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SAC Rate

Discussion in 'Technical Diving Specialties' started by BradMM, Sep 2, 2019.

  1. BradMM

    BradMM ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Texas
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    Still an interesting discussion but I found my mistake. Thx
     
  2. Alekseolsen

    Alekseolsen Angel Fish

    # of Dives: 50 - 99
    Location: Norway
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    I do feel sorry for you imperial guys when reading your formulas!

    I struggle to follow the imperial calculations, but do remember that the compressability if air reduces with pressure, so a 10L bottle at 300 bar contains roughly 2700 liters, not 3000.

    Could that explain your difference?
     
    NothingClever likes this.
  3. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,535
    1,102
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    that is a bit different because the term bar is tied to both depth and the volumn in a workable format. If imperial used our capacity in say cubic inches it would be some what the same. but we dont measure consumption in cubic inches. metric volumn is the same measurment used in consumption measuring. liters to liters ours is cubic inches to cubic feet. metric is so much better to do these calc's than imperial is. metric is gage bar times tank physical volumn say 15 liters. if you use 50 bar you used about 750 liters of gass. the same in imperial would be 750 psi * 10/28 equals ltrs. For those that can visualize it. take a s 80 .... 77 cuft per 3000 psi easier is divide by 3 and say 1000 psi per (about) 25 cuft. that makes 100 psi about 2.5 cu ft. You can do it any way you want to. Just make it easy so being narced a bit dont make it too hard. if your gage is marked in hundred psi tics then it would be 100 psi = 3000/77. what ever you do make the tank factor the same on all of your tanks. IE psi per cuft or cu ft per 100 psi. Standard is i believe,,,,, psi per cu ft so a s 80 should have a tank factor of (77 cu ft tank) 3000/77.4....
     
  4. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 500 - 999
    Location: SE TEXAS
    4,535
    1,102
    113
    tank actual volumn 80 vs 77.4 is different also on the 2 tanks.
     
  5. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    16,367
    7,856
    113
    @BradMM I don't think you used the actual water capacity or you would have found the difference right there.
    Faber HP80's are 10.2L water capacity
    AL80's are 11.1L water capacity

    That is fairly substantial when it comes to SAC calculations so I suspect you were using an incorrect value in the formula
     
  6. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,725
    10,216
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    Probably not. Compressibility doesn't matter much until you pass 240 bar, and 'murrican tanks aren't hydro'd for more than some 240-ish bar.
     
  7. Kmart921

    Kmart921 Nassau Grouper

    # of Dives: 100 - 199
    Location: Norfolk, VA
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    In the future, whenever doing conversions of any sort, look at the units when doing your calculations and everything makes a whole lot more sense.

    For example... my average SAC using an AL80 is around 14 PSI/min. An AL80 tank contains 77.4 cubic feet of gas at 3000 PSI working pressure. So to convert from SAC (psi/min) to RMV (cuft/min) you multiply 14 PSI/min x 77.4cuft/3000PSI, the PSI cancels out and you get 0.36cuft/min. You can repeat this calculation for any tank configuration you like and you should arrive at the same value for RMV.
     
  8. atdotde

    atdotde Solo Diver

    # of Dives: 200 - 499
    Location: Munich, Germany
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    I thought so, too but in particular for gas consumption it does: When real gas corrections matter – The Theoretical Diver

    BTW, imperial tank sizes are also often not actual sizes but marketing numbers somewhat related to the actual content.
     
  9. Storker

    Storker Divemaster

    # of Dives: 100 - 199
    Location: close to a Hell which occasionally freezes over
    13,725
    10,216
    113
    One of the many reasons I prefer the Euro standard for designating tank size: water volume and service (fill) pressure
     
  10. NothingClever

    NothingClever Barracuda

    # of Dives: 200 - 499
    Location: Red Sea and Atlantic Ocean
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    I’m so not looking forward to Imperial measurements when I return to the US.
     

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