NYCNaiad
Dive babble all day long
Regarding my earlier post about slinging a 13 cf pony:
Context: All my dives have been and will be non-solo recreational boat dives in warm, high visibility, low current Caribbean waters where we are usually doing an underwater loop that never goes very far from the boat, plus the occasional shore dive out to a reef and back. My max depth is generally about 100’ as I am almost always on EAN32 and I give myself a few feet of safety factor. My normal SAC rate is about .45 cf/min. I can (have done) CESA from about 50’ if needed. I have two herniated discs/sciatica and want/need to minimize the amount of weight I carry out of water. (Btw, diving is incredibly therapeutic for my back.) I have never seen anyone else on one of my dives carry a pony, but it gives me a little peace of mind.
Calculation:
At 100’ for one minute to get sorted out: .45 x 4 x 1 = 1.8 cf
Ascend from 100’ (4 atm) to 15’ (1.5 atm) at 30 fpm 2.75 ave atm x .45 cf/min x 85’/30 fpm = 3.5 cf
Safety stop at 15 ‘ (1.5 atm) for 3 minutes: 1.5 atm x .45 cf/min x 3 min = 2 cf
Ascend to surface: 1.25 ave atm x .45 cf/min x 15’/30 fpm = .3 cf
Total air consumption: 7.6 cf
If I speed up my ascent to, say, 45 fpm then we are talking 6.3 cf
If I eliminate the safety stop we are talking 4.3 cf
I am comfortable that a 13 cf will do the job for me if for some reason I become OOA. Plus I don’t want to carry the extra 3 lb of a 19 cf.
I'm guessing you probably haven't been in a situation where you panicked or were out of air. My air consumption is very good & I also have the same back problems you do. Your air consumption will change DRASTICALLY in an emergency situation. Mine did which is why the larger pony generally makes more sense. In good conditions, I have the boat hand me down my gear when I'm in the water & I hand it up before I get in. This way, I don't have to worry about the weight at all.