: good question, hopefully someone who knows max co2 level can answer

Probably not a relevant figure. You'll probably get bored with staying upside-down before you could build up that high a level.

: here's a guess along the way: if average xsection is abt 200sq in and bulkhd
: to blkhd is 60in get abt 7cu ft. minus body and equip say 4cu ft left
: over.

: one max breath is abt 4.5litre=.17cuft

I'll take those numbers and go with them. Different kayaks will of course have different areas with stored air. And it is rare to take a "maximum" breath. More often you'ld move about 2 to 3 liters of air in and out on each breath.

: so if we just breathe in and exhale into the water and let waterlevel rise in
: yak in perfect conditions w/ just enough water coming in would get abt 24
: full breaths.

More like 40 to 48, since we aren't expecting maximum lung effort.

: my guess in the closed system is that you could go for 1/3 of this, say 8
: breaths. could anyone answer better?

Well, actually, you'll go a LOT longer than that. Of the oxygen you inhale you only use a fraction, and you exhale the rest. That oxygen would be contained in the sealed area of the kayak, and would be available on the next breath.

The first-aid technique of mouth-to-mouth resuscitation relies on the fact that there is still plenty of good, vital oxygen in the air that you are exhaling -- more than enough to keep another person alive. Let's say your body uses 1/3 of the oxygen you inhale on each breath. I think the actual level is more like 1/4, but we'll assume that you are breathing heavily because you are doing some exhausting exercise. While each breath you take reduces the oxygen content slightly, let's assume that for a given 2 liters of air you can breathe it in and exhale it 3 times before the oxygen is used up. At that rate you would still be inhaling a low percentage of oxygen on your 120th breath. I tried forcing myself to breath in and out on a 4 second cycle (15 cycles a minute) and that was a rather fast rate -- but I'll use that number because you might be panting from overwork or excitement. At that rate you would have about 150 breaths in 10 minutes. So, you could probably breathe off of the air contained in the compartment for about that long.

The build-up of carbon dioxide, and the decrease of oxygen would cause you to breathe more rapidly, so I think you'ld get a pretty good warning about when to call it quits.

Now if you wanted to put in a canister of material to absorb the carbon dioxide, and a bottle of compressed oxygen you could go for a much longer time. The big problem here would be monitoring the gas mixtures so you would be safe, but in principle this is the same as the early (and modern) "rebreathers" that were used by divers before the introduction of SCUBA equipment. I don't recommend or encourage this. Misadjustments could lead to fatal accidents.

A few problems that might not have been thought of. The stored air is not going to get "refreshed" by itself. Just returning the kayak to an upright position will not magically replace the stored, stale air with a fresh batch. If you breathed from the stored air on the first roll, and then again on the 4th and the 8th, by the time you mext hit the air hose the oxygen content would be dipping fairly low. You would need to open a hatch after each use of your breathing space, and either leave it open for a few minutes, or fan it somehow. You want to get the air moving so that you got a good exchange.

Another problem would be the length of the hose. If it is more than 3 or 4 feet long (and I suspect it might be aobut that long) then it really doesn't matter how good the air is in the sealed chamber. You'll keep building up the level of carbon dioxide in the hose. If the volume of the hose is close to a liter, then that same liter is being sucked in and out. You would only get a small additional amount of air, and keep rebreathing a good percentage of stale air. That is just one reason why the snorkels used by skin divers are so short, and fairly wide.

: what percentage of co2 do we breath out, what ultimate percentage would stop
: us from trying one more roll?

Whatever it takes to kill you. Would you stop rolling if you were still breathing?

: would have to purge all the 'air' to do it again.

: (shawn, i think embolism is only a danger from greater depths than just
: upside down and only if pressurized air-more capacity that lung can carry
: is breathed in - like can free divers embolize if get the air from air
: pressure returned to ??)

Embolism is not a problem here.

: ((he he and what if our bottom exiting footpumps now could reverse? constant
: air supply??? espec if electric?))

Actually, that is a distinct possibility. Just make sure that any water that gets in has someplace to dump away from your breathing tube, and that your battery works well when inverted.

If you can pump in enough air to more than replace that which you are breathing then the excess will bubble out, and that will help to keep you from building up a high level of carbon dioxide.

There might be another advatage to having air pumped into the inverted boat. Any air that bubbles out of the sealed areas and comes into the (inverted) cockpit will displace water, raising the boat, and possibly aiding your roll. Someone would have to play with this to try out the technique and see exactly how much of an aid this would be.

Hope this helps

PGJ