Rebreather Discussion from Brockville Incident

[gianaameri]

I understand your point, however I maintain that even if you're overworking your scrubber you're not going to just suddenly spike your PPCO2 and black out. You're going to get incremental breakthrough during the period of work which, admittedly, could become cumulative and cause an issue over time - before that time you ought to notice that you're working hard and adjust, however - this comes back to lack of preparation and poor decision making of the diver (e.g., what are you doing in a scenario where you have to work that hard and don't have a DPV?).

I do not think you understand my point.

CO2 retention can occur without scrubber CO2 breakthrough. You need to watch Dr. Mitchell video link I posted.

---------- Post added July 9th, 2013 at 03:20 PM ----------

10-20 dives is a month of diving (or a good week), not a year. I don't believe anyone here was advocating rebreathers for use by recreational/vacation divers.

PADI and the rebreather industry is advocating rebreather diving for the average recreational diver.

 

[HIGHwing]

The difference is that to effect a complete diluent flush you will need a LOT of gas on CCR vs. just breathing the same gas from an OC regulator.

But that's my option and I can plan to take advantage of the diluent in varying ways. I can simply plan a nitrox mix that gives me a very workable PO2 to escape a cave using SCR mode. I can add a LP hose to my bailout gas, plug into the loop, and then use SCR mode to gas extend. By planning a Dil gas which is hotter in certain environments I could stay on the loop in a variety of circumstances. In fact, with a hotter diluent, I can run my CCR with the on-board bottles, regulators, solenoid, both handsets, and the heads up display failed. As long as the loop doesnt' flood, and the scrubber is working and I have a plumbable off-board gas, I'm still in business.

Whether I would stay on the loop is another discussion, but all this single failure and you die right there talk is nonsense. Everyone knows in aviation, diving, and medicine it's an error chain that causes the accident. I'm growing weary with every condition on a rebreather is fatal. It's just not true.

 

[gianaameri]

Again - you're not going to have this happen without user error on the part of the CCR diver either. There just isn't a scenario where five sensors and two independent PO2 monitoring devices fail simultaneously and you're left guessing at what's in your loop.

I dive with 3 O2 Sensors in the rebreather head from a sensor manufacturer connected to a single controller, and 3 O2 sensors from a different manufacturer in the inhale counterlung connected to a different pPO2 monitor, all sensors from different production batches and no sensor older than 12 months from date of manufacture.

Diversity and totally independent electronics increases redundancy and reduces risk.

Nonetheless:

1. Both electronics use common ground between the 3 sensors. A failure of the common ground system will kill all 3 sensors at once in one of the two systems.
2. 3 O2 Sensors in each system are in the same location. Common environment could cause all 3 O2 Sensors to fail simultaneously.

My system and your system offer more redundancy than the typical rebreather system (using only 2 or only 3 O2 Sensors), but I had event 1. above happen two times (due to poor quality cable from the manufacturer).

I suspect though a. you do not change O2 Sensors every 12 months from date of manufacture, b. you do not use different sensors from different manufacturers, c. your O2 sensors are all located in the same place in the rebreather - this means if you bail-out on a single O2 sensor failure you will be doing so a few times in a typical year.

If you do not bail-out on a single O2 sensor failure, then I suspect your system iintegrity is not as good as you think.

---------- Post added July 9th, 2013 at 03:39 PM ----------

But that's my option and I can plan to take advantage of the diluent in varying ways. I can simply plan a nitrox mix that gives me a very workable PO2 to escape a cave using SCR mode. I can add a LP hose to my bailout gas, plug into the loop, and then use SCR mode to gas extend. By planning a Dil gas which is hotter in certain environments I could stay on the loop in a variety of circumstances. In fact, with a hotter diluent, I can run my CCR with the on-board bottles, regulators, solenoid, both handsets, and the heads up display failed. As long as the loop doesnt' flood, and the scrubber is working and I have a plumbable off-board gas, I'm still in business.

Whether I would stay on the loop is another discussion, but all this single failure and you die right there talk is nonsense. Everyone knows in aviation, diving, and medicine it's an error chain that causes the accident. I'm growing weary with every condition on a rebreather is fatal. It's just not true.

I use N32 for diluent and carry a lot of bail-out gas. I could use SCR mode to exit the cave from max. point of penetration, but quite frankly I hardly see the need ever to do it because I carry a LOT of bail-out gas in the first place.

So, I see your point, share your view, could adopt your strategy if needed... but that will only work if I am conscious.

If there is a failure mode which renders me unconscious, I have none of those nice options.

My opinion is that given the dynamic of the fatalities, the dead knew they had a problem, but were unable to take corrective actions, and realistically that is the way I am going to go, if...

Bad gas is disabling.

 

[HIGHwing]

If there is a failure mode which renders me unconscious, I have none of those nice options.

My opinion is that given the dynamic of the fatalities, the dead knew they had a problem, but were unable to take corrective actions, and realistically that is the way I am going to go, if...

Bad gas is disabling.

I'm glad you see my first point.

My second point is that I think these statements apply equally to OC and CC. If you're breathing something that doesn't support life, just because it comes from an OC regulator does not imply/guarantee you have the percentage of consciousness required to take a corrective action. That's all I'm bringing to the table.

 

[gianaameri]

I'm glad you see my first point.

My second point is that I think these statements apply equally to OC and CC. If you're breathing something that doesn't support life, just because it comes from an OC regulator does not imply/guarantee you have the percentage of consciousness required to take a corrective action. That's all I'm bringing to the table.

Sure, you can breathe CO at home from your gas heater and die.

Bad gas kills in many ways in more than one circumstance.

The rebreather is unique though because despite the diver having done everything correctly - no mistake on the part of any one at any stage or point - the diver can pass out and die without warning.

I have provided various examples.

The most insidious one is CO2 retention, which is grossly misunderstood, and Dr. Mitchell has taken the opportunity to explain at RF 3.0 (see RF3.0 - CCR Physiology - YouTube )

By all means, theoretically CO2 retention can happen while sucking on a straw repeatedly while sitting on the sofa, but when it happens on rebreather it can kill you without warning.

Today's rebreather technology cannot provide protection to the rebreather user from CO2 retention.

This is not the only failure mode which can kill the user without warning despite the user doing everything correctly.

The interesting thing about CO2 retention is what happens on the arterial side.

There is research in Western medicine which points to this possibly causing the heart to go in fibrillation (due to blood pH being altered). There is also talk in Chinese Medicine of CO2 stopping the heart from functioning under some circumstances. Nothing definite in our application yet, but it would be interesting if medicine would devote some time and money to research the subject of increased arterial CO2 in diving physiology and the heart function.

Bottom line, CO2 retention (and this is just one example) may be the undetectable cause of several current unexplained rebreather fatalities (no user error required). We know it happens and has happened.

 

[HIGHwing]

I was at RF3.0 and attended Dr. Mitchell's discussion, so I'm quite familiar with the proceedings. I'm not asking you to make a case about how CO2 retention kills. All I'm asking you to consider is there can be insidious mixtures/fractions of gas in OC tanks capable of quietly incapacitating and killing a diver ever so softly.

We recently saw a well-respected diver fill two sets of doubles, label them, and transport them to the boat himself. One set was at 23%, the other at 36% which corresponded to the intended dives. He analyzed the tanks upon mixing, and on the boat. When the diver setup his gear for the 170ft dive, he accidentally installed his kit to the tank of 36% for the deeper dive. The diver was incapacitated at depth and drowned – witnesses said he just tox’ed and it was game over despite a rescue attempt.

We can review the deaths in Mexico last year from carbon monoxide in OC tanks. We can argue about whether undetected/unknown gas fractions in OC tanks are or are not more or less “dangerous”, “insidious”, or whatever.

I maintain a mixture incapable of sustaining life will eventually kill the diver unless an intervening event takes place. Your argument appears to be that a diver on CCR is less likely to respond to something s/he can’t feel, smell, or taste. I cannot agree with the piece of equipment being the difference in the circumstance of something you cannot feel, smell, or taste, and I’m happy to disagree with you.

Undetectable gas fractions have killed divers, and it will happen again. It doesn't really matter where the gas comes from be it a regulator, counter lung, lift bag, ceiling of a cave, or your BCD.

 

[doctormike]

Undetectable gas fractions have killed divers, and it will happen again. It doesn't really matter where the gas comes from be it a regulator, counter lung, lift bag, ceiling of a cave, or your BCD.

Again, saying this with respect, since pretty much everyone here is far more experienced than me... but most of this back and forth about the technical details seems to miss the larger point.

What we need are some statistics comparing OC to CCR for the dives that 98% of divers in the world are contemplating, especially when a large agency like PADI is promoting training to people with nothing more than EAN certification and 25 logged dives (the requirements for the rebreather diver course).

At the extremes of OC diving, I can certainly see the possibility that the safety advantages of CCR might outweigh the disadvantages, depending on the specifics. But once you start talking about dives involving multiple gasses and deco bottles, you are reducing the denominator by at least one or more orders of magnitude, so you are really answering a different question.

What people like me want to know - OC divers who are considering CCR - is what is the additional risk that I will be assuming given the non-deco, single gas dives that I am doing now. Mistakenly breathing a hypoxic bottom mix or ox-toxing by switching to too rich a mix at depth just isn't something that I have to deal with now, so to say that these types of accidents are just as much a problem with OC doesn't really ring true for the vast majority of certified divers.

And just saying that every accident is user error and that there is no way to make these devices "idiot proof" also doesn't really make the case to me. Sorry, complexity does matter with human operated machines, and at some point complexity itself makes the likelihood of skipping or incorrectly performing a single step become significant. If you have 30-40 steps that all have to be done exactly right, then it's not going to be long before even a very conscientious and well trained diver misses one. I don't know if that one missed step is potentially lethal or not, but it is concerning that the failure modes are potentially not as obvious as those of OC.

 

[Doppler]

What people like me want to know - OC divers who are considering CCR - is what is the additional risk that I will be assuming given the non-deco, single gas dives that I am doing now.

And that Mike is the 64,000 dollar question.

On the face of it, when asked by someone who seems to have similar goals and experience to yours, my stock answer is No.

If we move away from the generalizations though, there are several other questions I ask folks looking at CCR (or SCR) and the first ones are:

How many dives do you do a month?
Do you shoot pictures or shoot video... seriously?
Are you detail-oriented?
Do your friends consider you a risk-taker (read cowboy or cowgirl)?

But usually, my answer stays the same. In my opinion, and it ain't worth much, fun sport diving and rebreathers are not quite mutually exclusive, but close to it.

 

[doctormike]

And that Mike is the 64,000 dollar question.

$64,000..!! Oh, well then forget it. I didn't realize that CCRs were THAT expensive...

On the face of it, when asked by someone who seems to have similar goals and experience to yours, my stock answer is No.

If we move away from the generalizations though, there are several other questions I ask folks looking at CCR (or SCR) and the first ones are:

How many dives do you do a month?

Varies with the time of year, but since the local season started up again in April, I have done 35 dives.

Do you shoot pictures or shoot video... seriously?

It's the main reason I dive (see website in my sig)

Are you detail-oriented?

Hmmm... I suppose so, but maybe not as compulsive as some other divers...

Do your friends consider you a risk-taker (read cowboy or cowgirl)?

Not really.

But usually, my answer stays the same. In my opinion, and it ain't worth much, fun sport diving and rebreathers are not quite mutually exclusive, but close to it.

Thanks, I appreciate the advice!

 

[gianaameri]

I was at RF3.0 and attended Dr. Mitchell's discussion, so I'm quite familiar with the proceedings. I'm not asking you to make a case about how CO2 retention kills. All I'm asking you to consider is there can be insidious mixtures/fractions of gas in OC tanks capable of quietly incapacitating and killing a diver ever so softly.

We recently saw a well-respected diver fill two sets of doubles, label them, and transport them to the boat himself. One set was at 23%, the other at 36% which corresponded to the intended dives. He analyzed the tanks upon mixing, and on the boat. When the diver setup his gear for the 170ft dive, he accidentally installed his kit to the tank of 36% for the deeper dive. The diver was incapacitated at depth and drowned – witnesses said he just tox’ed and it was game over despite a rescue attempt.

We can review the deaths in Mexico last year from carbon monoxide in OC tanks. We can argue about whether undetected/unknown gas fractions in OC tanks are or are not more or less “dangerous”, “insidious”, or whatever.

I maintain a mixture incapable of sustaining life will eventually kill the diver unless an intervening event takes place. Your argument appears to be that a diver on CCR is less likely to respond to something s/he can’t feel, smell, or taste. I cannot agree with the piece of equipment being the difference in the circumstance of something you cannot feel, smell, or taste, and I’m happy to disagree with you.

Undetectable gas fractions have killed divers, and it will happen again. It doesn't really matter where the gas comes from be it a regulator, counter lung, lift bag, ceiling of a cave, or your BCD.

I check my tanks for O2, He, CO, and CO2.

Then I use a black indelible felt pen to mark them with my name and the MOD.

Sticky labels like duct tape have the habit of moving from one tank to another on a busy boat or in a busy cave.

So, YES, bad gas can happen, but NO, once the gas is in the OC bottle, then the OC bottle is not going to change that gas to something else.

The rebreather, instead, at any point in time and at any depth can incapacitate the diver (including, but not limited to from delivering bad gas) resulting in loss of consciousness and drowning - despite the diver having done EVERYTHING right to include his gas checks.

Did you hear the presentation from Dr. Concannon?

He believes that check-lists are responsible for the fatalities (none were found after the diver died, so he concludes the lack of a check-list killed the diver).

Do you use a check-list?