Sorb surprises

[JohnnyC]

@JohnnyC not a lot of CO2 production when you're using scooters...
797 is something like 150l/kg, so on those SF2 scrubbers, using our normal rule of thumb with an hour a pound you have 5 hours on that scrubber. It is able to theoretically absorb 330liters of CO2, knock off some efficiency, usually 70% for axials which is 230 liters. We usually use 1lpm for calculations of O2 for consumption which we know is high, but call it 230 minutes of burn time.

CO2 production for scrubber duration is calculated at .8lpm, 1.3lpm, and 2.5lpm ish during testing. .8lpm is kind of lazily swimming, 1.3lpm is something like that river dive, and 2.5lpm is basically sprinting and is not considered sustainable. IIRC NEDU uses 1.4lpm for their testing.

1lpm seems pretty conservative for a normal use case with the rebreathers, especially if for deep diving where a majority of the time may be spent on deco essentially not moving and the 1lb/hr is practical for that.

The 2.5kg can theoretically process ~375liters of CO2. Assuming maybe 80% efficiency is 300liters, and over 8 hours is .625lpm CO2 production. If most of it was that calm lake dive and/or dpv/deco type diving I'd say that's easily plausible

Oh I'm aware of the math, but it IS NOT comparable to Intersorb 812 (despite both being 8-12 mesh) or Spherasorb 408. In Simon's Sofnalime vs Spherasorb comparative study, breakthrough occurred 32% faster in the Spherasorb vs Sofnalime. They're doing those dives on blue kegs, not white..... It's certainly not a direct comparison as the Spherasorb is 4-8 mesh and Sofnalime is 8-12, and Intersorb is 8-12 mesh as well.

Translating into real world diving (warm water, lower CO2 production, etc.), I'm sure the difference is not that dramatic under real world operating conditions, but the fact remains that they're doing those dives on demonstrably less efficient scrubber material. The 8-12 mesh products do not produce similar results it would seem, although certainly less difference than 4-8 mesh. And I don't know why anyone would choose to dive 4-8 mesh.

rEvo is certainly comfortable with Intersorb though, so it's not like it doesn't have merit as a useful absorbent.

They're doing those dives without issue so obviously they are successful, but I personally don't want to be that close to the edge. To me it would be like planning bailout requirements on standard OC SAC rates as opposed to a worst-case CO2 hit scenario.

Study abstract for anyone else who wants to take a look: The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system. - PubMed - NCBI

Again, it's Spherasorb 408 vs. Sofnalime 797, so different manufacturers, different mesh. Keep that in mind.

 

[tbone1004]

@JohnnyC
I do wonder what would happen if the fine blue keg was tested instead of the big stuff.
Intersorb claims 150l/kg on 812, 128l/kg on 408 *same as spherasorb 408*, and 133l/kg on Sofnolime 797 so theoretically while different mesh size, it should be a direct comparison in terms of ability to process CO2. In terms of why they chose the 408, I suspect it is ease and cost of acquisition since it's used in the medical industry. I know I can get it substantially cheaper than 812 mesh dedicated for diving.

For anyone else looking out there, they got 58 minutes/kg for Spherasorb, and 76.5min/kg for Sofnolime or 30% longer duration.

I imagine the numbers would be very close if they tested 812 against 797.

 

[JohnnyC]

For sure I'd love to see a direct comparison between both 812 mesh products. Anecdotally, I get less time on Intersorb than Sofnalime. But anecdotes are like elbows and opinions and.....

I was visiting with Joel of LM fame and in his pleasant southern drawl said something to the effect of, "....and them boys are doin' it on the blue stuff! That stuff ain't for me."

 

[Wookie]

For sure I'd love to see a direct comparison between both 812 mesh products. Anecdotally, I get less time on Intersorb than Sofnalime. But anecdotes are like elbows and opinions and.....

I was visiting with Joel of LM fame and in his pleasant southern drawl said something to the effect of, "....and them boys are doin' it on the blue stuff! That stuff ain't for me."

Big fan of the stuff in white myself. I don't blow near as much around the deck. The aluminum deck.

 

[rjack321]

I have friends that do long ass dives in Eagle's Nest on a stock SF2 2.2kg scrubbers with Intersorb and I think they're nuts. However, I think scrubber design does play a part in the ability to do this. Theoretically, an axial design (like the SF2) will be less prone to breakthrough due to the increased dwell time through the scrubber bed (gas has gotta go through 7 inches of sorb). However, it's still a loooooooong time to be on one can of kitty litter, especially one that small. While the WOB is nicer on a radial, I don't think I'd push it that hard. The dwell time is just too short (around 2 inches of sorb all the way around) and you can overbreathe it much easier. If you could guarantee perfectly symmetrical gas flow around a radial it would be a different story, but I'm not convinced that it is possible. Fluid dynamics is a funny thing..

Actually the 7" is a bit misleading since the reaction front in an axial does not progress evenly across the cross sectional area. The active area in an axial is a cone shape, its much more even in a radial which is why pound for pound of sorb the radial will last longer.
A Look Inside Rebreather Scrubber Canisters, Part 1

 

[JohnnyC]

Actually the 7" is a bit misleading since the reaction front in an axial does not progress evenly across the cross sectional area. The active area in an axial is a cone shape, its much more even in a radial which is why pound for pound of sorb the radial will last longer.
A Look Inside Rebreather Scrubber Canisters, Part 1

True, it is conical, and in terms of overall duration, the radial will last longer all things being equal. However, in terms of bypass due to something like overbreathing or channeling, the scrubber bed on a radial that could allow bypass or be subject to channelling is much smaller than on a comparable (by volume) axial. Much less prone to issues caused by something like a poor packing job. The reaction front in an axial still has to pass through X" of sorb to allow CO2 into the loop, assuming starting from fresh pack. A radial it only has to pass through the smaller distance between scrubber wall and center tubing. Just because you have fresh sorb doesn't mean you aren't letting CO2 back into the loop.

Now in theory none of that will ever happen. We all tightly pack our scrubbers right? We never short fill axials right? We always double check out scrubbers after a bumpy ride right?

Hell, they both seem to work when packed right, and either way guys are putting more hours on both than I am comfortable doing.

Incidentally, I'm thinking about seeing if I can fit a radial in my SF2 just for gits and shiggles. I'm curious how it will end up performing.

 

[northernone]

Big fan of the stuff in white myself. I don't blow near as much around the deck. The aluminum deck.

Yellow sorb a no go too? Don't suppose potassium superoxide plays nicely with aluminum either.

 

[doctormike]

Along these lines, wondering what you guys think about this? Some of you may be in that thread already. Breakthrough, CO2 retention or both?

 

[Wookie]

Yellow sorb a no go too? Don't suppose potassium superoxide plays nicely with aluminum either.

I don't think it's the reaction with aluminum you have to worry about, it's the sudden exothermic reaction when it gets wet. When we used to through OBA canisters overboard following drills.....

 

[tbone1004]

Along these lines, wondering what you guys think about this? Some of you may be in that thread already. Breakthrough, CO2 retention or both?

when I read it earlier my gut says CO2 retention. It's near impossible to actually overbreathe a scrubber, and while breakthrough is possible, it is coincidentally timed with the increase in work load. I bet failure to fully exhale that caused CO2 retention