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Greg - 3/11/2014 1:21 PM 
Using the dive log data on DiveBuddy (over 3000 logged dives), I just figured out the average depth and time for most scuba divers...54 feet for 52 minutes. That’s using a standard AL 80 cu/ft tank (3000 psi).
To develop a baseline using this average, we can do something like this (figures are rounded)...
Time (Depth/33) = ?
52 (54/33) = 85
To make this average depth and time the baseline for all other calculations, we can set it to 100...
100 - 85 = 15 (15 will now be a constant used in the equation)
So using my three dives listed above, the calculations would look like this:
1. 60 (40/33) = 73 ... + 15 = 88 (88 is lower than 100, so on this dive, I didn’t breath as good as the average scuba diver).
2. 45 (60/33) = 82 ... + 15 = 97 (97 is pretty close to 100, so I did pretty good on this dive).
3. 25 (90/33) = 68 ... + 15 = 83 (83 is lower than 100, so I need to work on my breathing).
Here’s another sample, lets say I dove to 50 feet for 65 minutes...
4. 65 (50/33) = 98 ... + 15 = 113 (113 is a lot better than 100, so my breathing rate was awesome on that dive).
So the final simple calculation to determine my underwater breathing rate, only using depth and time, compared with the average scuba diver, with 100 as the baseline, would look like:
Breathing Rate = (Time (Depth/33)) + 15
Any number around 100 is average. Any number above 100 means you breath better than most scuba divers. Any number below 100 means you need to work on your breathing.
What do you think?
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We worked an equation like this in my advanced class for the same reason. The only thing the equation can’t equate for is how divers respond to stress such as low viz and temp. It’s still should be a good representative if always comparing to divers in your areas since you probably dive the same conditions.
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Agojo - 3/11/2014 8:45 PM
Nice calculations but I am not as concerned about a buddies air consumption rate as if they are a safe and good buddy. I have a 5’ octo hose and my buddy typically takes it for 10-15 minutes every dive. Gives us both practice with regulator exchange and still lengthens our dive time. I’ll dive with a newbie as long as they don’t stray further than a breath away.
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Above is a great discussion. My current goals and objectives include learning more about SAC rates, RMV rates, and understanding DIVE PLAN. I printed your notes above, and will review them in the next few days. Great topic.
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EskimoBluDay, Greg,
Yes, I agree that discussion about comparitive consumption rates are important. It takes experience to learn this. Any beginner diver should realize that somebody diving one tank has a need to come up earlier than the buddy with two tanks. And don’t cheat on this. Also, a beginner will consume more air than an advanced diver. Huffin and puffin vs streamlining. Yet, my quest for the math has been frustrating. Both dive buddies should have a pre-dive understanding of their inequality. Calculations are important. Standard formulas are critical. But, we have divers with wide range of math skills. Even good divers can have a bad day putting two and two together. So, let there be peace on earth about this. We need a comrehenaive and exhaustive document on this calculation. And then, true to the discussion, 20 or 40 examples can provide detail seperation between Dive Plans that are safe, and those that are very risky.
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The most accurate is to use the method Eskimo lays-out, and base your future predictions on your past performance (SAC-rates). Much like you would estimate the ’range’ you might get out of a tank of gas in your car based upon past MPG calculations. There will always be some variables even during a single dive, but I find my SAC rates run very consistent, which makes using my past SAC rates as a predictor for what I can do on upcoming dives pretty accurate. No reason to repeat the formula because Eskimo provided it above.
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PS - the only thing I might stress is that you always ’minus 500 PSI’ and the corresponding CF that would remain in your tank for contingencies before starting out. That way you are using a volume equal to starting PSI minus 500 PSI. Example: Most AL-80s actually hold 77.4 CF when exactly full at 3000 PSI. If you divide 77.4CF by 3000 PSI you get 0.0258, meaning 1 PSI = 0.0258 CF. If you multiple 0.0258 by 500 (ending PSI) you get 12.9 CF. Starting volume 77.4 CF minus ending volume at 500 PSI of 12.9 CF = usable volume on the dive of 64.5 CF out of an AL-80 (another reason to use HP S-100s). So, instead of having ’80’ CF available for the dive, you really only have 64.5 CF available for the dive. Divide that by your past/average SAC rate(s) and factor in the depth in ATA and you should be good to go. See ya U/W :)
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