• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

32% Dive plan examples by Hand

Discussion in 'Advanced Scuba Discussions' started by SentinelAce, Jul 21, 2018.

  1. stuartv

    stuartv Seeking the Light

    # of Dives: 200 - 499
    Location: Manassas, VA
    8,175
    3,809
    113
    Cool, but the OP said he's taking AN/DP (I think), which is a TDI course. I wouldn't think a TDI course would require you to use IANTD tables...
     
  2. SentinelAce

    SentinelAce Nassau Grouper

    # of Dives: 200 - 499
    Location: Ohio
    114
    22
    18
    I'm not sure what I'm doing wrong here. I planned a dive at 32% @ 120 ft for 20 minutes. I know the max dept is 110 at 1.4 but at 1.6 its 132'. Just testing. ATA is 4.6 which I got by 120/33+1. RMV is .7. ATM is 1.48. PSI/min is 32.35. I took .7 x 4.64 x 20 and get 64.4 cubic feet. My computer says 77 cubic feet. if I take 32.35 x 4.64 I get 150 psi per minute at depth or 13.3 minutes. What did I mess up?
     
  3. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
    8,307
    6,032
    113
    How did you get 0.7 and 32.35? What size tank are you assuming? When you say 1.48 ATM do you mean 1.48 PPO2? What computer is telling you 77 cuft?
     
  4. sea_otter

    sea_otter DIR Practitioner

    # of Dives: 200 - 499
    Location: San Jose, CA
    323
    244
    43
    Chat with your instructor - there's a bit more wrong here than the arithmetic. For one, 32% is not an appropriate gas to use on a 120 ft dive.

    Your math on calculating bottom gas consumption is sound. Consumption as SCR (0.7) x Depth in ATA (4.6) x Time in minutes (20) = 64.4 cubic feet. Your computer is adding in the gas needed to reach the surface, including mandatory stops.

    The other problem is that this calculation (including the higher number from your computer) does not give you enough gas to complete the dive safely. Contingency planning and sufficient gas reserves to both deal with a problem at depth and get yourself and a buddy (sharing gas) to your next breathable gas source (either a switch or the surface) must be factored in. If a gas switch takes you a couple minutes, plan for that too. Again, your instructor should cover this.

    Converting volume to pressure gets a little bit tricky, and I'm not sure where you're getting the 32.5 PSI/min number. I assume you're using that as an estimate of what you would consume at the surface, per tank, in pressure (which is a bit high based upon your other numbers - intentional conservatism is fine, but I suspect this is more a case of miscalculation). In the US, tank capacity is expressed as cubic feet of gas at rated full pressure. Your AL80s each hold approximately 77 cubic feet each, at 3000 PSI. This can be expressed as a fraction of 77 cf / 3000 PSI, or, simplified, we can write this as 2.5 cf / 100 PSI. This nice round (rounded down, making it a tad more conservative) number is referred to as a tank factor, and it can make calculations easier, especially when teams have different sized tanks. The formula to determine gas used in PSI (x100), at depth, is SCR x Depth in ATA x Time / Tank factor. I'll let you figure out the rest on your own.

    Hopefully this helps to clarify a little of the math, but there's a lot missing that the class needs to cover. This doesn't give any information on how to calculate reserve gas, deco gas requirements, what to do when your tanks start at different pressures, and how these calculations vary for a backmount diver in a mixed team. I'll optimistically hope you're just trying to get a jump on the homework assignments, and your instructor is not totally incompetent. But alarm bells are ringing if you're resorting to asking these questions on an internet forum.

    Safe diving.
     
    northernone likes this.
  5. SentinelAce

    SentinelAce Nassau Grouper

    # of Dives: 200 - 499
    Location: Ohio
    114
    22
    18
    yes I am just practicing. I'm still in the class and trying to grasp the math portion. Hes a great instructor. I am just behind
     
  6. SentinelAce

    SentinelAce Nassau Grouper

    # of Dives: 200 - 499
    Location: Ohio
    114
    22
    18
    This was done in the to get the RMV. I am using alum 80s. I am using a dive calc. I put in 3000 PSI, 20 minutes for depth at 120'. It games me 4.64 ATA and PSI/min or SCR of 32.35
     
  7. kelemvor

    kelemvor Big Fleshy Monster ScubaBoard Supporter

    # of Dives: 100 - 199
    Location: Largo, FL USA
    5,928
    3,130
    113
    Sure, for example the NAUI EAN36 table covers dives with as much as 39 minutes of mandatory decompression stop. The other NAUI tables also all cover deco. They don't cover mixed gas diving or accelerated deco on oxygen or anything else but simple decompression with a single gas. I don't know about you, but given the size of my sac I'd have to bring a lot of gas to do 140 minutes at 80' and then do a 39 minute deco stop at 15'.

    upload_2018-7-31_9-22-21.png
     
    IncreaseMyT likes this.
  8. sea_otter

    sea_otter DIR Practitioner

    # of Dives: 200 - 499
    Location: San Jose, CA
    323
    244
    43
    Gotcha! That calculator is telling you that if you spent 20 minutes at 120 ft, and in the course of that dive used 3000 PSI in a single Aluminum 80, you were breathing the equivalent of 32.35 PSI/min on the surface. The formula is gas used (3000 psi) / (Time (20 min) * Depth in ATA (4.64)). It's common to talk about these numbers in terms of volume because PSI varies depending upon the types of tanks you're using. This 32.35 PSI/min can be converted back to volume using the 2.5 tank factor 32.35 PSI * 2.5 cf/100 PSI = 0.81 cf/min. It's important to understand how these things are calculated rather than blindly using the calculators. The other thing this doesn't account for is the fact that you have two tanks, so remember, if you use 1400 psi from one and 1600 psi from the other, the input numbers need to account for this.

    The number you get from the calculator is probably not far from correct, but that's just a coincidence. To get that number for you, you'll need to go underwater, note your tank pressures, note your average depth, note your time, and use those real numbers to calculate how much gas you need at a given depth. Track those numbers across many dives in different environments. Understand your baseline. Most experienced divers will fall in the range between 0.5 and 0.75 cf/min. If you're learning a new configuration, it might be higher. If you're very relaxed and/or small/female, it might be lower. A decent guesstimate if you don't have real numbers is 0.75 cf/min (I was assuming that's where you got the 0.7 number).

    It's also useful to have quick tricks to make calculations more idiot-proof underwater, because I'm an idiot doing math underwater. In sidemount, when I'm switching tanks every 200 psi, I like to know how many minutes it'll take me to breathe 200 psi at a planned depth. You'll learn these sorts of tricks from the class. They take underwater practice to get right. You'll learn to be a bit less dependent on your SPG. It becomes a sanity check of mental expectations.

    Do ask your instructor for help if you don't understand this stuff. It's really important to know how the numbers fit together, but the math can make it a little daunting at the start.
     
  9. SentinelAce

    SentinelAce Nassau Grouper

    # of Dives: 200 - 499
    Location: Ohio
    114
    22
    18
    Thank you. I'm still not sure how you get the SCR or PSI/min. Or maybe I didn't understand what you typed there. 3000/20*120 ? or * 4.64 ? Neither give me 32.35 LOL. What am I missing there? Yes I can use the dive calculator but want to know how to do this by hand. You guys have been very helpful. Just practicing :)
     
  10. sea_otter

    sea_otter DIR Practitioner

    # of Dives: 200 - 499
    Location: San Jose, CA
    323
    244
    43
    Let's back up one step. Make sure to understand that the input numbers are made up. You did not spend 20 minutes at exactly 120 ft using 3000 PSI of gas in an AL80. You simply told the calculator that you did. :)

    But why not go through the math anyhow:

    Gas used: 3000 PSI
    Depth: 120 ft = 36.4 meters = 3.64 ATA (water) + 1 ATA (air) = 4.64 ATA
    Time: 20 minutes

    Let's first reduce time, since we're looking for a per minute value.

    Gas used per minute = 3000 PSI / 20 min = 150 PSI / min. This is how much we're breathing at depth, per minute, or our depth consumption rate.

    Now we need to calculate our surface consumption rate. Boyle tells us that gas compresses proportionally as we descend, and as a result, we know:

    Depth consumption rate = Surface consumption rate x Depth in ATA

    Plug in the numbers and solve for SCR:
    150 PSI / min = SCR x 4.64 ATA
    SCR = 150 / 4.64 = 32.3 PSI / min


    I'd challenge you to go figure it out with real numbers. Go for a dive. A shallow one is okay. Write down:
    • How much gas you use, in PSI (this is simpler with a single tank, but if you're sidemounting two, remember to add the gas used from right tank + gas used from left tank)
    • Your average depth - hopefully from your dive computer. Note that maximum depth or planned depth doesn't help us here, because we're not doing our entire dive at the bottom. We're hopefully coming back up breathing.
    • Your dive time
    From these numbers, you can figure out your SCR.

    Hint - here's an easy way to combine all the logic above into a single formula:
    Gas used = Consumption rate (SCR) x Average depth (in ATA) x Time

    Now you can figure out how much gas you need at a given depth.

    If you understand those basics well, you're in good shape to learn the more complicated scenarios. The next question becomes how much gas do you need if something goes wrong? And how do you know you have enough to get everyone home healthy and happy?
     
    Last edited: Jul 31, 2018

Share This Page