JohnB47
Contributor
Safety stops are considered optional--that's what distinguishes them from deco stops. If I were calculating my minimum gas needs in an out-of-gas situation, I would not factor in any deep stops, 1 minute stops every 10 feet, or a 3 minute stop at 15 feet. I would assume an ascent at 60 feet per minute to 60 feet, and 30 feet per minute to the surface.
It's up to you what assumptions you want to make. I assumed that in an emergency, I would use the whole tank rather than holding back 500 psi, but I decided that I would like the option to do a safety stop. You have to make some kind of assumptions in order to make calculations, and the point is that once you decide on what assumptions you are comfortable with, you can calculate the maximum depth from which you can make a safe ascent with a given size pony tank. When you do so, it becomes very obvious that 1.7 or 3.0 cu. ft. is not very much air, but that 13 cu. ft. is probably adequate for a bailout from normal recreational diving depths. If you dive deeper than that, maybe you should consider 19 cu. ft. or more. I would encourage anyone who is thinking of buying a redundant air supply to make the calculations rather than just buying whatever is currently popular.