Question CCR for recreational depths

[billgraham]

I think the point overall is PPO2 going to drop by x percent with each breath unless dil or 0-2 is added either by the machine or by the diver manually, and it doesn't take long to get to unconscious at 1 atm.

 

[AJ]

I think the point overall is PPO2 going to drop by x percent with each breath unless dil or 0-2 is added either by the machine or by the diver manually, and it doesn't take long to get to unconscious at 1 atm.

Got that, but when your setpoint is 0.7 it has to drop a lot to be dangerous. I have my controller warning if I go below 0.4. Going to 10% (=0.1) is quite a drop from 70% (=0.7). Or I am missing something here?

 

[rjack321]

Got that, but when your setpoint is 0.7 it has to drop a lot to be dangerous. I have my controller warning if I go below 0.4. Going to 10% (=0.1) is quite a drop from 70% (=0.7). Or I am missing something here?

When your counterlung volume is 4L at the surface and you are metabolizing 0.7L of O2 per minute it's about 4 mins to drop to non-life sustaining levels. If your orifice clogs or your solenoid quits near or at the surface. Even faster if your ADV is on and injects low fO2 dil as the loop volume drops.

At 30m depth the loop volume is still 4L but there's 16L of gas in there. Your ppO2 decays much slower if the solenoid quits or orifice clogs. If your ADV fires hypoxic gas its also going to be breathable at that depth.

Bottom line, mixed gas CCRs are particularly dangerous at or near the surface.

 

[The Chairman]

Can someone clearly explain why the risk is higher when shallow?

The big culprit is that we metabolize O₂. The second culprit is that we limit O₂ at depth, because at too high of a partial pressure it becomes toxic. The third culprit, is that above 20FSW or so, our PPO₂ can't get above 1.2atm which we have it set at, so to prevent losing all of our O₂, we turn off our O₂ solenoid. But, we're still metabolizing O₂ and we're off-gassing N₂, so it's very easy for our O₂ levels to go below levels needed to sustain life if you don't pay attention. People prone to not pay attention are not good candidates for a rebreather.

There's a longer answer, but I'm not up to trying to 'splain it at this point.

 

[tursiops]

There may be something to pure O2 CCRs for shallow dives, I don't have that specific experience but have entertained the fantasy of getting one. I think there are people here who dive them. If your "thing" is shallow water multi-hour macro photography dives I could definitely see the appeal of a pure O2 machine. But you would be truly limited to 20 feet.

Limited to 20 ft and to however many OTUs you can tolerate. At 15 ft you might get upwards of 2h on the O2 clock, once a day.What's the point? I can do that on an AL80, as many times per day as I want.

 

[undrwater]

Limited to 20 ft and to however many OTUs you can tolerate. At 15 ft you might get upwards of 2h on the O2 clock, once a day.What's the point? I can do that on an AL80, as many times per day as I want.

Yeah, but noisy bubbles.

I don't think I'm the right candidate for CCR, but the idea is still attractive. Perhaps someday I'll get a chance to try a double hose.

 

[The Chairman]

Perhaps someday I'll get a chance to try a double hose.

Kiss makes a semi-closed rebreather. It produces about a third of the bubbles.

 

[billgraham]

Got that, but when your setpoint is 0.7 it has to drop a lot to be dangerous. I have my controller warning if I go below 0.4. Going to 10% (=0.1) is quite a drop from 70% (=0.7). Or I am missing something here?

As someone mentioned earlier on, there have been a number of fatalities at or near the surface.

 

[The Chairman]

As someone mentioned earlier on, there have been a number of fatalities at or near the surface.

Ruh roh: reality just hit the fan!

 

[OTF]

Limited to 20 ft and to however many OTUs you can tolerate. At 15 ft you might get upwards of 2h on the O2 clock, once a day.What's the point? I can do that on an AL80, as many times per day as I want.

I suppose that's true, and there's the actual explanation why pure O2 CCRs are not very useful outside of niche applications.