Know your PCO2 (Why Rebreather have no CO2 sensor)

DarkSideOfTheMoon

granso:

although I overemphasized my original post to get some attention, my main
concern on a purely engineering standpoint is that you dive with a system
that has not a complete feedback on its critical parameters.

The O2 loop (I use the word loop, not in the sense of Rebreather technology but in
the context of dynamical systems) is properly handled with many redundant
sensors giving feedback to the system.

the CO2 loop is however open, with absolutely no feedback returned apart
how the diver feels.

An early warning on CO2 level would certainly be a good thing. It would mean to slow down your efforts or flush your Rebreather loop if you have to continue to work hard for example.

Some have suggested that CO2 sensors would make the dive more dangerous by giving a false sense of security. That is nonsense to me.

Ultimately, the diver makes the call. Electronics can go wrong. But in general, a well conceived system with redundancy and proper maintenance
will fare far better than the human brain, especially in the context of diving
where the brain itself is affected by the gas mix and can become part of the problem.

Don't get me wrong, we are free to take whatever risks we want and I respect that. It is possibe to dive safely without CO2 sensors, as long
as you stay within certain limits with regard to exertion.

Unfortunately, unforseen things can happen and require you to push those
limits beyond what you are used to and this is when things can go bad.

Mark Chase

Over breathing at depth 80m to be precise. Dispite an END of 25m i suffered massive narcosis put down to c02 retention due to the work load.

Skip breathing is another good way to get a hit but both my hits were over work at depth.

ATB

Mark Chase

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Abbo

Could anyone share an experience of a CO2 hit from over-exertion on a fully-functioning rebreather with properly-packed scrubber? I'd question DSM's characterization of rebreathers as delicate instruments where you have to keep your exertion to a minimum to avoid overloading the scrubber. I've had CO2 hits on OC from overworking the regulator, but feel I've worked my rebreather far harder and it has come out flags flying and asking for more.

Gilles

Can't say I 100% know this as fact. Like most other things, it is partially substantiated fact, this time based on my own experience pushing my scrubber (I have been interested in learning how to feel the onset of hypercapnia).

I have worked hard at depth feeling fine doing it (freshly packed radial), and done the same with used and nearly expired sorb (on either axial or radial), and I can definitely feel the difference. I can definitely work harder and longer using my radial over the standard axial. If one is inhaling a good lung full of gas, and still feeling like it's not enough, then it is probably the onset of HCAP. It is easy to deal with this a la BOV. Much more difficult otherwise.

Exactly the same feelings going to deep on air.

I dunno if a sensor could ever be as effective as your (or my) own nervous system at detecting elevated PCO2 levels.

Being gripped by fear (as what happens when PCO2 rises) tends to cause denial, and then paralysing fear, especially if one has to switch mouthpeice. Whenever I feel less than 95% comfortable, I'll just stop and twist that mouthpeice knob and breathe until I feel better, and then make a decision.

I don't know if I would encumber my system with yet another electronic loop to maintain and monitor. It seems too much cost/trouble for the "nice to have" value created.

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Gilles
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decoweenie

Not so long ago, I was young and stupid and also wanted to push the CK scrubber to find its limit. I am not going to post how far I have pushed it, but it was a long way past Jetsam recommendation.

Since I always carry adequate OC-bail-out and a BOV, my thinking was... "What is the big deal ? Just switch to OC if I don't feel right"...

...but it scares me thinking back especially now that I have seen first-hand how quickly someone could pass out from CO2.

It is your own life, but reading what you had written above reminds me of how I was thinking back then.

Perhaps you want to re-read post #23 and #25 once again...

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Gilles

I am thinking that I am better prepared to predict the onset of HCAP, than if I were to simply stick to the official protocol, and forever wonder why sometimes I feel great, and why other times it (Rebreather diving) feels really bad.

Subjectively, I feel a significant amount of discomfort, seemingly leading to terror, whenever;

1) going to deep on air
2) working too hard on used sorb
3) use the axial over the radial
4) insufficient pre-breathe of used sorb

In a way similiar for those who find it difficult to switch mouthpeices in the grip of HCAP, similiar is likely to occur in admitting something is wrong and signaling time to turn the dive. The induced fear screws mine and possibly others ability to make judgement and make correct action.

Tests on surface seem a little more unrepresentative as one is safe and does not sense fear (or much less of it).

The amount of unsubstantiated and cold fear I feel at depth, acts as my PCO2 monitor.

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decoweenie

I am not going to try to change your mind...

... but I just want to bring up the point that there are many different parameters affecting how the scrubber is working between different dives.

For example, unless you could control your loop volume EXACTLY on every dive, the fluctuation on the total loop volume will affect the amount of CO2 retention on your loop...

Dive safe!

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narked at 90

I've been watching this thread and decided not to say any thing but this morning i have changed my mind in the hope you dont make the same mistakes as my friend we put to rest last friday after getting him self killed on his home build rebreather and don't think because you have a commercial unit it will be different
you can not handle co2 and sorb is cheap so why do you want to push it?
#now don't get me wrong because those that know me know my history with rebreathers and the things i have done with them over the years to see what are the limits, and some of the things like giving my self a dry hit to see if i would recognise the onset of co2 before it takes hold underwater....i have had 2 co2 hits for real and i can tell you the dry hit dose not help in any way.
the 2 hits i have had have been different from each other and with different on sets.
my friend was a very capable diver much more so than most of the internet warriors on these boards but it would appear that was no help to him when the rebreather killed him,
i have been told my friend was pushing his scrubber to the limit and i know he thought he could tell the onset of co2....he was wrong......
please don't be the next person to find out the hard way about co2 and what it is capable of doing because you can not handle it...run away from it and stay as far away from the limit as you can, there are no second chances


it apeared my friend had no chance to bail out.

regards john routley

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iain-hsm

The maximum limit for C02 level in a rebreather loop is 0.5% (canister breakthrough) and as the perceivable level in humans is quite poor at just over 3% together with the breathing loop, tidal volume, work of breathing all effect the elapsed time after breakthrough (0.5%) before any perceivable C02 level increase alerts the diver, IMHO pushing C02 should be avoided.

In some divers (known as C02 retainers) by the time the high C02 is perceptible (breathing rate increases, pulse, WOB etc) the C02 level is around 5%. Due to the relatively small loop volume it is not long before this level increases further, until the WOB is stressing the heart, continue on the loop and the steady increasing level of C02 will result in unconsciousness. It is simply just a matter of time.

The history of C02 detection in commercial and military diving is pretty well known and our military CO2 monitor was discussed two years ago here:
http://www.rebreatherworld.com/gener...t=co2+detector


Blood gas C02 retention goes hand in hand with C02 increase and is another interesting subject, underlying autopsy reports of death by heart attack.

Gilles

It seems there is some misunderstanding.

The point being made is that there appears a significant difference between loss of conciousness as a result of HCAP and that from either hyp and hyper -oxia.

The former preceeded by significant discomfort, often it seems, followed by inability to proceed with corrective action (e.g. removal/replacement of mouthpeice). The latter 2, often without sensory or physiological warning whatsoever.

Of course it (HCAP) should be feared as much as Medusa's head, but there are warning signs that can be aknowledged beforehand, enabling better strength of mind when her serpent head begins to appear!

It is a known fact that our bodies try to ventilate excessive CO2 by increasing respiration. Do you have a sensor yet, more effective than an abnormal respiration rate (= comfort/discomfort level)? Is it significantly easier to maintain and use than a BOV? With exception to a few fittings/o-rings, BOV's seem a significant simplification to any Rebreather system acting as effective mitigation to HCAP, in addition to a number of other risk factors.

Adding a Spectrometer system to your Rebreather won't make the system as a whole any better. Just more complex and out of reach.

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