Rebreather Discussion from Brockville Incident

[flots am]

This thread highlights several points for me.1. Non-CCR divers and divers with very little CCR experience are frightened of CCRs.

Reports of unexpected silent deaths will do that.

Nobody is surprised when they hear "John Smith ran out of gas at 150', bolted for the surface and died." What they don't want to hear is "John Smith was happily diving, passed out and died, and never knew what was wrong"

2. People greatly overestimate the level of "complexity" in a rebreather.

RBs aren't complex, they are, however, unforgiving.

flots.

 

[gianaameri]

3. The lack of understanding of what it takes to really screw up on a rebreather is amazing (you don't just GO hypoxic, "POOF!" You've got to ignore your gas or cells pretty obtusely to screw that up)

The three things which get me is the level of misinformation provided to newbie and potential rebreather divers, namely:

1. Rebreather fatalities are under-reported (and God forbid open discussions in open forums).
2. Rebreathers are just about the only "life-support" product in this modern day and age which is sold lacking "Functional Safety" and nobody tells the buyer to get his/her "informed consent."
3. You point number 3 above.

If the O2 sensors malfunction, as has happened, you pass out and die despite the electronics and the scrubber working perfectly well.

If the CO2 removal system fails, as has happened, you pass out and die, despite the electronics and the O2 sensors working perfectly well.

If the WOB is too high causing CO2 retention, as has happened, you pass out and die despite electronics, O2 Sensors, and CO2 removal system working perfectly well.

If your O2 supply is deficient and ascend, as has happened, you pass out and die.

Unquestionably, the above does not happen all the times. When everything is going for you and 100%, rebreather diving is fabulous. You watch the O2 Sensors respond to O2 injections and have time to intervene manually if your target pPO2 is slightly off... but surely all the dead divers who passed out incapable of self-help in front of their buddies are testimony that when things are not right, unlike in OC where you can take corrective action, with a rebreather you pass out and drown.

Failure mode and consequences are entirely different from OC.

In addition to equipment risk, make a mistake, a human error, and the likely consequence is death.

 

[mathauck0814]

If the O2 sensors malfunction, as has happened, you pass out and die despite the electronics and the scrubber working perfectly well.

All five sensors? All at once? Please cite this accident which did not include a flood forcing the diver to bail out to Open Circuit.

If the CO2 removal system fails, as has happened, you pass out and die, despite the electronics and the O2 sensors working perfectly well.

Again - it just doesn't happen for no reason. If you push your scrubber limits and don't cycle as recommended, sure. Flying a plane without fuel is also ill advised. CO2 removal degrades over time. Please cite the example where someone correctly filled their scrubber with fresh sorb, got in the water and died on that dive.

If your O2 supply is deficient and ascend, as has happened, you pass out and die.

Only if you have a hypoxic diluent, in which case you'd also have a rich mix for bail out and offboard gas plumbing. There is zero excuse for this happening. You aren't going to "not know" that your O2 is not injecting as all of your monitoring will clearly show your PO2 dropping.

In addition to equipment risk, make a mistake, a human error, and the likely consequence is death.

I'm good with that. Do things correctly and live, do it incorrectly and live almost every time, do it incorrectly frequently or obtusely enough and die. Yep. Totally good with that.

 

[gianaameri]

All five sensors? All at once? Please cite this accident which did not include a flood forcing the diver to bail out to Open Circuit.



Again - it just doesn't happen for no reason. If you push your scrubber limits and don't cycle as recommended, sure. Flying a plane without fuel is also ill advised. CO2 removal degrades over time. Please cite the example where someone correctly filled their scrubber with fresh sorb, got in the water and died on that dive.



Only if you have a hypoxic diluent, in which case you'd also have a rich mix for bail out and offboard gas plumbing. There is zero excuse for this happening. You aren't going to "not know" that your O2 is not injecting as all of your monitoring will clearly show your PO2 dropping. This is the least likely of the scenarios you've called out.



I'm good with that. Do things correctly and live, do it incorrectly and live almost every time, do it incorrectly frequently or obtusely enough and die. Yep. Totally good with that.

You forgot the CO2 retention bit.

I wrote:

"If the WOB is too high causing CO2 retention, as has happened, you pass out and die despite electronics, O2 Sensors, and CO2 removal system working perfectly well."

Any comment?

On the O2 Sensor topic, see:

RF3.0 - O2 Sensor Technology for Rebreathers - YouTube

On the CO2 removal system failure, you can't have the one-way valve fail despite the scrubber being 100%? How often do you check visually your one-way valves? This is just an example.

On deficient O2 supply, I believe someone posted (incorrectly) most fatalities are due to diver forgetting to turn ON their O2 supply. Solenoid stuck closed on eCCR and you ascend is not that the same situation as an O2 cylinder valve closed?

The fundamental problem is that there are some failure modes which are not detectable by the diver, generally due to bad gas, resulting in loss of consciousness and drowning.

With OC we have a fixed gas supply and alternative supplies (i.e.the buddy in the PADI system) to go to in case of gas supply failure. You can attempt self-rescue.

 

[mathauck0814]

"If the WOB is too high causing CO2 retention, as has happened, you pass out and die despite electronics, O2 Sensors, and CO2 removal system working perfectly well."

Any comment?

Simply never had any issue with WOB. Though that could be unit specific so I can't speak to others' experience. It seems if you're diving a large production, commercially available and supported machine this shouldn't be an issue. Either that or you need to get on the treadmill...

 

[HIGHwing]

If the O2 sensors malfunction, as has happened, you pass out and die despite the electronics and the scrubber working perfectly well.

If the CO2 removal system fails, as has happened, you pass out and die, despite the electronics and the O2 sensors working perfectly well.

If the WOB is too high causing CO2 retention, as has happened, you pass out and die despite electronics, O2 Sensors, and CO2 removal system working perfectly well.

If your O2 supply is deficient and ascend, as has happened, you pass out and die.

With all due respect, you're glossing over a pretty large piece about technical diving that is an important component in this discussion. If I may... Anyone with a technical skill is subject to that skill degrading. I am quite certain there are many divers certified to various [technical/recreational] levels who have a current capability at this very moment which falls below the requirements and standards of the certifications they hold. To understand these accidents, it's imperative we realize human factors are a critical component to the vast majority of these cases. In my mind a diver jumping off a boat with a set of steel doubles shut off is in not much better of a situation than a CCR diver with the cylinders shut off. Some might even argue that since the loop PO2 is increasing with depth the CCR diver has more time to deal with a very similar issue unless the counter lungs are fully collapsed making breathing on the loop difficult or impossible. The question becomes how to increase your odds of doing everything right and diving at or above the level of your training on every dive. How do you implement currency or recent experience as part of your process?

People jump to aviation for a parallel to rebreather diving, which I find quite interesting. As a pilot and OC/CC Tec diver I can see why people would consider the anaolgy, but it falls short. Pilots are required to have recurrency or recent experience relevant to the aircraft/conditions being flown. While I don't want to see SCUBA Police, Government regulation, or anything similar, it wouldn't hurt to establish a currency guidelines within the community for technical diving most could agree upon. We have a concept in the community of workup dives, but who knows what that really means?

Tec divers absolutely have to practice. I see a common tendency in threads where OC divers condemn CCR divers for using the tool when it's not required. These same OC divers will jump in a pool session with the local dive shop just to shake off rust, or try something new (sidemount!). CCR divers should be encouraged to use the tools to gain practice, but we also have to adapt practices to make sense for the environment. IMHO a pool is a relatively dangerous place for a rebreather because the relative lack of pressure makes for a very unstable loop. Let's fix this problem by simply having a pool diluent containing O2, or 80% and pick a set point accordingly. Now the only issue we can have in the pool is hypercapnia. You can't tox, and you'd have to go out of your way to suffer hypoxia (especially if you are equipped with an ADV).

There are ways to create consistency. Checklists are a beautiful option and they are used extensively in the often paralleled industries of aviation and medicine. I find it interesting that OC Tec has checklists but few actually use them, whereas contentious rebreather divers almost make an issue of dutifully wearing their mask, or pinching their nose while they complete an out of the water pre-breathe.

Regardless of OC vs CC, if you inspire a gas which is incapable of supporting life, you can in fact become incapacitated and die. This issue is not somehow unique to CC diving, it's specific to humans. Clearly a lot of best practices are born from blood in technical pursuits the world over.

If you really distill all the back and forth about rebreathers, the breathing loop can essentially suffer four conditions, which are quite similar to OC.

CCR: Too much oxygen --> OC: Bad gas switch during deco

CCR: Too little oxygen --> OC: Jumped in on hypoxic back gas vice travel gas/ascend on hypoxic mix from last stop

CCR: Too much CO2 --> OC: Breathing technique, etc.

CCR: Too much water --> OC: Dislodged/poorly maintained regulator

I realize I am inspiring a breathing gas, and I know that not all gases are available for breathing under all pressures. I realize my body could not care what device gives me the gas I am breathing, but it does care about the dosages I’m prescribing myself. In my mind, a bad gas switch on an OC technical dive = CCR producing something I shouldn’t be breathing. OC has the NOTOX gas switch, CCR has the pre-breathe.

We can go back and forth about this forever, what matters is the diver is of sufficient skill and currency to handle the eventualities of technical diving commiserate with equipment and environmental variables for the dive at hand.

I think ignoring the human factor of “currency” is a large factor in technical diving accidents, and it should be a carefully explored subject in technical diving as we progress forward as a community. Education about community standards make rebreathers more approachable and a managed risk versus a wild death wish.

Best of luck in your diving pursuits.

 

[gianaameri]

In my mind, a bad gas switch on an OC technical dive = CCR producing something I shouldn’t be breathing.

There is a difference.

On CCR you are exposed to the risk of bad gas continuously and at any depth in both technical or recreational dives AND bad gas can happen unbeknown to the diver and the dive buddy without diver error.

On OC you are only exposed at the time of the gas switch. It requires a human error. The buddy can observe and intervene (i.e. DIR gas switch procedure).

On the issue of training and retraining, I think it will affect substantially the typical recreational diver (10 - 20 dives a year). Give him a rebreather, and skills and proficiency will be lost, almost guaranteed in the first year.

The technical diver would be less affected if he/she dives more frequently than 10 - 20 dives a year, particularly when it comes to rebreather assembly/preparation/checks.

---------- Post added July 9th, 2013 at 03:07 PM ----------

Simply never had any issue with WOB. Though that could be unit specific so I can't speak to others' experience. It seems if you're diving a large production, commercially available and supported machine this shouldn't be an issue. Either that or you need to get on the treadmill...

This will help clarify my point:

RF3.0 - CCR Physiology - YouTube

 

[HIGHwing]

There is a difference.

On CCR you are exposed to the risk of bad gas continuously and at any depth in both technical or recreational dives AND bad gas can happen unbeknown to the diver and the dive buddy without diver error.

Please explain how a complete dil flush is physiologically any different than breathing from an OC regulator. Please also explain how SCR mode on a CCR does not begin with known gas fraction, which is essentially your parallel for good gas.

In my next line of questions, I'm going to ask you to reconsider your assertion and have you discuss OC divers who where poisoned by bad fills. They were, in fact, breathing bad gas unbeknownst to the diver on OC SCUBA. I'm also going to point out a recent OC fatality from a diver who did everything right, except he mixed up the set of doubles for the intended dive because he was distracted on the boat. In this case, he was breathing a fatally high PO2. Again what's the difference?

BTW - I was in attendace at RF3.0 for the presentation you linked.

 

[gianaameri]

Please explain how a complete dil flush is physiologically any different from breathing from an OC regulator.

A complete diluent flush is the same as breathing from an OC regulator.

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.

I do not understand though the point you are trying to make with your statement in response to my statement:

"On CCR you are exposed to the risk of bad gas continuously and at any depth in both technical or recreational dives AND bad gas can happen unbeknown to the diver and the dive buddy without diver error."

 

[mathauck0814]

On CCR you are exposed to the risk of bad gas continuously and at any depth in both technical or recreational dives AND bad gas can happen unbeknown to the diver and the dive buddy without diver error.

On OC you are only exposed at the time of the gas switch. It requires a human error. The buddy can observe and intervene (i.e. DIR gas switch procedure).

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.

On the issue of training and retraining, I think it will affect substantially the typical recreational diver (10 - 20 dives a year). Give him a rebreather, and skills and proficiency will be lost, almost guaranteed in the first year.

The technical diver would be less affected if he/she dives more frequently than 10 - 20 dives a year, particularly when it comes to rebreather assembly/preparation/checks.

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.

This will help clarify my point:

RF3.0 - CCR Physiology - YouTube

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?).

Rebreathers offer far more benefit than they introduce risk (my opinion). While I agree that a thoughtless diver can get themselves into trouble, I disagree that the thoughtless diver is any less likely to hurt themselves on open circuit.