The imbalance of CO2 absorption of the rebreather scrubber

[YuchenEasonJiang]

I have learned that the absorption of CO2 on a rebreather scrubber is imbalanced as there will always be some dead spaces that remain unreacted.
I wonder why the scrubber works in that way?

 

[Dsix36]

I believe it is generally due to the specific flow characteristics if the rebreather (or other machine) that the scrubber is installed into. This can also be impacted by absorbant granular difference, packing discrepancies, moisture content and location, and orientation of all assembled parts. Back when I used color indicating sorb, I never once saw anything I would consider close to linear scrubber usage.

 

[YuchenEasonJiang]

I believe it is generally due to the specific flow characteristics if the rebreather (or other machine) that the scrubber is installed into. This can also be impacted by absorbant granular difference, packing discrepancies, moisture content and location, and orientation of all assembled parts. Back when I used color indicating sorb, I never once saw anything I would consider close to linear scrubber usage.

Yep that’s the point, why the usage of scrubber is not linear? My guess is that because of the buoyancy of exhale gas, CO2 tend to touch the upper side of the scrubber when diver is in trim position. But in the end, all scrubber should be used and there supposed to be no dead space, I just can’t figure out why.

 

[justinthedeeps]

Your diagrams don't match well-regarded other studies and simulations in this area, which show the flowfront as a cone in the center of the canister.

But it's just gas and flowfront dynamics, nothing weird going on here.

Go radial if it bothers you

If anyone is breaking through a canister(s) of 2+ kg sorb in three hours or less, take a serious look at several other things [trim, buoyancy, exertion, finning technique, loop integrity, sorb storage conditions, ...]. In spite of ISO training guidelines, this is far below the rated capacity of the scrubber for typical diving activity. And get a BOV.

 

[YuchenEasonJiang]

Your diagrams don't match well-regarded other studies and simulations in this area, which show the flowfront as a cone in the center of the canister.

But it's just gas and flowfront dynamics, nothing weird going on here.

Go radial if it bothers you

The diagram is from Molecular Ltd. Can you specify which studies and simulations show the flowfront as a cone?

 

[Wibble]

Logically, with the flow downward in a scrubber, the gas flow containing CO2 would be most in the area of least resistance and thus the scrubber would be consumed more greatly there. If it were a simple cylinder with no central tube, one would expect there to be a cone. Other shapes and factors would affect this area of least resistance and thus the resulting cone.

@Dsix36 mentioned using indicating scrubber (common in hospital usage) which would be very interesting to see if carefully "excavating" the used section.

Not mentioned yet is that the deeper you go, the more pressure there is and thus more gas molecules passing through the scrubber (the diver's lungs being a fixed size pump). It is said that this creates a larger active front -- not shown in the graphic above, but it would be like a greater angle of the green/blue line. The danger here is that the scrubber may break through quicker than if at a lower pressure. Also, during the bottom phase the diver would be more active and producing more CO2 than compared with the same diver during the decompression phase.

 

[Gone for diving]

Depending on orientation of scrubber and the diver... heat rises. That could change things...

 

[YuchenEasonJiang]

Logically, with the flow downward in a scrubber, the gas flow containing CO2 would be most in the area of least resistance and thus the scrubber would be consumed more greatly there. If it were a simple cylinder with no central tube, one would expect there to be a cone. Other shapes and factors would affect this area of least resistance and thus the resulting cone.

@Dsix36 mentioned using indicating scrubber (common in hospital usage) which would be very interesting to see if carefully "excavating" the used section.

Not mentioned yet is that the deeper you go, the more pressure there is and thus more gas molecules passing through the scrubber (the diver's lungs being a fixed size pump). It is said that this creates a larger active front -- not shown in the graphic above, but it would be like a greater angle of the green/blue line. The danger here is that the scrubber may break through quicker than if at a lower pressure. Also, during the bottom phase the diver would be more active and producing more CO2 than compared with the same diver during the decompression phase.

Thanks, but I’m still confused about the formation of the cone. Is there any articles discussing that?

 

[1c3d1v3r]

The reaction front is longer closer to the canister sides because of temperature differences. The middle of the scrubber is hotter and the reaction is faster making the reaction front shorter.

 

[broncobowsher]

Are we discussing axial or radial flow scrubbers?

I think beginners are under the impression that there is a line that defines the consumption of the sorb. Not so much a line, zone.

If you look at a rEvo and the dual scrubber. One scrubber is rated for 3 hours, but both together are 4½ hours. You can dive 3 hours, rotate in a new scrubber and go 3 hours again, rotate another in again and keep going. So clearly you can get 3 hours out of a single scrubber, why not 6 from two? The up front one is a safety margin.

Lets add in what depth does. It packs a lot more molecules of gas into the same volume. That is what happens when you pressurize gas. Do the grains of sorb get any closer? No.
So if you pick an exact spot in a scrubber, some space that has exactly enough room for 100 molecules between the 2 surfaces. At any exact moment there would be a 2 in 100 chance that a molecule can react with the sorb if the molecule is CO2.
Lets go down 30m/100ft/ 3 additional bar of pressure. The gap didn't change, but now there are 400 molecules squeezed into that gap, and still only 2 points of contact possible. 2 in 400 chance of getting a reaction. Thankfully we are not dealing with 2 points, we are dealing in surface area. And there is a lot of surface area on all those grains. And more once you consider that the sorb is a deeper reaction and not just a skin reaction. But still, the reaction front gets deeper as you go deeper. The scrubber that has a breakthrough at depth still has some life in the shallow deco.

Trying to put simple rules to dynamic events just isn't good. Either it skips the outlier, or dumbs stuff down to the lowest common denominator.

And my demo is flawed as it is tried to be dumbed down to only the major points.