What size do you recommend?/pony/bailout/backup tank?

[eponym]

Here is how I figure it out. Anyone agree/disagree with my method? Lets say you need to get up from 130ft [snip]

I'll respond to two points you made.

First, I would never go anywhere near -130 fsw while relying on a pony to get me to the surface in the event of a primary air system failure. Not even in clear warm water. That has as much or more to do with narcosis as with air supply. I'm not saying that you claimed this was a safe approach: I just don't want to be misunderstood as advocating the practice.

Second, your math can be simplified. For an ascent from X to Y, use the midpoint of that range to calculate the atmospheres. For example, going from 100 to 0, the average is 50 and so the average ATA for the ascent is 1+50/33 or 2.5 atmospheres absolute. Multiply that by the time the ascent takes and by your estimated respiratory rate under stress. Then figure and add in the air you'll breathe while assessing the situation and deciding to abort, and the air you'll breathe at your rest stop(s). Then please remember that this is only a theoretical value and does not guarantee anything.

Fin on,
Bryan

 

[geraldp]

I don't know about calculations, but I remember reading an article in a magazine about pony bottles. The article describe a test they did to determine what size pony was appropriate. It stated a 13 cu. ft. bottle could bring a diver from 130 ft. to the surface in an emergency controlled ascent.

I wish I could remember the article. I think it was in Sports Diving.

This April Scuba Diving magazine had an article about spair airs and the h2Odyssey bail-out bottle. In ScubaDiving's tests the felt they could get to the surface from 132 feet via an Emergency Ascent using a 6 cu. ft. bail-out bottle, and still have a breath or two left over. See Article .

So I doubled that and got a 13 and mounted it alongside my main tank. After reading here I should have gone with a 19. That should allow me to get in a safety stop.

Jerry

 

[teknitroxdiver]

Lots of people here (including myself) have reccommended stage-mounting the pony. But here's a question: with a smaller one like a 13cf, could it be belt mounted like an argon bottle/canister light?