Deco on CCR

[MechDiver]

Question based on a high level of ignorance concerning CCR operation.

Is it possible, or normal, for a CCR diver to deco out of a deep (>200fsw) dive by using the CCR alone, or do you do like we do on OC and use stages/deco bottles? I would think in the former, that there would not be enough gas carried internal to the rebreather to allow for that.

Like alot of scuba questions, I realize the answer to this may be quite involved, but I'm just curious based on a "scuba guys gab session" the other night.

Thanks,

MD

 

[MikeFerrara]

I don't dive a rebreather but it is possible and it is done. At least some CCR (if not all) are constant PPO2 so you can end up with less decompression obligation in the first place. The PO2 can also be adjusted on the fly so it can be used to accelerate decompresion also.

 

[MechDiver]

I know about the constant PPo2, but am having trouble with the amount of gas held in the rebreather itself vs what it takes to deco out of these dives on OC.
As I said, no answer in scuba is easy :upset:

MD

 

[MikeFerrara]

I think knowinf what a typical cc sac is would answer that question. If nobody helps out here I'm going to be with some ccr divers this weekend and I'll ask them.

 

[caveseeker7]

Yes, usually deco is done in CCR mode. CCRs are way more efficient with their gas, especially at depth. The tanks you see on most CCRs (3ltr) give you hours of dive time, so there is plenty there for deco. Only O2 is used while at depth, diluent during descent to make up for lost volume and during a diluent flush. For most CCRs, scrubber time and O2 toxiety are the limiting factors, not gas.

Using OC during ascent/deco would take away the benefit of the ideal mix/constant ppO2 that CCRs offer.

If you see CCR divers using stages, they are usually for OC bailout only. Some CCR divers change diluent mixes, however, heliox/trimix at depth, air closer to the surface.

Check the 'Some real questions' thread here, Madmole explains his tank set-up.

 

[madmole]

The big leap in understanding CCRs is that the O2 you use each breath is constant, you need x molecules pr breath. Now this leads to a great situation on a rebreather, you breath the same amount of O2 molecules at 300ft as you do on the surface and at 100ft that amount of molecules are contained in 1/10th of the amount of gas they are at the surface

So the upshot is that you use less gas at depth on a CCR. In fact the comparison to OC is 100:1 at 50M. So those little 3L 200 br tanks acyually hold 10 hours of gas for the average diver. Enought to do plenty of Deco, and then go and do the same dive another 4 times

On average I use 30 bar of O2 and diluent per dive (and cos their 3L its only $10 for a fill)

The side slungs are purely for emergency bailout or changing diluent on the way down (not really needed)

Also as youre at a constant PPO2 and not a steadily decreasing level like the OC guys, you have less deco to do as well. ALthugh we can fly any PPO2 we want

 

[MechDiver]

Caveseeker7 and Mike,

Thank you for the info. That makes sense.

MD

 

[MechDiver]

madmole once bubbled...

On average I use 30 bar of O2 and diluent per dive (and cos their 3L its only $10 for a fill)

Okay, I knew it was good, but had no idea there was that much difference.

Thanks much.

MD

 

[madmole]

200-300 trimix dives on OC and the money you wasted on gas would have paid for an Inspiration

 

[MechDiver]

madmole once bubbled...
200-300 trimix dives on OC and the money you wasted on gas would have paid for an Inspiration

I'm in the Yellow Pages right now looking for a PADI Inspiration class

MD