Potential Safety Improvements in Rebreather Design

[Scubagolf]

Show me one CCR manufacturer who will deliver a unit to anyone who hasn't been certified on it. "As far as is possible" is meaningless word-salad that doesn't demarcate anything. And while this isn't the forum for it, your critique's phrasing suggests to me that you really don't want to argue the finer points of negligence and products liability with me.

Happy to, in the appropriate forum. I have been in the trenches defending manufacturers in product liability lawsuits in California for over 30 years.

 

[descent]

... The counterlungs are attach up top by a "Y" shaped "yoke" that attches to the H-frame via the backplate & wing bolts, The CL's are held on by a double side flap of velcro, with the CL velcro sandwiched in between ...

Let's assume for a second that the counterlungs needed to be be cleaned individually and then reattached. Let's further assume a truly geometry-challenged person was putting them back on, and they pushed and shoved until the lopsided-looking things settled in place backwards over the harness.

It sounds like the velcro sandwich would allow a left counterlung to be securely attached to the right side of the harness.

Are the edges of the velcro blazoned with special matching fabric colors, to indicate a misplaced CL?

... [On the Prism2] the threads on the DSV & corrosponding loop hose are keyed differently from one side to the other. You would not be able to completely screw the DSV onto the wrong hose without probably destroying the threads & you would not get a successful & proper positive or negative.pressure test ...

I see. Many thanks for your careful explanation.

Now suppose the counterlungs have been reversed, but the hoses (being keyed) stubbornly refuse to go on incorrectly. Suppose they are installed correctly by trial and error.

Would a distracted diver then notice if the loop and the DSV were upside-down? Does the mouthpiece have an indication for "up"?

Also, would it be possible to attach the over-the-shoulder hoses to the incorrect fittings on the head, or are the head hose fittings keyed as well?

 

[Brad_Horn]

Here is one: One of our divers encountered a Prism diver who thought that by filling up his scrubber half way is helping him to conserve sorb! Wonder who trained him! So if this guy had died, it was not because of manufacturing error, but because of poor training. Just like "baking" sorb or disturbing the sorb bed otherwise.

I don't think that even the Apoc would be able to remedy this.

Claudia?, If you raise the question on a specific thread, then I can answer in detail on behalf of OSEL. If questions on the Apoc are posted on other threads, it drowns out discussion on other units which do not offer the same functional-safety features.
The short answer is that on the Apoc it cannot be incorrectly assembled (in a diveable state) and it is impossible to half fill the scrubber as the solid-state Micropore EAC is used.
Micropore - Extendair on Vimeo
https://www.youtube.com/watch?v=JET2OdrjDlA

 

[Doppler]

Second lesson to be learned and this I posted before long ago - a 5 minute pre-breathe is insufficient to detect CO2 problems - do 10 minutes minimum.

I am sure most readers here are aware of the recent study on the effectiveness of pre-breathing to detect the presence of CO2 completed by Dr. Simon Mitchell.

His initial presentation was made at EuroTEK this past summer, and we await the full paper. In the meanwhile, here are the major points in Simon's own words.

"The primary outcome was comparison of the proportion of subjects who terminated the prebreathe in each condition.

No subjects terminated when a normal scrubber was in place (as you would expect).

25% of subjects did not terminate the prebreathe when there was no scrubber present despite dramatic changes in the physiological parameters.

90% of subjects did not terminate the prebreathe in the partial failure condition despite significant CO2 break through and some changes in the physiological parameters.

The changes in physiological parameters were fascinating and helped considerably with interpretation of the above results, but I will not discuss those at this stage because this will form much of the discussion in the paper. Once the study is published we will be able to exploit the significant educational potential of those results.

The obvious conclusion is that even in a study where there would have been a high expectation of scrubber problems among subjects, the 5 minute prebreathe had only mediocre sensitivity for detecting complete absence of a scrubber, and extremely poor sensitivity for detection of a significant partial failure. We therefore believe that it is not a valid intervention. I hasten to add that this is NOT to say that there should be no prebreathe. The prebreathe also gives the user the opportunity to ensure that other systems (like the oxygen controller) are working correctly. But it does not need to be 5 minutes long in the belief that this allows detection of problems with the carbon dioxide scrubber."

Certainly something to think seriously about.

 

[boulderjohn]

All the checklists I have seen are stupid long with superfluous lawyer written stuff in like there like "fill O2" and "check for damage"

This creates lengthy checklists which then get ignored or not used. To avoid this I made my own. No its not manufacturer approved but making the checklist actually useable and legible is key to actually using it every time, which is how you stay alive.

I fully agree with this. As someone who has taught educational practices, one of the thinks I long argued was that overly lengthy and precise scoring guidelines for assessments mean that scorers will ignore them and go on gut feeling. It is also what is wrong with people who tell new computer users to "just read the manual." Too much information is the same as not enough information. They need to have the key and indispensable functions spelled out for them.

Just last year weren't you lamenting how your scooter couldn't be trimmed in freshwater? Having an un-trimmable scooter released for consumers is an indication of the thoughtfulness of those engineers and honestly I think its an enormous oversight. I wouldn't be surprised if some CCRs have the same types of issues, for instance I have heard that one CCR has 1 absolutely critical oring and a dog hair in it can lead to CO2 bypass. That's the kind of single point failure that needs to be designed out or at least made very difficult to create.

Yes, that was me, and it is another good point. When I talked with the technician in the company about that (and other problems I was having with it), the person with whom I spoke did not realize that taking a scooter that was negatively buoyant even when unweighted into a cave was a problem, and the problem was worse when the scooter went out of trim without a weight in the nose. When I told him the very simple and almost undetectable design change it would take to change the volume the tiny bit needed to correct that, he acted like it was a revelation. He said he would pass it on to the people responsible for that sort of thing.

 

[gianaameri]

I am sure most readers here are aware of the recent study on the effectiveness of pre-breathing to detect the presence of CO2 completed by Dr. Simon Mitchell.

His initial presentation was made at EuroTEK this past summer, and we await the full paper. In the meanwhile, here are the major points in Simon's own words.

"The primary outcome was comparison of the proportion of subjects who terminated the prebreathe in each condition.

No subjects terminated when a normal scrubber was in place (as you would expect).

25% of subjects did not terminate the prebreathe when there was no scrubber present despite dramatic changes in the physiological parameters.

90% of subjects did not terminate the prebreathe in the partial failure condition despite significant CO2 break through and some changes in the physiological parameters.

The changes in physiological parameters were fascinating and helped considerably with interpretation of the above results, but I will not discuss those at this stage because this will form much of the discussion in the paper. Once the study is published we will be able to exploit the significant educational potential of those results.

The obvious conclusion is that even in a study where there would have been a high expectation of scrubber problems among subjects, the 5 minute prebreathe had only mediocre sensitivity for detecting complete absence of a scrubber, and extremely poor sensitivity for detection of a significant partial failure. We therefore believe that it is not a valid intervention. I hasten to add that this is NOT to say that there should be no prebreathe. The prebreathe also gives the user the opportunity to ensure that other systems (like the oxygen controller) are working correctly. But it does not need to be 5 minutes long in the belief that this allows detection of problems with the carbon dioxide scrubber."

Certainly something to think seriously about.

It was a well known fact (by true and serious rebreather divers) well before Simon's good research.

See, for example, "PREBREATHING DURING CLOSED-CIRCUIT DIVING APPARATUS SET-UP INEFFECTIVE IN ASSESSING SCRUBBER EFFICACY" pg. 268 here: http://media.dan.org/RF3_web.pdf .

Simon's research was sparked by heated debate on an internet forum discussing "Fatality on Mk VI in Portugal - April 2013" (back in 2013) when I was attacked for posting that pre-breathe should be minimum 10 minutes.

I say it again: "Pre-breathe should be minimum 10 minutes."

Lesson learned well before this fatality, but to date ignored by training agencies and manufacturers.

A 10 minute pre-breathe could have saved this one life (no need for CO2 Monitor... but best to have it as well).

 

[Doppler]

... My local dealer wouldn't even let me take the [rebreather's] head home with me after it arrived, but shipped it directly to my instructor.

Ron: that's as close to standard practice as we have. There is certainly a issue with rebreathers purchased used online and "punter to punter" but as far as I am aware, all manufacturers are very reluctant to ship a diveable unit directly to the end-user without prior training ON THAT UNIT.

 

[sunnyboy]

Here is one: One of our divers encountered a Prism diver who thought that by filling up his scrubber half way is helping him to conserve sorb! Wonder who trained him! So if this guy had died, it was not because of manufacturing error, but because of poor training. Just like "baking" sorb or disturbing the sorb bed otherwise.

I don't think that even the Apoc would be able to remedy this.

I'm sorry, but this cannot go unchallenged. The Prism (and Prism 2) use a radial scrubber. You cannot pack it half full and have a usable rebreather. The gas path would effectively bypass any scrubber material completely. What you have is a CO2 recirculating machine.

I doubt anyone could breath from such a rebreather for even a few minutes without getting massive CO2 problems.

 

[Dr. Lecter]

I'm sorry, but this cannot go unchallenged. The Prism (and Prism 2) use a radial scrubber. You cannot pack it half full and have a usable rebreather. The gas path would effectively bypass any scrubber material completely. What you have is a CO2 recirculating machine.

I doubt anyone could breath from such a rebreather for even a few minutes without getting massive CO2 problems.

Exertion level will play a huge role in this, but I think you'd be surprised how long you can breathe off a scrubber-less unit before you notice much of anything.

 

[tstormdiver]

If somebody posts the Hollis Prism 2 counterlung volume and we can make an assumption on her weight and work rate, then we can also derive the pCO2 estimate at 14 meters (but kind of pointless to do so).

The Prism2 has 2 different size counterlungs. The 3.5 liter set & the smaller 2.5 liter set. I assume (don't know for sure) it is the combined volume. The 3.5 liter size is standard.