Decompression - CCR vs OC

[tbone1004]

How does decompression work with CCR when the offgassed nitrogen that you are exhaling into your loop is simply re-inhaled?

I get the theory during OC - You breathe in (for the sake of argument) 100% O2 at 6m and breathe out 95% O2, some CO2, and your offgassed N2. The cycle is repeated until equilibrium occurs (enough) and you surface.

However, when you are on a CCR, N2 is not removed from the loop like CO2 is, so it seems to me that the diver is simply re-inhaling it. Not only that, because it's not scrubbed out, the percentage of N2 increases with each breath.

The thought exercise came up when I asked whether or not a CCR rig can be used as an O2 source in remote areas when there isn't enough O2 to sustain a high flow rate for long periods of time.

what has been said above, but there is also a bit of a fun party trick on rebreathers for determining when you have nearly completed decompression. When you are coming up from dives that have a significant decompression obligation the difference in inspired vs. absorbed partial pressures of inert gas are fairly large. This causes the inert gas to dump into the loop and the ppO2 falls fairly rapidly as the loop volume grows. Since we want to maintain min-loop volume you vent that excess gas out and a bit more and add O2 back to bring the ppO2 back to where you want it. As you near the end of your decompression time at that stop the gases are starting to equalize and the frequency with which you have to vent gas reduces considerably until you have nearly equalized and that is about the same time that your deco computer will tell you to go to the next stop or surface.
If you dive with a heated vest and as is fairly common on longer cave dives, leave the battery canister with your O2 bottle, as soon as you plug in and turn the heat on you notice a huge amount of gas start dumping into the loop as your offgas efficiency improves with the improvement in blood circulation through the skin as it warms.
It's pretty neat to be able to observe the decompression process through the loop volume increase and subsequent ppO2 decrease. Also super interesting to see how individuals offgas differently as one buddy is usually done before the other, often by a significant amount.

 

[OTF]

You're asking about in-water recompression, which can be an option for a well-trained, experienced, and properly equipped team. There are established protocols for it. It can be useful in remote areas where the time required to evacuate a diver to a recompression facility could be detrimental. The paradox is that the divers with DCS who would most benefit from immediate recompression (i.e. the sickest ones) are also the ones most in whom IWR is the riskiest.

Best regards,
DDM

I took the question to be about administering O2 on the surface before getting to a chamber, for more efficient oxygen use compared to breathing off a medical cylinder. Not IWR.

CCR setpoint on deco may be a little higher than the time-averaged PPO2 on OC, but 1/4th (or even 1/2) the deco time means you choose your OC deco gases very poorly.

air, air, and more air is indeed a poor choice of deco gasses, but not an uncommon dive plan and I think a fair comparison to illustrate the largest delta a CCR can have. Admittedly on a reasonable length square profile dive, compared to an OC diver with ideal bottom mix and a rich deco gas, the marginal decompression advantage of CCR is small.

 

[duke dive medicine]

I took the question to be about administering O2 on the surface before getting to a chamber, for more efficient oxygen use compared to breathing off a medical cylinder. Not IWR.

Good callout. I read IWR but the OP may well have meant surface O2.

Best regards,
DDM