Mike Veitch:
Oops a misunderstanding of sorts. Wanted to know diff of SAC and computer
If you dive a lot, particularly if they are similar profiles, then it all becomes intuitive. I suspect that's the case with you.
I use a simplified air consumption calculation to see if a proposed dive profile makes sense, the same way I'll look at an RDP or wheel to see if a certain multilevel profile makes sense from a decompression point of view. It may sound complicated in this post, but once you've done it a few times, it's almost automatic.
Take the case where I'm on a boat in Maui and the DM says "OK, the profile is going to be 130' for 7 minutes, 80' for 10 minutes, then 50' for another 15 minutes." Is this profile within air NDLs? Does an AL80 hold enough air for me to do this dive? Running the profile through the wheel or my flat table equivalent quickly shows that the dive is within NDL. Air consumption takes a bit more calculation. An assumed consumption rate, or SAC is a vital part of that calculation.
I do calculations in a slightly different fashion than I've seen taught .... I convert all depths and times to the equivalent times on the surface. In 1 minute at 100'/4ata you will breathe out of your tank the same amount of air that you would use in 4 minutes on the surface. I just call this 4 "surface minutes" or 4 ata-minutes. Similarly 5 minutes at 66'/3ata would consume 5*3 = 15 ata-minutes.
An AL80 holds 77.4cu ft. If you hold the last 500psi in reserve, that leaves you 65cu ft. If you have an SAC of 1 cubic foot per minute, that 65 cu ft obviously would last you 65 minutes on the surface. At 0.75CFM SAC, it would be about 90 minutes on the surface. Doing the whole dive on the surface isn't much fun, but you can readily see that 90 minutes of air on the surface is equivalent to 90/2 = 45 minutes at 33' or 2ata; or 90/3 = 30 minutes at 66'/3ata.
OK, but it's not quite that simple. If I went to 66' for 30 minutes, then I won't have any air allocated for my ascent and stops (other than that 500psi backup reserve). Let's see how many surface-minute-equivalents a standard ascent/deep stop/safety stop will take (One advantage of figuring out "surface minutes" instead of cubic feet is that one can easily adjust for different SAC later).
My most common ascent is something along the line of:
1 minute at 40'
1 minute at 25'
3 minutes at 15'
with 30fpm ascents between stops.
40' is 2.2 ata. 25' is 1.76ata, 15' is about 1.5ata. The ascent is 4/3 minute at 20' average depth.
2.2+1.8+(3*1.5)+(4/3*1.6) = 11 ata-minutes. In other words, I will consume the same amount of air during my 40' and above stops and ascent as I would hanging out on the surface for 11 minutes. NOTE: you only need to do this calculation ONE time, using whatever deep stops, ascent rate, and safety stop you choose.
Similarly, you can do the calculations for the ascent prior to starting this 40' and shallower sequence. For example, a 30fpm ascent from 100' to 40' will take 2 minutes at an average depth of 70'. 2 minutes * 3ata = 6 ata-minutes. Obviously, an ascent to 40' from shallower than 100' will take less air.
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OK, now that we've done these preliminary, 1 time only calculations, you can start putting it all together.
An AL80 is 65cf after subtracting a 500psi reserve. 65cf/0.75cfm= 87 ata-minutes of air. My ascent and stops from 40' upward will take 11 ata-minutes, leaving 76 ata-min for the dive. Let's see if 7@130, 10@80, 15@50 is a doable profile.
130'(5ata) for 7 minutes uses 35 ata-minutes.
80'(3.4ata) for 10 minutes uses another 35 ata-minutes
50'(2.5ata) for 15 minutes uses another 38 ata-minutes.
ignore 50' to 40' ascent.
Total of 108 ata-minutes vs. 76 available. Ain't gonna work.
OTOH, 65cf at 0.5cfm = 130 ata-minutes. or about 120 minutes before stops, so at 0.5cfm that profile will work.
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Note that this sort of calculation also works backwards.
I need 11 ata-minutes for my normal ascent and stops from 40'. At 0.5cfm this is 11*0.5=5.5cf. In an AL80, there are 39psi/cf, so I'll need a bit less than 6x40=240psi for my ascent and stops. I want to be at 40' with 740psi
It's easy to redo with a different SAC.
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It may sound complicated, but
1. A lot of this stuff only needs to be calculated one time, and
2. in practice I use a lot cruder rounding off than shown in this example.
3. After you have done this a few times, you may find yourself putting together your dive "backwards"..... 500psi on the surface, add 240psi for stops 40' and above, 120psi for my ascent 100' to 40' -- I need 860psi for my normal ascent and reserve from 100'. (My alternate calculation of my rock bottom for air sharing with an excited buddy sets a higher min ascent pressure in this case, but rock bottom is a whole other discussion).
