OC bailout - why not CCR bailout?

[KISSer]

Originally Posted by Dominic

Heh. Here's an even simpler one - keep the inside of your wing disinfected & store some scrubber material inside it. Hey presto, instant & very simple rebreather

Why bother with the disinfectant? Given a choice between lung disease and breathing salt water, I know which way I'd vote

[NotDeadYet]

You don't even need the scrubber.

Exhale into your wing, inhale, exhale, inhale, dump the gas, add some via the inflator, repeat. You'll probably get a damn sore head but it does work. The same can be done in a RB loop if the scrubber has failed. You could even do it with diluent if the electronics had failed and the scrubber still worked, effectively turning it into a SCR.

Bail-out rebreathers have been used in saturation diving systems for years. The diver uses surface supply gas but must carry a SCUBA system for emergency use. On deep dives the OC SCUBA requirement means big bottles, not good for getting in and out of a bell or clambering about structures. There are a few RB systems to replace that, IIRC they are SCR units.

I was doing a research project to look at RB bail out systems on long, deep dives. I'm not so sure about it as a first response system, you want something you know will work. For a RB bail-out you need to know that the loop is always instantly breathable, that it is functioning, that there is adequate volume, that the scrubber reaction will start, that it hasn't flooded, etc. My own thoughts would be to bail out OC which you know works (generally) and then get the back-up RB unit going to get you wherever you wanted to go. I can't see this being less hassle on most dives but there are applications where it could work (long cave dives).

The other approach which I would prefer is to engineer out the failure modes of rebreathers and build something ultra reliable that would survive a failure. The Cis-Lunar approach was to have one big RB, then they moved to three RBs in one unit and then finally took the approach of engineering a fault-tolerant rebreather. Did it work? Cis-Lunar are out of business...

The double SCR approach is not a bail-out system. They dive SCRs like an OC diver would dive an independant twinset. 15min on one RB then switch, 15min on the other then switch, and so on. The double EDO-04 system is used without any OC bail-out. I've used the system as I considered buying one. It is however extremely heavy (I'm guessing 60-100kg depending on what drive bottles) and very big, if I was without a scooter I'd rather be on OC than try swimming that bugger!

I'd say rebreathers had more ways out in a failure than OC. But the key is knowing that you have a failure in the first place. With OC it is instant. It's only when you start looking at stuff like this that you realise the happy-clappy sales pitch is just a pitch, these things kill easily.

Cheers,

Stuart

[Simon A]

Why not have a back up breather?
One of the reasons as I understand it is that the reaction front in the scrubber needs to be up to temp for the scrubber to work efficiently (hence the requirement to pre-breathe the scrubber prior to the dive) . Also the deeper you go the less efficient the scrubber is (related to gas density and permeability of scrubber material)
So in cold water at depth, it may not be possible to get the scrubber up to working temp fast enough to make it a viable bail out solution.

(usual disclaimer: A rebreather is a quick way to kill yourself. All the information above is purely my own understanding, and relying upon it for your own use is liable to kill you.)

Simon A

[nigelH]

Originally Posted by Simon A

...the reaction front in the scrubber needs to be up to temp for the scrubber to work efficiently...

Not actually true. The chemistry is very energetic, works cold and generates heat. The prebreath is a safety check to see things are working. I want to see and hear things operating and a prebreath does that but the scrubber is not one of my worries. Unless I've forgotten to put it in the canister three minutes isn't going to tell me anything about it.

nigelH

[Simon A]

Nigel,
Thanks for the correction
As I understood it, in very cold water there was the potential for so much energy to be removed from the loop that the reaction was basicaly a non starter , but a working scrubber generated enough heat to keep working.
Do you have any sources with further info on the Chemistry?

Regards
Simon A

[nigelH]

Originally Posted by Simon A

Do you have any sources with further info on the Chemistry?

Not to hand. Martin Parker of AP put some stuff on the Inspiration remailer on this a while ago that basically summed up as 'if you can dive the scrubber will work'. The reaction is strongly exothermic - you have to get your hands on a recently used scrubber and realise that all that heat was generated by a lot less than one gram a minute of CO2. What it does like is some water but it can pick that up as you breath and promptly releases more of its own.

nigelH

[Mdemon]

Can scrubbers get too hot? Is there a benefit to insulating them?

[Phil Ennis]

Originally Posted by Mdemon

Can scrubbers get too hot? Is there a benefit to insulating them?

Are you going to knit a nice little jacket for your scrubber.

[wreckweasel]

Originally Posted by Mdemon

Can scrubbers get too hot? Is there a benefit to insulating them?

You can actually make them steam!

As for insulation.. well, I'd work on my bucket chemistry premise that reactions are faster at higher temps, so insulation should improve the reaction rate.


Going back to the bailout breather debate, its something myself and a few mates have been thinking about for a while. As far as I can see SCR is a reasonable compromise. It lets you work with minimal worry about starting the loop up in a viable state (just dil inject), you can use it without PO2 monitoring (at a push) and its relatively easy to build. The only bit to manage is maintaining internal pressure (ADV+OPV+collapsed CL). then obviously you want it to be multi gas capable... good old swagelock

/Zak

[bang-on]

You can actually make them steam!

As for insulation.. well, I'd work on my bucket chemistry premise that reactions are faster at higher temps, so insulation should improve the reaction rate.


on the subject of heat i have just ordered the dave thompson stainless case from dave townsend,my ap case has finally disintegrated and dave's case seems ideal for larger cylinders.
i was going to insulate the scrubber with a compressed neoprene cover to preserve some of the heat in the scrubber,welcome with higher he mixes that can feel a lot colder.
any comments?
cheers
barrie