I've read about as far as I can without adding a comment (I'm about half way through this discussion). I have worked in product safety in the semiconductor industry, and worked as EHS Manager in the chemical industry (Process Safety Management). I also am a diver since 1959, although I have purposely stayed completely away from rebreathers. I dive mostly solo, in rivers, and do not feel that rebreather technology is where I want it at my age (almost 69, later this month). I do have some information to pass along which may help this discussion.Odd,... Since it has been shown that nearly all rebreather fatalities have been shown to be the result of operator error, not machine design, whether, because of being a inexperienced or careless or such. Good training & having the discipline to follow accepted checklists would go a massive way in minimizing risks. For that rebreather to have been in the shape it was in for the accident to occur, there just about had to be red flags that were not noticed or ignored when/ if the pre- dive checks were done.
The above quote relies on training of the diver to ensure safety. Well, in the Hierarchy of Controls (what we in professional safety use to try to mitigate hazards), this is what we call an "Administrative Control." The hierarchy goes like this:
Elimination > Substitution > Engineering Controls > Administrative Controls > Personal Protective Equipment
Administrative controls (training) do work, but are subject to breakdowns due to a number of potential problems, such as fatigue, peer pressure, inattentiveness (distraction), sleep deprivation, "hurry up" sickness, etc. In short, while vital, training can only do so much. We see human errors occurring during training and for about 6 months after training, until the person is really attuned to what (s)he is doing. Then we also see human errors increase after about 6 years of familiarity with the job/task/situation.
Rebreathers are what we would term "safety critical" items, and because of this, single-point failures are normally engineered out of the system. This is a part of the profession of Reliability Engineering. This can be done by providing redundancy, sensors to sense an impending failure, or engineering so that the system cannot be improperly installed so as to fail. There are a number of tools to determine whether controls are necessary, and if so the types of controls. The analysis can be a Failure Modes and Effects Analysis, Fault Tree Analysis, a What-If Analysis, and a number of other techniques.
The point here is that if a rebreather does have a single-point failure potential, it is not safe as a life safety system.
SeaRat
John C. Ratliff, CSP, CIH, MSPH
CSP = Certified Safety Professional; CIH = Certified Industrial Hygienist.
---------- Post added December 2nd, 2014 at 11:20 PM ----------
This is an example of a single-point failure of a safety critical item leading to a fatality.The beauty of the checklist that doesn't work is that it can be easily modified by the manufacturer so that it does work. Look, this isn't teaching 17 year old sailors to run a reactor plant, this is teaching adults who presumably want to be there to dive a bit of scuba gear. So if what you infer by your reading of the coroners report is that the loop was assembled backwards, the counterlungs were put on the wrong side, and the unit still passed positive and negative, a concept I find a stretch, but then, I'm not certified on a Hollis, nor will I be, then a simple checklist modification may be in order, and Hollis should be able to do that by sending out a notice to all P2 users. Namely, step 6A would further identify the inhale counterlung with a description of what it looks like. Tammy says that the counterlungs don't look the same at all.
It's fine that you don't use a checklist. I've had a number of conversations with Leon regarding checklists, and our common ground is that they work if properly done, but most folks get too smart for them very quickly. Sadly, I had a very good friend and crewmember die on his meg. A checklist would have saved him, because he couldn't possibly have passed the negative pressure test with his particular problem. He installed his O2 selenoid incorrectly.(emphasis added, jcr)
SeaRat
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