urge to breathe and co2 scrubber

Ben Field

This thread reminded me of a thought I'd had ages ago... I wonder if there is any merrit in monitoring the diver rather than the rebreather in terms of oxygen addition, PPO2 level, CO2 level and rebreather operation etc?

Obviously that would be a hugely more complex task than a few sensors in the rebreather!

It strikes me that typically when you are using a machine to control something you monitor what you are controlling not the machine (and thereby assume the thing being controlled is working correctly)

Just thought I'd that one out for debate...

BEN

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nigelh

Fitting a new sensor would be a whole new prospect...

Oneday maybe we can work at brain level and I can get an implant that will mean I am conscious of my blood O2 and CO2 levels. Come to think of it let's do total inerts and pressure so I can take a class in deco theory, learn the numbers and really do deco-on-the-fly.

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nigelh

DonalWalsh

"End Tidal" CO2 sensors are used in anaesthetic circuits to give an indirect measure of dissolved co2 in he blood,facilitate the mechanical control of ventilation and consequently the PaCO2 (partial pressure of CO2 in arterial blood). "normal" levels for end tidal co2 are known. Even now these sensors do become inaccurate as moisure builds up on them in these "not quite" closed circuits so I doubt they'd be much use to home builders. Also its telling you something about the diver but not the gas he/she's about to breath. That said it would be an interesting experiment to rig one to the inhale side of the loop...Then there come the mechanical problems of measuring it accurately at pressure which are well beyond me but discussed very knowledgably elsewhere on this site

wizbang

On a similar tangent, how do O2 sensors go with Carbon Monoxide.?
If haemoglobin has such a high afinity for CO and can take a long time under ambient conditions to be displaced it prompts me to ask;

How CO affects our O2 sensors.?

It might just be the composition of haemoglobin that gives the increased affinity and that the chemical compostion of O2 sensors bares no resemblance, but I thought I would ask.

I don't even know (haven't bother to look/research) the composition of O2 sensors.

Some people leave their 'breather in the garage with cars running at different times, I thought it might be pertinent.

dteubner

See http://www.rebreatherworld.com/gener...stick-1st.html for some somewhat surprising results when Ian actually measured the CO2 in a real rebreather dive.

Dave T

dteubner

O2 sensors work due to the fact that a current is produced when oxyen is added to a substrate containing potassium hydroxide and two metals (usually gold and lead). The amount of oxygen affects the rate of oxidation and hence the current produced.

CO is a reducing gas and so will should have no affect at all on an oxygen sensor. In fact very high levels of CO will lower the output by a tiny bit (about 1 part per million per 10% CO). This means that in an atmosphere of 10% O2 and 90% CO an O2 sesnor would read an alarmingly low 0.099993 instead of 0.10.

So in atmospheres containing an amount of CO that won't kill you dead straight away the effect is too small to measure

See -
http://www.teledyne-ai.com/tsfaq/faq_sensors.html#q17

Dave T

babar

There are also transcutaneous oxygen and CO2 detectors that we use sometimes in the hyperbaric chambers. They work pretty well, but they take 5-10 minutes to equilibrate, and only really give you information on the area of the body the probe is on. We use them to estimate the viability, and "capillary bloodflow" usually in an extremity. This can give us an idea of whether certain wound will heal. Not particularly useful for diving, allthough we do use them an higher pressures. They need to have a tight skin seal.

another oxygen detector used clinically is the 'pulse-ox'. It measures wavelengths of light in blood. It measures the percentage of hemoglobin in blood that is bound to oxygen. versus un bound. On the surface normal would be 95-100%. On a rebreather, I don't think it should ever be less than 100%. There have been experiments done on free divers with units adapted to water. So they are made. This might be a useful early warning system, it could alarm if the O2 sat drops below 99 or 100%. What do you think?

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Babar
Evolution Plus
Sport kiss

Freef

When you are monitoring the Rebreather loop, you are monitoring the gas you are breathing, in the same way an OC dive does when they use nitrox or trimix.

Hmmm, RoboDiver? At least you'd need less lead to sink!

Usually true, I can't think of any examples at the moment.

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David.

Currently owner of two differently sized ankles.

DonalWalsh

Yes I thought that was a very interesting discussion- it made me think that ultimately the variation in co2 production breath to breath was so great that perhaps trying to measure it in the loop was pointless anyway. Someone under anaesthesia isn't exerting as sucl so there isn't such spiking in the exhaled gas. Perhaps transcutaneous detectors of co2 might ultimately be more reflective of a divers PaCO2 but that info might come too late to function as an early warning of scrubber failure