Let's assume that two experienced and fit divers are doing a wreck dive to 80ft. They breathe much better than I do. Instead of 90psi per minute at depth, they breath 40psi per minute at depth. Lets say they are doing a normal AOW dive like the Spiegel Grove. They are 200ft from the ascent line when one loses an o-ring at the regulator. He's in a bubble shower and trying to shut it down, his buddy realizes he has a problem, and assists in shutting the valve down, and gives his buddy the octopus on a 40" hose. This is a day with a bit of current so they go to find the anchor line. Let's say the donating diver began the donation with 1200psi. To go 200ft to the ascent line is approximately 50fpm swimming. This is pretty hard with only a 40" hose between them, but it's a standard speed for cavers to swim so we'll leave it at that. So when they reach the ascent line its been 4 minutes. From 80ft to 50ft takes a minute. From 50-20 is another minute, they do a 3 minute safety stop. They take another half a minute to get ther fins dealt with and hit the surface.
I count a 6 minutes before they get to their safety stop, and 3.5 shallow. Their combined breathing rate at depth was 80psi per minute. In 4 minutes getting to the line they breathed 320psi of the 1200 available. So they have 880 when they hit the line. They lose another 80 or so on the first minute of ascent. So now 720. They are coming shallow so lets say they only burn 50 coming to toward the stop, so that's 670psi. Lets say that now they are breathing at 15psi per minute each instead of the 40 at depth. So that is a combined 30psi for the 3 minute safety stop or another 90psi total on the safety stop. They have 580psi available to surface with.
