Decompression - CCR vs OC

[Heavydpj]

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.

 

[Whitrzac]

Offgased nitrogen and helium make up a small percentage of gas volume.
And the loop is usually flushed with fresh gase a couple times on deco.

 

[L13]

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.

If the %N2 increases, the %O2 decreases below the setpoint and more 02 is added. If there is enough N2 that %O2 can not be restored, the diver or the counter-lungs vent removing N2 (or He).

 

[LCoolio]

It may be reinhaled, but due to the differential of partial pressure on ascent, the N2 is not reabsorbed like on OC when using not 100% O2. As the others have said, as off gassed N2 builds up in the loop and drops pO2, the loop is vented and more O2 added.

 

[Heavydpj]

So theoretically, then, it should be possible to use a CCR to provide long-term oxygen therapy to a DCS patient. Is that a feasible thought process?

 

[OTF]

So theoretically, then, it should be possible to use a CCR to provide long-term oxygen therapy to a DCS patient. Is that a feasible thought process?

Yes, and this is indeed sometimes done to "extend" the oxygen supply in an emergency, depending on the severity of DCS and distance from definitive medical care. Great questions!

As an aside, because CCRs provide ideal mix at every depth including high PO2s on decompression, the decompression advantages can be outstanding. Like quadrupling no-decompression bottom times at moderate depths, or quartering deco times. But before you get to excited, they do make up for it by sucking to dive in other ways.

 

[Heavydpj]

I’m finding that out now. Doing my KISS Sidewinder course, lol.

 

[inquis]

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.

 

[berndo]

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.

You can buy a wenoll emergency o2 system that is made exactly for that purpose.

Wenoll Oxygen Circle Pack

The Wenoll re-circulation system allows providing 100% oxygen concentration during longer periods – e.g. over 8 hours with a 2,5 liter oxygen cylinder (200 BAR). This closed circuit Medical Oxygen Rebreather can be attached to any DAN multifunction regulator.

www.daneurope.org

 

[Duke Dive Medicine]

So theoretically, then, it should be possible to use a CCR to provide long-term oxygen therapy to a DCS patient. Is that a feasible thought process?

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