How Far Do You Push Your Scrubber?

divetheworld

Absolutely.

I dont know my scrubber usage because I have no idea of my CO2 production rates. So........
I dont push it, but I do take it to the line sometimes. Otherwise, Mr Chase and I think alike.
Everyone knows my position on this one, £2.00 extra usage, 100% extra risk.

Bollox to that.

Mike, could you expand on your experience/knowledge on the clumping and how you find it relative to usage? Thanks.

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sensor330

So far I've been sticking with the manufactures recommendation on this one. Although I probably would exceed it only if I were doing deco and only if the canister was fresh at the beginning of the dive. This is only my opinion and I do not stating it so others get the ideal it is safe practice. I haven't yet had any dives of this duration but when I do it will be on a Rebreather that will fulfill the requirements of the dive.

I got a question that pertains to how long you can keep the sorb if it partially used though. Maybe I wasn't paying attention during my Inspiration course or maybe the instructor missed it. Anyways I usually get out diving once a week and sometimes I may only get one dive with a duration of 45-60min would it be safe for me to seal the canister for no more than 7 days and then reuse the canister? I would naturally be conservative side if I where to use it again though.

Scott

bgpartri

I'm much more likely to change my scrubber early rather than late.

This is just my opinion.

If I look at the differences in scrubber duration versus flow rate, it appears that scrubber is much less efficient with higher flow rates. Here is a table from W.R. Grace. The actual conditions aren't necessary for the point, but it is with 600 grams of "High Performance" scrubber in a modified Mark VI canister (1):

Hours---Flow Rate (cu.ft./hr)
5.0-----22.0
3.0-----25.0
1.4-----26.5

If I've got the math right, 22 cu.ft./hr is 10.41 lpm and 10 lpm is about the respiration of a person sleeping. 22 cu.ft./hr is a respiratory minute volume of about .37 cu.ft. I might get .5 cu.ft./min on deco, but I'm more like .6 on a relaxed swim. That's about 36 cu.ft./hour. And I can do 72 cu.ft./hour for a while.

What I get from that is that at high exertion I get less dwell time and worse scrubber efficiency.

Part two is that you probably can't detect a CO2 hit at high exertion levels:

"There was a comment made about the ability as a diver to detect CO2. I need to clarify that. I don't mean to speak against the speaker here, but unfortunately Spec War Com paid me and my laboratory to explore the possibility of whether a SEAL could detect elevated CO2 levels, whether he was resting or whether he was exercising. I can tell you for a fact that if you're exercising, but the time you begin to observe something is wrong, you're probably on the verge of passing out."
John Clark, Experimental Diving Unit. (2)

With those two things in mind, here are a couple of scenarios:

I do a one hour dive, then plan a second dive. I go down the shot line and swim around the wreck for an hour or so, then turn to go back to the shot line. Ooops. The current has come up and I'm working hard to get back to the shot. Do I stop working to avoid a CO2 hit, or just let go and hope they find me at the surface, or ... I started diving rebreathers to remove pressure during dives, not add to it.

Here's another scenario. I'm diving off Hornby Island looking for 6 gill sharks. We didn't see a single shark yesterday, or on the first two dives today. It's the third dive of the day, and I see shark after shark. But I have to leave because I didn't change my scrubber. The open circuit guys do a two hour dive and come back raving.

I'm in a cave and things are going well. I only have a an hour on my scrubber and the cave's only about 150' deep, so I don't change my scrubber. Unfortunately, I am seeping water, and I don't notice. Most modern rebreathers are very water tolerant. But at the point of maximum penetration I start to pant. I immediately flush and I feel fine. Oh Oh. Now I'm in a bailout situation because I wanted to save $10. But the way this trip with airfare and liveaboard has cost about $4000 for the week.

I'm not saying you should change scrubber after every hour. But I never go beyond the manufacturers limits. If in doubt I change.

Does this make me chicken little?

1. W.R. Grace, Sodasorb manual of absorption, 1999
2. Proceedings of the Rebreather Forum 2.0, 1996

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http://www.shearwaterresearch.com

Good judgment comes from experience. Experience comes from bad judgment.

Mike

Chatting with a couple of guys with a bit of experience about this last weekend, and the consensus was that it's not an issue, even if the scrubber is not sealed. Indeed the primary concern was with mold growing on sealed hot moist scrubbers.

At the moment I seem to be doing around 5 hours of diving a month, so my scrubber get's changed every month whether it needs it or not. And it probably spends at least a 1/4 of the time unsealed (ie in the rebreather with the hoses unattached). This is on a Prism, not inspo/evo, but sorb chemistry remains the same...

Mike

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Open ....... Closed
Mind ........ Loop

LS_DK

Hey Fil.

No, I did not try this on my course.
I would think it requires a doctor to be present as well, in case something goes wrong.

I have tried CO2 buildup a couple of times. The most severe one was a full blown CO2 hit with all the signs and symptons you probably have read about previos posts on this subject.
The more "normal" CO2 buildup case happens when I excersise hard underwater, like e.g. carrying an achor across the sea floor or swimming against current. The "normal" case gets me nose breathing.. That is just so extremely annoying .. I have concluded it is due to me not ventilating my lungs properly. Sometimes I have to swim around holding my nose for 10 minutes until it goes away again.. Doesnt look pretty..

Cheers,
Lasse

CCR900

Funny you should say that. Many, including myself, suffer from this on occasions on longer divers, and probably when the diver is tyring.

As you said it takes time and patience to get your breathing rhythm back, best done by sitting still for a few minutes which is also going to lower your breathing/O2 burn rate.

Is 'mast suck' a documented excess CO2 sign or are we just guessing (rightly or wrongly).

Fil

LS_DK

Guessing... I guess...

I can get it in the beginning of a dive if I excersise hard... So, for me atleast, it is directly linked to workrate...
You are right, if I stop moving and concentrate on ventilating, then it disapears faster.. I once thought loop flushes helped making it disapear faster as well.. But I found it was due to me ventilating via the type of deep breathing you do during a loop flush that actually made the difference.. No reason to waste that lovely expensive trimix diluent...

Thanks,
Lasse

dave t

Mike

I think it does make a difference if the loop is sealed or not, most sorb is produced with a built in water content sofnolime has 18% water content for example. If that moisture is allowed to dry out (open loop) the sorb will cease to function

Dave

Tom F.

I never had that experience, mabe it is due to the fact that I NEVER work hard

Tom F.

dave t

I think the reason for this is the fact that breathing from a Rebreather is much more natural than OC so the body tries to revert to its natural breathing technique which is nose breathing, when you stop and concentrate the breathing pattern goes back to underwater mode. I dont think it has anything to do with too much co2 in the loop!

Dave