How to tell how much of your scrubber you used after a dive

[LFMarm]

Yes, that was what was being discussed above. You can safely assume a 1:1 ratio of O2 consumed to CO2 generated and is also the reason that many of us use Bar gauges on our O2 bottles *that is if you actually use gauges on your O2....*, so if you have 2L bottles and start the dive at 200bar, then you have 400L of O2 on board and once you consume 120L of O2 then you can assume the scrubber is done. As I did say in post #18, you can track this way however it is slightly conservative given O2 flushes once you get to deco and any inefficiencies you have in your diving. Those inefficiencies are primarily tied to really lean diluent mixtures and/or sawtooth profiles where you are adding a lot of oxygen in manually to compensate for lean diluent. You can go down rabbit holes to try to optimize gas mixes and all that nonsense but I wouldn't personally bother trying to get that close and for "normal" conditions I use 1lb/hr and call it a day. Slightly less if I know I'm going to be huffing and puffing, and I'll stretch it a little bit if I'm doing really deep stuff where most of the time is spend essentially napping on decompression, but those are extenuating circumstances.

Makes a lot of sense. I will use these as guidelines and from time to time try to triangulate with O2 consumption (indeed measured in bar as all my gases) and depth of the scrubber front to spot if anything seems out of place.

 

[Tassi Devil Diver]

This why I love the RMS on my rEvo no rules of thumb, gestimates, educated guesses etc, after a dive I have a very good indication of my remaining scrubber time AND i can track it in real time during the dive, works so well.

A fairly comprehensive evaluation of the Revo RMS and Inspo temp sticks was done a few years ago, it was surprising how accurate they were in predicting CO2 breakthrough. The study can be downloaded at

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6526050/pdf/DHMJ-49-48.pdf

Conclusion: When operated at even shallow depth, temperature sticks provided timely warning of significant CO2 breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure.

The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another.

 

[LFMarm]

This why I love the RMS on my rEvo no rules of thumb, gestimates, educated guesses etc, after a dive I have a very good indication of my remaining scrubber time AND i can track it in real time during the dive, works so well.

A fairly comprehensive evaluation of the Revo RMS and Inspo temp sticks was done a few years ago, it was surprising how accurate they were in predicting CO2 breakthrough. The study can be downloaded at

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6526050/pdf/DHMJ-49-48.pdf

Conclusion: When operated at even shallow depth, temperature sticks provided timely warning of significant CO2 breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure.

The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another.

Great technology but with 25% error rate I would still work estimations on the side.

 

[Dr Simon Mitchell]

Great technology but with 25% error rate I would still work estimations on the side.

Hello LF Marm,

I'm not sure where you get the "25% error rate" from. Happy to discuss that.

That paper which Tassi Devil linked to is a big help in answering the question you have posed here and in another thread about scrubber duration. If you look at the 'low-ex" data in the temp stick study you can see that in experiments designed to mimic real world dives (where there is moderate exercise initially, then relative rest during decompression) the scrubbers were lasting well beyond the manufacturers recommended limits (which are based on high exercise testing in cold water). This reflects the experience of many real world divers who are more or less forced to exceed manufacturers limits because of the length of their dives, and where experience teaches them that this works. I don't disagree with those on this thread who have recommended sticking strictly to manufacturer limits - that's obvious a conservative and safe choice, but the pragmatic deep wreck diver in me knows that this is not always possible. Taking your unit beyond manufacturer scrubber limits is obviously a calculated risk, which the temp stick study suggests can be ameliorated by using a temp stick. What you would not want to do has exercise hard on a scrubber that is nearly exhausted.

Kinda thinking about the Dave shaw..

As others have pointed out, breakthrough was not necessarily the problem in the Dave Shaw event. You would probably find the attached analysis of that accident interesting.

Simon M

 

[LFMarm]

That paper which Tassi Devil linked to is a big help in answering the question you have posed here and in another thread about scrubber duration.

Thank for the response Simon. I will go and read the full article.

I'm not sure where you get the "25% error rate" from. Happy to discuss that.

25% is 2 scrubber depletion cases that were not avoided out of the 8 tests mentioned above. Most likely not statistically relevant but it looks to me there are situations where the temp stick may not be conservative.

That being said, I would definitely use a temp stick if it was available on my CCR but it is not. So I will need to go with manufacturer recommendation and do some triangulation to see how conservative that is for the type of diving I do.

1. If you didn’t have a temp stick, what is the best strategy to safely extend manufacturer scrubber duration in easier diving conditions (warmer, less effort)?
2. Would you suggest to monitor O2 consumption and assume a 1:1 ratio of O2 consumed to CO2 generated and compare that with the L of CO2 that the scrubber is rated to capture?

 

[Zack-Bloom]

Now, I've never heard of anyone doing this, mind you. But, theoretically, you could mix up all your Sofnolime, take out a small representative sample, heat it up in a sealed chamber, and measure (somehow) how much CO2 was released. That could give you an indication (at least on a relative scale) of how much you used the canister on the last dive.

 

[happy-diver]

Dude have you got any more JC videos

 

[flymolo]

Dude have you got any more JC videos

Haha

 

[rjack321]

Now, I've never heard of anyone doing this, mind you. But, theoretically, you could mix up all your Sofnolime, take out a small representative sample, heat it up in a sealed chamber, and measure (somehow) how much CO2 was released. That could give you an indication (at least on a relative scale) of how much you used the canister on the last dive.

Sadly you cannot. For all practical/useful CO2 measuring purposes, this is not a reversible reaction.

 

[Snusmumrik]

Now, I've never heard of anyone doing this, mind you. But, theoretically, you could mix up all your Sofnolime, take out a small representative sample, heat it up in a sealed chamber, and measure (somehow) how much CO2 was released. That could give you an indication (at least on a relative scale) of how much you used the canister on the last dive.

Such a brilliant idea! (sarcasm)
Theoretically, you could mix up all your Sofnolime from scrubber, take out a small
representative sample, weight it, soak it in any kind of acid (vinegar, citric acid, etc)
and somehow collect all gas bubbles to measure volume of CO2 and make some math
to determine overall absorbed CO2 volume per kg or lb (damn those imperial units).