Are rebreathers getting safer over time?

[Akimbo]

... When descending diluent adds oxygen at at rate proportional to your descent rate (unless you are using a completely inert gas for dil)...

Coffee on the monitor, and not the happy kind.

Don't try this at home, kids.

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Pure Helium diluent is what we used for rebreathers… for 300'+ saturation dives. Shallow training and testing was a PITA though. It is great on descent unless you have a loop leak. Pure Helium was also used on the ElectroLung when it was introduced in the late 1960s.

 

[wedivebc]

Pure Helium diluent is what we used for rebreathers… for 300'+ saturation dives. Shallow training and testing was a PITA though. It is great on descent unless you have a loop leak. Pure Helium was also used on the ElectroLung when it was introduced in the late 1960s.

Yes Canadian Navy uses it in the CUBA rebreather as well.

 

[kr2y5]

If you inject too close to the sensors, there's no mixing. If you inject before the scrubber (pretty common) then it takes a while to get mixed air to the sensor again....so it's sensing low PO2 a lot and will keep dumping O2 in if your response time is too low. Too high, and you'll never get your PO2 right. You want instant information, which can't happen due to the challenges above.

Thanks, this is enlightening. So, you are saying that the delay in mixing gas, and the resulting lag in sensor readings, are high enough that it becomes difficult to maintain setpoint on ascent purely in response to the real-time pressure and ppO2 signals, and that because of this, a manual action well ahead of the time is required on behalf of the diver... that makes sense. I see that the cruise control analogy is not really accurate, since the feedback from the speedometer presumably comes on a much faster time scale.

So, it sounds like getting the gas to mix much faster would be a major step forward towards a more reliable fully automated rebreather... have any improvements happened in this domain recently?

 

[victorzamora]

Thanks, this is enlightening. So, you are saying that the delay in mixing gas, and the resulting lag in sensor readings, are high enough that it becomes difficult to maintain setpoint on ascent purely in response to the real-time pressure and ppO2 signals, and that because of this, a manual action well ahead of the time is required on behalf of the diver... thank makes sense. I see that the cruise control analogy is not really accurate, since the feedback from the speedometer presumably comes on a much faster time scale.

So, it sounds like getting the gas to mix much faster would be a major step forward towards a more reliable fully automated rebreather... have any improvements happened in this domain recently?

Not just the delay, no. The delay plays a huge part in not being able to compensate quickly enough. On ascent, there are many more factors than just the delay. One of them is the delay, obviously. You can ascend a long way before the solenoid fires. Another is gas expansion. When you start expanding, the gas in the loop expands and pins your counterlungs full. The main thing you seem to miss is that PO2 varies with depth given a constant FO2. So, you set your PO2 in your handset...right? Let's say you want a PO2 of 1.0. At 33ft, the gas in your loop is ~EAN50. If you descend, the PO2 comes up. If you ascend, PO2 goes down. Get to the surface and your PO2 is 0.5. You set a PO2, your O2 cells measure PO2....but the gas is "constant" FO2, not PO2. So if you ascend or descend aggressively you FO2 remains unchanged with a large depth change, drastically changing your PO2.

 

[kr2y5]

Another is gas expansion. When you start expanding, the gas in the loop expands and pins your counterlungs full. The main thing you seem to miss is that PO2 varies with depth given a constant FO2. So, you set your PO2 in your handset...right? Let's say you want a PO2 of 1.0. At 33ft, the gas in your loop is ~EAN50. If you descend, the PO2 comes up. If you ascend, PO2 goes down. Get to the surface and your PO2 is 0.5. You set a PO2, your O2 cells measure PO2....but the gas is "constant" FO2, not PO2. So if you ascend or descend aggressively you FO2 remains unchanged with a large depth change, drastically changing your PO2.

I understand that, but as far as gas expansion, the pressure reading is presumably instantaneous, so O2 could presumably be added in real-time? You say that it is hard to compensate fast enough. Why is it hard? Is it because the smallest amount of gas that can be reliably injected in response to decreasing pressure is so large, that that alone bounds the reaction time from below? I am not trying to disagree with you... just trying to get to the bottom of where actually the challenge lies. Thanks!

 

[rjack321]

So, it sounds like getting the gas to mix much faster would be a major step forward towards a more reliable fully automated rebreather... have any improvements happened in this domain recently?

I had no idea victor dove a CCR...

The mixing of gas is not the problem. The problem is that if you ascend too fast you can't keep up with the drop in ppO2 due to the expansion of the loop. You could inject O2 nearly continuously and STILL not keep up with the expansion and resulting drop in ppO2. Mixing faster is not the problem, the runaway ascent is the problem. Slow down (~30ft/min) and the "problem" is solved.

There are "fully automated rebreathers on the market now - and they are not "safer". Several people have perished on the Poseidon already and its a recreational fully automated CCR.

 

[wedivebc]

Not just the delay, no. The delay plays a huge part in not being able to compensate quickly enough. On ascent, there are many more factors than just the delay. One of them is the delay, obviously. You can ascend a long way before the solenoid fires. Another is gas expansion. When you start expanding, the gas in the loop expands and pins your counterlungs full. The main thing you seem to miss is that PO2 varies with depth given a constant FO2. So, you set your PO2 in your handset...right? Let's say you want a PO2 of 1.0. At 33ft, the gas in your loop is ~EAN50. If you descend, the PO2 comes up. If you ascend, PO2 goes down. Get to the surface and your PO2 is 0.5. You set a PO2, your O2 cells measure PO2....but the gas is "constant" FO2, not PO2. So if you ascend or descend aggressively you FO2 remains unchanged with a large depth change, drastically changing your PO2.

If you are ascending so fast that the PO2 drops to dangerous levels you have much bigger problems than low PO2

 

[KenGordon]

There are "fully automated rebreathers on the market now - and they are not "safer". Several people have perished on the Poseidon already and its a recreational fully automated CCR.

What extra automation does a Poseidon have over say a JJ or a Vision?

 

[Germie]

I dive eCCR too. Inspiration Vision. Yes, the unit has some disadvantages, it is more bulky than a twin12, but I can now do long long longer dives.
And yes any ccr costs more time to prepare and to clean after a dive. If I am in a hurry for a local dive, then sometimes I take my twinset and not my CCR because of the hurry. Hurry and ccr is not a good couple.
I don't have a CO2 sensor, I know they are available. I don't like a BOV, that is too bulky. These things are personal things.
I have dived my unit now a couple of times over 130m and dives up to 5 hours. That are all things that are impossible to do on oc. To reach the bivacplace in the Resselcave I need to bring another scrubber for the way back. But on oc you cannot do this dive.
Did my unit never fail? I had 2 times that with new batteries 1 of the 2 controllers falled off during a dive. And I had 1 time at 132m that I mentioned I was breathing faster than normal. In all cases I could finish the dive without bailout to oc. In the case of faster breathing at 130m, it was ow so we went up again and all things where solved around 110m. In the other cases I decided to run the unit manually and switch to low setpoint so the solenoid had no need to fire to safe the batteries for the controller that was still working. You only need PO2 readings then.
During my first course I had several times cell issues with the bad cells, but that is now solved.
CCR diving is in 1 way easier: no gasswitches, just watch PO2. But in other ways it is more complicated. Bouyancy on lungs, drysuit and maybe wing, what to do if things goes wrong, etc. But I like it. And I take sometimes OC because that fits the dive better, it depends on the dive I want to make. I use my CCR in caves, deep dives (wrecks, reefs), shallow to look for carp fish, etc.
Deco is deco, on oc and on cc, so you have to wait for the stops, and have to control your stops.

I know some people with a se7en and they have problems with cells when doing a second dive. The vision can have this with cell 2 if transported on the back. But the poseidon is soooo sensitive with this. And I don't like the scrubbers you have to use. They are already filled. You don't have to fill yourself. But all things are personally. There are things I don't like on a JJ and on a vision too. Same with other brands of ccr.
Your own car or horse is always best.

 

[ajduplessis]

I am not a CCR diver, yet I struggle to understand why you say these dives can't be done on OC? Do you not carry bailout for these dives? I know OC gas for a 5 hour dive is massive for the full run, but don't you still need the gas on OC to complete a safe dive if bailout switch is required?