Cheap, disposable CO2 sensor?

jasondrake

Seems to me that reliable ppCO2 monitoring is fairly desireable. A few encouraging attempts using temp sticks are being made. But they don't deal well with a breakthrough scenario. And it seems that infrared doesn't deal with water well, and sucks up power and lets be honest probably dollars too. And do we really want MORE moisture-sensitive electronics in our loop?

So I had a thought. Paramedics use chemical CO2 sensors in emergency breathing loops. The small disposable chemical channel turns black in the presence of CO2. Fast, and in very low ppm. Many of us have used Draeger tubes using the same principle when testing compressor air.
In a rebreather we don't need to know the ppCO2 at all. We just need to know if there's ANY co2 in our inhalant. There should be zero. So could a cheap, disposable chemical co2 sensor be mounted on the inhalant/upstream side of a mouthpiece with perhaps a viewing window so it would become instantly clear if we had co2 building in the loop. Doesn't seem that this is too big a challenge. And clearly chemicals which work well in moist environents already exist - paramedics use them.

So whadya think?

Jason

sven

Jason,

I think the problem is the speed at which a CO2 buildup occurs and impacts on a diver's ability to react. Most of the sensors are way too slow to be of any help. A chemical sensor would be even slower than some of the electronic ones (which normally take minutes! to show rising CO2). That's where infrared sensors shine - they are quick. So if anyone solves the other issues (power drain, problems with humidity in the gas), we will still have to wait for a CO2 sensor which satisfies a Rebreather diver's requirement. That is the reason all RBW members are looking forward to finally see a workable design from one of the many vendors who have announced CO2 sensors for their breathers. Vaporware until proven to work.

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whoizrob

I'm a paramedic and I have a good amount of experience using these chemical detectors so I'll chime in here. THEY SUCK. They are *not* moisture resistant, any vomitous, medication, or water that comes in contact with the paper and the detector is shot. Useful duration of the chemical reaction is mesasured in minutes not hours (keep in mind their primary use is to determine correct endotracheal tube placement, NOT long term gas monitoring- that's what capnography is for). Finally shelf life is the pits unless you are able to protect them from any ambient atmosphere and wear and tear outside of the package. We've had tons of these detectors fail due to pin holes found in the foil wrapper. The only posistive thing I can say for them is that their reaction time is INSTANT, and you get color change with each breath. The Draeger tubes you mention are one use only and are not quantitative, only qualitative and could only be used in a go no go situation and I am unsure of how sensitive they are to being exposed to loop gas for long periods of time. Many of the Draeger tubes are time sensistive and once you break the seal you must draw your sample or else they go bad quickly in a matter of minutes (I also do a little bit of HAZMAT work on the side and get to play with these tubes regularly)

Realistically speaking, in my experience capnography is THE way to go. Yes it is currently expensive but I am sure costs will go down in the near future as capnography becomes the gold standard in the medical industry. Also the use of IR has gotten very much refined in the past few years where moisture is less of an issue (obvious mucus chunks and the like will still foul a sensor but humid loop gas won't). Finally capnography is great because it can be used to quantitatively monitor CO2 levels. You can monitor exhaled gas or loop gas, determine a base level and watch as levels rise, possibly alarming past a certain preset. If you do a google search you can find boards and sensors which are DIY friendly for reasonable cost. I'd jump into it but I am not as electrically savy as some here and my captial is somewhat limited. Anywho, I feel confidant that CO2 detection is not as far away as some might think for the Rebreather crowd and will, no doubt, become as indispensable a tool as our O2 sensors.

HTH,

Rob

Mdemon

Padowan and I were talking through something similar a while back. His solution was as you describe; a tube near the mouthpiece containing sofnalime which changes colour with CO2. I've read similar ideas elsewhere.

This sounds a bit more doable than the thing you were describing.

If WOB wasn't an issue, then I think it would have its uses. Namely, should CO2 be present, colour will start at one end and move to the other. But! That won't be instant, just quick depending on CO2 levels. This may be due to over-breathing instead of full-on breakthrough. Either way, the biggest and first hurdle for a diver to deal with is that of knowing that there is something wrong.

Even if the CO2 makes its way past the indicating slime and starts affecting the diver, the diver doesn't have to waste time wondering what's up, they can react instantly to a very clear indicator. "Tube turned purple - bail out now!!!"

So why not? Suck it and see. If it works, the only thing required is to make sure the diver swaps out he slime at every cannister change. Just make sure you have some Inspo scrim holding it in place as you are now in cocktail territory...

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jaap

No, I don't agree that there should always be zero CO2 in the inhaled gas. Sure the scrubber is there to remove CO2 but low levels of CO2 are not a big issue and can be expected during normal operation.

I think this can be a problem for some sort of colourimetric (change of colour) chemical indicator. I could be hard to distinguish between the slow and continuous exposure to a low level of CO2 slowly giving an indication over a loong dive and a slightly faster indication from a real and dangerous gradual build up of CO2.

I feel that one can also get a rather unharmful transient CO2 hit early in the dive if the scrubber has not been prebreathed properly or during prebreathing (for example if the scrubber is very cold at the start). Or if one performes to hard a work during a shorth period. AFAIK colourimetric CO2 indicators are not reversible in short enough timespans to allow for indication of a transient CO2 hit that is not nessecarily a reason to bail out completely.

But it could be a neat thing and should be investigated if someone has not already done it. There are quite a lot of ways of making a colorimetric CO2 indicator so it should be possible to make something that is cheap and functional.