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These calculations are fine, assuming that the average depth was 15.7 m.Hey guys: I just calculated a recent dive that I did (from memory of my logs):
(3000 psi in - 500 psi out) / 27 minutes = 92.59 psi per minute
15.7 m / 10 + 1 = 2.57 ATA
92.59 / 2.57 = 36 psi per minute
There are a couple problems with this:the RMV that we could use for this (a 100 ft3 tank): baseline 95 cuft / 3000 working psi (leave a 500psi buffer) = 0.031 cuft per psi
36 * 0.031 = 1.116 RMV
You need to correct your gas consumption for the ambient pressure at which it is breathed.Now I know that I breathe about 1.116 cubic feet per minute. I'm going to dive a 100 cubic foot tank (so call it 95 feet). I know that my SAC is going to be around 36 psi/min at the depth I plan to visit, so should I just say 95 cubic feet / 1.116 cubic feet per minute and solve for minutes? Because that says I have 85 minutes of air!
These calculations are fine, assuming that the average depth was 15.7 m.
There are a couple problems with this:
1. I don't know of any 100 cuft tanks that have a service pressure of 3000 psi.
2. You shouldn't allow for a 500 psi buffer/reserve when calculating your RMV.
Re-working your numbers with a more realistic tank choice...
Assume AL80 with capacity of 77.4 cuft and a service pressure of 3000 psi.
77.4 cuft / 3000 psi = 0.0258 cuft/psi
36 psi/min * 0.0258 cuft/psi = 0.93 cuft/min
An RMV of 0.93 cuft/min is rather high for an experienced diver under low work-load conditions, but 0.93 cuft/min is very reasonable for a novice. You didn't mention the work-load for this particular dive, so that may have elevated the RMV somewhat.
You should calculate RMV for all of the dives in your divelog. This will give you a handle on the range of RMV under various dive conditions. Expect the RMV to be elevated for higher work-load dives.
Use these data to arrive at a guesstimated RMV for dive planning purposes. When working the numbers, you should allow for sufficient gas reserves for an estimated "stressed" RMV -- something that might occur in a true emergency. Also, consider that you are carrying your buddy's gas reserve. You should have enough gas to get you and your buddy
to the surface at any point during the dive.
You need to correct your gas consumption for the ambient pressure at which it is breathed.
Don't forget that your gas isn't just for the bottom phase of the dive. You need gas to descend, ascend, conduct stops, and serve as an ample reserve for your buddy.