Rebreather Discussion from Brockville Incident

[Larry7895]

Thanks for your prompt response, I'll have a look at the references you suggest. Regards.

 

[gianaameri]

Thanks for your prompt response, I'll have a look at the references you suggest. Regards.

Post 51 has a number of other reference documents you can find on Google on the subject.

This is a pretty good link as well:

http://www.lix.polytechnique.fr/~ra...-11-23-RauzyAntoine-SafetyIntegrityLevels.pdf

 

[tridacna]

What's scary to me is that lately it seems that pretty much ever time I hear of a CCR death it's a male over 50. Now there's a study.

Please explain what that study will accomplish? Perhaps establish that older people die more often than younger ones? Or that older and therefore probably richer people can afford to buy these expensive contraptions? You must work for the government. Get a study funded. They love those sorts of things.

 

[shoredivr]

Rebreathers are more complex than OC and a relatively new technology. The technology is new for all intensive purposes when galvanic O2 sensors and electronics were added to a traditional rebreather.

This was back in 1998 with the Electrolung: see Electrolung

From the link:
"Sensors of this type don't wear out so they were hard wired into the circuit. Unlike galvanic sensors they don't use oxygen but rather just respond to its presence. They work equally well submerged so the effect of any condensation is negligible. A drop of water fully covering the end of the sensor would only slow the response time."

So the designer and maker of the Electolung states that the Electrolung O2 sensors were not galvanic, which is in contrast with your statement.

*edit* the link was quite interesting.

 

[flots am]

Then there's a design flaw in OC.

I never said there wasn't.

Just because the current valves are popular doesn't mean they're as good as they can get.

Many gas connections are "on when connected and off when disconnected". It's standard for Natural Gas quick-disconnects.

flots

 

[gianaameri]

From the link:
"Sensors of this type don't wear out so they were hard wired into the circuit. Unlike galvanic sensors they don't use oxygen but rather just respond to its presence. They work equally well submerged so the effect of any condensation is negligible. A drop of water fully covering the end of the sensor would only slow the response time."

So the designer and maker of the Electolung states that the Electrolung O2 sensors were not galvanic, which is in contrast with your statement.

*edit* the link was quite interesting.

I must be wrong then on that point.

Who is the one in the history of rebreather development to first use galvanic O2 sensors and why the change since the Electrolung?

Sent from my HTC Desire C using Tapatalk 2

 

[shoredivr]

Found this, dated 1994, from Analytical Industries, but this seems just for the sensors themselves.
http://www.aii1.com/PDF/dv_s_app.pdf

*edit* Used on the US Navy MK16 rebreather since 1998.

 

[dreamdive]

Many more things kill CCR divers.

Just to cite an example, a rather common one, is failure of the galvanic O2 Sensors.

Then there is failure of the electronics.

These failures can be intermittent.

Then there is failure of the CO2 scrubbing system.

The key is that most failure modes in OC are detectable by the diver and the diver can take corrective action. Where the diver takes no action or the wrong action (i.e. panic), then this can and usually does result in fatality of injury.

Conversely, when a CCR fails, the diver, we see this in the pattern of the fatality, becomes unconscious and drowns, unable to effect self-rescue, and the buddies, despite their proximity, are unable to help.

We see this also in the most recent CCR fatality not yet picked up by the Scubaboard accident discussions:

"At the time of the accident George was at a depth of 14-15 meters decompressing from a dive in the 60m range. George suddenly became unconscious. He was spotted within seconds and the closest team member got to him immediately. He was completely unresponsive and not breathing. As this happened just below the habitat we moved him into the habitat and tried to revive him with no success. We recovered George later in the night."

The above was from a recent post on CCRX.

Rebreathers are more complex than OC and a relatively new technology. The technology is new for all intensive purposes when galvanic O2 sensors and electronics were added to a traditional rebreather.

This was back in 1998 with the Electrolung: see Electrolung

Since then, everybody who tried struggled to make the system reliable enough.

There are various ways to measure risk.

One is statistical looking at the fatality rates. Although we do not have precise data (i.e. the denominator), it is strikingly obvious that CCR fatality rates are very large.

This is what led A. Fock to conclude in his RF3.0 paper and presentation that CCR is 10 times more risky than OC - see also Rebreather diving: ?Killing Them Softly? | Diver Magazine .

Then there is the engineering approach which can use reliability measures or SIL Levels.

SIL Levels measure Probability of Failure and in particular the Probability of a Dangerous Failure.

If you have two products, one with a SIL Level of 1 and the other with a SIL Level of 4, then you can say that the SIL 4 product is safer than the SIL 1 product because measured in accordance to the methodology required in the international standard EN61508 (that is what is referenced in the rebreather standard Clause 5.13.1 of EN14143:2003) the SIL 4 product has a lower Probability of a Dangerous Failure.

So, SIL 1 is more risky or more dangerous according to this methodology than SIL 4.

"Functional Safety" is applied to many product in our daily life. It is just not stamped on the product, but where a product can endanger life it is a means of ensuring a reasonable or acceptable level of safety.

Rebreather can kill without warning. A very dangerous failure mode.

A SIL 4 rebreather (the best level of Functional Safety) would still be not 100% safe, but it would offer a very high level of safety and protection to the user.

A SIL 4 rebreather would be safer/better (and likely more expensive) than a SIL 1 rebreather.

Unfortunately, despite Clause 5.13.1 of EN14143:2003 requires a rebreather to attain at least a SIL 1 level of Functional Safety, the rebreather industry has to this date been unable to deliver to the general public one such electronic rebreather - that is a rebreather with at least a SIL 1 level of Functional Safety.

Rebreathers do not meet the standard EN14143:2003 against which they are benchmarked and certified (hard to believe YES, but this is true).

There was a blunder, an innocent blunder some time ago, and this one slipped through.

Based on the intuition and work of Dr. Fock and the application of engineering SIL level calculations, the conclusions are the same.

Rebreather are very dangerous machines. There is no two ways about it.

They are fun to dive, but they are dangerous.

A poster asked what needs to be done to make them at least SIL 1 or safer?

The industry cannot deliver a SIL 1 electronic rebreather. The technology is not there for this at this point in time.

That is where we are now. We can only face the fact and accept it. No solution.

Rebreathers are not a tool safe enough for the typical OC recreational diver. Stick to OC Air and Nitrox for dives in the recreational range (safer and cheaper).

I stopped posting here and instead been enjoying the caves in Mexico on CCR which incidentally our OC buddy can't keep up with.

Reading these posts with interest, I keep wondering: is gianaameri really Alex Deas, a friend or an associate of his?????

 

[gianaameri]

I stopped posting here and instead been enjoying the caves in Mexico on CCR which incidentally our OC buddy can't keep up with.

Reading these posts with interest, I keep wondering: is gianaameri really Alex Deas, a friend or an associate of his?????

Nope.

Dr. Alex Deas is just another manufacturer who can't yet build and deliver to the general public a recreational electronic rebreather to the EN14143:2003 standard, but he is sure trying hard and wish him luck.

He reports on Facebook he is one very important critical component short to achieve his goal of a SIL 3 rebreather (the actuator to kick in and auto-switch a diver to OC before he passes out and drowns).

Believe, believe....

Sent from my HTC Desire C using Tapatalk 2

 

[mathauck0814]

This thread highlights several points for me.

1. Non-CCR divers and divers with very little CCR experience are frightened of CCRs.
2. People greatly overestimate the level of "complexity" in a rebreather.
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)
4. People greatly underestimate the level of task loading and likelihood of screwing up gas on open circuit. I posit that it's entirely more likely that a diver would swap to the wrong gas on open circuit than that a rebreather diver would sufficiently screw up their PO2 and tox.

As to why people perceive folks to be defensive? I think it's more likely that CCR divers are just tired of hearing it. Meanwhile, I'll enjoy my longer NDL, easier deco and cheaper trimix while all you guys revel in the relative "safety" of your gurgly dive toys.