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Help preparing for exam

Discussion in 'Advanced Scuba Discussions' started by Merryoceaner, Aug 28, 2019.

  1. Protondecay123

    Protondecay123 Barracuda

    # of Dives: 0 - 24
    Location: Apex of Moronia
    437
    303
    63
    So start with the General Gas Equation (P1xV1)/T1 = (P2xV2)/T2. Solving for V2 will give you V2= ((P1xV1)xT2) / (T1xP2) using metric units V2= (1.6 ATM x 4L x 277.4 K) / (302.4K x 2.77 ATM)= 2.12 L which is pretty close to D. Maybe there's some rounding off going on on my part and the freshwater to saltwater conversion I have for pressure may play into it (10.34 m/ATM). I don't have a great calculator. Here is a link that may be helpful. https://www.ehs.ucsb.edu/files/docs/ds/physics.pdf
     
    Merryoceaner likes this.
  2. BackAfter30

    BackAfter30 Nassau Grouper

    # of Dives: 50 - 99
    Location: Denver
    70
    61
    18
    Yep, sorry, I don't want to sound like I am piling on, but I agree that these are science problems. You need to understand the underlying science/math. Or, you need some permanent wetnotes with enough notes and examples that you can find the right equations to plug numbers into to work out problems like these.

    Question: do you recognize that 40 and 41 don't require a calculator to solve? Those can be solved with very basic "in your head math" if you are clear on the principles behind those questions. That might or might not be true for 44 also but I'd need paper and pencil to write that one out to know for sure.

    If this isn't clear to you then your best bet might be to find a friend with a science background who can tutor you up a little on the physics and math here. I'd say that all of the replies here are accurate, but I'd understand if they are not helping you get to where you need to be.

    BTW, the NOAA Physics of Diving doc (linked from my alma mater) referenced above has an example exactly analogous to 44 on page 2-14.
     
    ishmaelcat, Merryoceaner and mwerle like this.
  3. MargaritaMike

    MargaritaMike Divemaster ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: On a non-divable lake in SE Texas
    881
    775
    93
    I may be stupid but I did a quick calculation on #44. I said to myself, "Self there is a 70' difference. That's about 2 atm difference. One atmosphere would half it to 2 liters. Another atmosphere would half it again to 1 liter. Allow a difference for the temp change. so guess answer a. Worst case, it's answer b." Then I said to myself, "Self, if you have to solve this problem before you can go have fun diving, then you need to find a new hobby."

    Cheers -
     
    Nick Ward, 1isNone and Jcp2 like this.
  4. Zef

    Zef Divemaster

    1,909
    1,309
    113
    Are the answer choices listed for question #44 correct? I calculate 2.09 or 2.08 depending on how one rounds.

    Here is my math:

    P1 = 6.1M....(6.1/10)+1 = 1.61ATA
    V1 = 4L
    T1 = 29.4c....29.4 + 273K = 302.4K

    P2 = 18.3M....(18.3/10)+1 = 2.83ATA
    V2 = ???
    T2 = 4.4c....4.4+273K = 277.4

    (1.61) x (4) = (2.83) x (V2)
    302.4................277.4

    6.44 = (2.83) x (V2)
    302.4.........277.4

    6.44 (277.4) = (2.83) x (V2)
    302.4

    1786.456 = (2.83) x (V2)
    302.4

    5.9076 = (2.83) x (V2)

    5.9076 = V2
    2.83

    2.087 = V2

    V2 = 2.087L

    -Z
     
    Merryoceaner likes this.
  5. uncfnp

    uncfnp Solo Diver

    # of Dives: 500 - 999
    Location: North Carolina
    6,104
    4,576
    113
    40 and 41 you can answer with simple math and an understanding of the concepts (that is if I have the right answers :wink:). No complex equations needed

    40. They keep this simple by keeping the info in pressure readings. So diver uses 30 psi a minute at the surface but this diver is at 90 feet so we need to know the rate at depth. For this we need the atm at 90. So divided 90 by 33 = 2.7 then add 1. That gives us 3.72 as atm at depth. Now multiple his surface rate by the atm at depth, 30 x 3.72 = 111 psi at 90 feet.

    Next we need to know how much gas he used so 3000 starting pressure minus the ending pressure of 1000 = 2000 psi used.

    Now just divide the 2000 psi used by his rate at depth 111. That gives answer d) 18 minutes.
     
    Merryoceaner and Zef like this.
  6. uncfnp

    uncfnp Solo Diver

    # of Dives: 500 - 999
    Location: North Carolina
    6,104
    4,576
    113
    Now to reason out 41.

    To calculate the gas in any tank you just need the tank size, rated pressure and current pressure. In this case the tank is an 80, the rated psi 3000 and current pressure 2200.

    First you need the “magic number.” This number is different for different size tanks and rated pressures. Instead of memorizing a list just remember how to calculate that number...divide the size by the rated pressure. In this case 80 divided by 3000 = .026

    Now use this number to calculate the current gas volume 2200 x .026 = 58.6. So answer b) 59 cf
     
  7. Zef

    Zef Divemaster

    1,909
    1,309
    113
    I agree with your calculations and results but when computing the metric equivalent it does not hit on the number of liters indicated for answer "b"....it comes close...I just don't know what figures the exam writer used to come up with the calculations. Here is mine:

    2265 = 10.942
    207

    10.942 x 152 = 1663.184

    -Z
     
  8. Merryoceaner

    Merryoceaner Angel Fish

    # of Dives: 100 - 199
    Location: Saint Petersburg
    6
    2
    3
    Divemaster
     
  9. Merryoceaner

    Merryoceaner Angel Fish

    # of Dives: 100 - 199
    Location: Saint Petersburg
    6
    2
    3
    They do :) 59 cubic feet to be exact! I think I was so tired to realize it was a simple ratio lol! Thanks for your help :)
     
  10. Zef

    Zef Divemaster

    1,909
    1,309
    113
    is this for NAUI divemaster?

    -Z
     

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