Question CCR for recreational depths

[The Chairman]

Rebreather's are not for everyone.

No, they are not.

The conundrum with all closed circuit rebreathers in recreational depths is that they are at their highest risk when shallow.

This isn't true. I meant to address this and forgot. Pure O₂ rebreathers and those that have their dil shut off with an O₂ flush are fine at shallower depths. When I do Blue Heron Bridge, I simply don't turn on my dil unless I pass the 20FSW range by the trench. Then I turn it off on the other side of the "trench" when I'm back around 20FSW. BTW, during long decos, it's kind of cool to see my O₂ levels drop after an O₂ flush. It's indicative of the N₂ leeching out of your body.

But my main draw is still the absence of bubbles. That is simply awesome!

 

[L13]

The conundrum with all closed circuit rebreathers in recreational depths is that they are at their highest risk when shallow. This is particularly true of mCCR's that are not able to actively maintain a ppO2 setpoint.

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

 

[Dsix36]

The O2 molecules are not as dense when shallow as they are when compressed at depth. This means that the PO2 will decay much faster and there is a real possibility of going hypoxic, passing out, and drowning. Normally the solenoid will fire and keep it breatheable but without close monitoring you might not notice a malfuncion in time. Several very experienced RB divers have died either on the surface or very shallow.

 

[inquis]

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

Speculating, but perhaps because the PO2 drop for a given depth reduction is greater the shallower you start? It seems difficult to me to be that inattentive where this would be an issue, but apparently it can happen. A PO2 of 0.32 atm at 66 ft turns into 0.16 atm at 16 ft. Then there's the fact that buoyancy increases faster for a given depth reduction (normal Boyle's law stuff), so it's easier to be "behind" on adjustments.

 

[billgraham]

Yes, your PPO2 goes up when you descend, and goes down when you ascend. So let's say you're at .5 at 33 feet, just by going to the surface you'll be at .25. Take a breath or two and you're at .14 or whatever. Lower than the .21 your body needs to stay conscious.

 

[formernuke]

.21 your body needs to stay conscious.

Actually that's between .18 and .16 to stay concese

 

[billgraham]

Actually that's between .18 and .16 to stay concese

Ha ha, at one point I became curious about this years ago because we were diving 10/50 as a diluent, and I sat on the couch with a regulator on a 19 full of it. I'll be damned if I didn't keep going for three or four minutes, but I suppose I must have been breathing through my nose unconsciously! And obviously not working.

 

[lowwall]

Actually that's between .18 and .16 to stay concese

That will be news to anyone who lives above 7000'/2100m.

Ha ha, at one point I became curious about this years ago because we were diving 10/50 as a diluent, and I sat on the couch with a regulator on a 19 full of it. I'll be damned if I didn't keep going for three or four minutes, but I suppose I must have been breathing through my nose unconsciously! And obviously not working.

PO2 of .1 is approximately the top of Mt. Kilimanjaro or Denali.

The climbing death zone, where low O2 levels will kill you within a couple of days no matter how much acclimatization time you have, is generally considered to start at 8000m or a PO2 of .0734.

Death zone - Wikipedia

en.wikipedia.org

 

[inquis]

Actually that's between .18 and .16 to stay concese

Or even less. No issues here with a PO2 of 0.14 while snowboarding on a 10,000 ft mountain. Pike's Peak (a popular tourist attraction around here) is at 14k with a PO2 of 0.12 atm.

 

[inquis]

The O2 molecules are not as dense when shallow as they are when compressed at depth.

I'm missing how this is true since the fraction of O2 is greater when shallow for a given PO2. My understanding is that the density should be constant (assuming PO2 is maintained). Unless you're talking about an intentional difference in setpoint?