Potential Safety Improvements in Rebreather Design

[Brad_Horn]

the other side that is supportive of improvements in reliability and safety, but still feel that there will never be a level of improvement that will remove the need for a trained diver with checklists and a high degree of care to ensure safe operating.

I am not sure if this is a side; rather the common norm!

A crown strap would not have helped here. It would merely have kept the mouthpiece in. If she died of drowning in seawater yes. She drowned on her own gastric fluids.

You still however accept that having it fitted is a potential safety improvement - yes? A very simple and cheap potential safety improvement that is already in the CE standard as a requirement, required of all military rebreathers in western navies and one that was again flagged at the RF3 conference. REBREATHER FORUM 3 CONSENSUS STATEMENTS | The Underwater Marketing Company Blog
In relation to certainly this second Prism2 fatality and indeed a number of other incidents, it's fitment may have prevented both the diver and rebreather flooding and minimised complications during the recovery/rescue of the diver.... if the rebreather is recovered unflooded your surely also going to have a better chance of conducting an accident investigation!!!

 

[descent]

... as simple and inexpensive design feature like left-hand and right-hand thread could have prevented this fatality [Jillian Smith, Jersey C.I., UK].

Sometimes safety modifications are only obvious in retrospect.

I don't think that is the case here.

I often sit with other engineers in meetings, discussing product features. Sometimes we say no to a proposed new feature because adding that item would be "overkill". What we really mean is that even though the new idea is a nice one, none of us can make a good financial case for spending the extra money today.

We are always punting features downstream. We build up lists of desired improvements, fixes, and occasionally, even a genuine innovation. We revisit old lists, argue over what features will go into each release, and finally add new functions when a business case becomes obvious.

We live in a weird fog of the past, the present, and several possible alternative futures, always trying to choose the highest quality path through the fog while spending the smallest amount of money and causing the least amount of discomfort to our customers.

The ground is always shifting under our feet, and we always feel like we are not moving fast enough. Every time there is a serious problem, we doubt our own competence, even after a history of many successes.

Many times, proposed features get shot down because they add complexity. Engineering a complicated design is hard enough. Add complexity is rarely welcome without some obvious benefit.

In the case of the Prism 2, I don't think modifying the tees and the counterlungs would have required unreasonable effort to redesign, source parts for, and QA. Testing a new set of parts that physically resemble the old ones would be challenging at first, but identifying these difficulties is the entire point of the exercise.

I would be surprised to find that the improved safety provided by a modification like this had been discussed and punted downstream as uneconomical. Still, despite long safety traditions in other nautical and many aerospace applications, it seems that opposite-threaded fittings for mirror-image subcomponents are not included on new Prism 2 rebreathers.

Given the demonstrated excellence of other parts of the system, I confess: I am stumped, and wonder "why not?'

If I were a Hollis customer -- potential or current -- I would want to know that the Prism rebreather design had received a top to bottom post-accident review, with special attention to integrating the safety-specific concerns and opinions of outsiders.

Ideally, a few of those outsiders would come from other nautical backgrounds, and perhaps aerospace as well.

 

[John C. Ratliff]

Okay, I've sat on this discussion for a while now, but now will talk a bit about what I, an outsider with no rebreather experience, see as deficiencies for the rebreathers. Here are the ones I've identified during these discussions.

--High on my list is a lack of a CO2 indicator. Rebreathers have oxygen indicators, most times redundant (or triple redundant), but nothing to indicate carbon dioxide poisoning potential. Both can kill, and recent deaths (there have been several) underline this deficiency.

--Continued buoyancy problems. Because of the breathing loop, rebreathers are positively buoyant, but if they flood, there is an instant negative buoyancy. When I went over a scenario about using a buddy line with one rebreather user, he stated that he would not be tied to another rebreather diver because of this negative buoyancy, and the increased work (with a resulting potential of CO2 poisoning for the rescue diver, who is also using a rebreather). If this is true of all rebreather divers, then all rebreather divers dive virtually solo.

--Rebreather technology for removing CO2 is still back where it was in Hans Hass' days. There has got to be a better way (and apparently there is at least one newer alternative).

--Lack of automatic redundency. There is one loop, and the diver is left to determine when to switch to OC. The diver is already in a condition of CO2 poisoning, with resultant lethargy and confusion, yet has to make this determination on his or her own. If someone wants me (or other experienced OC divers) to think about rebreathers, then they need to sense when the scrubber no longer is working, and make the switch automatically. Some rebreather divers may cringe at this, and say that they depend upon their training and monitoring of their own physiology to do this, but that is not a reliable way of coping with this safety-critical failure mode. There should be no single-point safety critical failure potentials in a life support system.

Observations

Rebreather divers rely upon their training and checklists. This training is extensive, and the divers are told by instructors that the rebreathers are trying to kill them. They need to be hyper-vigilant about their own bodily reactions, even though those reactions themselves compromise this ability.

As a safety professional, with over 35 years experience in both occupational safety and product safety, we rely upon a concept called the "Hierarchy of Controls." Those controls go from the most effective to least effective control measures, and look like this:

Elimination > Substitution > Engineering Controls > Administrative Controls > Personal Protective Equipment

Note that training, the use of checklists, and procedures are all administrative controls. They are effective as long as the followed without exception; but there are always exceptions.

There are several different models of accident causation. One part of one is by Dan Peterson, his Second Causation Model. Note that it states that "Management sets vision, has values, makes decisions." So too do companies who design life support systems, and the instructors who teach divers to use them. Rebreather divers have created their own culture, which is somewhat different from that of OC divers. But in doing so, they have put some blinders on concerning the equipment that they are using (rebreathers). Because of this, the rebreather diving community is going to have to change its entire outlook in order to make rebreathers better. This includes new engineering designs, which overcome the limitations I've outlined above.

Note that Mr. Peterson talks also about "Traps." In my view, the rebreather of today has many of these traps, and they need to be eliminated. There is also a person's "capacity," with a "load," in a "state." Today's rebreather diver is asked to ascertain his own state, in a high load situation, with a diminished capacity. To me, that is simply not acceptable.

SeaRat
John C. Ratliff, CSP, CIH, MSPH
NAUI #2710 (retired)
(CSP = Certified Safety Professional; CIH = Certified Industrial Hygienist)

 

[KevinNM]

In the case of the Prism 2, I don't think modifying the tees and the counterlungs would have required unreasonable effort to redesign, source parts for, and QA. Testing a new set of parts that physically resemble the old ones would be challenging at first, but identifying these difficulties is the entire point of the exercise.

I would be surprised to find that the improved safety provided by a modification like this had been discussed and punted downstream as uneconomical. Still, despite long safety traditions in other nautical and many aerospace applications, it seems that opposite-threaded fittings for mirror-image subcomponents are not included on new Prism 2 rebreathers.

Given the demonstrated excellence of other parts of the system, I confess: I am stumped, and wonder "why not?'

I suspect, based on the comments of the people diving a Prism 2, that the particular arrangement built in the accident is something that someone who was intimately familiar with a Prism 2 would never even consider as a possible option. Since the people who design the system and the reviewers who certified it are very familiar with the system they never considered this mistake as something anyone could do and hence they took no precautions against someone doing it.

 

[Dr. Lecter]

Today's rebreather diver is asked to ascertain his own state, in a high load situation, with a diminished capacity. To me, that is simply not acceptable.

SeaRat

Stick to OC and its limits, then, where you clearly belong. The rest of your post shows a fundamental misunderstanding of what's possible vs. what's best tech for CCRs - there's not a better CO2 scrubbing or detection system floating around out there that just hasn't quite made its way into commonly dove units (Brad's vaporware claims not withstanding) and the idea that we're going to turn control of the loop over to an automated system would be uproariously funny if you didn't seriously think it would be a good idea.

Then again, maybe you can work with Poseidon on their next CCR and come up with something suitable for CCR DSDs.

 

[John C. Ratliff]

Stick to OC and its limits, then, where you clearly belong. The rest of your post shows a fundamental misunderstanding of what's possible vs. what's best tech for CCRs - there's not a better CO2 scrubbing or detection system floating around out there that just hasn't quite made its way into commonly dove units (Brad's vaporware claims not withstanding) and the idea that we're going to turn control of the loop over to an automated system would be uproariously funny if you didn't seriously think it would be a good idea.

Then again, maybe you can work with Poseidon on their next CCR and come up with something suitable for CCR DSDs.

Dr. Lecter, I have to ask, are you an expert in the rebreather? I ask because your profile states you've been using a rebreather since 2014, which is less than a year. I'd also like to point out that this is the time period when a person who undertakes a hazardous activity is most at risk (the other being when the person has more than six years experience). I'm also curious about what your doctorate is in?

Now, the other thing is that in order to build a better rebreather, which is the point of this thread, you must encourage those of us who have been diving a few years (I'm now starting year 56 diving) to use the product. If we, who have been analyzing hazards for many, many years, state that something needs improvement, you need to listen.

Concerning putting a device together in the correct manner, that is mandated by product safety protocols in many different industries. The device is manufactured so that it cannot inadventently put together incorrectly. If it can, that is called a design defect. If it can be assembled incorrectly so as to produce a fatal safety hazard, that is a single point safety critical failure. There is something called the ROPES Process, which has eight steps for safety:

1. Identify the Hazards
2. Determine the Risks
3. Define the Safety Measures
4. Create Safe Requirements
5. Create Safe Designs
6. Implement Safety
7. Assure the Safety Process
8. Test, Test, Test

This link has the following for the topic "Hazards":

• Release of Energy
• Release of Toxins
• Interference with life support or other safety-related function
• Misleading safety personnel
• Failure to alarm

Note that there are several hazards that are pertinent to rebreather deficiencies, including "release of toxins," "interference with life support or other safety-related function," and "failure to alarm." The failure to alarm in a state which is life-threatening, such as used up or bypassed CO2 scrubber, is a major problem for rebreathers. This is not just me stating the obvious (to product safety personnel), but part of a well-defined safety discipline. I have, in the above two posts, done a partial safety evaluation of overall rebreather technology. This should not be taken lightly, or dismissed as "stick to OC." When a rebreather can meet this criterion, it will be ready for more divers to use:

• (Minimal) No hazards in the absence of
faults
• (Minimal) No hazards in the presence of
any single point failure
– A common mode failure is a single point
failure that affects multiple channels
– A latent fault is an undetected fault which
allows another fault to cause a hazard...

Your reply above is consistent with a safety culture amoungst rebreather divers who accept design problems, and denigrate others who point these out.

SeaRat
John C. Ratliff, CSP, CIH, MSPH
NAUI #2710 (retired)
(CSP = Certified Safety Professional; CIH = Certified Industrial Hygienist)

 

[Dr. Lecter]

I am most certainly not an expert in rebreathers, though I do have significantly more experience with them than you do, having exceeded 100 hours of diving in the 200'-300' range on one since switching over from OC. Also unlike you, I don't make the mistake of thinking my professional experience carries over well to lecturing an industry and segment of the diving world about how they need to do things, when I have no understanding of the technology they use or the diving they do.

I don't dispute your various "is" statements about CCRs (though if you were being objective you'd identify many of the same issues with an OC technical diving rig, and you are mistaken in thinking there is better technology that is viable for CCR use but not being used) - merely the relevance of your subjective risk/benefit evaluation from those "is" statements to anything of any importance to CCR divers. I do not make, sell, or offer training on rebreathers; I could not care less whether you find them too risky to use for your diving in their current state.

I do not like my CCR as compared to OC, and I am quite aware of its limitations and the insidious yet rapidly deadly nature of some of its fault modes - indeed, I have very recent experience with such a fault, and while it was manageable I have no illusions about there having been much distance between my surviving and my not surviving the bailout and ascent. I dive a CCR because it is the only available tool that realistically allows me to do the dives I do; I dive the CCR that I do, with the bailout and DPV that I do, because I think the system as a whole offers me the best possible risk/benefit combination.

If CCR manufacturers want to listen to you and try to build a better mousetrap that's safe enough for you, that's their business. Several have tried to take their units in the direction you're pushing, and they are certainly more popular with a certain, largely recreational, segment of CCR divers. Others have different requirements and will accept different risks to do the dives they want to do.

You are not being "denigrated" - you are being told that your premises are not news to those of us who dive rebreathers, and that your conclusions about them are not relevant to anyone but yourself.

 

[John C. Ratliff]

...You are not being "denigrated" - you are being told that your premises are not news to those of us who dive rebreathers, and that your conclusions about them are not relevant to anyone but yourself.

Okay, Dr.(?) Lecter, you say that rebreathers are copacetic, but the title of this thread is "Potential Safety Improvements in Rebreather Design." You are arguing for the current designs, not for potential future improvement. I am telling you what it would take for me to try rebreathers. Do you have anything to add to this particular discussion?

And, by the way, I do have issues with technical OC diving configurations too. It's just not the topic of this thread.

SeaRat

 

[Dr. Lecter]

No, I'm arguing for intelligent discussion of improvements to the current design. That would be the opposite of a clueless (as to CCRs) expert (not as to CCRs) attempting to translate his apparently substantial experience in a tangentially-relevant field into absolute statements about what needs to happen w/r/t CCR design and what is/is not acceptable regarding inherent risks in design...all without first doing the necessary due diligence to understand whether what he's saying is the way things should be is possible without (a) as-yet unavailable technology, and/or (b) sacrificing capabilities desirable (or even necessary) for certain kinds of dives.

Since the handle "Dr. Lecter" is apparently too confusing for you, please note that while I enjoy excellent Sauternes and occasionally indulging in sadistic whimsy, I am neither the fictional character portrayed by Sir Anthony Hopkins nor a medical doctor.

 

[hroark2112]

Every diver makes his/her own decision about their level of acceptable risk. For some, it's no more than 5' away from their buddy, in less that 50' of water. For some, it's 300' solo dives. For others, it is diving a CCR. It's an individual decision, and provided I'm not putting another diver at risk, my decisions are my own. I chose the brand & model of rebreather based upon my own preferences and my own bias against letting a machine make all the decisions and have all the control.

Anyone who disagrees with my decisions regarding what I dive and how I dive it is wasting their brain function. I don't care what anyone else thinks about my diving decisions. The closest I come to giving a damn about it is input from my instructor and certain requests made by my wife.

I'm sure someone has a chart about that somewhere