Know your PCO2 (Why Rebreather have no CO2 sensor)

silent running

Hi Clive, it's true that there is a certain amount of trust involved with any decision, much less the decision to dive CCR. If there was no risk at all, what decision would there be to make? But one can limit the amount of trust needed by some simple tests/checks.

What is the date of your sorb manufacture and where did you buy it? Do you know your O2 supply purity/%? Is your calibration procedure reliable? Can all of your cells hit 1.6 at 6M? How often and to what degree do you check your system while diving? Do you carry some BO and have quick access to it? Do you know your unit well enough to recall all the pre and post dive procedures without the manual? This is all stuff which becomes 2nd nature to a CCR diver and doesn't even take that much time or effort after the first few dive days.

I have tried to find an instance of bad sorb coming from a keg that was still in date and have not found one. I've even asked the Sofnolime rep here on Rebreather World if there's any other way beyond being out of date or freezing/baking to compromise good sorb, no answer. I buy my sorb from people who know how it should be handled and are aware of the route it's taken from the manufacturer to get to them. I've also had all my kegs shaked/dropped and stowed in the cold hold of transcontinental flights with no ill affects.

So am I trusting by nature? Not so much. But I am more comfortable as a CCR diver than say Christopher Walken playing Russian roulette in The Deer Hunter or getting on a motorcycle again? Yes. -Andy

granso

Back to the original post,
DarkSideoftheMoon (in case I'll call you DSM if you don't mind ) could you go a little bit further and explain why should the disaster be waiting?
As a PhD engineer have you done some calculations on the reaction probability of CO2 and scrubber, some fluid-dynamic simulations of what happens in the canister?
Or is it an educated guess given the possible mistakes the diver can do when assembling the unit?
thnx

granso

decoweenie

Do you know as fact or hear-say ?

I have done a real-life hypercapnic and hypoxic pool exercises (with full support, years ago before we knew better) as part of my PSCR course, and could tell you that you don't always feel hypercapnia.

And I have rescued a hypercapnic diver who passed out before he could finish the signal of "let's go up" (i.e. simple thumb up).

I am not in the medical field, but those are my real-life experiences...

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fubari

I've been following Tom Rose's progress with keen interest.
Here are a few links...
CO2 Detection (rbworld article)
Co2 Sensor At Dema (thread from Dec 2006)
specs (pdf)

bgpartri

Here is a quote from the Proceedings of the Rebreather Forum 2.0 1996.

John Clark, NEDU:
Quote
".. Spec War Com paid me and my laboratory to explore the possibility of whether a SEAL could detect elevated CO2 levels, whether he was resting or whether he was exercising. I can tell you for a fact that if you're exercising, by the time you begin to observe something is wrong, you're probably on the verge of passing out.

I've ridden a bicycle at very high CO2 rates and finally, at some point, I decided, "Well this is probably about enough," and then I tried to move, tried to dismount the bicycle and was completely incapacitated."
End Quote


This reminds me of the oxygen triangle. You need oxygen, and you can never be sure that all of your gear is perfectly clean, so you have to control the heat.

With CO2, we don't have any sensors shipping yet, so it seems prudent to try very hard to avoid high exertion. Since exertion also seems to be bad for deco, it seems to be win-win.

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silent running

Hello Bruce, while I don't go looking to exert myself diving CCR, I do find that most of the diving I do requires it for some period. I dive in areas with lots of current, which is where the fish usually are, and usually I have to fin hard to get to a good spot for watching. On some dives, I'm at a constant low level exertion, swimming into a current. On a few rare occaisions, I've had to work at max exertion to avoid being blown down or off a site in potentially dangerous situations. So unless I want to quit CCR diving in these places, I can't avoid it.

As for deco, I don't doubt that exertion under say 6 ata is not the best idea as it relates to deco, but are OC divers cautioned during training about high exertion adversely affecting their deco? It's an honest question, I don't know as I came to CCR very soon after I started diving. I would think that they would be in a worse deco position than a CCR diver after max exertion as their inert gas uptake is not occuring on a gradual smooth curve as a constant PO2 diver's is.

I don't remember being warned during training about CO2 hits from over exertion or it's affect on deco, though it does seem common sense to minimize it it. CO2 build up was discussed as it relates to gas density on deeper dives and the increased WOB which results. I was taught that as long as I was within the manufacturer's recomended depth and duration, CO2 buildup was not an issue. Though again, a simple understanding of deep diving physiology/physics would lead one to avoid excessive exertion at depth. But of course the question becomes-how much avoiding can any of us who do dive in open water reasonably do and is the scrubber able to handle high CO2 production? These questions bring me back again to the importance of having the lowest possible baseline WOB in a CCR. -Andy

cramerdn

This is how it was explained to me when I did my course. Even with a reliable sensor, what will it help? By the time you react to the alarm you will already be out. Or the alarm will have to be set much lower, so you are right back where you started, changing sorb conservativly to never get close. I don't need sopmething else draining the battery on the unit to tell me to do what I already do.

But hey, I dont have a PhD in engineering, but I did saty at a Holiday Inn Express last night.

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bgpartri

I think they should be. Exertion as contributing factor to decompression problems has been suggested for a long time. Some research by Michael Powell at NASA comes to mind.

That's not quite the point I was trying to make. What I was saying is that it appears that if you get a high CO2 event during exertion you would not be able to detect it in time.

A lot of people get minor symptoms if they get a moderate CO2 event while at rest. But if it happens under heavy exertion you may get little or no warning.

Bruce

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Good judgment comes from experience. Experience comes from bad judgment.

Mark Chase

[quote]
Dave Shaw was at massive depth running very high narcotic depths and he had assembled his CCR incorrectly which resulted in further 02 issues.

QED its totally irrelevant


You can bail out to CCR but its not a easy and its expensive.

OC you have no PP02 sensors and also no C02 sensors. I have suffered two C02 hits and they were both on OC

A co2 monitor will be very welcome in CCR diving but its also an option to stay within the endurance of the scrubber. For most of us this is really not an issue.

ATB

Mark Chase

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Is it supposed to make that noise ?

I took my unit to the dive shop and demanded they bolt on every thing that would fit.

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decoweenie

Can you expand on how please ?

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