Question CCR for recreational depths

[L13]

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 math doesn't work for this answer. if your pp02 setpoint is the same (or similar) at the surface and at 30m, you have exactly the same O2 in your loop at both depths, only the nitrogen changes, you metabolize it at the same rate, and it lasts the same amount of time. At say a ppO2 of 1.0 at the surface you have 4L of oxygen, and 0L nitrogen. at 0.7L/min it lasts ~4 min. With the same ppO2 of 1.0 at 30m, you have 4L of oxygen, and 12L of nitrogen for a total of 16L, and at 0.7L/min it still lasts the same ~4min.

 

[L13]

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.

This make more sense. If you are operating the system differently (O2 solenoid turned of near the surface) I can see how there is a risk of inattention resulting in death. This risk would be mitigated by any of the constant O2 mass flow solutions out there (mCCR etc.).

 

[rjack321]

The math doesn't work for this answer. if your pp02 is the same at the surface and at 30m, you have exactly the same O2 in your loop at both depths, only the nitrogen changes, you metabolize it at the same rate, and it lasts the same amount of time. At say a ppO2 of 1.0 at the surface you have 4L of oxygen, and 0L nitrogen. at 0.7L/min it lasts ~4 min. With the same ppO2 of 1.0 at 30m, you have 4L of oxygen, and 12L of nitrogen for a total of 16L, and at 0.7L/min it still lasts the same ~4min.

Swap the surface ppo2 to a realistic 0.7. 4L x ppo2 0.7 (=fo2 of 0.7) = 2.8L of O2. 2.8L / 0.7 L/min = 4mins (convenient math) to completely gone.

You don't run a ppO2 of 0.7 at depth, a realistic ppO2 is 1.2. You 4L x ppo2 1.2 (fO2 = 0.3) x 4ata = 4.8L of O2 in there. 4.8L/0.7lpm = 6.8mins to completely metabolize it. About 71% longer to become hypoxic.

Net, if you actually dive CCR you'll notice the decay at depth is much slower because there is both more of it and you start off further away from hypoxic in the first place (usually 1.2-1.3 ish).

This does not apply to O2 rebreathers that the OP originally tried because they have nothing in them but a small fraction of N2 that is actually offgassed into the loop from the body (similar to an astronaut prebreathing O2 before a space walk) and the rest is O2 (assuming its decently flushed before use). If your O2 rebreather breaks or fails you quickly run out of loop volume (or have far too much loop volume) before the loop becomes dangerously hypoxic. The inerts in a mixed gas CCR, or the diluent added, much more easily creates a hypoxic loop and this risk is particularly high at or near the surface - which is what happened to Brian Burgee in HI (and many others over the years). At depth the risks flip and the toxicity of O2 is a much greater risk than the threat of hypoxia - which is never zero as evidenced by the Ginnie Springs mCCR hypoxia fatality at 125ft ~3 years ago.

 

[rjack321]

This make more sense. If you are operating the system differently (O2 solenoid turned of near the surface) I can see how there is a risk of inattention resulting in death. This risk would be mitigated by any of the constant O2 mass flow solutions out there (mCCR etc.).

Solenoid battery dies
Solenoid sticks off
Orifice clogs
O2 was never turned on in the first place
ADV on and firing and the orifice or solenoid can't bring the O2 up to breathable levels.

There are many risks of CCR on or near the surface. For every attempt to reduce these risks a tradeoff is made somewhere else.

 

[L13]

Solenoid battery dies
Solenoid sticks off
Orifice clogs

Most of these seem to be equal risks, shallow or deep.

O2 was never turned on in the first place

This seems like a beginning of dive risk, not a shallow risk.

ADV on and firing and the orifice or solenoid can't bring the O2 up to breathable levels.

I would think this would only be a risk if the DIL was chosen for a deep dive, which shouldn't be the case if not intending to go deep.

There are many risks of CCR on or near the surface. For every attempt to reduce these risks a tradeoff is made somewhere else.

Most of those tradeoffs (that have been expressed so far) appear to be with regard to operation or performance at depth. If you aren't planning on going deep they would no longer be tradeoffs.

In other words, it looks like the main reason that current rebreathers have an increased risk shallow is that they are optimized for greater depth, and not something inherent about rebreathers as such at shallow depth.

 

[broncobowsher]

My understanding of most of the O2 rebreather designs started life as a niche non-diving market, mine rescue for example. Broadened the market and added underwater.

I see a few places where pure O2 rebreathers make sense. Under water filming in a pool/aquarium would be a really good one

But a regular mixed gas (even if just air and O2) is still very enjoyable in recreational settings. Several years ago I did the U352 with a bunch of bubble blowers. I could be the first in the water and the last out. I could enjoy the dive more, never chasing the SPG to be back on the boat before running out like everyone else. The most annoying part was listening to everyone else blowing bubbles. Sometimes when the boat was in the right spot, I could listen to the pings from the sonar.

Some boats locally that do a 2-tank dive at the same location will let the rebreathers have a single long dive. I'll spend the surface interval that the bubble blowers have in the water enjoying not being in a crowd. When they start coming back down, I head up. Might be only 1 dive, but I spend 50% more time in the water then they did and enjoyed it way more.

 

[211ratsbud]

It may only be slightly less than common, but 10/50 is used by quite a few divers in my experience due to it being, heliar, and rather inconsequential with regard to helium usage to begin with. I don't know the protocol for diving with that mix near the surface or at the surface aside from always have a breathable gas in your loop.

But the tradeoff to a CCR is that wreck sites change frequently on a charter where you may expect 150+' dives weather could change and be on a 80' wreck or less. I bet some bring back up Dil but not all as the philosophy of "the gas maker is on your back" is pretty prevalent.

 

[kensuf]

It's about the number of molecules of oxygen in the rebreather and the fact that your body only metabolizes a certain number of molecules per minute regardless of what depth you're at. As you go deeper, to maintain the volume you need to add more molecules of gas. So at 330' you'll have 10x the amount of molecules in a bag as you would at the surface.

Since your body metabolizes the same number of molecules of oxygen per minute, regardless of depth, you run a higher risk of going hypoxic when there are fewer oxygen molecules in the breathing loop to begin with.

 

[rjack321]

My understanding of most of the O2 rebreather designs started life as a niche non-diving market, mine rescue for example. Broadened the market and added underwater.

More or less yes. In years past people converted O2 units or SCR units (like the dolphin) to mCCRs. But that is less and less common nowadays.

 

[broncobowsher]

More or less yes. In years past people converted O2 units or SCR units (like the dolphin) to mCCRs. But that is less and less common nowadays.

I think the availability of good modern professionally designed and built rebreathers is key. 10-15 years ago a rebreather was pretty much unicorn material. They did exist, but you never saw one. If you did, it was something really special. Right place, right time, right connections.

These days, it isn't that hard to find. I've run into them without looking. Was at the Blue hole in New Mexico 2 summers ago on a weekday and the only other person in the water had (I think) a JJ. By the time I paid my pass and got back they had already left. I know several shops that have added some rebreather to the sales floor, and not that long ago none of them had it. When I got certified the idea of Nitrox was something from the devil. Unless you were some commercial diver, all anyone had access to was air. Times have changed