New to the diving world and happy to be here. I finally did it and celebrated my certification by purchasing a few books to grow my knowledge and understanding to this new world. It seems all of the diving agencies promote safety, but not all promote a redundant air system. Feel free to chime in and share your thoughts on a secondary air source. I'm thinking about adding a 19 cu ft pony alongside my Faber steel 100 cu ft. Although I, too, believe in being safe, despite the added expense I see a pony as a smart investment.
Just use 1 cu ft per min as your consumption rate, and you can easily see & quickly comprehend the following:
At the surface, 19 cu ft would last 19 mins.
At a depth of 33 ft, 19 cu ft would last 9 mins.
At a depth of 66 ft, 19 cu ft would last 6 mins.
At a depth of 99 ft, 19 cu ft would last 4 mins.
To ascend from 99 ft properly would take you 3 mins. This would consume about 6 cu ft of a 19 cu ft bottle. That also tells you that the smallest bottle you could effectively get to the surface with from 99 ft without stopping at 15 ft, would be a 6 cu ft bottle. However the smallest bottle that I would ever recommend for dives to 99 ft would be at least twice that volume.
With a 19 cu ft pony bottle, that would leave you with 13 cu ft with which to do a safety stop at 15 ft, after your ascent. You could wait 8 mins at 15 ft with that much gas, even at an elevated stressed 1 cu ft per min consumption rate. You should have plenty of extra air with a 19 cu ft bottle, although you do not have time to be delayed at 99 ft for whatever reason while using it either.
Most normal RMVs are in the 0.5 to 0.75 cu ft per minute range. But when something goes wrong, a stressed breathing rate of 1 cu ft per min in the examples above is a reasonable estimation. Plus it makes the calculations in US Imperial for a pony bottle contingency easier to figure and see. . .
