CO2 monitoring (and the X-CCR)

[uwxplorer]

Hi guys,
We have just finished a fairly comprehensive investigation of how accurately they achieve this in both the Inspiration and Revo rebreathers. The data are about to be submitted for publication, and I cannot reproduce them here. However, it is a fair summary to say that temp sticks work well. I will describe those data in detail once they are published.

Simon M

I will be curious to read about the conditions of this study when available, as my (and some my buddies) anecdotal field evidence is that the rEvo's rMS prediction algorithm output is kind of a crapshoot. So much so that I don't plan to replace my rMS sensors now that they have failed after a few years of moderate use. Not that they are advertised as consumables, mind you, but the fact is that much about everything in a RB has to be replaced at some point (this last bit for info to the OP).

 

[Tassi Devil Diver]

We have just finished a fairly comprehensive investigation of how accurately they achieve this in both the Inspiration and Revo rebreathers. The data are about to be submitted for publication, and I cannot reproduce them here. However, it is a fair summary to say that temp sticks work well.

I too will be interested to read this paper when it is published. After 5 years of diving the rEvo and close to 400hrs on the unit I have full confidence in the predictions of scrubber time the rEvo RMS gives and have found it to be reliable. Though the durability of the probes has been frustrating, however I am persevering due to the value the RMS system provides through the scrubber duration information, which no other system is able to provide.

 

[KevinNM]

Until that comes out John Clarke has a bunch of interesting stuff on his site, if you haven't seen it it might be worth a look. Here's one, but there are a lot more.
Knock Yourself Out (Carbon Dioxide – The Diver’s Nemesis)

 

[Jeremy Williams]

I think that's what we're talking about here. Although 'End Tidal CO2 monitoring' has a specific medical definition, the systems I linked are (at least attempting) to directly measure the CO2 content in the loop. This is different than the systems that attempt to measure the scrubber temperature to achieve an indirect measurement of scrubber effectiveness.

Okay, so the explanations were helpful. Thanks for those. I can only give myself partial correctness for the statement above.

I now understand that there is a key difference implicit in 'End Tidal' CO2 monitoring which is that it is attempting to measure the CO2 in the last part of the expired gas in an attempt to approximate the ppCO2 that is INSIDE THE DIVER. What these rebreather loop CO2 sensors are doing is definitely not that. Their function is much more similar to the O2 sensors in that they are attempting to measure the ppCO2 of the soon-to-be-inspired gas IN THE LOOP.

But in many ways the digression into 'End Tidal' measurements isn't helpful. It sounds like a really great feature. But you can't buy a rebreather with that feature, and by all accounts such a rebreather isn't forthcoming.

Does anybody have ANY experience with these in-loop direct CO2 sensors? I get the idea that there may be precious little time between an increase in inspired CO2 and substantial impairment of the diver. Are the sensors good enough to make a positive difference in the safety of the unit?

It surprises me that there isn't more interest in this technology... One way to look at rebreathers is that there are really just three major problems with breathing gas... too much or too little oxygen are both measured by the (often three) O2 sensors. The only other really major problem with breathing gas is too much carbon dioxide. Part of me feels like I'm looking at SPGs and asking if they are better than 'J' valves. I feel like if the CO2 sensor in the loop doesn't generate many false positive alarms that it MUST then be worth substantial consideration. It's a big unknown 'if' to me...

 

[JohnnyC]

That's the problem with current CO2 sensing and temperature-based breakthrough estimations, they're just not that accurate or reliable. I know a couple people who have had CO2 hits despite zero warning whatsoever from either their RMS system or the temp stick. Again, zero indication that there was any CO2 issue and they (fortunately) bailed out and survived. I know several others who have had alarms go off on a freshly packed scrubber 5 minutes into the dive with no other indicators that there was any kind of CO2-related issue.

I will be curious to see the results of the study that Simon is a part of. In my experience with my own units, and friends who have units with CO2 monitoring, I am not confident in the equipment, and knowing that the only CO2 monitor is my own brain, I'm much more wary and will bail out much quicker. Your brain isn't a computer, and your computer isn't a brain. Doing things like keeping track of deco obligation is the realm of the computer. Keeping track of your own physiology and your physical state is the realm of the brain. As soon as you start trying to abdicate one responsibility to the other, you can run into issues. So until there is an accurate and reliable way of monitoring CO2 that is truly beneficial, I put much greater faith in my ability to bailout than have a computer convince me it is safe to stay on the loop when I'm feeling otherwise.

Look at it this way, most rebreathers use 3 or more O2 cells because of the relative unreliability of galvanic O2 measuring sensors. We know their failure modes, and can track cell health. CCR's with CO2 monitoring have one single avenue of CO2 estimation. How can we verify the information is correct? How can we track the accuracy and reliability of them from a single data point? The answer is, we really can't. Now that doesn't mean that it's not worthwhile to have some form of CO2 monitoring, however reliance as an early warning system seems irresponsible at best. A temp stick won't indicate scrubber bypass, only a theoretical front, and a loop sensor may not indicate an issue despite rampant CO2 retention by the diver. Ultimately, it's up to the diver to be aware and respond accordingly. There's a reason that end tidal CO2 monitoring is the goal, because it's really the only reliable way to indicate a failure state that doesn't rely on the diver responding to a physiological condition.

 

[KenGordon]

Okay, so the explanations were helpful. Thanks for those. I can only give myself partial correctness for the statement above.

I now understand that there is a key difference implicit in 'End Tidal' CO2 monitoring which is that it is attempting to measure the CO2 in the last part of the expired gas in an attempt to approximate the ppCO2 that is INSIDE THE DIVER. What these rebreather loop CO2 sensors are doing is definitely not that. Their function is much more similar to the O2 sensors in that they are attempting to measure the ppCO2 of the soon-to-be-inspired gas IN THE LOOP.

But in many ways the digression into 'End Tidal' measurements isn't helpful. It sounds like a really great feature. But you can't buy a rebreather with that feature, and by all accounts such a rebreather isn't forthcoming.

Does anybody have ANY experience with these in-loop direct CO2 sensors? I get the idea that there may be precious little time between an increase in inspired CO2 and substantial impairment of the diver. Are the sensors good enough to make a positive difference in the safety of the unit?

It surprises me that there isn't more interest in this technology... One way to look at rebreathers is that there are really just three major problems with breathing gas... too much or too little oxygen are both measured by the (often three) O2 sensors. The only other really major problem with breathing gas is too much carbon dioxide. Part of me feels like I'm looking at SPGs and asking if they are better than 'J' valves. I feel like if the CO2 sensor in the loop doesn't generate many false positive alarms that it MUST then be worth substantial consideration. It's a big unknown 'if' to me...

If you read more rebreathery forums and FB, especially UK ones, you will hear a lot of people complaining about the APD CO2 sensors giving false alarms.

When I was considering which unit to buy I was attracted by the Inspo and the Sentinel for all these electronic features. In the end I bought a JJ which has nothing extra but which lives in a very robust metal tube. I am not kept up at night by co2 worries but more by my own skills.

The JJ head comes off easily so drying it and so the O2 cells is easy, apparently the XCCR is better still. The things that make life better or worse with one CCR or another are probably not the ones you, as a novice prospective buyer, will be thinking about.

 

[Jeremy Williams]

If you read more rebreathery forums and FB, especially UK ones, you will hear a lot of people complaining about the APD CO2 sensors giving false alarms.

When I was considering which unit to buy I was attracted by the Inspo and the Sentinel for all these electronic features. In the end I bought a JJ which has nothing extra but which lives in a very robust metal tube. I am not kept up at night by co2 worries but more by my own skills.

The JJ head comes off easily so drying it and so the O2 cells is easy, apparently the XCCR is better still. The things that make life better or worse with one CCR or another are probably not the ones you, as a novice prospective buyer, will be thinking about.

Yea I got the idea that reviews were mixed on the Inspo sensor. But I doubt I’m looking in the best forums. I believe that the RedBare is the descendant of the Sentinel which also had a CO2 sensor. I can find very little commentary on the CO2 sensor in it. I believe the X-Ccr is a descendant of the Hammerhead and that this is the first model in that line with such a sensor. But the firmware to enable it is pretty new and there is literally zero on it.

I think your comment about “not knowing” is such a real problem. So few people have a rebreather. Perhaps only a few thousand recreational units world-wide today? And so very few of those people have more than one to compare a significant amount of experience themselves. The best I think most people can hope for is a couple of ‘try’ dives and if they are lucky then a few more ‘try’ dives on maybe one or two or three different units. And then (as far as I can tell) almost nobody gets to build any significant time on a rebreather at all before committing to a brand/model via a large purchase. And everybody wants to feel like their expensive rebreather is the best.

 

[uwxplorer]

I too will be interested to read this paper when it is published. After 5 years of diving the rEvo and close to 400hrs on the unit I have full confidence in the predictions of scrubber time the rEvo RMS gives and have found it to be reliable. Though the durability of the probes has been frustrating, however I am persevering due to the value the RMS system provides through the scrubber duration information, which no other system is able to provide.

Confidence based on what type of orthogonal measurement (which is what the upcoming study will include, I presume)?
If this is based on the fact that you never had a CO2 hit, well, read JohnnyC's post above.
My lack of confidence is due to the lack of internal consistency of the prediction from dive to dive, but this may be due to the fact that many of the dives I did with it were less than an hour long (rEvo, in its short description of the system, says that it takes a long time to reach a reliable prediction).
But I don't want to make this a rMS bashing thread. I am really over it, even though I'll read the study with attention.
The point is that the idiot-proof rebreather (an APOC thread just popped up a few weeks ago) doesn't exist and probably never will. A lot of dives (read most) are done without CO2 sensors or temp stick, and nobody dies. Most of the non-medically related fatalities tend to be due to users donning their units trusting that nothing bad could happen to them, instead of checking, rechecking and triple checking stuff before getting in the water (and while in the water). That's that obsession with not committing a stupid error which will keep you alive, not the expensive (and fragile) bleeding edge gadget...

 

[Tassi Devil Diver]

Confidence based on what type of orthogonal measurement (which is what the upcoming study will include, I presume)?
If this is based on the fact that you never had a CO2 hit, well, read JohnnyC's post above.

Confidence based on nearly 400 hrs of dive time on the rEvo. No I have never had a CO2 hit and I regularly run the RMS down to the last few minutes. I track the RMS RCT and RST time in my dive log and have found the RMS scrubber time predictions to be reasonably consistent, for both short and longer dives.

 

[KenGordon]

Yea I got the idea that reviews were mixed on the Inspo sensor. But I doubt I’m looking in the best forums. I believe that the RedBare is the descendant of the Sentinel which also had a CO2 sensor. I can find very little commentary on the CO2 sensor in it. I believe the X-Ccr is a descendant of the Hammerhead and that this is the first model in that line with such a sensor. But the firmware to enable it is pretty new and there is literally zero on it.

I think your comment about “not knowing” is such a real problem. So few people have a rebreather. Perhaps only a few thousand recreational units world-wide today? And so very few of those people have more than one to compare a significant amount of experience themselves. The best I think most people can hope for is a couple of ‘try’ dives and if they are lucky then a few more ‘try’ dives on maybe one or two or three different units. And then (as far as I can tell) almost nobody gets to build any significant time on a rebreather at all before committing to a brand/model via a large purchase. And everybody wants to feel like their expensive rebreather is the best.

This week I had a buddy who left his Sentinal at home and dived OC. The deepest we went was 56 and we could have gone to 65 for sure and maybe a bit more, so generally better on CCR than OC.

The CO2 sensor on the Sentinal was one of the things he ranted about, as well as the various wireless bits and how it might turn itself on in the garage complaining of low ppO2.

Find an instructor (or several) who can teach several and talk to them. Talk to people on FB, here TDF. Keep in mind there is a regional bias. Round here having a Meg sets you out as a weirdo. I guess that isn’t true everywhere.

In the wild I see a lot of Inspos, a fair few JJs, the occasional Revo (oddly they seem to dive in packs) and very little of anything else. This week there were 4 JJs (two GUEish style) and about 6 or 7 Inspos, some in G boxes so they half look like a JJ. This is two boats and 17 divers in all. The rest were on twinsets with a deco stage. Compare that to the machines that get talked about on here.

I am pretty happy with my JJ, the ADV can be a bit annoying now and again but I would buy another if this one isn’t on the baggage reclaim when I land. I’d probably look at an XCCR for 10 minutes but dismiss it as trendy and failing the ‘devil you know’ test. My JJ is stock except for the bailout valve, I don’t think any of the Inspos were stock.

Do keep in mind that excessive technology can just be complication and faff. A CCR is already a long way in that direction for OC. Stuff like the NERD and the APD version (HUS I think), co2 sensors, temp sticks, and so forth really are more things to break.