Are rebreathers getting safer over time?

[tbone1004]

In the ten years I have been diving, nothing about rebreather technology has changed.

That's just simply not true. In the last 10 years the biggest change is going from analog systems to digital which represents a huge advancement in the safety of the units. While you still are reliant on the O2 cells, and in that you are 100% correct that cell technology hasn't changed, the movement from analog to digital systems allows the units to operate in a more reliable manner and get more accurate values out of the O2 sensors. Fiberoptics feeding the HUD's, computers in the units with independent systems to monitor etc etc is all a very huge leap in technology from the old analog units.

Regarding CO2, there are three things to consider about CO2 and monitoring. Is my CO2 hit self induced and due to my body not being able to get rid of the CO2, or am I overworking the scrubber, or have I exhausted the amount of media?

One is the most common, guys are overworking their bodies and the scrubbers can keep up but their bodies can't get rid of it, bailing out won't fix this.

Two is rare in this day in age especially with the CE rated units, you're just not going to overbreath the new scrubber designs, sorry.

Three is operator error for you violating the published run-time on the scrubber, also very rare to see.

CO2 monitoring I wouldn't worry about, new O2 cells are currently being researched to give more reliable cells, but by going digital they have a lot more freedom in how the cells are handled since they don't have to worry about interference and what not so you're seeing new designs in the heads that help protect the cells. The units overall are much safer than they were 10 years ago, that's just a fact, the analog designs are going away and while the basic rebreather design is the same as it was 100 years ago, the monitoring is what is important, and as long as the operator is able to stay responsible enough to operate the unit, the unit isn't actively trying to kill them anymore.

 

[Akimbo]

In the ten years I have been diving, nothing about rebreather technology has changed...

More accurately, eCCRs have not fundamentally changed since Walter Stark’s Electrolung was introduced in 1969. There have been a significant number of small refinements, especially in electronics, but the basic design is unchanged.

CO2 absorbents are a little better, but not that much. Oxygen sensors are not very different than what was widely used in the early 1970s. CO2 sensors are much better but still leave a lot to be desired. This isn’t all that complicated, allow salt water to get to most of the internals and your rebreather is toast.

 

[dreamdive]

That's just simply not true. In the last 10 years the biggest change is going from analog systems to digital which represents a huge advancement in the safety of the units. While you still are reliant on the O2 cells, and in that you are 100% correct that cell technology hasn't changed, the movement from analog to digital systems allows the units to operate in a more reliable manner and get more accurate values out of the O2 sensors. Fiberoptics feeding the HUD's, computers in the units with independent systems to monitor etc etc is all a very huge leap in technology from the old analog units.

Regarding CO2, there are three things to consider about CO2 and monitoring. Is my CO2 hit self induced and due to my body not being able to get rid of the CO2, or am I overworking the scrubber, or have I exhausted the amount of media?

One is the most common, guys are overworking their bodies and the scrubbers can keep up but their bodies can't get rid of it, bailing out won't fix this.

Two is rare in this day in age especially with the CE rated units, you're just not going to overbreath the new scrubber designs, sorry.

Three is operator error for you violating the published run-time on the scrubber, also very rare to see.

CO2 monitoring I wouldn't worry about, new O2 cells are currently being researched to give more reliable cells, but by going digital they have a lot more freedom in how the cells are handled since they don't have to worry about interference and what not so you're seeing new designs in the heads that help protect the cells. The units overall are much safer than they were 10 years ago, that's just a fact, the analog designs are going away and while the basic rebreather design is the same as it was 100 years ago, the monitoring is what is important, and as long as the operator is able to stay responsible enough to operate the unit, the unit isn't actively trying to kill them anymore.

I agree with it all and want to add something:

In the "old days" and older design units only allow for 3 O2 cell monitoring. Meanwhile, newer units are allowing additional cells being added. That is another improvement in my book.

 

[muzikbiz22]

a very experienced diver who uses a lot of Helium declare that he will never get into rebreathers

Yep there's a lot of those out there. Rebreather experts that have never dove a rebreather. They're are extremely proud of the fact that a couple of hours of diving, costs them $200 in gas, a car load of tanks, and about 4 to 8 regulators/2nd stages, etc. My advice to you is ask a rebreather diver about rebreathers. I can tell you this, I'm a rebreather diver, and there is no better, funner way to dive. The rebreather community is growing, and is the future.

 

[kensuf]

I agree with it all and want to add something:

In the "old days" and older design units only allow for 3 O2 cell monitoring. Meanwhile, newer units are allowing additional cells being added. That is another improvement in my book.

What about the Se7en? They seem to think you only need two, and the argument for their verification system is compelling.

I don't think adding more sensors, going to five, six, seven, or whatever, does much except to make an already complex system more complicated. I would expect a five sensor system to have the same human element issue, complacency in cell verification and replacement cycles, if not more so.

 

[Akimbo]

I agree with it all and want to add something:

In the "old days" and older design units only allow for 3 O2 cell monitoring. Meanwhile, newer units are allowing additional cells being added. That is another improvement in my book.

That's an improvement? That’s just an admission that O2 cells aren’t up to the task. They are fine in the hospitals they are made for, but not so great at elevated pressures in salt water. Same for CO2 absorbent.

IMHO, the root problem is the same as commercial, military, and recreational diving all have always had. The market size is too small for the level of innovation necessary. Engineers look at the problems and quickly realize that the risks of developing an ideal solution can never be paid back. That leaves them with adapting whatever they can find in other industries to address the problem.

 

[tbone1004]

What about the Se7en? They seem to think you only need two, and the argument for their verification system is compelling.

I don't think adding more sensors, going to five, six, seven, or whatever, does much except to make an already complex system more complicated. I would expect a five sensor system to have the same human element issue, complacency in cell verification and replacement cycles, if not more so.

Similar to CO2 there are three schools of thought. You have two cells and you default to "Bail-out" if they disagree. The Se7en is first and foremost a recreational rebreather, if they disagree, you bailout call the dive and replace the cell. It's fine for Rec breathers where SOP is just bailout and call the dive, insufficient for proper diving, though Poseidon offers a third cell holder for you to connect external monitoring that way.

The next is the 3-cell thought where you have three cells and as long as two agree, you're ok to finish the dive. May turn the dive if in a cave or start your ascent, but as long as two are agreeing with each other you don't bailout. May bail-out and do a dil-flush to verify before hopping back on the loop or may stay on the loop during dil-flush, but you verify two of the three cells that way and finish on the breather.

The third school of thought imho stems from people not truly understanding/trusting the electronics and are operating under "more must be better" and throw 4+ O2 cells in there to get the most monitoring they can. Personally I think this is excessive and is why 3 cells is standard. Voting logic indicates that with proper rotation of cells, the odds of two being similarly wrong is low enough to not be cause for more, if you are concerned, just bailout, that's what you're carrying it for.

The important thing here is you have two options, trust the electronics, or be paranoid. If you don't trust them, you probably shouldn't be diving a breather because the benefits have just been thrown out the window by you not being able to enjoy the experience because of your paranoia. You need a healthy respect for the units, they are more complex than OC, but if you trust the electronics to do what they're supposed to and monitor them appropriately they are much safer than they used to be when they were mCCR's, then analog eCCR's, both of which required significant active input from the diver. These new units are able to self-diagnose issues *including verification of signal integrity from the sending units to the receiving units which is HUGE*, report these issues to the diver, and are far more autonomous overall. This is good.

Expanding on what Akimbo said, the need for improved O2 cells and scrubber media is the biggest setback right now. The introduction of digital systems has brought the units into this century finally, but the media and cells are still the limiting factors, but they are adapting. The next big break will be in a new O2 cell design, potentially followed by new scrubber media, luckily those will not require substantial redesigning of the units.

 

[PfcAJ]

The o2 cell problem stems from trying to stay on the unit. There's been at least one death due to cell failure when TWO cells read the same thing, one was different. The 'different' one was the correct one, the other two were reading incorrectly.

In my mind, it doesn't matter if you have 2,3,5, or however many cells. If one is reading something abnormal get to a known source (OC) and head for the surface. Its just not worth it.

 

[TSandM]

A number of years ago, Andrew Ainslie posted an essay on voting logic. His proposal, which made a great deal of sense to me, was that if you have three cells and they don't agree, you really cannot be sure which cell or cells are in error. BUT you can pretty much ensure your safety, if you plan your deco around the cell showing the lowest O2 percentage, and your MOD on the cell showing the highest.

I don't understand the concept that going "digital" changes much of anything. The cells aren't digital -- their output is still analog, and has to be converted. The cells are the flaw, and my understanding is that, with the prior major supplier going out of business, the reliability of the cells is felt by many to have decreased.

Don't get me wrong. I think we all take risks by going underwater in the first place. I think it's likely that the risks are higher when you are using an anesthesia machine with no mass spec to supply your breathing gas. If you are meticulous, disciplined, and utilize conservative decision-making, those risks may be acceptable, especially if you are driven to do dives where OC is no longer logistically reasonable. I have a hard time saying that one's desire to see a wreck at 300 feet isn't worth risking one's life, because I swim long distances into caves, where a major problem could be lethal.

I just don't see that, in my diving lifetime, the safety of rebreathers has fundamentally changed. They are still unstandardized. They still rely on a far from perfect monitoring technology. There is still no consensus on how bailout needs to be managed. And they remain quite intolerant of human error, something which is not amenable to eradication in systems involving humans

 

[cathal]

Interesting statistic from over this side of the pond. Last year we had 6 diving fatalities. 5 were on OC while just 1 was on CCR. Is this an indication of the improvement in CCR technology and safety procedures around diving CCR's, I can't categorically say, probably is. Why though have I yet to hear anyone over here worry about how dangerous OC is in light of these 5 fatalities. Is this user risk perception bias? Of course.