Prebreathing Survey

[doctormike]

I don't have a JJ manual but for every manufacture I have actually seen, this isnt the case. They'll say their scrubber is good for X minutes at Y depth in Z temp. Often 40m on air at 4C with 1.6lpm CO2 production i.e. the CE test parameters. This is not diving (there's no ascent, nobody works that hard recreationally, 4C is rare).

The JJ test dives do incorporate ascents.

You are required to interpolate that to your diving.

Well, not sure what you mean by this. The manufacturers don't require you to interpolate anything, they just say that here is our test data that shows breakthrough times with this specific set of circumstances. Of course, they can't state anything regarding different circumstances that they haven't tested.

Interpolating to your own diving is not deviance - it might be art I'm not sure.

Look, I don't mean to be argumentative about this - pretty much every CCR diver in this forum is more experienced than me. But I'm not saying anything absurd here, or telling you how to dive.

Saying that the test data is way to conservative and you should just come up with your own breakthrough guidelines isn't the best message for a new CCR diver. Sure, maybe you have thousands of hours on a unit and you know that you routinely push it from X to Y with no hypercapnea, then you are comfortable running your scrubber for Y. But that's in your very specific circumstances with your personal metabolic profile - it's simply not generalizable. If it was, then the manufacturer would publish an algorithm that says plug in your water temperature and your workload and your own CO2 production rate and this will give you your real scrubber time. But they don't do that.

So maybe it's an art form, but it is deviating from published guidelines. I don't know about other CCRs but I can't find anything in the JJ manual that waffles on that. When I look at that CO2 breakthrough curve, it takes off pretty quickly soon after 180 minutes. Yeah, I'm sure that I could save some sorb by running it longer in warmer water, and if I was doing very long dives I might have to rethink all of this, switch to a non CE canister and decide what my particular risk tolerance was. But again, all I'm saying is that for my diving that I am doing now, with run times less than 3 hours, I'm fine dumping the sorb after that.

It's sort of like FDA clearance. The absence of evidence isn't the evidence of absence. Flonase is FDA approved down to 4 years of age, and Nasonex is FDA approved down to 2 years of age. Does that mean that if you give a 3 year old Nasonex you will be safe but if you give them Flonase they will have a problem? Probably not. What it means is that the manufacturer of Flonase didn't spend the money to establish that lower age approval because it probably wasn't worth it from a business standpoint.

At what point does breakthrough happen on the 2.5 kg scrubber in 25C water? Dunno, but if you have the money, you can run the same tests and find out.

 

[rjack321]

I have no problem if you want to do 3hrs on 5.5lbs of sorb. Or 2 hours, or 6 hrs, all your choice. I know "normalization of deviance" is all the rage due to Gareth's writings, that and Dunning & Kruger effect. But using those terms like a weapon is not cool.

Taking the CE test data alongside instructor, manufacturer and other diver's experiences to derive a reasonable sorb duration for actual dives (and sticking to it) is not what NASA did prior to the Challenger disaster whatsoever. So be careful what long established practices you attack and how. This is not normalization of deviance even if the answer is "different" or "wrong". This is just different divers coming to different conclusions from different datasets. The appeal to some CE lawyer in Brussels as the ultimate authority is not very healthy or even accurate - even if it is conservative.

In 25C water you would have at least 6 hrs on that scrubber and still be 3 sigma below the (tight) breakthrough distribution. If you aren't able to accept tens of thousands of FL and Caribbean CCR dives and instructors as evidence of that oh well, we are working from different datasets and you prefer the 3 CE tests. That's up to you.

 

[rjack321]

And btw its not just water temperature differences. My normal metabolic rate is way below CE test rates (and everyone else's is too). My 0.6 to 0.7 L/min O2 is less than half the CE machine. That doesn't mean my sorb duration doubles, because the scrubber ends up being cooler and less efficient. But nobody swims as hard as the CE machine for hours and hours.

 

[doctormike]

Dude, it's not a "weapon". I'm trying to learn, sorry if you took this the wrong way.

I was explaining why I dumped my sorb after 3 hours. It's deviating from the manufacturers guidelines. You are probably correct in saying that it's fine to do that in this or that circumstance. All I'm saying is that that statement is not generalizable.

This is also exactly why I started this thread a while ago... There are a lot of variables, that's the point, and it would be nice to have some reproducible standardized data about real world diving that are better than just saying that everybody guesstimates their scrubber duration, and you are naive if you don't do that.

 

[taimen]

I dump my sorb after 3 hours too. It is a good limit for one days diving and probably safe guesstimate for repetitive dives too.
But I never thought that the published 180 min dive profiles would represent a limit for scrubber duration issued by the manufacturer or really even a guideline. They are test results in certain conditions. There are no published safe time limits for scrubbers.
During the course we spent quite a lot of time discussing real life examples of scrubber use, durations and problems. Rebreather manufacturers like JJ are not able to do extensive laboratory testing of scrubbers and even less capable of doing all the legal work required for publishing their own guidelines. That is why they only publish a couple of profiles with very large safety margin (in near CE test conditions). It would be nice to have better data, but this is what we have to live with.
Radial scrubbers have also their own issues, axial is thought to be more reliable. There are some very experienced rebreather divers who think that extending 2,5 kg axial scrubber use might be a better deal than the radial scrubber. They have the radial scrubber so it is not just being cheap.
Note that this whole discussion includes estimating metabolic and ventilation rates. During a long deco dive the scrubber duration is extended by decoing relatively shallow with low oxygen consumption rates. Setting new standards based on available data and experience is not what normalization of deviance in negative sense means. Technical diving is full of this kind of decisions based on guesstimates. Repetitive diving makes it very difficult the estimate scrubber limits.
The published test profiles give no indication that it is safe to do several dives totaling 3 hours, although that is how it is usually interpreted with good results in real life.
Thus it is a bit naive to anchor to 180 minute duration.

 

[doctormike]

I know "normalization of deviance" is all the rage due to Gareth's writings, that and Dunning & Kruger effect. But using those terms like a weapon is not cool.

Again, I’m not trying to be difficult here, I usually appreciate your thoughtful posts, and I hope that you don’t take this thread the wrong way. I think that the phrase “normalization of deviance” has ruffled some feathers. But try to understand what I am saying here.

In 25C water you would have at least 6 hrs on that scrubber and still be 3 sigma below the (tight) breakthrough distribution. If you aren't able to accept tens of thousands of FL and Caribbean CCR dives and instructors as evidence of that oh well, we are working from different datasets and you prefer the 3 CE tests. That's up to you.

If you have that data, that would be great, that was exactly what I was looking for with this thread. But the point is that I haven’t seen that data, and I don’t think that it exists (at least, not for the JJ). Data would be something like formalized testing (e.g. the CE stuff), or even a published report that conformed to some sort of scientific standards. For example “we reviewed profiles and scrubber times from 1000 warm water rebreather dives, here are our methods, here is what we found”. But that’s not the same thing as saying that thousands of Caribbean CCR divers push their scrubbers beyond the published manufacturer’s standard. Yes, that’s true! And I’m NOT saying that these people are diving unsafely. But what I AM saying is that you can’t take that statement and tell me that X is the time to scrubber breakthrough for an average diver doing an average dive in 25C water.

I mean, scrubber breakthrough due to pushing a stack exists, right? People do get CO2 hits from overusing their scrubber, right? So those people were going beyond the unknown limit for their particular dive. All I’m saying is that without any actual data to tell me when my scrubber is going to fail, I don’t know what that limit is. So that’s why I’m sticking to limits that I do know.

Sure, an experienced rebreather diver doing warm water dives is over time going to know that he can dive more than 3 hours on a well packed scrubber. But that’s not a generalizable thing to be published. And the next less experienced diver in a bit colder water who is working a bit harder and has a higher baseline CO2 production shouldn’t take home the message that the published limits are without meaning.

 

[cathal]

This is not normalization of deviance even if the answer is "different" or "wrong". This is just different divers coming to different conclusions from different datasets. The appeal to some CE lawyer in Brussels as the ultimate authority is not very healthy or even accurate - even if it is conservative.

In 25C water you would have at least 6 hrs on that scrubber and still be 3 sigma below the (tight) breakthrough distribution. If you aren't able to accept tens of thousands of FL and Caribbean CCR dives and instructors as evidence of that oh well, we are working from different datasets and you prefer the 3 CE tests. That's up to you.

I would agree broadly with your approach if it wasn't for the fact that scrubber breakthrough can be a critical faiIure on a rebreather. Guestimates or subjective extrapolation of scrubber life based on water temperature and or divers experience etc. is not sufficient for a life safety system.

 

[rjack321]

I would agree broadly with your approach if it wasn't for the fact that scrubber breakthrough can be a critical faiIure on a rebreather. Guestimates or subjective extrapolation of scrubber life based on water temperature and or divers experience etc. is not sufficient for a life safety system.

Yet here we are, basing "life safety" decisions on 3 data points under (in theory) highly conservative conditions OR alternatively looking at the body of evidence that a very typical 5.5 to 6 lbs of sorb in warmer water (sometimes 20+C warmer), with a much lower exertion rate, can last far longer than what the manufacturer's lawyers will allow them to print.
And some CCRs/scrubbers publish no CE data at all or only 1 data point. And many are modified, although not usually the scrubber flow dynamics. The CE tests (not just for CCRs) are just terrible representations of reality. There are tens of thousands of unsealed regs in use around the world, CE would say that theses are forbidden in <10C water yet in practice this isn't the case either.

 

[stuartv]

Sure, an experienced rebreather diver doing warm water dives is over time going to know that he can dive more than 3 hours on a well packed scrubber.

How? You are diligent in sticking to 3 hours. When you have another 1000 hours on your unit, if you continue to be diligent about 3 hours, how would you ever know that you can do more than 3 hours?

It seems to me that the only way people ever know these things is by first pushing the limits into unknown territory. You won't know if you can dive more than 3 hours until you've tried it. Multiple times.

Or, you can take a preponderance of anecdotal evidence, from very experienced people to whom you attribute some credibility, and begin to push your own limits where it's not 100% "unknown". Just be prepared to FIND those limits....

It seems to me very much like figuring out what works for you when it comes to NDLs, Gradient Factors, etc.. You start with a published "limit" (e.g. PADI tables, or Shearwater Conservatism on High, or whatever), get experience with it, learn what you can from other people's experience, and then gradually expand your own body of experience to figure out what works for YOU.

 

[rjack321]

Or, you can take a preponderance of anecdotal evidence, from very experienced people to whom you attribute some credibility, and begin to push your own limits where it's not 100% "unknown". Just be prepared to FIND those limits....

On both of my units I was advised of the scrubber life by instructors (and in the case of my Meg again by the actual builder on a trip we did together) and in neither case was that number derived from the CE test number. I stick to those recommendations not the CE values, the Kiss has no CE testing anyway. In theory I might survive a CO2 hit but most people don't if they are at any depth at all - so no I have no desire to experiment.