Sorb surprises

[fsardone]

What I think you are arguing (hard to know exactly because you are mixing depth with time) is the fCO2 is 0 at the start, 0.01% after 20mins, 0.02% after 40mins, 0.03% after 60mins etc etc. That is not how a scrubber works.

I believe I did not argue nothing of the sort.
ppC02 will stay zero until breakthrough. Which by the way is the literal meaning of the word
What I am saying is that breakthrough will happen sooner at depth.
The article you linked is at constant depth (I did not notice what depth, if it was specified at all) what I am discussing is a real dive: deep for the fun bit shallower for the boring deco ....

Maybe I was not clear.
At higher depth due to the fact that gas is denser the amount of CO2 coming from the metabolism is the same it is more diluted therefore you need to increase the depth of scrubber to cross for all of the CO2 molecules to come in contact with sorb and be adsorbed.
This means that since you use up sorb from the entry point of the gas, if the remaining portion of active scrubber is less than the lenght needed AT THAT DEPTH you have breakthrough. If you decrease depth you might still have enough unused (which can become active) scrubber to prevent breakthrough. This is why time and depth are linked.

If you have a temp stick and do a deep dive you will see how the active scrubber portion is function of depth.
The next 3 images are the screen shot of my reb logging software. This was a dive to about 58 meters 30 minutes bottom time and approx 90 total run time. Scrubber had been used, in a previous dive, 30 minutes at 54 meters I then bailed out to OC for deco (I was just training nothing dramatic) so this scrubber had 70 minutes left below 50 meters and 150 minutes usable scrubber time (according to manufacturer manual and tests).

You will notice (in the lower right of the pictures) the bar of the active portion of tthe scrubber. It runs from from left to right and you need to imagine gas entering from left.

First image: During the surface/shallow portion The scrubber is active from the beginning only 2 blocks.

Second image: At depth the initial portion of the scrubber is unable to react and cools down (two blocks off) buth the bar is longer (4 blocks) and starts from the third.

Third image: once I get back up in the shallow the initial portion of the scrubber heats up again, block 2 becomes hot again. What you do not see here is the last part of the scrubber cool down but it does. It will not receive CO2 to react with although it takes a long time to cool down because the gas is hot from reaction, but in a further shallow dive you would see that.

This is what I am arguing. And it is chemistry and physics that mixes up scrubber duration and depth, not me.

Hope this makes it clear. Recommend reading the book I mentioned in my previous post.

Cheers

 

[Superlyte27]

So, out of scrubber time at 200' but plenty of scrubber when you get back to 50'? Bwuahaha

 

[fsardone]

So, out of scrubber time at 200' but plenty of scrubber when you get back to 50'? Bwuahaha

This is what people doing testing and certifing rebrether have written.
And also what is in Mastering Rebreathers ....

Why you believe it is wrong?

cheers

 

[DA Aquamaster]

@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

I plan for 1 lpm of O2 use for scrubber life calculations and 1.25 lpm O2 use for O2 planning purposes - the extra 0.25 lpm is based on actual measured use on cave dives and covers normal loop adjustments with depth changes, bumping up the PPO2 during deco, and an end of dive O2 flush at 20' to check for current limited sensors and linearity during the ascent.

I also used the same assumption of 0.8 liter of CO2 produced for each liter of O2 metabolized. Molecular products quotes a "typical usable capacity" of 150 liters of CO2 per kilogram of Sofnolime 797 absorbent. Intersurgical quotes the same 150 liters per kilogram for Intersorb 812, and 128 liters per kilogram for Spherasorb 408.
Converted to pounds that is 68.18 liters per pound for the 8-12 mesh products and 58.18 liters for the 4-8 mesh.

Assuming 100% scrubber efficiency, an average O2 use of 1 lpm and average CO2 production of 0.8 lpm with 5.6 pounds of 8-12 mesh would give a maximum of 381 liters of CO2 that could be absorbed, or 476 minutes. 4-8 mesh would give a maximum of 325 liters of CO2 absorbed, and 406 minutes.

However, I also assumed what I thought was a pretty conservative 70% efficiency for an axial scrubber, which reduced the maximum dive time for 8-12 mesh to 380 minutes and 324 minutes for 4-8 mesh.

Then I did some testing to see how well the match and underlying assumptions held up in 68-70 degree F water under normal swimming conditions. Over time, keeping notes on work load and scrubber duration I worked up to a maximum of 6.5, hours, with about 1 hour of that in more or less high flow conditions (mainline Ginnie), about 0.5 hours in deco conditions and the remaining 5 hours under normal swim speed conditions. That's 390 minutes. However, I felt that there was no margin for increased workload above a normal swimming rate for that last 30 minutes and stopped pushing any farther at that point. Consequently, 6 hours is as far as I actually plan to dive my 5.6 pound axial flow scrubber with 8-12 mesh.

The real world results did validate the math and the underlying assumptions, using some high flow and minimal deco to give a reasonably conservative "average" condition. However it also indicated that the 70% efficiency assumption was not conservative at all, but rather pretty much on the nose.

I know one well respected diver who put 11 hours on the same 5.6 pound scrubber using 8-12 mesh, but he only recommends 8 hours, and I strongly suspect that includes a significant percentage of time on a scooter and on deco, so I'd be very careful in pushing past the 6 hour mark on a 5.6 pound scrubber, not to far off the 1 hour per pound of 8-12 sorb rule of thumb. And of course I'd scale the dive time back by half in cold water (35-40 degrees), even with the scrubber located inside the counter lung where it probably stays a little warmer.

 

[rjack321]

...so I'd be very careful in pushing past the 6 hour mark on a 5.6 pound scrubber, not to far off the 1 hour per pound of 8-12 sorb rule of thumb. And of course I'd scale the dive time back by half in cold water (35-40 degrees), even with the scrubber located inside the counter lung where it probably stays a little warmer.

Are you actually diving your sidekick in <40F water?

The Meg axial scrubber is 6.5lbs. I have done 2x 200ft dives in a day with it in 38F water (30mins on the bottom of lake huron). The 90mins of deco was mostly 40-64F. So both depth and temp vary quite a bit, making figuring out "how close to breakthrough am I?" a challenge. But Leon was doing the same dives and if the manufacturer is willing to do it I figure that was endorsement enough of the practice/duration.

 

[DA Aquamaster]

Are you actually diving your sidekick in <40F water?

The Meg axial scrubber is 6.5lbs. I have done 2x 200ft dives in a day with it in 38F water (30mins on the bottom of lake huron). The 90mins of deco was mostly 40-64F. So both depth and temp vary quite a bit, making figuring out "how close to breakthrough am I?" a challenge. But Leon was doing the same dives and if the manufacturer is willing to do it I figure that was endorsement enough of the practice/duration.

No, I don't have the same desire I used to have to dive cold water, thus the "I'd" in the sentence. It's not uncommon in the great lakes or in a deep alpine lake to have 35-38 degree bottom temps below 100', and then have 2 or more thermoclines above that with surface temps of 65-70 degrees. As you note that adds some more variables to the effects of temperature on scrubber life. However, since the longest deco stops occur in shallow water with warmer water, it's not even remotely the same as a dive at a constant 40 degrees.

 

[northernone]

Sorb Storage: New research released!

First link I've seen to the research.

Thank you to those who made this knowledge available to us.

Cameron

 

[Dr Simon Mitchell]

Hi Cameron,

Yes, I was about to post about this myself.

The Eurotek fund paid the early release fee for the paper, so it is now available via the link you have provided.

POLLOCK NW, GANT N, HARVEY D, MESLEY P, HART J, MITCHELL SJ. Storage of partly used closed-circuit rebreather carbon dioxide absorbent canisters. Diving Hyperbaric Med. 48, 96-101, 2018. doi: 10.28920/dhm48.2.96-101.

Have a read and I am happy to discuss any questions or comments you may have.

We did discuss this at the previous Eurotek, but this year will be presenting our new data on the efficacy of temp sticks. Eurotek is on in Birmingham, UK, the first weekend in December.

Simon

 

[rjack321]

Thanks for doing this work and writing it up in such an accessible manner!

I'm fascinated by the finding that the 2 dives had a cumulative runtime slightly longer than the single dive duration.