Bailout Breather - do you NEED independant tanks....

jradomski

the rebreather reaction is not instaneous.. IT DOES start up very quickly IF there is moisture present.. if you just pack the scrubber and wait to go onto it at depth the abosrption that you don't get could be enough to kill you...

Since you are an inspiration diver you should have seen this.. when you first go on the scrubber (virgin) after a fill and you watch your controllers when you first start the you typically see the po2 drop to .5 or so within a few minutes (faster in a warm climate) the unit has no problems holding .65-.7.. Its co2 that is there lowering the po2, the injection rate doesn't change.. If you have a huge surface area it might be enough on the surface to not effect the loop but at depth very little co2 is debilitating.

now dont change your scrubber after a dive then go back on the loop, the unit can hold setpoint almost from the start...

I have seen traces from a mass spec (multiple times) that show this exact thing.. on startup no co2, then a slight rise, rise.. rise.. the it starts dropping quickly then its none. up until the scrubber starts failing with a slight rise, then rapid rise... this is for an axial scrubber... for a radial scrubber its similiar except throught the dive on a radial scrubber you have slight amounts of co2 at random times then none...
on a primed scrubber it starts up very quickly it all depends on a sufficient amount of moisture to "bind" with the co2 to form carbonic acid.. a virgin scrubber has just enough to start but not enough to sustain a person without a rise..

at 10 atas you only need .005% co2 to be over the alowable limit!

I would use a bailout rebreather.. I wouldn;t necessarily switch to it during the dive but I would do a serious prebreathe to get plenty of moisture in the loop..

If you don;t have enough moisture you better have enough gas to do a flush or two until the loop is stable..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Mdemon

There is a good thread on this on the List and also right here on Rebreather World about just this.

http://www.rebreatherworld.com/t184-.html

Unofficially and completely off the record, I spoke to "someone" at APD who told me that this just wasn't an issue. He's left a unit at 70m for 24hrs and switched straight onto it with absolutely no problem. He also pointed out that the Royal Marines don't even bother with a pre-breathe! There was also mention of detailed CO2 testing at 4oC at 70m in the test rigs at a lpm of something ridiculous, like 250! Again, no perceivable CO2.

My uptake on it was that I'd be happy to switch to a bailout breather cold. BUT, as Joe points out, I would pre-breathe it at the surface first for safety (channeling etc), to raise the moisture level and to start the reaction. I would expect this to be a belt-and-braces additional safety thing. For added safety, a switch every 45mins or so (at 4oC) would keep the temperature up.

Just my 2bar's worth...




Caveat: Newbie - take with pinch of salt!!!

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onetime

I was thinking of it more inline with verifying that the back up was functioning properly and there wasn't some failure upon entry or achieving depth rather than the warming up being critical. Just more of a possible added benefit.

Drmike

Sounds like that myth has been busted. IIRC it was MartinP that told me a pre-breathe is unecessary from a 'starting of a reaction' viewpoint.

Steve

Personally I feel they are mandatory if the bailout breather is to be your sole source of bailout.



I like to think of it in terms of OC diving. If one is diving deep enough to require some form of redundancy then the prudent diver opts for twins over one big single tank as a reg failure can wreck your day.



If we jump back to the CCR and design a twin CCR, if we opt for the tanks being common to both loops, a simple O2 reg failure will place you in exactly the same spot as someone with a single CCR with the same failure.



You really have to sit down and work out the likelihood of each component failing and what you can do about it. The two bits that have failed on dives that scared me were an inhale flapper valve and a burst O2 line.



Thoughts?

wedivebc

You're saying that your loop could contain a PPCO2 as high as .2 ? That would be almost 100 times the CO2 found in the atmosphere. Sounds a little hard to believe you could breath that.

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Cheers,

Dave....

Man is the only animal burdened with the knowledge he will eventually die