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How to calculate total air consumption along ascent?

Discussion in 'Basic Scuba' started by magicfx, May 11, 2021.

  1. boulderjohn

    boulderjohn Technical Instructor ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
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    dd2ed3015afc9a5726a82ef4db2be38f--libraries-html.jpg
     
    OTF, Bob DBF, Charles2 and 2 others like this.
  2. loosenit2

    loosenit2 si respiratio sub aqua amet

    # of Dives: I'm a Fish!
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    not generally. Sac is generally defined as psi/min whereas rmv is generally vol/min. Rmv is more useful because it allows you to compare or transition between dissimilar tank sizes. My sac is different when using an al 80 then when using double Hp 100s but my rmv remains the same
     
  3. Wibble

    Wibble Contributor

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    Psi/min is meaningless, you breathe a volume of gas and metabolise a volume of oxygen. A 3 litre rebreather cylinder compared with a 12 litre twinset which has 8 times the volume

    Is RMV the existing medical term?
     
  4. Centrals

    Centrals Barangay Pasaway

    # of Dives:
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    If the SAC and ascent rate is NOT constant then there is absolutely no chance for an definitive answer.
    How about time you spend on various depth?
    How many unknowns you have there?

    The easiest, quickest and simplest way is to use the RMV/SAC at 10m(15L/min x2) as a start.
    Multiply the 30L/min by the total time, including safety stop, required to surface.
     
  5. Angelo Farina

    Angelo Farina Marine Scientist

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    Your instantaneous gas consumption rate, Vdot, in normal-liter/minute, is given by the following function of depth d (in meters):
    Vdot = 15*(d/10+1).
    Depth is ranging form 10 meters to 0, in one minute. So the instantaneous depth d will be a function of time t (in minutes):
    d = 10 * (1-t).
    We replace d in the previous formula, getting:
    Vdot =15*[10*(1-t)/10+1]
    Vdot =15*(2-t)
    Now you integrate the instantaneous gas consumption rate Vdot over time:
    V = integral from 0 to 1 minute of Vdot*dt
    Doing the indefinite integral you get:
    V = 15*(2*t - t^2/2)
    as t ranges from 0 to 1 minute, you get:
    V = 15*(2-1/2) = 15*1.5 = 22.5 liters
     
    Raphus and inquisit like this.
  6. Centrals

    Centrals Barangay Pasaway

    # of Dives:
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    To have any real meaning to apply RAV/SAC in this calculation you need to know the breathing cycle of that diver per min.
    So if the diver needs 15 cycles then one breath = 1L

    From 10m to the top in 15 breath at 1L per breath with pressure adjustment will give you the answer.
     
  7. dmaziuk

    dmaziuk Regular of the Pub

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    Or you could short-circuit it as (30 + 15)/2 = 45/2 = 22.5
    Figuring out why that works out is left as an exercise to the reader.
     
  8. Centrals

    Centrals Barangay Pasaway

    # of Dives:
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    If the diver requires to breath 10 times over 1m ascent increment from 10m to the top.
    1.5L per breath on the surface.
    10m = 3L
    9m = 1.9 x 1.5 = 2.85L
    8m = 2.7L
    7m = 2.55L
    6m = 2.4L
    5m = 2.25L
    4m = 2.1L
    3m = 1.95L
    2m = 1.8L
    1m = 1.65L
    Total sum = 23.25L
     
  9. Wibble

    Wibble Contributor

    # of Dives: 500 - 999
    Location: UK
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    We need the answer to 16 decimal places for complete irrelevance.
     
  10. Centrals

    Centrals Barangay Pasaway

    # of Dives:
    Location: Hong Kong
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    If the diver took one breath at 10m and another one 30sec later at 5m before reaching the surface after exactly 1 min.
     

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