All this talk about SAC and RMV is interesting, but I'm not convinced it's all that helpful in gas planning. Keep in mind that the numbers are normalized for depth and altitude (by the computer). My SAC sitting on the sofa is 0.25 cf/min. My Icon usually reports a RMV about 0.6 to 0.7 for most dives. However, when I divide the tank volume by the dive length in minutes - which is the actual amount of gas used - the number is significantly higher. I put together a spreadsheet with dive length, max depth, average depth, computer calculated RMV, and actual gas used. I graphed every pair of data points and found there is no corellation with dive length, max or average depth. Even the computer reported RMV doesn't corelate perfectly. Here is a table of some Roatan dives with computed RMV (col 1)and actual gas used (col 2):
0.52, 0.83
0.58, 1.16
0.6, 1.2
0.6, 1.2
0.61, 1.2
0.61, 1.18
0.61, 1.41
0.61, 1.23
0.63, 1.31
0.64, 1.16
0.67, 1.12
0.69, 1.19
0.7, 1.18
0.7, 1.28
0.7, 1.44
0.71, 1.29
0.72, 1.39
0.74, 1.35
0.83, 1.76
1.21, 1.98
Mean 1.29, Median 1.215, mode 1.2 - for me actual gas used is far more useful in gas planning than a theoretical calculation.
No. . . This is how you figure and track actual gas used:
Given a RMV of 15 liters/min per ATA with a 15 liters/bar cylinder (steel HP119 tank), which translates to a
1 bar/min per ATA pressure Surface Consumption Rate, then all you need for monitoring multi-level gas usage are the depths in ATA, the time you spent at each multi-level depth, and then you can figure out your consumption for each particular depth, confirming it with an SPG or AI reading. ("AI" is an optional dive computer "Air Integration" electronic digital pressure data logger display function with either direct high pressure hose connection, or a wireless transducer/transmitter to a regulator's first stage HP port).
So you're at 30 meters depth; that's 4 ATA (30 divided-by 10 plus 1 equals 4 ATA); you stay 5 minutes. 1 bar/min per ATA multiplied by 4 ATA multiplied by 5 minutes equals 20 bar consumed. Confirm with SPG or AI, your delta remaining pressure reads 20 bar less --so if you start with a tank pressure of 200 bar, the SPG or AI should indicate "180 bar". (200 minus 20 is 180 bar).
You then ascend to 21 meters depth; that's 3.1 ATA (21 divided-by 10 plus 1 equals 3.1 ATA); you stay 10 minutes. 1 bar/min per ATA multiplied by 3.1 ATA multiplied by 10 minutes equals 31 bar consumed. Confirm with SPG or AI, your delta remaining pressure reads 31 bar less --SPG or AI should indicate "149 bar". (180 minus 31 is 149 bar).
You then ascend to 15 meters depth; that's 2.5 ATA (15 divided-by 10 plus 1 equals 2.5 ATA); you stay 30 minutes. 1 bar/min per ATA multiplied by 2.5 ATA multiplied by 30 minutes equals 75 bar consumed. Confirm with SPG or AI, your delta remaining pressure reads 75 bar less --SPG or AI should indicate "74 bar". (149 minus 75 is 74 bar).
Finally ascend to 6 meters depth; that's 1.6 ATA ( 6 divided-by 10 plus 1 equals 1.6 ATA); you stay 10 minutes. 1 bar/min per ATA multiplied by 1.6 ATA multiplied by 10 minutes equals 16 bar consumed. Confirm with SPG or AI, your delta remaining pressure reads 16 bar less --SPG or AI should indicate "58 bar". (74 minus 16 is 58 bar).
Do a slow ascent to the surface --on the surface inflate your BCD/Wing/Drysuit and you know even before looking at your SPG that you have 50 bar remaining in your tank.
With repetition, experience & persistent rote practice-- you will know what your SPG nominally reads at depth after an arbitrary time interval like five or ten minutes. Or, in the case of an AI, your total running gas remaining estimate over a five or ten minute interval should be similar to the algorithm's displayed value.
The point is do not take for granted the calculations of AI --the features & functions of AI (like Air Time Remaining or real time SAC rate displays) are ancillary to what you already know how to do in your head. . .
(If the expected SPG or AI reading is 30% or more than you figured, then that indicates you are physically working & breathing harder than normal, or have a leak somewhere in your reg/gear set-up, and should consider aborting the dive).
