CO2 Monitor (End-tidal) - good news

[Simon Mitchell]

Dave,

I think we have to accept that we have an infra-red CO2 sensor that will accurately measure CO2 in a sample of gas presented to it in the rebreather environment. Iain Middlebrook has confirmed this. That being the case, you have to do one of three things in your project:

Either:

Find a way of putting the sensor in the mouthpiece. This would be the ideal option because you would be able to measure inpired CO2 during inspiration, and the end tidal CO2 during expiration. The presence of CO2 during inspiration means your scrubber is failing, and the end tidal CO2 will tell you if the diver is hypercapnic from any cause (CO2 rebreathing or CO2 retention). Indeed, with the combination of inspired and end tidal readings you can diagnose the cause of hypercapnia and formulate a sensible response. If the end tidal CO2 is high (ie the diver is hypercapnic) and there is CO2 in the inspired gas then the probable cause is scrubber failure and bailing out is the correct response. If the end tidal CO2 is high and the there is no CO2 in the inspired gas then the probable cause is CO2 retention and reducing both exercise and your work of breathing is the goal. Bailing out off a good loop may not be the best course of action in this setting.

Why has this not been done? Size of the sensor probably. You really would need a gag strap to hold your mouthpiece in place! Arne Sieber is working on solid state CO2 sensing technology that will be small enough to put in a mouthpiece but this is still a way off I think.

Or:

Find a way to sample gas from the mouthpiece to a sensor in the back pack. In our anaesthesia circuits this is achieved by a pump that draws gas at about 200ml/min through a fine capillary tube to the sensor unit. This is effectively the same as measuring CO2 at the mouth and has all the advantages outlined above.

Why has this not been done? Power required for the pump probably. I have always harboured a suspicion that a clever engineer might be able to devise a way of doing this using the divers respiratory effort to power a pump device but this is way outside my area of expertise. I don't think it would be easy.

Or:

Stick with Alex's original method using direct measurement at the end of the hose whilst acknowledging its limitations and potential inaccuracies. This is obviously the least desirable option because of the potential inaccuracies and the constraints they place on interpretation of the numbers. However, I must acknowledge the fact that in our published simulation of his technique it worked fairly well at higher tidal volumes (breath sizes). Alex appeared to change it after the original debate (leaving the sensor where it was, but applying a complex mathematical algorithm to derive an end tidal CO2 from the reading). To be honest, this seems even less likely to work. There has been a lot of talk about it having been tested but no data has been presented and quite frankly, based on the sticking oxygen buttons, flying off lids and leaking components etc on the oxygen units, I find myself doubting the credibility of the test program.

So there it is. If I were a handy person like yourself, the first thing I would do is have a look at all the sensors that appear capable of doing the job (Iain has strong opinions on this and can advise you), check out their dimensions, and evaluate the plausibility of incorporation in a mouthpiece.

Simon M

 

[Simon Mitchell]

and the bottom line is that they don't work very well when they get saturated with water vapor. This was the self-same issue that the apoc presented.

Hello Dave,

You must have posted this whilst I was writing mine.

Is this true?? This has never been acknowledged by anyone from DL or OSEL. I have always been prepared to accept their claims of having sorted such problems at face value.

Simon M

 

[Dave Sutton]

Greetings Simon,

To your first post: My "dream" solution was actually the second one you mooted: A capillary tube running along the main exhale hose (coaxial internally or parallel externally) drawing a sample of gas from the mouthpiece to the sensor. I don't think that the pump power would amount to much... considering that we already carry sufficient power to run heat vests, HID lights, and a camera. One might even use a small counterlung mounted coaxially within the main counterlungs as a mechanical bellows to move a gas sample into the sensor plenum. I'm scratching my head on this... but remote sensing seems the key, and as you say we want to measure the gas both pre and post inspiration. I am also scratching my head as to the possibility of using two sensors, one at either side of the flapper valves to try to derive the same data. There are a lot of possibilities.


To the second post: "No Clue why really" and I *certainly* do not speak for DL (and have had no contact with them in almost three years) so please place no authoritative value in my laymans observations. I had not long enough access to the fully implemented pod to really draw any conclusions... There were several niggles with it, and frankly the C02 sensing portion of it was not what I was concentrating most of my attention on (PP02 measurement and audible annunciation of same was what I was really looking at in detail... and that actually worked like a dream).

My gut feeling is that C02 sensing technology is dodgy in the environment that we are discussing. My gut feeling is that none of the cells are really designed for the high helium, 100% saturated humidy, hyperbaric environment. That's a general observation. Nothing to date has proven itself to me... but I categorically do not want any comment I make here to be taken as authoritive. I see on YD that someone who obviously has no other hobbies worth participating in has already run this morning to SGS with a formal query based on his ludicrous assertion that "Dave Sutton reported that the C02 sensing of the apoc does not work" after a post I wrote yesterday about something that I merely suspected three years ago. That's just plain stupidity.... so methinks in the face of this wholesale miscatagorization of ideas being shared that it's not my place to make further observations. Place the value of a guy leaning on the bar drinking a beer and talking to his mates on my comments and leave it at that. This is not a scientific journal, and should not be taken as one.

One observation before I end my involvement in this once and for all: It's very sad that people who have been hands on with the hardware no longer feel free to discuss it, based on an entirely hostile intellectual environment. That's the exact opposite of what we try to encourage in the scientific method, isn't it? And to be a bit personal, without trying to insult but simply to state the facts and to get it off my chest once and for all: Your own published ridicule of my very off the cuff and entirely unscientific statement three years ago that was related to the *mechanical* aspects of the rig when I said "it all worked" has caused me much grief and has subjected me to much ridicule... It's hurt me personally and professionally. It was taken out of context and has gained a life of it's own, and I do not appreciate the problems it has cost me. All I was trying to do was to share what I had seen with people who I believed were as interested in new technologies as I was. Once burned, twice shy. So please understand that I am not terribly inclined to say much more on the subject of the apoc. It's just another of about 50 different rigs that I have been privileged to dive, and is just one of about 25 that are in my technical study collection. Let's talk about how to get C02 monitoring into a Meg or a rEvo and I'll be happy to participate. The apoc is a closed subject to me.




Dave


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[Igor P]

Mabe an idea combining your writing/toughts and Gian idea placing CO2 sensor.
The inhale side placement could be same as Gian tryed (it measures inhaled mix good enought I belive), but the exhale probably should be remote sensing using small capilare tubing connected with sensor chambre.
So half job is solved, needs just solving exhale part which could be driven by diver exhale and could not use any pump.

I am thinking to try/simulate this aproach if/when I get CO2 sensor in my hands.


Best,

Igor P

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[Dave Sutton]

Hi Igor,

I'm still contemplating basic architecture, one cell or two. Need to get cells first, for form factor and power requirement study, and then look at the ways to exploit them in a usable way. After mechanical integration into the selected platform is done, we can worry about the electronic processing of the signals. One thing after the other. Lots of questions, few answers.


Best,

Dave

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[Igor P]

Yup, agree. Still lot of questions, but lot of useful ideas too.

And yes, first task is getting a decent sensor. I was thinking to use same type as Gian.

When I have it/them in my hand I will think further. But ideas already are there..

Hi Igor,

I'm still contemplating basic architecture, one cell or two. Need to get cells first, for form factor and power requirement study, and then look at the ways to exploit them in a usable way. After mechanical integration into the selected platform is done, we can worry about the electronic processing of the signals. One thing after the other. Lots of questions, few answers.


Best,

Dave

.

Best,

Igor P

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[Simon Mitchell]

Greetings Simon,

To your first post: My "dream" solution was actually the second one you mooted: A capillary tube running along the main exhale hose (coaxial internally or parallel externally) drawing a sample of gas from the mouthpiece to the sensor. I don't think that the pump power would amount to much... considering that we already carry sufficient power to run heat vests, HID lights, and a camera. One might even use a small counterlung mounted coaxially within the main counterlungs as a mechanical bellows to move a gas sample into the sensor plenum.

Great, I'd love to see someone do it, and would offer what support I can give to anyone to takes it on. If you decide to get serious about this then stay in touch.

I am also scratching my head as to the possibility of using two sensors, one at either side of the flapper valves to try to derive the same data. There are a lot of possibilities.

If you have a sensor with a fast response time (which you will need whichever way you decide to measure end-tidal CO2) then you really don't need two sensors: just one in the mouthpiece (between the flapper valves) which measures CO2 during both inspiration and expiration.

To the second post: "No Clue why really" and I *certainly* do not speak for DL (and have had no contact with them in almost three years) so please place no authoritative value in my laymans observations.... snip...Place the value of a guy leaning on the bar drinking a beer and talking to his mates on my comments and leave it at that.

OK.

One observation before I end my involvement in this once and for all: It's very sad that people who have been hands on with the hardware no longer feel free to discuss it, based on an entirely hostile intellectual environment. That's the exact opposite of what we try to encourage in the scientific method, isn't it?

Absolutely Dave, I completely agree. I suppose the only qualification I would put on your comment is that producers of the hardware are almost 100% responsible for creating that "hostile intellectual environment". But I do agree it is highly regrettable.

And to be a bit personal, without trying to insult but simply to state the facts and to get it off my chest once and for all: Your own published ridicule of my very off the cuff and entirely unscientific statement three years ago that was related to the *mechanical* aspects of the rig when I said "it all worked" has caused me much grief and has subjected me to much ridicule... It's hurt me personally and professionally. It was taken out of context and has gained a life of it's own, and I do not appreciate the problems it has cost me.

This, of course, is regrettable too. All I will say is that there was an established debate in which my own professional credibility was being seriously questioned, and you were unfortunate enough to wade into it in a manner that seemed to strongly support those debating me. People can judge for themselves how "off the cuff" it looked (indeed, someone has reproduced your original post on the matter over on YD). My aim was to challenge rather than ridicule you. I too have copped a lot of abuse and innuendo (albeit from a very small group of individuals) along the way, and the fact that subsequent events have supported concerns about the Apoc voiced by my colleagues and I (which I knew to be valid back then) might help explain my "reactivity" on the issue.

Let's talk about how to get C02 monitoring into a Meg or a rEvo and I'll be happy to participate.

Like I say, if you have a go at this and I can offer any assistance then I will give it unreservedly.

Simon M

 

[Clare]

Gentlemen (and I really can use that word this morning) - you excel yourselves.

I've spent the last two days having tests (going back this morning for an MRI) and just got a temporary diving ban (and a driving ban too) so am heading off to Jamaica with a good book. Nice to see I'm leaving this place in a good atmosphere.

 

[gianaameri]

And this is where we probably differ, because in my estimation the sensor is the only real remaining technical issue. The one that Gian was playing with is the same as the one that Golem has, and is probably the same one that Gurr used, and the bottom line is that they don't work very well when they get saturated with water vapor. This was the self-same issue that the apoc presented. So, until that's "really" solved my guess is that we have a "bottleneck in the tech".

I'm hoping I am wrong though.


'Bout exhausted this.... time to pack for diving tomorrow.



Best,

Dave

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Golem uses a 5% end-of-scale sensor.

I use a 2% end-of-scale sensor which is more accurate at the low end which is what we are really interested in measuring (on the inhale side).

All others (i.e. Gurr), use an Amphilogic sensor which does not hold calibration as well and is less impervious to humidity than the one I experimented with, in my opinion.

At any rate, this is old stuff now, as I have moved experimenting with dermal Sentec sensors following the recent scientific research published on the subject.

 

[Igor P]

And what are your findings?


Best,

Igor P

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