Why depth has an effect on sorb capability to scrub CO2

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fsardone

Solo Diver
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Location
Rome, Italy
# of dives
I just don't log dives
Hello everybody,
I am starting this thread because it is obvious, from answer in another thread, that people, even experienced ones, ignore the effect depth has on the capability of sorb to scrub carbon dioxide. Since this has potential to hurt people I thought appropriate to say something since I am seeing something.

The topic has been discussed in rebreather forum here:
Why does depth matter for scrubber lifetime

And one of the post report a very good description in layman terms which was copied from the defunct divermole site and I am going to repost here.

What I am trying to achieve is raise awareness.

Happy reading
Fabio


Here on Zorg, we abducted some humans to test your resistance to CO2 and the efficiency of our patented CO2 grabbing demon chamber.
We took a human and connected a hose to them. The hose supplies gas and has one-way valves. The exit of the hose goes into a box. Inside this box are 1000s of little demons. These demons adore CO2. They will grab a passing molecule of CO2 and hang onto it for the rest of their lives. They can only hold one each. After the CO2 demon box there is another box with different demons inside - these count the number of O2 molecules you have used and replaces them.
We observed that humans when in a steady state consume the same amount of O2 per breath, regardless of the pressure we subjected them to. When given 100 molecules of our gas, they would use 4 molecules of our oxygen and turn this into 3 molecules of CO2 and 1 molecule of water vapour.
So in the test, with 100 molecules of gas in the loop. The human push/pulled this through the box with the CO2 demons in it. Every breath, 3 lucky demons grab a CO2 molecule each and are happy for the rest of their lives. We repeated this for many of your earth hours, pushing 100 molecules of gas through the CO2 box at a nice steady rate - the happy demon front line progressed linearly through the CO2 demon box until eventually they are all happy. At that point, the loop gas has some CO2 in it and we observed that the humans started to show signs of unease, panic and general ill-feeling. They eventually died a rather uncomfortable death.
To continue our experiments, we abducted more humans and carried on, this time we subjected them to a pressure of 2 bar. This is the same as being under 10 metres of your water. There is now 200 molecules of gas in the loop, but the human still only uses 4 molecules of O2 and turnes these into 3 molecules of CO2 and 1 water vapour. Each breathe pushes 200 molecules through the CO2 demon chamber, so the demons have to work faster to grab the CO2 molecules and die happy. Sometimes a front-line demon misses, but the 2nd line catches it OK. This carries on and eventually all the demons are happy, then as above, the human dies painfully and horribly from CO2 poisoning.
We needed to do more experiments, so we continued with our abduction programme. Now we're testing to 90m. There are now 1000 molecules of gas in the loop, but as observed before, then humans still only take 4 molecules of O2 out and metabolises these into 3 of CO2 and one of water with each breath, However, the poor CO2 demons now have 1000 molecules of gas going through their chamber like a hurricane, and in those 1000 molecules there are still only 3 molecules of CO2! It's now very hard for the demons to catch a CO2 molecule and hang on to it! The front-line demons have a real hard time catching the CO2 molecules and a lot more pass further down the line to be caught by the latter ones. Eventually, the front-line demons are full, but still the latter ones need to work to catch the CO2 and there will come a stage where there aren't enough latter ones who can catch the CO2 fast enough, so some will get through. Eventually so many will get through that the human starts to notice it and dies horribly as before - even when there are still some unhappy and empty CO2 demons left.
Continuing our experiments with more abducted humans, we test again at 90m, but then we decide to ascend the human to some depth where the number of molecules in the loop is much less, so each breath the CO2 demons have more of a chance to catch the CO2 molecules left.
 
Continuation of previous post with references:

Shearwater research:
The CO2 Scrubber in a Diver’s Rebreather
Gas density / depth. With increasing depth, the gas passing through the scrubber gets denser and therefore cools the absorbent more, making it less efficient. At very great depths the rate of gas diffusion decreases. Simply put, other gas molecules are in the way of the CO2 on its way to the absorbent.

Ambient pressure diving:
https://www.apdiving.com/en/wp-content/uploads/rebreather-inspiration-classic-manual.pdf
The INSPIRATION’s depth limit is governed by three factors. The first is the gas used as the diluent, the second is the volume of bail out/emergency breathing gas and the third limit is the greatest depth at which the rebreather has been formally tested - 100m, (depth is a significant factor affecting the duration of the scrubber).
....
The CO2 duration was determined using the DERA/QinetiQ unmanned test facility at Alverstoke. Test conditions: Water temp: 3 to 4oC, Breathing rate: 40 litre/minute, CO2 rate: 1.6 litres/min. These rates have been previously determined by QinetiQ, as an average breathing rate, averaging out work and rest cycles.
Multiple Dives
The INSPIRATION’s scrubber can be used for multiple dives, providing the Sofnolime is not soaked during a dive, bearing in mind the total timed used must not exceed 3 hours (for CO2 produced at 1.6 lpm ).
Effect of Depth
In trials depth has proven to significantly reduce the scrubber’s CO2 absorption capability.
Rule No 1 - Dive Planning The Sofnolime must be replaced after 3 hours of use for CO2 produced at a rate of 1.6 lpm
Rule No 2 - For dives deeper than 20m, the diver must leave the bottom when the total time breathed from the unit reaches 140 minutes (for CO2 produced at 1.6 lpm). e.g. If dive 1 is for 100mins and the 2nd dive is deeper than 20m, the bottom time of the 2nd dive must not exceed 40 mins. Check the decompression times for the 2nd dive to ensure the dive durations, when added together, do not exceed 3 hours!
Rule No 3 - For dives deeper than 50m the diver must leave the bottom when the total time breathed from the unit reaches 100 minutes (again this applies to CO2 produced at 1.6 lpm.). e.g. If dive 1 is for 90mins and the 2nd dive is deeper than 50m, the bottom time of the 2nd dive must not
exceed 10 mins. Check the decompression times for the 2nd dive to ensure the dive durations, when added together, do not exceed 3 hours!
 
My simple take on it is quicker to read.

The deeper you go the denser the gas becomes.
The denser the gas becomes, the more molecules of each gas is in the same space.
Eventually there are more molecules of CO2 bombarding the sorb and it does not have enough available area to capture all of it and some will get through.

Extremely simple way to explain it and missing a lot of the complexities, but I understand simple much easier.
 
My simple take on it is quicker to read.

The deeper you go the denser the gas becomes.
The denser the gas becomes, the more molecules of each gas is in the same space.
Eventually there are more molecules of CO2 bombarding the sorb and it does not have enough available area to capture all of it and some will get through.

Extremely simple way to explain it and missing a lot of the complexities, but I understand simple much easier.
So doing things like using the JJ extended range canister for deep as well as long dives would tend to help?
 
My simple take on it is quicker to read.

The deeper you go the denser the gas becomes.
The denser the gas becomes, the more molecules of each gas is in the same space.
Eventually there are more molecules of CO2 bombarding the sorb and it does not have enough available area to capture all of it and some will get through.

Extremely simple way to explain it and missing a lot of the complexities, but I understand simple much easier.
And why would there be more CO2 molecules at depth?
:)
 
Take 2 grains of sorb. Space them 100 molecules of gas apart from each other. Repeat a few million times.
As a molecule of gas goes through the gap there is a 1 in 100 chance it will touch a grain of sorb and be absorbed.

Now increase pressure. The physical gap remains the same. But instead of 100 molecules fitting in that gap there are 200, 300, 1,000, whatever. So the odds of a single molecule of CO2 brushing up against some sorb is now 1 in 1000 instead of 1 in 100. You need a bigger scrubber with lots of fresh sorb to catch that CO2 that is hiding in the dense gas.

Actual numbers will be different. My BS numbers were chosen out of simplicity to show why it is and how increasing gas density means more scrubber is needed or it needs to be derated.
 
And why would there be more CO2 molecules at depth?
:)
I think what he meant is more gas molecules total. The number of Co2 molecules is diluted and makes it harder for them to find a reaction site on the sorb
 
I’m wondering if dual scrubber designs (i.e. Kiss Spirit) might give you a better chance at mitigating this issue? Thoughts?

Also wondering if any scientific tests have been done on this issue with EAC’s?

Would sorb be better at depth or would an EAC?

Damn, now I’m going to think about this crap all afternoon.
 
I’m wondering if dual scrubber designs (i.e. Kiss Spirit) might give you a better chance at mitigating this issue? Thoughts?

Also wondering if any scientific tests have been done on this issue with EAC’s?

Would sorb be better at depth or would an EAC?

Damn, now I’m going to think about this crap all afternoon.

As I recalled the EACs are "more consistent" in duration but the durations they get are shorter than an equivalent loose sorb. Which makes sense because for a given scrubber volume EACs have more inert material holding them together so less absorption sites.
 
https://www.shearwater.com/products/peregrine/

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