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As a novice diver, how do you determine reasonable numbers for a rock bottom calculation?
Here's how I do it in my AOW class ...
Descend to 33 fsw. Take note of your tank pressure. Do a hard swim, maintaining a depth of 33 fsw for 5 minutes. Take note of your tank pressure again.
Wait a few minutes for your breathing rate to return to normal.
Take note of your tank pressure again. Swim normally at 33 fsw for 10 minutes. Take note of your tank pressure again.
For each of those two swims ...
- Determine your actual gas usage (subtract end pressure from starting pressure).
- Divide by 2 (you're swimming at 2 ATA's and want to "normalize" gas usage to 1 ATA.
- Divide by the number of minutes of the swim (you want a "usage per minute" SCR)
- Multiply by the baseline (cubic feet per psi) of your cylinder
This will give you an SCR based on cubic feet per minute for each of the two swims. The "kicking hard" swim is your WORKING SCR ... the "swimming normally" swim is your RESTING SCR.
Actual consumption is somewhere between those two figures, based on conditions ... but the two numbers will give you a rough idea of what you can expect to use ... analogous to "city mileage" and "highway mileage" for your car.
Once you know working and resting SCR, you plug them into the calculations for ascent depths and times as previously described ... it's simple grade-school level arithmetic.
My students have been doing this in AOW class for nearly 10 years now ... I've yet to encounter a student who had any problems at all comprehending either the math or the reasons why it's worth the effort to get this information ...
... Bob (Grateful Diver)