Square scrubber and hypercapnia

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sprockjohnson

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Location
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I'm putting this in the Rebreather Diving section but it could also go into the accidents and near misses section. However, I think it belongs more in Rebreather Diving as it involves a question about scrubber canister design.

I have a Sport KISS and for those of you not familiar with the design it utilizes a square shaped scrubber canister which slides down into the metal case of the rebreather. There are no springs or any spring-like material used to compress scrubber material after it has been filled and you simply have to fill, tap, fill, etc. until the scrubber material is level with the bottom of the black plastic connectors on top of the scrubber which the opv/ adv pod and the sensor pod screw into. Any compression of the scrubber material occurs due to these pods sitting just below the bottom of these connectors when inserted. The openings for the filling of the scrubber are round and do not extend to the edges of the top of the scrubber canister, leaving approximately 1/2 to 1" of space around the openings to the corners.

Yesterday I did a dive at a freshwater spring and at a depth of 162' and a runtime of around 5 minutes began to experience what I'm fairly certain was hypercapnia. I was able to bailout to the necklaced regulator fed by an AL80 and ascended to about 70', after which my respiratory rate slowed down to a more normal rate. My ppo2 was a maximum of 1.00 at the bottom and after bailing out and checking again after I had calmed down it was at approximately 0.70 at the 70' depth. I attempted to go back on the loop to see if it was just a fluke and within about 30 seconds determined it was not and began to experience the same symptoms and again had to bailout to OC.

As a side note I was wearing a Garmin G2 as a backup computer and it was able to log my heartrate and showed an average of 94 bpm during the dive with a maximum of 109 bpm, which was much lower than I would have expected.

While leaving the dive site and speaking with another rebreather diver on the way out he brought up an interesting point which is where my question lies-

The square scrubber design obviously has corners. Specifically corners near the top which can be hard to verify whether they were completely filled with sodalime regardless of how much tapping is done. I checked my scrubber after the dive and after pulling the pods off the top I noticed a very small amount of unfilled space near these corners. The scrubber material just below where the pods screw into appeared to have been slightly compressed as intended. It did not appear like there were any visible channels going down from these corners which could assist in a breakthrough.

Anybody know if this was likely the reason for the CO2 retention? Is this a design flaw inherent to the square scrubber design or more likely my mistake with packing the scrubber? Just curious for the future with my unit and also wondering about the square design which appears to be incorporated in the Sidewinder 2, although that design has a spring system to compress the material.
 
Unlikely. The air/co2 still has to go through the sorb even if there's a dead space on top.

Its possible if it was underfilled that you would get channeling in a horizontal position, more related to not having any tension on it vs square canister.

It could be a multitude of other factors as well.
 
What diluent? Duckweed in the DSV perhaps? How was the scrubber media distributed when you removed it?
 
What diluent? Duckweed in the DSV perhaps? How was the scrubber media distributed when you removed it?
Air diluent. I know it's deeper than recommended but I have done this dive several times with my rEvo 2 mCCR with similar profiles and have never experienced this.

No duckweed at this site (Buford Spring), but it did have three water moccasins and an otter swimming near us.

When I removed the scrubber material it was dry, no clumps, and the material appeared slightly compressed below the pods, however I was able to see a very small amount of empty space (perhaps 1/8" or so) up in the corners of each side.

Also, I did a second dive on a friend's rebreather (Titan eCCR) with no ill effects. I think it was packed from the same keg of sodalime but on a different day.
 
I’d say it’s probably CO2 retention from a combination of diving air way beyond what it should be and the high work of breathing if the classic (super dense gas at those depths will reduce ventilation significantly, and resulting in increased CO2 retention). Once CO2 starts to build up, it’s hard to eliminate quickly, you’ll feel a bit better on OC, but symptoms can return when going back on the loop due to the higher work of breathing and increased dead air space in the mouthpiece.

In general, please don’t dive CCR on air to those depths (especially in and overhead!!!). There’s just far to much evidence to show its a terrible idea, no matter what unit you are on. Just because nothing went wrong on previous dives doesn’t mean you’ll always have the same result, especially when something unexpected happens and you need to manage a problem while significantly impaired. Helium is cheap for CCR.
 
Could be a DSV/BOV issue.

165ft on air on any rebreather is well beyond ALL industry safety standards and recommendations
 
Thank you all for the replies. I had wondered if gas density might be a factor.

DSV appeared to be functioning well and mushroom valves were sealing like they should.
 
KISS Sport has a design WOB of ~5.3J/L at 40m, which is roughly twice the CE limit for safe breathing performance. See graph on page 64 https://www.opensafetyglobal.com/Safety_files/FMECA_OR_V4_180821.pdf

Based on that and your workload at depth finning, you quite likely pre-planned to breakthrough your scrubber at that depth before you got in the water...
 
I had wondered if gas density might be a factor.
And being narced probably didn't help much either.

Didn't the Sport have a shallower max deph recommendation than the classic for some reason?
 
Mushroom valves? Did the "telephone" breathe test pass on the loop?
 
https://www.shearwater.com/products/swift/

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