Scuba Discussions' started by John Bantin, May 8, 2019.

  1. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,234
    9,633
    113
    *checks Wikipedia, does some simple arithmetic*

    About 2.4 cl.

    Can't say, as I've never driven for 14 days non-stop.
     
    D_Fresh likes this.
  2. chrisch

    chrisch Solo Diver

    1,062
    340
    83
    Part of a teacup. (Why you would put whisky in a teacup I don't know, foreigners put an e in it :wink:)
     
  3. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,234
    9,633
    113
    Top part, bottom part, left side part or right side part?
     
  4. Mustard Dave

    Mustard Dave Rebreather Pilot

    # of Dives: I just don't log dives
    Location: Manchester, UK
    546
    378
    63
    Why? If I know the volume of the tank and the current pressure, I can easily calculate the contents at any time.
     
    Storker likes this.
  5. John Bantin

    John Bantin Dive Shop

    # of Dives: 5,000 - ∞
    Location: London
    221
    366
    63
    If you have a 12-litre cylinder filled 232 times (232 bar) you've got 12x232 litres of gas (2784 litres)... but if you don't know what a litre is (2.2 pints) you're still none the wiser!
     
  6. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,234
    9,633
    113
    Imperial or US?
     
    John Bantin likes this.
  7. chrisch

    chrisch Solo Diver

    1,062
    340
    83
    It's a thousand ccs. However the relative volume of gas in a full cylinder depends upon the ambient pressure outside - if you empty the tank in space it is many times greater than 2784L of gas in it. Much better to use a measurement like Mole.
     
  8. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,234
    9,633
    113
    You're kidding, right? No matter if my tank contains air (molecular mass: 28.8), EAN36 (molecular mass: 29.4) or TMX18/45 (molecular mass: 17.9), I'm breathing between 15 and 20 liters per minute. So surface liters (or bar-liters, or cubic feet) is a much better measurement.

    Tank water volume x tank pressure = surface liters of gas
    Surface consumption x pressure = consumption at depth
    Consumption at depth / tank water volume = pressure decrease per time

    For me, the advantage of using tank water volume and pressure rather than surface capacity is that it takes service pressure and tank factors out of the equation when you calculate min gas and remaining bottom time ("At this depth I'm spending about 6 bar/min, and I've got 60 bar left to min gas. That's ten minutes until I should start ascending"). My calculations are the same no matter whether my tank is rated to 200 bar, 232 bar or 300 bar.
     
  9. MikkelBC

    MikkelBC Instructor, Scuba

    # of Dives: 200 - 499
    Location: Denmark
    209
    106
    43
    ... if they are both filled to their rated maximum pressure. This seems useful to me if you are in the process of purchasing a cylinder, but less useful while diving.
    If I have an LP100 cylinder on my back and look at my pressure gauge, then there is no way to calculate the remaining volume of gas without including the max. rated pressure in the calculation. If I knew the water volume of the cylinder, then it would be a simple multiplication to find the remaining volume of gas.
     
    Storker and John Bantin like this.
  10. Mustard Dave

    Mustard Dave Rebreather Pilot

    # of Dives: I just don't log dives
    Location: Manchester, UK
    546
    378
    63
    Why do you need to know that a litre is 2.2 pints? If you work in metric, you will measure your SAC in litres. This also makes it easy to calculate your true air consumption - SAC X 1 at the surface, SAC X 2 at 10 metres, and then add one to the multiplier every additional 10 metres.

    Working in litres makes sense for divers, as one litre of water equates to one kilogram of weight on the surface, and one litre of air underwater equates to one kilogram of lift.
     

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