Apocalypse Rebreather

[ruiner]

Maybe it is better with the devil you know, and just go with the measured peak CO2 values.
Oskar

Thanks for pointing out the document, I had seen it but missed the discussion of CO2 measurement. As far as I can understand the chapter 6.3 about CO2 measurement, that is the actual way they do it and end of exhale CO2 is in addition to that.

Apparently most of the relevant documentation is confidential to DL so none of the actual test arrangements or results are not shown. I too doubt very much that EN standard would specify tests that would address these concerns. I would think that CE testing would be mostly interested of verifying that there aren't false negatives (true CO2 higher than measured/calculated).

They claim that volume weighted average CO2 measurement has 3% error margin. Perhaps some one who knows the topic better can tell if that is accurate enough to detect scrubber breakthrough early enough. That is, I suppose, the most important function of the whole measurement.

With the auto-bailout I would be also interested to see that there aren't disproportionally high number of false positives. They acknowledge that there are scenarios where this can occur like while photographing. Still, false positives are more tolerable than false negatives.

I find it odd and regrettable that DL and OSEL has choose to ignore the offer that Simon Mitchell made.

 

[jimmy71]

Perhaps some one who knows the topic better can tell if that is accurate enough to detect scrubber breakthrough early enough. That is, I suppose, the most important function of the whole measurement.

This was my thoughts on this. I do not care as much about end exhalation of CO2 as much as inhalation. It is my understanding from the CO2 monitoring in the sentinel that scrubber eliminates close to 100% of the CO2 passed through it assuming it is functioning correctly. If CO2 is present on the inspiratory limb, you have a problem regardless of the cause.

Am I missing something?

Jimmy

 

[Dr Simon Mitchell]

This was my thoughts on this. I do not care as much about end exhalation of CO2 as much as inhalation. It is my understanding from the CO2 monitoring in the sentinel that scrubber eliminates close to 100% of the CO2 passed through it assuming it is functioning correctly. If CO2 is present on the inspiratory limb, you have a problem regardless of the cause.

Am I missing something?

Jimmy

Hello Jimmy,

Yes. I was interviewed on this subject at Eurotek, and I explain the merits of monitoring inhaled or exhaled CO2 here:

Submerge Productions : HD Dive Videos - Videos

I you watch that and have any further questions I would be happy to answer them.

Simon M

 

[jimmy71]

Dr Mitchell,
In the OR we are aggressively manipulating the ventilation of patients either with narcotics, muscle relaxation, or mechanical ventilation. The primary reason one would be concerned with etCO2 if when a patient has no spont ventilation. Obviously the shift in the CO2 curve with narcotics is a whole different issue.

It was my understanding that the inadequate MV you are talking about is because divers usually are not letting there body control MV, but consciously overriding the natural drive. One would believe that this issue should be self limiting as once the narcotic effects of the CO2 overcome the diver, they resp pattern would no longer be voluntary and they would develop a Kussmaul pattern to compensate for the acidosis.

If the issue is a simple how to be determine if the MV if adequate to remove the CO2 being produced, it seems like we could utilize the same technology that the rate modulating pacemakers use. Or even easier just a simple heart rate monitor like the uwatecs. As the heart tries to compensate for the increase workload, the MV should increase or the person will be building up CO2. If the scrubber is working as displayed on the inhalation limb, then all TV beyond the dead space would be removing CO2. It seems like what we are worried about/wanting to monitor is MV. The passive measurement of MV via etCO2 in this setting seems difficult and kind of missing the point.

When someone is using a Salter cannula, you put little faith in the actual number as it is not a closed system, but it is enough to know with competent auto-regulation the body will reach a steady state. Is this auto regulation violated during diving?

So once again I have to admit that I feel like I am missing something. Yes highly accurate etCO2 would be great, but the limitations seem that the time and money woudl be better spent elsewhere.

Very interesting discussion and I thank you for your time.

Jimmy

PS not proof read for spelling or regional differences in terms and such.

 

[Oskar Frånberg]

Thanks for pointing out the document, I had seen it but missed the discussion of CO2 measurement. As far as I can understand the chapter 6.3 about CO2 measurement, that is the actual way they do it and end of exhale CO2 is in addition to that.

Apparently most of the relevant documentation is confidential to DL so none of the actual test arrangements or results are not shown. I too doubt very much that EN standard would specify tests that would address these concerns. I would think that CE testing would be mostly interested of verifying that there aren't false negatives (true CO2 higher than measured/calculated).

Hi
My understanding of the document was that the mentioned algorithm is the only way they are estimating the end tidal CO2, but hope fully someone with insight could clarify this. The fist line says that they are directly measuring the volume weighted mean expired CO2, to do this one have to mix the whole exhalation and then measure. Otherwise they are measuring the CO2 change over time and without a flow it is difficult to determine the volume weighted average. This would add jet another step to the calculations and thereby increase the risk of an error.

In the EN14143:2003 end tidal CO2 monitoring is not mentioned. There are tests for inspired CO2 measurements which specify that the system should have a limit deviation of ±3 mBar in all conditions. To test this there are 1) visual inspection, basically checking the documentation. 2) Exposure to known CO2 partial pressures to check the accuracy, there are slight variations here if it is a monitor for inspired CO2 or if it is an active warning device. 3) Practical performance, 5 diver testing three complete sets, this is basically an ergonomics test. The manufacturer, the test house and the notified body do of course have discussion on which other tests to be performed but there are non mentioned in the referred standard.

If the issue is a simple how to be determine if the MV if adequate to remove the CO2 being produced, it seems like we could utilize the same technology that the rate modulating pacemakers use. Or even easier just a simple heart rate monitor like the uwatecs. As the heart tries to compensate for the increase workload, the MV should increase or the person will be building up CO2. If the scrubber is working as displayed on the inhalation limb, then all TV beyond the dead space would be removing CO2. It seems like what we are worried about/wanting to monitor is MV. The passive measurement of MV via etCO2 in this setting seems difficult and kind of missing the point.

If I read you correctly you suggest measuring the minute ventilation compared to the oxygen consumption instead. I suspect that monitoring the alveolar ventilation would do the job just as well as a etCO2, but that would first of all require knowledge of the dead space volumes, a respiratory rate sensor and then a measurement of either the minute ventilation or the tidal volume. The reason deep life is estimating the RMV is as they describe it “Attempts to measure RMV directly using pressure or flow sensors have not produced reliable sensor data, so these indirect relationships became essential”. In DCSC/ISmix, Interspiro uses the bellows to determine the tidal volume but that requires a special type of bellows arrangement. So my thoughts hear is that since they are basing an automatic bailout on this a direct measurement of the peak CO2 maybe the best choice.

And then we have the next questions as to what an adequate threshold for this bailout would be and to what extent it adds value.

Oskar

 

[Dr Simon Mitchell]

Dr Mitchell,

So once again I have to admit that I feel like I am missing something. Yes highly accurate etCO2 would be great, but the limitations seem that the time and money woudl be better spent elsewhere.

.

Hello Jimmy,

I think what you are missing is the importance of the change in the behavior of the respiratory controller during diving. A combination of: increased gas density and work of breathing, higher PO2, and higher PN2 all may contribute to a variable (between individual) tendency to permissive hypercapnia. In other words, the respiratory controller becomes much less sensitive to rising levels of CO2 and there is failure to increase ventilation in the normal manner. The diver thus "retains" CO2. It is not dissimilar to the "shift in the curve" caused by narcotics that you mention in your post. In rebreather diving, the scrubber might be working well, the inspired PCO2 is zero, and yet the diver could be retaining CO2 to dangerously high levels. This will obviously not be detected by a CO2 monitor on the inhale limb. Indeed, the only way to detect it reliably is to measure expired CO2. Your suggested alternatives would be far too imprecise.

With the above in mind, allow me to say that I believe the intent of the Apoc manufacturers is spot on and I applaud them for that. However, there are significant and (to date) unresolved concerns about the way they are going about it. I thus get very annoyed when Brad trumpets their R&D as beyond criticism without acknowledging the substantial debate that has surrounded this particular issue. Even worse is his quoting of testing to specific standards which probably don't even address these concerns as some sort of proof that the issue doesn't exist.

There is a substantial body of literature on this, and David Doolette and I have recently published a comprehensive review. I offered this to Oskar and if you would like it too please let me know.

Simon M

 

[tech_diver]

I am not a rebreather pilot but I have been a diver for 30 years and in the SCUBA business for 18 years. I cannot remember the mountains of prototypes, mockups, computer-generated images of ghost rebreathers I've seen that have never happened. The rebreathers that did make it to the market did so after years of struggling for their life.

So obviously, years ago, when the Apocalypse Rebreather first announced they have a rebreather that offers:

"Rebreather monitor, including retained CO2 as well O2, flood and breathing rate monitoring, meeting EN61508.. Automatic bail out Dive planning and logging. " at "prices so low, as to be accessible for all who currently use Open Circuit"

It is understandable I would be skeptical.

However, I was extremely intrigued by the CO2 monitoring function. CO2 is a very stable gas and difficult to test for. It was my understanding that reliable CO2 analysis like spectrophotometric techniques had issues dealing with humidity and changing relative pressure of the sample gas. Not to mention the issues discussed on this board about the actual sampling of the breathing loop gases.

I did meet a researcher once who told me he was looking at nanotube technology applications and gas detection. Maybe after all these years there had been a real breakthrough. I want a $3000 rebreather that does all that.

One of the documents they released was a stress test for a sapphire lens of an infrared transmitter so apparently it's some type of spectrophotometry. The documents I'm really interested in are the ones relating to the accuracy of their CO2 monitoring system which are marked as not released. There are documents released showing that their dive computer and CO2 monitors comply with standards for electromagnetic interference.

It seemed kind of strange but maybe there was a breakthrough. I really want a $3000 rebreather that does all that.

The retail pricing they were talking about amazing. I think it was under $3000. As someone who works building dive equipment I couldn't believe it. But maybe there was a breakthrough. Hey! I really, really want a $3000 rebreather that does all that.

Then I encounter the Deep Life document, "How Rebreathers Kill People." which tells us that "users and governments are turning a blind eye" and "Technologies are available that would have prevented every one of these incidents."

I think, "Holy Crap! There is some sort of sweeping conspiracy!" All these years in the dive business and no one told me.

The document analyzes rebreather accidents and their causes. Each cause is addressed by a design in the Apocalypse rebreather. The second incident was particularly interesting where a diver off Scapa Flow in June experienced a problem with a control system on a rebreather and the causes listed in the report were that the software/electronics engineer behind the system "had no engineering qualifications or formal training whatsoever" and the systems project leader "had no formal education after the age of 16". I've never seen a cause on an incident report like that.
I'm still not clear on the relationship between deep life and the Apocalypse rebreather. But hey, maybe there's been a breakthrough. I still really want a $3000 rebreather that does all that.

So anyway, here it is years later. People are pissed off, some want their money back. I didn't put down a deposit. But I still really, really want a $3000 rebreather that does all that.

 

[Brad_Horn]

Even if recent tests revealed that our concerns have been dealt with adequately, it would not change the paper at all. If you read my post again more carefully, the simple study we conducted was a response to comments made by Alex on line almost 2 years ago.

G'Day Simon, No worries. I presume the fact that your results were based of information published on an online forum 2 years ago rather then testing of the actual product was made clear in the report and your presentations.

Having said that, I would be only too happy to publish favourable results in a second paper if given the opportunity to test the actual unit (see below).

So you have published and presented unfavourable results based on speculation about the actual performance of the DL design then?
Seems that while your paper was peer reviewed and appropriately scientific you have stated the performance of say a SAAB car by testing a FORD which seems fishy....

That brings me to your claim that CO2 detection in the APOC has been tested to the standards you quote. That is all well and good if that testing addresses the potential inaccuracy we highlighted.

You said it needed to be independently tested, I simply pointed out it already had been to what appears to be a very rigorous level.

So here is a simple question Brad, and all it requires is a simple categorical yes, no, or don't know:

Has CO2 been measured using human subjects and properly calibrated analysers deployed simultaneously at the mouth and pod (end of hose) with the latter compared to the APOC pod reading over a range of tidal volumes from 500ml to 2000ml?

Properly calibrated analysers would be a yes as they include calibration details for their mass spec in their reports, pg 19 in the below one.
Don't 'know' on rest..., but from the following I would guess yes.

As report on Fault_Study_CO2_Bypass_110105
- Breath by breath measurement of CO2 was done
- They used zero end of inhale CO2 at the mouth to confirm that the flapper fault wasn't present.
- In EN 14143:2003 it stipulates that the Volume Weighted Average CO2 be measured during the scrubber endurance test “at the mouth”. So ALL testing of the iCCR would mean that they are measuring the CO2 simultaneously at the mouth and pod...
- The Notified Body required the VWAI CO2 measurements to be taken at each point of the dive, including at the start of the dive, to ensure the end-of-inhale contained zero CO2.
- Apparently measuring end-of-inhale CO2 prior to the inhale one-way valve gives false scrubber endurance and VWAI CO2 results in many rebreather designs when this fault condition occurs.
- They advise that in practice the low tidal volume alarm is not triggered, because if the diver does not metabolise enough oxygen, then the PPO2 in the rebreather will rise in all Deep Life O.R. designs (all use orifices to control flow), causing an oxygen warning, then alarm: this occurs well before the tidal volume drops anywhere near 300ml. This high PPO2 occurs in the iCCR at a tidal volume of less than 1 litre per minute with a RR of 14. Which appears to indicate that for 1/3 of the range your worried about the diver may well be on OC.
- During manned dive testing, the CO2 alarms were triggered in the following circumstances:
1. Before starting a weld, the diver would hold his breath, causing the first exhale to have a high CO2 level, tripping the alarm. This false alarm source was removed by increasing the averaging period.
2. Divers talking for long periods using intercoms, would reduce their tidal volume, which increased the calculated end of tidal CO2.
3. Some flapper valve combination's caused the CO2 warning to trigger.
- The gas fraction was measured in the following seven points of the breathing loop. 1. Mouth (MOUTH)
2. Exhale channel immediately downstream of the exhale one-way valve (EXH) 3. Centre of the exhale counterlung (EXH_CL)
4. Inside the base board bell membrane immediately prior to the scrubber (SCRB in)
5. Scrubber output tube (SCRB out)
6. Centre of Inhale counterlung (INH_CL)
7. Inhale channel immediately upstream of the inhale one-way valve (INH)
- Errors in the phase delay compensation would cause fundamental errors in determining Volume Weighted Average Inspired (VWAI) CO2, and in assessing the Deep Life CO2 sensor: the CO2 Lissajou figure would be incorrect. To eliminate this source of error, the phase delay at each measurement point was calibrated.
- The Deep Life CO2 monitor measures the same average CO2 as that measured by the mass spectrometer just upstream of the scrubber, but displays 7.0 % SEV when the consumption of 1.78lpm is simulated: this is correct. The actual RMV is 40lpm, with a 20 bpm RR, but the oxygen metabolised is 1.78lpm STPD at 1.6lpm, so the calculation used by the CO2 monitor concludes the RMV is 49lpm, and the tidal volume is 2.45 litres. Applying these numbers to the formula described for determining the peak CO2 in the previous section of this report, results in 7.0 % SEV of CO2, and the actual number measured using the mass spectrometer is 6.9% SEV.

If so, show me the data.

Why, you apparently have a lab with mass spec and capability to conduct testing of CO2 sensors in rebreathers designed for underwater use to a suitable standard proving that the DL design doesn't work without even testing it!

In addition, I will once more in public repeat my offer to conduct this testing with you present, and my promise to publish the results be they favourable or unfavourable to the APOC. I would like them to be favourable, because we all want to see this system work. All you need to do is provide me with a moutpiece, hose and CO2 pod. What could be fairer than that?

Noted.

Out of curiosity, did you notice the same flapper valve fault that DL have reported in your testing?
What depth did you do the testing for your paper too?

In any event, the key point is this: DOES IT WORK? So far, I have not seen any data that addresses this question, and as mentioned above, I doubt that the testing Brad quotes addressed the issue. And the second key point is: IT WOULD BE EASY TO DEMONSTRATE WHETHER IT WORKS OR NOT, AND I HAVE OFFERED TO DO IT FOR THEM. Any controversy around this issue could be made to go away in a weekend's work for me, yet I have not been taken up on this offer. It makes me slightly suspicious.

Simply out of curiosity what is your test setup and how does it compare with what DL have published they used?
What would your testing give DL considering the only controversy seems to be generated by you et al?

Agreed, and we said that in the paper. I reiterate that the paper was a response to an obfuscation of the fact that low tidal volume breathing could become problematic at some level of breath size. Having said that, and as we point out in the paper, tidal volumes around 500ml could become relevant in some situations in diving.

What has the response been from the committees that draft the CE and NORSOK standards as well as the likes of NEDU to this information?

However, there are significant and (to date) unresolved concerns about the way they are going about it.

Rather then saying, interesting wonder how they did it and ordering one to trial, you seem to be saying it can't be done, but it can be if I am consulted....

I thus get very annoyed when Brad trumpets their R&D as beyond criticism without acknowledging the substantial debate that has surrounded this particular issue. Even worse is his quoting of testing to specific standards which probably don't even address these concerns as some sort of proof that the issue doesn't exist.

Simon, I still have yet to see any testing proving that the DL engineering doesn't work as intended. I seriously would like to see valid criticism of DLs R&D with calibrated test results of the ACTUAL product in question backing up the criticism. Also other manufacturers and interested parties like yourself publishing testing of rebreather designs to the same level so folk have a choice....
Am still sourcing a copy of your complete paper, though I understand from your talk, what you have pointed out is that end of tidal CO2 is not related to the mean CO2, when one does not measure tidal volume.

Kind regards
Brad

 

[Dr Simon Mitchell]

Brad,

Debating you on line regarding this matter is essentially a waste of time. You either do not understand the issues, or it is your intention to deliberately obfuscate them. This is clearly illustrated by the comments in your most recent post. The only reason I persist is that your gobbledegook dressed up as technical information has the potential to mislead other readers on this forum. So...

I presume the fact that your results were based of information published on an online forum 2 years ago rather then testing of the actual product was made clear in the report and your presentations.

Brad, the way you trip yourself up really makes me laugh. You know perfectly well that at the time the debate about this CO2 monitoring configuration took place the Apoc was alledgedly about to be shipped. Dont' try to deny this; I can easily dig out the relevant posts from the RBW forums. So, are you saying that the information I was given then is no longer relevant? Put another way Brad, are you saying that the CO2 monitoring configuration / algorithm has been modified since that time when the unit was just about to be shipped?? What modification Brad? Why? On the basis of our debate perhaps? And if the unit has not been modified, then what is the point of raising the "2 years ago" issue?

So you have published and presented unfavourable results based on speculation about the actual performance of the DL design then?
Seems that while your paper was peer reviewed and appropriately scientific you have stated the performance of say a SAAB car by testing a FORD which seems fishy....

Brad, you can test whether a spark plug ignites petrol using either a Saab or a Ford. We were illustrating a simple principle. That principle will hold true for the Apoc or any other rebreather tested in the same way. It is entirely predictable from simple physics and physiology. When the tidal volume is low enough the CO2 in the end tidal alveolar gas will be diluted to some extent by mixing with mouthpiece and anatomical dead space gas in the exhale hose of a rebreather. There is no point in arguing about it so stop trying. The most important issue is how small does the tidal volume have to be for the inaccuracy in direct measurement of CO2 at the end of hose to become a concern. We address this in the paper (actually in reasonably favourable terms for the Apoc). A secondary issue is whether an adequate compensating algorithm can be / has been developed for the Apoc.

You said it needed to be independently tested, I simply pointed out it already had been to what appears to be a very rigorous level.

This is rubbish. The rigorous testing is no use if they don't do the right test. A patient has syphilis. I order a test for every disease in the world except syphilis. I can rightly claim that the patient has undergone rigorous testing, except that I will miss the actual diagnosis because I didn't do the right test.

Properly calibrated analysers would be a yes as they include calibration details for their mass spec in their reports, pg 19 in the below one.
Don't 'know' on rest..., but from the following I would guess yes.

As report on Fault_Study_CO2_Bypass_110105
- Breath by breath measurement of CO2 was done
- They used zero end of inhale CO2 at the mouth to confirm that the flapper fault wasn't present.
- In EN 14143:2003 it stipulates that the Volume Weighted Average CO2 be measured during the scrubber endurance test “at the mouth”. So ALL testing of the iCCR would mean that they are measuring the CO2 simultaneously at the mouth and pod...
- The Notified Body required the VWAI CO2 measurements to be taken at each point of the dive, including at the start of the dive, to ensure the end-of-inhale contained zero CO2.
- Apparently measuring end-of-inhale CO2 prior to the inhale one-way valve gives false scrubber endurance and VWAI CO2 results in many rebreather designs when this fault condition occurs.
- They advise that in practice the low tidal volume alarm is not triggered, because if the diver does not metabolise enough oxygen, then the PPO2 in the rebreather will rise in all Deep Life O.R. designs (all use orifices to control flow), causing an oxygen warning, then alarm: this occurs well before the tidal volume drops anywhere near 300ml. This high PPO2 occurs in the iCCR at a tidal volume of less than 1 litre per minute with a RR of 14. Which appears to indicate that for 1/3 of the range your worried about the diver may well be on OC.
- During manned dive testing, the CO2 alarms were triggered in the following circumstances:
1. Before starting a weld, the diver would hold his breath, causing the first exhale to have a high CO2 level, tripping the alarm. This false alarm source was removed by increasing the averaging period.
2. Divers talking for long periods using intercoms, would reduce their tidal volume, which increased the calculated end of tidal CO2.
3. Some flapper valve combination's caused the CO2 warning to trigger.
- The gas fraction was measured in the following seven points of the breathing loop. 1. Mouth (MOUTH)
2. Exhale channel immediately downstream of the exhale one-way valve (EXH) 3. Centre of the exhale counterlung (EXH_CL)
4. Inside the base board bell membrane immediately prior to the scrubber (SCRB in)
5. Scrubber output tube (SCRB out)
6. Centre of Inhale counterlung (INH_CL)
7. Inhale channel immediately upstream of the inhale one-way valve (INH)
- Errors in the phase delay compensation would cause fundamental errors in determining Volume Weighted Average Inspired (VWAI) CO2, and in assessing the Deep Life CO2 sensor: the CO2 Lissajou figure would be incorrect. To eliminate this source of error, the phase delay at each measurement point was calibrated.
- The Deep Life CO2 monitor measures the same average CO2 as that measured by the mass spectrometer just upstream of the scrubber, but displays 7.0 % SEV when the consumption of 1.78lpm is simulated: this is correct. The actual RMV is 40lpm, with a 20 bpm RR, but the oxygen metabolised is 1.78lpm STPD at 1.6lpm, so the calculation used by the CO2 monitor concludes the RMV is 49lpm, and the tidal volume is 2.45 litres. Applying these numbers to the formula described for determining the peak CO2 in the previous section of this report, results in 7.0 % SEV of CO2, and the actual number measured using the mass spectrometer is 6.9% SEV.

I think what you really meant was "no".

Why, you apparently have a lab with mass spec and capability to conduct testing of CO2 sensors in rebreathers designed for underwater use to a suitable standard proving that the DL design doesn't work without even testing it!

Brad, another categorical question (although history tells me such questions are a waste of time with you). Have you actually read the paper? Yes? No? I suspect no because virtually every component of this statement is wrong. For your information: We did not use mass spec; we did not test a sensor (we tested a monitoring configuration); we did not test to any standard (we simply illustrated a physiological principle); we did not say that the DL design doesn't work; and we did test the DL design, but just not using the actual DL components. I would be very happy to send you the paper so you can put yourself out of your misery.

Out of curiosity, did you notice the same flapper valve fault that DL have reported in your testing?

No

What depth did you do the testing for your paper too?

Read the paper. Depth is fundamentally irrelevant to the principle we were trying to illustrate, though it would be interesting to perform the same experiments at depth.

Simply out of curiosity what is your test setup and how does it compare with what DL have published they used?

Read the paper.

What would your testing give DL considering the only controversy seems to be generated by you et al?

How about relevant and credible results. Look Brad, if they are convinced it works, then fine. Release it.

What has the response been from the committees that draft the CE and NORSOK standards as well as the likes of NEDU to this information?

I don't know about CE and NORSOK. What is your point? The paper was only published a month or so ago. If it ever has impact with these bodies I doubt that it will happen overnight. I am not at liberty to quote my colleagues at NEDU on line.

Rather then saying, interesting wonder how they did it and ordering one to trial, you seem to be saying it can't be done, but it can be if I am consulted....

No Brad, I think you are mixing me up with someone else you know. Please feel free to use your encyclopaedic knowledge of internet forums and posts to find a single one where I claim to have the answer to this conundrum if consulted. I can define the difficulties for you, but I don't have the answers. There is a chap called Arne Sieber who also publishes in the scientific literature (as opposed to unreviewed self published pdfs) who is creeping closer to the breakthrough. If you want the solution, watch his progress; don't consult me.

Simon, I still have yet to see any testing proving that the DL engineering doesn't work as intended.

No, of course you don't because for all intents and purposes it still does not exist. Not in the wider world of independent researchers who have original concerns like mine. And just for the information of forum readers, both Brad and DL have promised me access to a unit to run the relevant tests. This has never materialised.

Am still sourcing a copy of your complete paper

I will send you one.

though I understand from your talk, what you have pointed out is that end of tidal CO2 is not related to the mean CO2, when one does not measure tidal volume.

No, what we have shown is that end tidal CO2 measured at the mouth (which is the true value) may be higher than end tidal CO2 measured at the end of the exhale hose (where the Apoc pod is) because of dilution of the exhaled alveolar gas when it mixes with anatomical and mouthpiece deadspace gas in the exhale hose. Predictably, this effect is most pronounced at low tidal volumes. You believe that DL have a measurement algorithm that compensates for this potential problem, but I have yet to see any evidence that it works. The only way to demonstrate this adequately is to perform the simple experiment outlined in my previous post. It would seem this has not been done.

Simon M

 

[Brad_Horn]

G'Day Simon,
I owe you an apology with my comments regarding your paper, it is very neutral.

It would appear to be the takeaway message from your talk that twists it. See comments towards the end of this presentation about it 2010 NACD Cave Summit - Jill Heinerth (47 minutes) on Vimeo

That said, saying your paper is neutral, I don't think you would get any great difference of a result if you utilised just the Apoc DSV, exhale hose and CO2 pod. Your study only appears to be looking at part of the system that is used to give the CO2 readout on the Apoc.

You know perfectly well that at the time the debate about this CO2 monitoring configuration took place the Apoc was alledgedly about to be shipped. Dont' try to deny this; I can easily dig out the relevant posts from the RBW forums. So, are you saying that the information I was given then is no longer relevant? Put another way Brad, are you saying that the CO2 monitoring configuration / algorithm has been modified since that time when the unit was just about to be shipped?? What modification Brad? Why? On the basis of our debate perhaps? And if the unit has not been modified, then what is the point of raising the "2 years ago" issue?

To the best of my knowledge there has been no change to the CO2 monitoring side of the Apoc since it was launched, other then it going through a lot of detailed testing which you can read about just as well as I can.

What I do know is that there was a lot of information perhaps relevent to your pet topic that was left out of that RBW disclosure by DL.
I understand that DL started work on a CO2 monitor for a CCR 10 odd years ago. First inhale and more recently moving to exhale which is fitted to all their models.

I presume your aware that the commercial unit has the dual CO2 sensors, ontop of the dual scrubber which also features temp stiks, hence the widebore EAC design. Extrapolate your test to take the measurement from the top of your scrubber and DL still have this working accurately by all their reports and their client acceptance trials....

Brad, you can test whether a spark plug ignites petrol using either a Saab or a Ford. We were illustrating a simple principle. That principle will hold true for the Apoc or any other rebreather tested in the same way. It is entirely predictable from simple physics and physiology. When the tidal volume is low enough the CO2 in the end tidal alveolar gas will be diluted to some extent by mixing with mouthpiece and anatomical dead space gas in the exhale hose of a rebreather.

Put in those terms, no arguement.
What I am saying, is there is more then what you have tested at play. If it was as simple to measure end-tial CO2 as put a Co2 sensor in the loop, I would have thought this would have been done. What your study misses, is the engineering, be it mechanical or coding to give the end-tidal CO2 measurement that your after. This missing link wasn't published in the RBW posts IIRC.

I can't prove that the Apoc does measure to the level your after, as I obviously have not lab tested it myself. I can based on having dived it and seen it do what I expect when others have dived it look at the DL reports and accept them as valid until proven otherwise.

The most important issue is how small does the tidal volume have to be for the inaccuracy in direct measurement of CO2 at the end of hose to become a concern. We address this in the paper (actually in reasonably favourable terms for the Apoc). A secondary issue is whether an adequate compensating algorithm can be / has been developed for the Apoc.

How small does it have to be?
You would appear to be looking at just the CO2 end tidal aspect in isolation. Based on what I have read from their reports DL have looked at the dynamic situation. If the diver can not safely be on the loop at the time, if the tidal volume is that low, does it really matter other then from an academic position?
The compensating algorithm appears to be openly published, is it adequate?

This is rubbish. The rigorous testing is no use if they don't do the right test.

So why is this NOT then currently in the tests and/or standards if it is so important?

I think what you really meant was "no".

I could only point you to where the information is published that answered your question with a yes.

Read the paper. Depth is fundamentally irrelevant to the principle we were trying to illustrate, though it would be interesting to perform the same experiments at depth.

Copy depth is irrelevant to the principle, as you have said its a matter of physics etc

On one hand I am reading reports about a company testing their product at 0-100+m and it working in all respects, independently audited by muliple notified bodies with measurement by the gold standard of a calibrated mass spec.
Understand the next stage is the audits to get CE certification to 350m using the same equipment, CO2 monitor and rebreathers.

On the other hand your et al paper, sole focus is the surface component. It doesn't take into account the actual method used to get the CO2 %SEV as displayed on the pod or dive supervisers panel. Sure you have stated the priciple which is as I understand a known. Surely to scientifically criticise a method used, you have to replicate that method?

I don't know about CE and NORSOK. What is your point? The paper was only published a month or so ago. If it ever has impact with these bodies I doubt that it will happen overnight. I am not at liberty to quote my colleagues at NEDU on line.

If this is important for dive safety, then I would expect it to be a factor in the applicable standards and or testing in the future!

There is a chap called Arne Sieber who also publishes in the scientific literature (as opposed to unreviewed self published pdfs) who is creeping closer to the breakthrough. If you want the solution, watch his progress; don't consult me.

Will be a very interesting development to watch when it is shown working in a helium atmosphere at a PPO2 of greater then 1.0. At a cost of Euro700k or so, wonder who will be funding the development to put that tech in rec rebreathers!

And just for the information of forum readers, both Brad and DL have promised me access to a unit to run the relevant tests. This has never materialised.

Also for disclosure for forum readers I was directed to not assist with physical access to a unit... I did wonder why at the time!

No, what we have shown is that end tidal CO2 measured at the mouth (which is the true value) may be higher than end tidal CO2 measured at the end of the exhale hose (where the Apoc pod is) because of dilution of the exhaled alveolar gas when it mixes with anatomical and mouthpiece deadspace gas in the exhale hose. Predictably, this effect is most pronounced at low tidal volumes. You believe that DL have a measurement algorithm that compensates for this potential problem, but I have yet to see any evidence that it works.

The DL report on flapper valve failure would have to have been audited by the notified body before granting CE. To audit it, as I understand it they need to physically see the testing advised in that report. You might not have seen any evidence that it works, though it appears others have!

http://www.deeplife.co.uk/or_files/Fault_Study_CO2_Bypass_110105.pdf
We have the report stating that the mass spec reading basically matched the pod display 7.0 vs 6.9 at a tidal volume so this appears to meet your first requirement that the pod is accurate.
We have that due to high PPO2 in the loop from the constant O2 flow, your bailed out at a tidal volume of less than 1 litre per minute with a respiratory rate of 14. So once again the accuracy of the end-tidal CO2 at low tidal volumes may be academic, if the diver isn't on the loop for other reasons!

The only way to demonstrate this adequately is to perform the simple experiment outlined in my previous post. It would seem this has not been done. Simon M

It will be very interesting, when you do test an Apoc exactly when you hit the boundary limits and it bails you off!
Will also be interesting seeing your trial conducted in a wet chamber to 100m.

Kind regards
Brad