CO2 build up during 65m (210 ft) dive.

[beester]

Isn't there a third reason to bail out -- Super high PPO2?

thanks for sharing.

Slightly off topic. But in my opinion while a super high PPO2 is certainly cause for immediate action (specially if you didn't see it coming), it's not a reason to go immediately off the loop and on bailout. Depending on the cause (and the troubleshooting) you have time to still remedy the situation. A sudden spike in PPO2 is not going to immediately give you an hyperoxic seizure and you can quickly drop the PPO2 by doing a diluent flush, which will buy you time to then troubleshoot the issue (checking if it's a stuck solenoid, controller issue, mav stuck, etc, etc, etc)

A hypoxic incident (super low PPO2) on the other hand is for me cause for an immediate bailout, before I do anything else. Because I could be seconds from blacking out.

Others obviously may have different opinions.

 

[Dr Simon Mitchell]

Hello Beester,

Thanks for sharing your experience. These discussions are extremely valuable in that they focus thought on some very important safety issues.

I agree with Brad and marsh9077 that the most likely cause of your CO2 problem was scrubber break though and inhaled CO2 on the basis of a hard working dive at moderate depth using a partly exhausted scrubber.

I think those of us involved in moderately deep and long dives would all be prepared to admit that we have exceeded manufacturer-prescribed scrubber durations on a fairly regular basis, in part because we don't have much choice in order to do the dives. The reason we don't get into trouble is that those prescribed durations are based on fairly aggressive testing protocols which, as you point out, represent exercise and ventilation levels that are not sustained throughout a typical deco dive (in which there is a period of moderate exercise early followed by a period of relative rest during deco). Therefore, a typical dive can likely be longer from a scrubber point of view than suggested by tests used to develop recommended limits.

I can provide some objective evidence of this from one of our recent publications looking at temp stick function. We did two sets of simulated dives at surface or minimum pressure (3 or 6m) using two dive protocols. One aimed to replicate a rebreather test regimen with ventilation at 45 L/min and CO2 addition of 2L/min (approximately equal to 6 MET work) throughout the 'dive'; and the other aimed to replicate a more typical dive with those latter 6 MET parameters for 90 minutes (to simulate work at the bottom), followed by 2 MET parameters (ventilation 16.5 L/min and CO2 0.67 L/min) to simulate resting deco. The end-point in both cases was break through to an inhaled CO2 of 1 kPa. We used an inspiration rebreather which has an identical scrubber size (2.6 kg) to your JJ.

The following is the result for 8 trials at the constant 6 MET exercise protocol. You are probably not interested in the temp stick changes so ignore all the colours, but the top of each bar represents the time to break through to 1 kPa of inspired CO2 (around 180 - 190 minutes in all cases).

Contrast that with the graph below which is the result for 5 trials using the protocol to simulate a more realistic dive (6 MET work at depth, 2 MET during deco). You can see that break through (again, to exactly the same end point of 1 kPa) occurred at 400 to 500 minutes.

So, that is why we get away with exceeding the manufacturers recommended scrubber limits. This is something that rebreather divers doing long dives have to make a considered risk vs benefit decision about. It is a calculated risk (which can be partly mitigated by carrying plenty of bailout gas). It works so long as the long tail of the dive really is at rest. What you DONT want to do is exercise hard again late in the dive when your scrubber is substantially used. Again, as Brad and Marsh have suggested, to at least some extent, that is what you did by using it again after a 130 minute dive the previous day. If that entire second dive had been at rest it probably would have been OK. For 65m dives involving work I would definitely start each dive with a new scrubber.

The full paper [1] describing the above experiments can be found here.

Your gas density was about 4.4 g/L. This does not rule out CO2 retention because of increased work of breathing, but it is certainly within the recommended limits.

Simon M.

1. Silvanius M, Mitchell SJ, Pollock NW, Frånberg O, Gennser M, Lindén J, Mesley P, Gant N. The performance of 'temperature stick' carbon dioxide absorbent monitors in diving rebreathers. Diving Hyperb Med. 2019 Mar 31;49(1):48-56.

 

[GF99/99]

Hello Beester,

Thanks for sharing your experience. These discussions are extremely valuable in that they focus thought on some very important safety issues.

I agree with Brad and marsh9077 that the most likely cause of your CO2 problem was scrubber break though and inhaled CO2 on the basis of a hard working dive at moderate depth using a partly exhausted scrubber.

I think those of us involved in moderately deep and long dives would all be prepared to admit that we have exceeded manufacturer-prescribed scrubber durations on a fairly regular basis, in part because we don't have much choice in order to do the dives. The reason we don't get into trouble is that those prescribed durations are based on fairly aggressive testing protocols which, as you point out, represent exercise and ventilation levels that are not sustained throughout a typical deco dive (in which there is a period of moderate exercise early followed by a period of relative rest during deco). Therefore, a typical dive can likely be longer from a scrubber point of view than suggested by tests used to develop recommended limits.

I can provide some objective evidence of this from one of our recent publications looking at temp stick function. We did two sets of simulated dives at surface or minimum pressure (3 or 6m) using two dive protocols. One aimed to replicate a rebreather test regimen with ventilation at 45 L/min and CO2 addition of 2L/min (approximately equal to 6 MET work) throughout the 'dive'; and the other aimed to replicate a more typical dive with those latter 6 MET parameters for 90 minutes (to simulate work at the bottom), followed by 2 MET parameters (ventilation 16.5 L/min and CO2 0.67 L/min) to simulate resting deco. The end-point in both cases was break through to an inhaled CO2 of 1 kPa. We used an inspiration rebreather which has an identical scrubber size (2.6 kg) to your JJ.

The following is the result for 8 trials at the constant 6 MET exercise protocol. You are probably not interested in the temp stick changes so ignore all the colours, but the top of each bar represents the time to break through to 1 kPa of inspired CO2 (around 180 - 190 minutes in all cases).

View attachment 734234
Contrast that with the graph below which is the result for 5 trials using the protocol to simulate a more realistic dive (6 MET work at depth, 2 MET during deco). You can see that break through (again, to exactly the same end point of 1 kPa) occurred at 400 to 500 minutes.

View attachment 734237
So, that is why we get away with exceeding the manufacturers recommended scrubber limits. This is something that rebreather divers doing long dives have to make a considered risk vs benefit decision about. It is a calculated risk (which can be partly mitigated by carrying plenty of bailout gas). It works so long as the long tail of the dive really is at rest. What you DONT want to do is exercise hard again late in the dive when your scrubber is substantially used. Again, as Brad and Marsh have suggested, to at least some extent, that is what you did by using it again after a 130 minute dive the previous day. If that entire second dive had been at rest it probably would have been OK. For 65m dives involving work I would definitely start each dive with a new scrubber.

The full paper [1] describing the above experiments can be found here.

Your gas density was about 4.4 g/L. This does not rule out CO2 retention because of increased work of breathing, but it is certainly within the recommended limits.

Simon M.

1. Silvanius M, Mitchell SJ, Pollock NW, Frånberg O, Gennser M, Lindén J, Mesley P, Gant N. The performance of 'temperature stick' carbon dioxide absorbent monitors in diving rebreathers. Diving Hyperb Med. 2019 Mar 31;49(1):48-56.

@Dr Simon Mitchell Thanks for sharing this article, very very interesting! I actually had not read this one yet. Much appreciated for sharing with the community.

 

[beester]

Hello Beester,

Thanks for sharing your experience. These discussions are extremely valuable in that they focus thought on some very important safety issues.

I agree with Brad and marsh9077 that the most likely cause of your CO2 problem was scrubber break though and inhaled CO2 on the basis of a hard working dive at moderate depth using a partly exhausted scrubber.

I think those of us involved in moderately deep and long dives would all be prepared to admit that we have exceeded manufacturer-prescribed scrubber durations on a fairly regular basis, in part because we don't have much choice in order to do the dives. The reason we don't get into trouble is that those prescribed durations are based on fairly aggressive testing protocols which, as you point out, represent exercise and ventilation levels that are not sustained throughout a typical deco dive (in which there is a period of moderate exercise early followed by a period of relative rest during deco). Therefore, a typical dive can likely be longer from a scrubber point of view than suggested by tests used to develop recommended limits.

I can provide some objective evidence of this from one of our recent publications looking at temp stick function. We did two sets of simulated dives at surface or minimum pressure (3 or 6m) using two dive protocols. One aimed to replicate a rebreather test regimen with ventilation at 45 L/min and CO2 addition of 2L/min (approximately equal to 6 MET work) throughout the 'dive'; and the other aimed to replicate a more typical dive with those latter 6 MET parameters for 90 minutes (to simulate work at the bottom), followed by 2 MET parameters (ventilation 16.5 L/min and CO2 0.67 L/min) to simulate resting deco. The end-point in both cases was break through to an inhaled CO2 of 1 kPa. We used an inspiration rebreather which has an identical scrubber size (2.6 kg) to your JJ.

The following is the result for 8 trials at the constant 6 MET exercise protocol. You are probably not interested in the temp stick changes so ignore all the colours, but the top of each bar represents the time to break through to 1 kPa of inspired CO2 (around 180 - 190 minutes in all cases).

View attachment 734234
Contrast that with the graph below which is the result for 5 trials using the protocol to simulate a more realistic dive (6 MET work at depth, 2 MET during deco). You can see that break through (again, to exactly the same end point of 1 kPa) occurred at 400 to 500 minutes.

View attachment 734237
So, that is why we get away with exceeding the manufacturers recommended scrubber limits. This is something that rebreather divers doing long dives have to make a considered risk vs benefit decision about. It is a calculated risk (which can be partly mitigated by carrying plenty of bailout gas). It works so long as the long tail of the dive really is at rest. What you DONT want to do is exercise hard again late in the dive when your scrubber is substantially used. Again, as Brad and Marsh have suggested, to at least some extent, that is what you did by using it again after a 130 minute dive the previous day. If that entire second dive had been at rest it probably would have been OK. For 65m dives involving work I would definitely start each dive with a new scrubber.

The full paper [1] describing the above experiments can be found here.

Your gas density was about 4.4 g/L. This does not rule out CO2 retention because of increased work of breathing, but it is certainly within the recommended limits.

Simon M.

1. Silvanius M, Mitchell SJ, Pollock NW, Frånberg O, Gennser M, Lindén J, Mesley P, Gant N. The performance of 'temperature stick' carbon dioxide absorbent monitors in diving rebreathers. Diving Hyperb Med. 2019 Mar 31;49(1):48-56.

Dr Mitchell,

This is very very useful information. Is the publication public, can it be shared?

Thank you for sharing your thoughts! During this study was there also a temperature variable or was the temperature kept constant (and what temperature)? I understand that's probably not very relevant for a study on the performance of temperature stick monitors but in the context of this dive it might be ;-)

Can I ask another question? I've always presumed that scrubber breakthrough is final. Meaning, once breakthrough happens your scrubber is basically done and cannot revert back to a state where it will be removing CO2 at optimal levels. However during this dive after the dil flush (which removed a lot of CO2 in the loop) and slowing the workrate and breathing (CO2 production) it seemed the scrubber was again able to efficiently remove CO2 when the CO production rate was lower.

Looking back, the type of diving this year which involved dual use of scrubber in repetitive dives was always the same profile (50-70m range up to 50' bottom time), 2nd dive on the next day, temperature range always the same. 16 dives were done like this by me (but there were multiple teams in the water running similar profiles) and the last one is the one we are discussing now. Anything deeper than this ment we renewed the scrubber for every dive. I can see also why during this dive nobody ever had any CO2 issues, because with the exception of the last dive, the worklevel on those dives was kept low (no real tasks except documenting), always using scooters, etc.

Then the last dive happens and on a half used scrubber I'm cutting through 2 inch rope cables and towing them and attached netting away. Hence high breathing rate, lower temperature, depth, used scrubber... and there you go!

As I already stated I'll share this info and will adjust my procedures regarding scrubber use.

Thanks again for sharing.

 

[Heat Miser]

Slightly off topic. But in my opinion while a super high PPO2 is certainly cause for immediate action (specially if you didn't see it coming), it's not a reason to go immediately off the loop and on bailout. Depending on the cause (and the troubleshooting) you have time to still remedy the situation. A sudden spike in PPO2 is not going to immediately give you an hyperoxic seizure and you can quickly drop the PPO2 by doing a diluent flush, which will buy you time to then troubleshoot the issue (checking if it's a stuck solenoid, controller issue, mav stuck, etc, etc, etc)

A hypoxic incident (super low PPO2) on the other hand is for me cause for an immediate bailout, before I do anything else. Because I could be seconds from blacking out.

Others obviously may have different opinions.

I’m on a rEvo so it’s different and the RMS (temp sticks) help a lot with original problem.

But on the rEvo high pp02 bail out, (ie really high) hypercapnia dil flush then bail, hypoxia quick dil flush and bail.

 

[cathal]

I’m on a rEvo so it’s different and the RMS (temp sticks) help a lot with original problem - When they work

 

[Heat Miser]

@cathal - When they work


I haven't had problems with my temp sticks but they are fairly new - 2019 model. I understand when they first came out there were problems and people had to replace them fully with the better version of the tempstick.

I was recently recommended to clean the scale off with CLR (an Australian Calcium Lime Rust remover) and they came up sparkling.

The other tip I would recommend is when doing the build, just before you close the lid just flick the controller to Scrubber>Temperature to make certain they are measuring the temperature right. Sometimes they aren't put in straight or the they don't sit right in the lung (particularly the inhale scrubber). No temperature - they aren't working and you can adjust the scrubber, to make certain they are seated properly.

 

[boriss]

I'm still new and learning, but hopefully this adds to the conversation. If nothing else, I'll be wrong and learn from the experts. I will be using colloquial terms, as I'm not a scientist in this field.... and what I'm saying could be completely ignorant as well.

I've read several papers about CO2 absorption, channelling, performance, etc. Most based on CFD, but a couple that are based actual testing (including a few of Dr. Mitchell's ). Here's an example of one such paper, which I'll use to present my point: https://www.cit.ie/contentfiles/seanf/Carbon Dioxide Absorption and Channelling in Closed Circuit Rebreathers Summary.pdf

If my understanding is correct, then we need to consider how we look at the scrubber usage/performance in terms of total CO2 absorption as well as the bandwidth of absorption, or absorption at a particular point in time (the green line below).

So what do I mean by this? When the scrubber is fresh the bandwidth is high... because the first "layer" of sorb will absorb the CO2, but if work goes up then the next layer will pick up the "slack" and so on. So we can get away with a higher work-rate (CO2 production) when the scrubber is freshly packed. However... as we use up the sorb we have fewer and fewer "layers" effectively being used and so the chance of CO2 breakthrough is much higher as time goes on.

Here's the way I picture @beester scrubber bandwidth, based on what's described:

In other words, had this exercise been performed with new sorb the probability of CO2 breakthrough would be lower.... or there would be more active sorb to deal with the "problem."

So as long as my understanding is correct, then I think we need to look at/measure scrubber usage in the same way we look at multi-level dives vs. square profiles. Meaning... if the second dive isn't as deep, isn't as cold, won't have the same level of CO2 production, then reusing the scrubber poses little risk to CO2 breakthrough. In this case, it seemed that after 130' of usage the available bandwidth of the scrubber, was compromised given the planned dive (temp/depth/workload/etc).

Seems like a rather simple course of action is to evaluate the planned dive relative to the bandwidth (vs. minutes remaining) and repacking or reusing the scrubber accordingly. OP came to that conclusion on his own, but I must say I'm thankful for the discussion (and vulnerability for posting this stuff), @beester as it's a great opportunity for others to learn.

Thanks and if I'm wrong anywhere, please correct me.

 

[Dr Simon Mitchell]

Dr Mitchell,

This is very very useful information. Is the publication public, can it be shared?

Hello Beester,

Yes, the fully paper is freely available at the link I provided in the previous post. Anyone can download it. The link takes you to the PMC webpage with an XML version of the paper, and there is a tab 'pdf' on the upper right where you can download a pdf.

Thank you for sharing your thoughts! During this study was there also a temperature variable or was the temperature kept constant (and what temperature)? I understand that's probably not very relevant for a study on the performance of temperature stick monitors but in the context of this dive it might be ;-)

The rebreathers were immersed in water at 19oC and this was kept constant. Yes, it is true that the numbers could be different in colder (or warmer) water depending on the degree to which the scrubber canister is insulated from the water. I suppose that segues into another important point that I should have made in my previous post: the duration numbers in that paper should not be interpreted as 'guidelines' for different types of dives. The data illustrate an important point, but the durations should not be interpreted as guidelines.

Can I ask another question? I've always presumed that scrubber breakthrough is final. Meaning, once breakthrough happens your scrubber is basically done and cannot revert back to a state where it will be removing CO2 at optimal levels. However during this dive after the dil flush (which removed a lot of CO2 in the loop) and slowing the workrate and breathing (CO2 production) it seemed the scrubber was again able to efficiently remove CO2 when the CO production rate was lower.

Yes, this is correct and fairly well summed up in @boriss' post. I refer to it as 'dynamic absorption capacity' rather than 'bandwidth' but the principle is the same. If you have used (for example) the bottom half of your scrubber, it now has only half its previous distance over which passing gas can have CO2 removed. If you now work hard, breathe harder, and thereby accelerate the flow rate of gas through the scrubber, there is a much higher probability that some CO2 will make it to the downstream side of the scrubber without being removed. However, if you then rest, reduce your ventilation, thus slowing the rate at which gas is passing through the scrubber, it is once again able to remove all the CO2 before the gas passes downstream. Obviously the more of the scrubber that is exhausted, the less 'room to move' you have with ventilation rates before there is a problem. Eventually the scrubber will stop working even at very low flow rates. Hope that makes sense.

Looking back, the type of diving this year which involved dual use of scrubber in repetitive dives was always the same profile (50-70m range up to 50' bottom time), 2nd dive on the next day, temperature range always the same. 16 dives were done like this by me (but there were multiple teams in the water running similar profiles) and the last one is the one we are discussing now. Anything deeper than this ment we renewed the scrubber for every dive. I can see also why during this dive nobody ever had any CO2 issues, because with the exception of the last dive, the worklevel on those dives was kept low (no real tasks except documenting), always using scooters, etc.

Then the last dive happens and on a half used scrubber I'm cutting through 2 inch rope cables and towing them and attached netting away. Hence high breathing rate, lower temperature, depth, used scrubber... and there you go!

As I already stated I'll share this info and will adjust my procedures regarding scrubber use.

Thanks again for sharing.

Yep, there's your answer.

Simon

 

[cathal]

@cathal - When they work


I haven't had problems with my temp sticks but they are fairly new - 2019 model. I understand when they first came out there were problems and people had to replace them fully with the better version of the tempstick.

I was recently recommended to clean the scale off with CLR (an Australian Calcium Lime Rust remover) and they came up sparkling.

The other tip I would recommend is when doing the build, just before you close the lid just flick the controller to Scrubber>Temperature to make certain they are measuring the temperature right. Sometimes they aren't put in straight or the they don't sit right in the lung (particularly the inhale scrubber). No temperature - they aren't working and you can adjust the scrubber, to make certain they are seated properly.

My comment was based on my own experience of a lot of temp stick failures which turned out to be expensive to replace. One other failure mode to bear in mind is if you tighten the scrubber nut too tight they can pop out of the probe as they are only glued in place…happened to me on a couple of occasions.