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

[Tassi Devil Diver]

^ I have had my rEvo nearly 10 years in the first three years when it was under warranty, I had 13 temp probes replaced. Since around 2018 I have had no issues with temp probes, I was on the verge of giving up on the RMS having replaced a few more at my own expense.

I have not heard of or experienced the "pop out of probe" failure mode.

 

[broncobowsher]

^ I have had my rEvo nearly 10 years in the first three years when it was under warranty, I had 13 temp probes replaced. Since around 2018 I have had no issues with temp probes, I was on the verge of giving up on the RMS having replaced a few more at my own expense.

I have not heard of or experienced the "pop out of probe" failure mode.

Stories like that are the exact reason I did NOT get RMS with I got my rEvo back in 2018. Nobody knew that they finally fixed the RMS probe problems. If I knew they really were fixed, I probably would have got it. But years of horror stories of repeated failures was enough for me to spend the money on a NERD instead.

 

[beester]

Hi Boriss, thanks for putting the effort in visualising this. It makes it picture clear and helps alot! So far when evaluating reusing a scrubber I've only looked at total scrubber time and of course depth/temp (which of course impacts bandwith or as Dr. Mitchell calls it 'dynamic absorption capacity'). I never really took into consideration workload, so when we planned this dive the scrubber planner felt safe because it had worked in similar profiles/depths/runtime/water temp, so why not now. Maybe there was also some normalization of deviance at work here on my part.

Well very clearly because I was unknowingly on the 2nd dive in the deep part already close to the scrubber limit and added workload and breathing pushed it (luckily temporarily) over the threshold.

Dr Mitchell, noted to of course NOT take the results of your study as a hard figure to use in calculating scrubber time... too many variables at play ;-)

The above has clarified a lot. In the beginning, when I was getting pummeled left and right on my "obvious mistakes" I felt that sharing this experience might have been a bad idea but looking at the full post I'm glad I did and maybe someone may learn from this as well.

On my part I've shared the story but also the full topic here on scubaboard with a good friend of mine who is a high level GUE instructor and currently writing an operating procedure on ghostfishing (net cleaning) for GUE rebreather divers. The incident but mainly the responses here from everybody involved might prove very useful!

Thanks everybody for sharing and reading!

 

[Brad_Horn]

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.

Beester, to expand on a point that Simon flags. Rebreather manufacturers have a pretty in depth knowledge, about their scrubbers unique performance envelope, from their R&D engineering process. This comes from the mass of reporting generated, when they conduct thorough unmanned testing of their rebreather, at the full swathe of different gases, depths, temps and workloads that it's designed to operate within the performance envelope of. Done as part of a formal Test & Evaluation process to both Validate and Verify the design before any manned test diving and then retail is even a consideration.

A great historical example of this is Peter Ready, mentioning that he only found out that his original PRISM scrubber performance was sub-par when he had it tested in cold water, after originally only conducting warm water testing. Which necessitated him going back to the drawing board.

There is nothing stopping this rather useful scrubber duration data at various tempts, depths and workloads, being publicly releasable and included in the units user manual to enable safer pre-dive planning. I'm sure a simple request to JJ would be quite enlightening. And I'm sure Simon has done the same for his unit.

As example, if you refer to pages 112-114 of https://www.opensafety.eu/manuals/OR_Apocalypse_User_Manual_110505.pdf you can see that all other criteria remaining unchanged for a 90m dive, simply going from a workload of 22lpm to 40lpm RMV - as you possibly did - halves the available scrubber duration and would have a considerable impost on your pre-dive planning if diving that particular rebreather.

In essence, what you experienced working removing fishing nets at depth, is no different to what you would already have planned for, in covering loss of your scooter at depth, and potential need to suddenly fin hard.

Note that while the Open Safety SRB uses a 2.2kg Micropore EAC, that equates to 2.6kg of granular sorb, its testing is also conducted at the mouth as opposed to the inhale breathing hose. As this takes into account the deadspace of the BOV it articulates a lesser duration than other rebreathers testing but was required of Open Safety for CE certification to EN14143. And it probably doesn't need to be said that the scrubber duration results for one rebreather aren't transferable to another make; even if they use the same weight of sorb.

As an aside, figures 5-7, 5-8 and 5-9 in the below link show the scrubber exceeding 0.5%SEV breakthrough, shortly into the dive and then backing off, before going back through 0.5%SEV a second time. AFAIK this is unique to a Micropore EAC but it does potentially visually show what you experienced across your two dives.

https://www.opensafetyglobal.com/Safety_files/DV_OR_ScrubberEndurance_Retest_SRB_101215.pdf

 

[Dr Simon Mitchell]

Hello again Beester,

Here is another illustration of the difference in duration in the same scrubbers due to differences exercise patterns during a dive, this time showing the break through curves.

In this study we were comparing granular absorbent with the extendair absorbent cartridges in the Optima rebreather which is designed to take either. It seemed an ecologically valid comparison on that basis, and the canister and granule weights were virtually identical (2.15 kg cartridge vs 2.09 kg granules). Five granular and five extendair canisters were run in each of the two simulated dive exercise patterns described previously. The first, simulated moderate exercise at 6 MET throughout the dive until breakthrough is more like the typical test protocol used to establish scrubber limits. The second, simulated moderate exercise for 90 min followed by relative 'rest' to emulate deco at 2MET until breakthrough would be more representative of a typical real world dive. The results are shown in the figure below.

You can see that the two canister types are not dramatically different in performance (extendair cartridge slightly better), but there is a huge difference in duration between the two exercise patterns. Another illustration of how real world dives could be different to testing protocols, but I repeat that using a canister over a long dive in this way is a calculated risk. For example, in the simulated resting deco phase somewhere around the 200 minute mark, if we had increased the ventilation and CO2 addition back to 6 MET , there would probably have been breakthrough straight away. So, you don't want to have to exercise hard unexpectedly during a resting deco that has extended beyond the recommended scrubber limit.

The full paper [1] describing this experiment is available on Pubmed Central.

Simon

1. Gant N, van Waart H, Ashworth ET, Mesley P, Mitchell SJ. Performance of cartridge and granular carbon dioxide absorbents in a closed-circuit diving rebreather. Diving Hyperb Med. 2019;49(4):298-303.

 

[RedSix]

I really like this very productive discussion here! I really like the very scientific approach and especially the picture of @boriss!
I would like to point on something here:
Before the dive, the scrubber has been used for 130min. 15 to 20 min into the next dive (with parts of it with high workload) @beester experiences problems with CO2.
But: I think we all agree, that beester wasn't so high in working, that he got close to the simulated workload during the factory tests of the JJ. And when beester had his issues with the CO2 he was something like 150min of scrubber usage, so still within the 180min of usage JJ at maximum recommends. And for sure this 150min of usage before weren't as high as the factory tests simulated.
So actually I am not sure, if this CO2 hit has been (partly) caused by the scrubber. When the hit occurred, beester was still within the manufacturer specification.

 

[cathal]

^ I have had my rEvo nearly 10 years in the first three years when it was under warranty, I had 13 temp probes replaced. Since around 2018 I have had no issues with temp probes, I was on the verge of giving up on the RMS having replaced a few more at my own expense.

I have not heard of or experienced the "pop out of probe" failure mode.

I’ve had mine 10 years as well and I would estimate a similar RMS probe failure rate but with the difference being that I experienced failures of the probes up until recently. I then made an adjustment to the RMS and it has never given me any issues since - I turned it off completely.
Also maybe it’s just my bad luck but I experienced the top nut off the probe popping off on at least three occasions. The seriousness of this failure mode was yet another reason to get rid of the RMS. Now I just use an M8 stainless steel bolt which unlike the RMS probes, cannot be over tightened.

 

[inquis]

when beester had his issues with the CO2 he was something like 150min of scrubber usage, so still within the 180min of usage JJ at maximum recommends. And for sure this 150min of usage before weren't as high as the factory tests simulated.

Wasn't there a difference in depth or temperature for the 180 min spec compared to the actual dive?

 

[RedSix]

Wasn't there a difference in depth or temperature for the 180 min spec compared to the actual dive?

Yes, that is correct. For the 100m dive it something like 20min @ 100m, decompression stops starting roughly 40 m till 6m, from minute 65 on on 6m till 230minutes. Breakthrough starting at 170 to 180min.
For the 40m dive it is something like 40 minutes at 40m, 5min @ 15m, and then 9m till minute 230. The scrubbers testet, breakthrough also startin arround 170 to 180min. There is a third test at 6m only, but with two phases of elevated breathing rate (5min at 75l/min, 3.33l/min CO2 injection rate, each), rest 40l/min with 1.6 l/min CO2 injection rate. Breakthrough startin at something like 170 to 180min.

 

[justinthedeeps]

Thanks for giving this account & analysis of the phenomenon of "overbreathing" a partially used scrubber. It seems like it could be lethal under much worse circumstances like intense currents, entrapments, surf zones etc

A notion that I find convenient is that any scrubber will scrub "100%" of your CO2 up to a certain threshold (or rate), but "0%" of everything over that--though it's probably a more curvy math & transition.

For a partially used scrubber that threshold is presumably lower, but still above the basal metabolic rate. That explains why many of these large scrubbers can work up to 6+ hours, even in cold water, for a relaxed diver encountering no exertion.

I've experienced the same(?) phenomenon with volatile organic compound (VOC) filters when painting boat hulls. An old cartridge set can seem to be working just fine at first for low levels, but bam, all the extra toxic fumes come right through as soon as the concentration runs over a threshold. Maybe I'll carry a bailout! (And put on a fresh set of ~$10 scrubbers...)