Is there a simple solution to CO2 break through?

onetime

I know the discussion of CO2 break through can get technical fast. Some members here can whip out charts, quote research, and detail the methods of measuring and warning for CO2 presence, and that's a good thing.

But, as I sit here, reading through a bunch of threads discussing recent (and past) deaths, unanswered questions, and potential fundamental flaws of Rebreather designs I wonder if we aren't missing some simple solution(s) to one potential flaw.

There's no doubt that most Rebreather makers take scrubber design very seriously and a properly packed scrubber combined with following sorb replacement guidelines from the manufacturer will cut the odds of CO2 break through immensely. However, with the very real potential that CO2 is the starting point for some of the deaths, posts about the presence of CO2 after the scrubber in some cases (I believe I recall this discussion from one of AD_Ward's insightful posts), and the reality that many manufacturers are testing scrubber duration in a, potentially, flawed way (constant flow of CO2, and/or testing at surface pressures) would there be a benefit to the integration of a secondary, smaller scrubber after the main RB scrubber to "mop up" residual CO2 when exertion or other factors allow CO2 into the inhale segment of the loop?

e.g., an inline scrubber placed in or after the inhale counterlung (on rigs with IH CL's, of course), the inhale hose, or even at nearer the inhale portion of the mouthpiece? Perhaps even something as simple as filling the inhale hose with sorb. Obviously there are drawbacks to this like an increased risk of a caustic cocktail as the sorb is closer to your mouth, probably increased WOB, and more weight on one side of the kit, but having a little extra assurance that CO2 that might slip by the main scrubber may well be worth it.

Thoughts? Ideas on how much sorb might be necessary to provide an adequate protection from this "stealth CO2"?

Genesis

How about this....

A small (inline) cannister with color-indicating sorb in it.

With an LED and phototransistor mounted in there.

The idea here is NOT to absorb a significant amount of CO2 - it is to provide a "you're screwed!" breakthrough warning quickly enough so you can bail before you pass out. The monitoring could either be integrated into existing electronics or stand-alone - it wouldn't take much in the way of power as the LED doesn't have to be on all the time - a pulse emission every 10 seconds or so is fine.

I'm thinking a 2x2 section of tube with the sorb and sensor in it. It would hold almost nothing, and thus be cheap to change - you could even make it up as an "inline cannister" with QDs so you could carry multiple ones for a given day's diving, and swap to a new one for each new dive (you have to know there's new sorb in there, because the violet color fades after a while - even though the sorb is still consumed!)

What 'ya think?

onetime

I think it could go either way, a safety to absorb CO2 is one way, a "you're screwed" is the other way.

Not real clear on the phototransistor's purpose, how does it monitor CO2? I know there are people working on CO2 monitors for new units but retrofitting existing units probably won't be part of their business plan so what you're describing may hold potential.

As far as the sorb color, how quickly does it change (don't have a clear can on my KISS so I just see the after) in the presence of CO2? If it lags significantly then it won't supply enough lead time to warn you.

I like the thoughts though.

Mdemon

[Caveat: Newbie!]

Damn! That's my cunning plan blown! (And IIRC, there is a French design being used which pretty much does this already.)

I am in the process of doing just what you described. Two home-made scrubber cartridges in one scrubber, electrickery to indicate to the diver when the second cartridge is taking over. Playing with size to get an acceptable WOB.

The benefits are just as you say - the chance of a breakthough/overbreathe is now miniscule. However, there is another benefit... (this is theory - haven't tested this yet...)

Dive your unit until you get the "beep" to say scrubber 2 has taken over.
Look at your watch!
At the end of the dive, swap scrubber 2 to 1 and put a new scrubber in at 2.
As a guestimate, subtract the time between getting the beep and getting out of the water on the last dive from 3 hours - this gives you an idea of what's left on that scrubber in case the electronics die.

The benefit here is that you will use every bit of scrubber 1 and will never be chucking unused slime away - without the need for a temp stick.

The other benefit is that you can dive beyond the beep - so now have 2 x 3hr scrubbers = 6 hours, if you want to push it. Stick a temp stick in there and you have 8+ hours I guess?

I'm just playing at the moment - there may be all sorts of pitfalls I haven't come across yet. Reliability of electronics, size of scrubber, life of slime in cartridges etc etc. But as was mentioned before, why not address the CO2 issue head on?

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EBT

How about a simpler idea. rather than multiple containers, we have one container on which we run a "safety margin". We then rely on divers having the brains not to overrun the "safety margin", because it can lead to "death".

("death" being something to avoid, since it interferes with the bodies ability to consume red wine).

I know, its radical, but it just could work!


On a serious note, the CO2 sensor debate is one where all wacky ideas should be welcomed, in the hope of cracking it one day!

Personally, Im a fan of the small animal trapped in a clear lexan section of breathing loop... when it croaks, its time to go. Now if only you could train it not to fart....

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Genesis

I don't think it would be enough absorbant to keep you from getting screwed - the dwell time would not be sufficient. This is just an indicator - kinda like the CO alarm in your house - get the hell out if it rings until you figure out what's up!

The LED is chosen to be a color that is absorbed by the colored (used) sorb. As such the phototransistor will normally show a "spike" when the LED is fired. That value (initialized when the unit is turned on) is stored; when it diminishes by some percentage, the alarm sounds. Basically the PT tells you the color change is occurring - which means the sorb in the secondary cannister is being exposed to CO2 (e.g. breakthrough)

The reason for the breakthrough isn't known but it really doesn't matter. Whether you screwed the pooch packing it, the rocks were bad or you simply overstayed your welcome you get a warning that you're now breathing a potentially unsafe gas.

I think this is significantly safer than a temp stick, because while the stick tells you where the temperature front is, it does not tell you if CO2 is exiting the stack. This does, because the only way that secondary cannister can react is if CO2 reaches it.

onetime

That doesn't address break through caused by hard breathing or flaws in the testing method that determines the safety margin (the constant flow of CO2 typically used in testing).

onetime

Gotcha on the LED/phototransistor now. Interesting.

As far as whether it could be enough sorb, I guestimate a pound may well fit in the inhale hose. Dwell time may not be the greatest but the presumption is that the main scrubber was doing at least part of its job and most of the scrubbing would occur there.

paulraymaekers

hm, hm, not that I'm so proud of my country, but it's BELGIUM design

outside view:
http://www.fr.smartgroups.com/pictur...lbumID=5753865

double scrubber: picture of an old rEvo model, just to show the scrubbers:
http://www.fr.smartgroups.com/pictur...dernum&Dir=ASC

temperature monitoring of the scrubbers gives indication when the second takes over MOST of the scrubbing

regards
paul

Genesis

I was thinking more of something along the line of the size of a 2" PVC coupling that would go "in line" with the inhale side hose......