Channeling - Defined...?

Abbo

Forgive me, but is there any evidence of channeling having been the primary cause of any fatalities? There has been so much hushed talk of tapping and rotating and chanting that the process of filling a scrubber is beginning to sound like an arcane freemasonic ritual. Of all the things that worry me about rebreather diving, and rightly there are many, channeling is none of them.

kmev

Channeling was the least of my concerns until it happened to me on my meg axial. On dives approaching 3 hours I was getting an inch-wide purple strip on the top of my scrubber – all the way from top to bottom, with a corresponding section of loose sorb. I never noticed any CO2 effects, probably because the end of these dives involved a lot of low-exertion deco, but I was obviously concerned.

I assumed I was doing something wrong with packing it, so I tried everything, including settling the sorb on top of a vibrating compressor, but nothing prevented the channeling on long dives. Settling during transport was not the problem.

On taking a closer look at the canister, I discovered several radial cracks emanating from the brass button on the “wagon wheel”. These cracks were causing the lid to hang up on the scrubber wall and allowing the settling of the sorb during long dives. I had ISC make a new wagon wheel and I haven’t had any problems since.

I had assumed that packing a spring-loaded scrubber was foolproof and channeling was extremely unlikely, but it can happen.

I’ve seen discussion that indicating die in scrubber is a bad thing. I disagree.

mempilot

Tim,

When packed properly and stored vertically, a Meg's scrubber spring takes up any compacting that may take place. If you store the Meg horizontally after it is packed, this compacting caused by vibration and jarring can cause a channel to form along the longitudinal axis at what is at that point the highest part of the scrubber.

Leon, and all the Meg instructors I've talked to, insist on a tight fill and tightly compressed spring, as well as maintaining the Meg vertical during transport, both in the car and on the boat.

Any WOB issues claimed with tightly packing a scrubber have not been noticed by this diver. Maybe on a scientific test, the lab results show a slightly higher WOB, but the diver may not notice any difference.

Tim, Jill Heinerth will be teaching you all about proper packing, storage, transport, and channeling as it pertains to the Meg axial scrubber.

In my opinion, if you pack it properly, transport it vertically, and don't do dumb stuff like repacking it, then you will not have problems with channeling.

See you on the boats.

Eric

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Eric Stadtmueller, otherwise known as, MEM "Da Pilot"

mempilot

This is a big no no. This rigorous vibration can break down the shape and size of the grannules. This shape and size is important to the way the grannules allow gas to pass in and around them. This was specifically discussed in my MOD1 class.

Sounds like this was the root of the problem, exaberated by the vibrating compressor.

I'd venture to say that you may not have been packing it properly based on the above statements.

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Eric Stadtmueller, otherwise known as, MEM "Da Pilot"

kmev

Settling on the compressor was an attempt at a solution, not my normal way of packing.

mempilot

Gotchya.

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Eric Stadtmueller, otherwise known as, MEM "Da Pilot"

kmev

Have you ever looked closely at Grace Sodasorb? Not a lot of consistency in grannule shape there.

mempilot

Yep. I don't use it. It breaks down into dust quickly as well. I use 812 Sofnolime. Very distinct grannule patterns. I think that it breaths better (noticeably), and especially if it is moist after several hours.

When throwing out Sodasorb after a dive, the whole scrubber seemed to be one piece. When throwing out Sofnolime, most of the grannules fall apart like uncooked rice with some clumping. I like scenario two better from a WOB standpoint. This may be BS to some, but it's my opinion.

**added** BTW, with the new lower competive pricing between FillExpress and Rebreather World Store, Sofnolime is the no-brainer in my mind.

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Eric Stadtmueller, otherwise known as, MEM "Da Pilot"

silent running

Hello mempilot, 6-12 sodasorb is less consistant in it's granuale size because it has a wider variation in it's size of granuals by design, hence the designation 6-12 as opposed to the finer mesh of 8-12 sofno.

And a bigger grannual will give a lower WOB as there will be bigger gaps btw the granuals and less duration as well. If my memory serves me right, fine mesh sorb is not recomended for the Drager axials due to the increased resistance. Dragersorb is, I think, 3-5 mesh.

So all that being said, I think overpacking an axial is indeed possible and would be quite a bad thing during say a period of maximum exertion, even a very short period. As any CO2 hit starts with the diver retaining CO2, I would think that an overpacked, hard to breathe scrubber is probably the fastest way to get a CO2 hit.

As for settling, I believe this is a hazard as well. Very few people I know pack their scrubbers right before dive and many units wind up bouncing around in a RIB or a car trunk for some time. I have heard some older CCR divers mention this problem as the main cause of channeling right after a very badly packed scrubber, which is rare for anyone but a beginner. The Prism scrubber is actually packed upside down and when put in a unit, it will actually pack itself tighter while sitting upright in a bouncing vehicle. The entire scrubber is also held above the bottom of the scrubber housing by a spring, which further decreases the chance of settling. Lots of thought went into this design to preventing settling/channeling, so obviously SMI think it's a serious issue... -Andy

caveseeker7

Actually, DrägerSorb is 5-9 (Tyler) mesh

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Cheers
Stefan



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