Can CCR's be safe?

[floridabob]

Nevermind...

[DA Aquamaster]

/....And lastly, diving a rebreather has its own "first rule" where in recreational scuba it is to "never hold your breath", in rebreathers it's "don't be cheap." This was reinforced by the discussion of changing cells (some people push the limits there) as well as sorb, and other consumables, o rings, and other ####.

Basically, to be safe don't be a dick and think you know better than the people who make the rebreathers.

Garth

Unfortunately some rebreather manufacturers are closed mouth about scrubber duration, probably out of concern that if they quote a figure someone will rely on it, and cack themselves by not considering water temperature or higher exertion rates. As a result, scrubber duration is often an open question with not much information from the rebreather manufacturer.

The chemistry of the absorbent reactions are all understood and some sorb manufacturers publish data on the amount of CO2 which can be absorbed per Kg of absorbent. That data can in turn can be equated with the amount of O2 metabolized, and in turn used to approximate the CO2 produced based on O2 used (.8L of CO2 for every L of O2 metabolized), but the pieces that are missing are the actual efficiency of the scrubber design itself with a particular absorbent under different temperature and exertion conditions.

However, knowing that 2.4 Kg of Sofnolime 797 will absorb a total of 360L of CO2, and that you produce about .8 L of CO2 for each liter of metabolized O2 doesn't tell you much about actual scrubber duration when you don't know:

a) the efficiency of the scrubber design (axial, radial, etc) in the specific unit,
b) the effects of lower water temperature on that efficiency, and
c) the effects of higher respiration rates and the resulting reduced dwell time in that particular scrubber with a particular absorbent.

Some manufacturers provide scrubber duration test results, but they are often next to useless as the test conditions are such that the results are extremely conservative - very cold water and CO2 amounts that reflect exertion levels that are so high that a diver could not sustain them anyway.

I understand their hesitation given all the variables and the litigious society we live in, but the end result is that the usual test results produced from that overly conservative, worst case approach are effectively meaningless to divers in most conditions and are probably ignored in many cases.

For example, except for quoting dive times of 2, 3 and 4 hours in 39 degree, 50 degree and 75 degree water respectively when using a Micropore 5" cartridge (which is I suspect a Micropore estimate with the liability resting on them), KISS doesn't address duration on the Sidekick CCR at all. They mention Sofnolime 408 on page 21 and Sodasorb HP (another 4-8 mesh sorb) on page 40 of the manual as "the" recommended granular absorbent, but they post no times for granular absorbent.

I hear 3 hours as a quoted duration but with no source to back it up and no qualifications in regard to temp or exertion rates. We've done 4 hours in warm water with Intersorb 8-12 mesh under moderate workloads (but including time on deco with basically no exertion) and I know a couple divers who've done 5 hours on the KISS sidekick scrubber, under warm water and I presume light workload conditions with 8-12 mesh, but I don't know what brand. The end result is that divers often end up being their own crash test dummies as there is a lack of reliable information.

Consequently, while I totally agree that pushing sorb to the limit is a bad idea, it would be really helpful if rebreather manufacturers would publish some useful data to help divers better decide what a reasonable limit might be.

Aircraft manufacturers publish detailed take off and landing performance data charts based on weight and density altitude conditions. A similar approach from rebreather manufacturers showing scrubber duration times based on graphs of a range of water temperature and exertion rates would be equally helpful.

[Dsix36]

I think the discussion was spot on regardless of whether they build cells or not. I think us divers can get caught up in i know how to do this better so I make the rules and this can get us into trouble.

I don't claim to be the most experienced diver at all so I simply was trying to take the information I heard at the conference and apply it to the original question. It would probably be better if another person who was there could recount what they heard too.

Garth


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You actually condensed it pretty accurately. I do not recall them calling you a dick though.

[kwinter]

You actually condensed it pretty accurately. I do not recall them calling you a dick though.

No, but it sounds like the comment about not being cheap might have been aimed in your direction.

[Dsix36]

No, but it sounds like the comment about not being cheap might have been aimed in your direction.

I am not cheap. I recently bought a Reference RS and now a Defender. I am frugal and wise with my spending

[kwinter]

Hmmmmm......... I recall a certain someone writing:

"I can't frickin' stand it. I have Teledyne sensors in my rEvo and have had some awesomely good luck with them. #1 is 18 months old, #2 is 15 months old, #3 is 9 months old, #4 (actually an AI cell) is 6 months old.

I just put them all in a cell checker to test them again. They are all spot on in regards to linearity.

It just "tweeks my melon" to have to throw out a cell that is still working. It is not the cost that pisses me off, I just have a problem throwing away something until it is completely fracked."

[Dsix36]

Hmmmmm......... I recall a certain someone writing:

"I can't frickin' stand it. I have Teledyne sensors in my rEvo and have had some awesomely good luck with them. #1 is 18 months old, #2 is 15 months old, #3 is 9 months old, #4 (actually an AI cell) is 6 months old.

I just put them all in a cell checker to test them again. They are all spot on in regards to linearity.

It just "tweeks my melon" to have to throw out a cell that is still working. It is not the cost that pisses me off, I just have a problem throwing away something until it is completely fracked."

I rest my case. Even back then I said that it was not the cost that pissed me off, but throwing out something that still works that did.

[floridabob]

One might question wise with regards to a Reference.

DA... While I am relatively certain you are simply regurgitating other's data I have become curious of your personal rebreather experience.

What unit or units and the hours on them. I am wondering if there's more hours on your posts or the loop.

Bob