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+--- Thread: list your pony bottle size options / reasons (/showthread.php?tid=4571)
Re: list your pony bottle size options / reasons - jleech - 09-09-2009
Quote:So to play the \"out of air\" scenario from 130' coming up at 1' every 2 seconds (Not counting the 1 minute half stop at 65' or the 3 minute safety stop at 15' for now) it would take me 4 minutes and 20 seconds to ascend 130'. Now add 4 minutes for the half stop and safety stop, and I would be looking at 8 minutes and 20 seconds to get to the top total time. Round up to 9 minutes to be conservative and my sac rate could be as much as 2.0 cu ft per minute and I would still have a little gas left in a 19 cu ft pony bottle.
Actually, the math is quite a bit more complex than this. The idea of SAC rate is that it is your "Surface" consumption rate. You need to factor in the additional pressure associated with diving.
Using your example, the easiest math is done like this:
Ascent from 130 to Surface: 4.33 min
Average Depth from 130 to surface = 65ft
65/33+1=2.97 ATM
4.33*2.97*(SAC rate)= volume of gas used
Deep Stop at 65 ft: 1 min
65/33+1= 2.97 ATM
1*2.97*(SAC rate)=volume of gas used
Safety stop at 15 ft: 3 min
15/33+1= 1.46 ATM
3*1.46*(SAC rate)=volume of gas used
(4.33*2.97)+(2.97*1)+(3*1.46)=(V1+V2+V3)/(SAC Rate)
20.21*(SAC Rate)=volume used
SAC rate=volume used/20.21
If 19 cuft, then SAC rate would need to be 0.94 or less
With an aluminum 80, and a SAC rate of 0.6 cuft/min, you would use 450 psi out of the tank using these numbers, which seems reasonable.
Re: list your pony bottle size options / reasons - DetectorGuy - 09-09-2009
Thanks man, I feel like a goof! :-[ I need to crack the books open again.
Re: list your pony bottle size options / reasons - SCUBA Dog - 09-10-2009
No problem! I think they should have included the units next to the window to eliminate that confusion. I don't know if you have one or not, but the SAC Rate Calculator used to come with an informational booklet that explained how to use it, with some sample problems. Let me know if you don't have a copy and I will see about getting a copy to you.
Re: list your pony bottle size options / reasons - arcFlash - 09-10-2009
By the way, anyone thinking they can do the SAC test sitting on the couch to get the rest number, it doesn't work. First, I've tried it and you can't use enough air in an hour to get accurate numbers and second the physiology of the thoracic cavity and esophagus changes while diving. Respiration is at best 83%.
"... The Tidal volume decreased during immersion, average only 83% of the surface value. Hense, Immersion was associated with a marked stiffening of the respiratory system. In all, there were three factors effecting pulmonary functions during immersion: the FCR reduction, the change in the thoracic configuration and the diaphragm configuration, and the stiffening of the respiratory system."
It's interesting to note that the consumption numbers for L/min of gas and this marked decrease in respiration efficiency lead the quoted author to the conclusion that one is unlikely to over-breath the regulator but they over-breath their pulmonary system.
If you like reading really dry white papers this is an awesome read. I've read it twice.
quoted:
Re: list your pony bottle size options / reasons - wukuu - 09-10-2009
When I am calculating my SAC I try to do either a 30' or 60' dive and take a measurement when I have no load, sitting around or so, a measurement when I am doing standard swimming, and a third when I am under heavy stress. If I am then trying to figure out what kind of extra air to carry I tend to use my heavy consumption, because if I am using it, things probably hit the fan and there will be a bit of stress there. Better to have a bit to much air then not enough.