Suitability of varying rebreathers for cave diving

decoweenie

I believe the same instructor had a student stuck at the entrance for 10-15 minutes (on the way out) doing a light-out exit exercise. The student was using twin-18 (in Truffe) but that was all he had. He was a bigger guy (not fat, more muscular).

I got out earlier since I was the first one to go on the exercise, didn't even touch my twinset on the restriction on the way in or out (may be need to drink more beer ?). The trick was to get as low as possible and don't get your arms underneath. When stuck, people panicked and push up thus making themselves more stuck.

Anyway, I finished having lunch by the time everyone got out. And the student refused to go back for the second dive so we had to move to a different site.

lizardland

I'm not going to the Congress, I can't be doing with that many divers in one place

It's basic cave diving theory... dive within the gas you have available.

My Rebreather has been rigged so far with 12's and 7's (manifolded). That gives me a theoretical equivalence of either 3x20's or 3x12's as the OC gas is for a one way trip (i.e. out). Manifolded sets so in theory (lots of that about) it takes a lot of f*** ups to lose all that gas. Once I get comfortable on it what i really want to do is leave the backgas entirely for bailout and drive the RB from stages. The whole point of it is that I want to cart one twinset, a keg of slime, some O2 and a few stages of drive gas on the 1000mile round trip rather than 2-3 twinsets, J bottles, compressor, stages, etc. I'd rather fill one bottle and drink six beers at the end of a day instead of filling six bottles and having one beer. It's as much a logistical thing as any other reason.

As for scooter failures, as I only have one then I'm not using it anywhere I can't swim out of. It's not complicated to work out what the limits are and dive within them.

Cheers,

Stuart

ROB DAVIE

Probably the best and most concise answer concerning the logistics of diving that I have ever seen posted!

Rob

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[SIZE=2]"CC Rebreathers will become a viable part of tech diving [U]WHEN PIGS FLY[/U]!!"--GI3[/SIZE]

Crazyduck

For me the environment (the cave) itself dictates what I am going to be using.

I have a buddy that dives an Inspiration in Cow - that has to be interesting at times.


Cave is definitely a specialty and sub-group but more than anything rebreathers provide cave divers the one thing they don’t have on OC- Time.

Time to find the line.

Time to wait out some of the silting

Minimum percolation in the cave


Is there a reason people are trying to force themselves through these restrictions?
Take the tanks off and swim them through!

The rebreather with respect is a different story.


This is were I like the meg, the profile Rocks!

If I was diving a Inspiration I would tempted to take it out of the case and mount the scrubber and guts in a u-shaped piece of sheet steel and cover it with ballistic nylon. That way you protect the canister and cabling and mount it to a backplate and harness. I have seen back plates with slits on the sides (deep sea supply) to mount tanks. Or heck drill in mounting holes for tiger gear adapters. You should be able to seriously cut down the profile of the unit. (Clearly this is my opinion and probably has tons of flaws.)



Logistically compares to open circuit – no comparison.

Later, Andrew

AM

I am interested to know what effect there would be from changing the exhaust position without also changing the design to divert water to the bags. Unless changing the exhaust is just to counter the risk of the bags getting flooded?

There is still the problem of water going into the down pipe and soaking the lime on long dives. Tampax/trek towels notwithstanding.

Re the gas addition - curious about the idea of adding via the manifold, I would have considered this a risk. A second gas inlet is possibly less risky. I am considering a switching block.

I will be interested in feedback from France.

Regards

AnneMarie

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decoweenie

For Truffe, the "restriction" is only at the entrance. The only way in/out of this cave.

It is a beautiful cave as excellent viz is almost guaranty most of the time. You probably have seen shots by Gavin Newman of the system. Also, it is a shorter and shallow dive so it is a good warm-up dive to begin the week, or cool-down before going home. As well as a nice practice dive...

Personally, I can't understand why I hear stories of problem with this "restriction" since of all of the time I dived there, I think I touched the tanks probably once or twice getting in.

decoweenie

I have 2 blanking plugs that I made to plug the outlet to the CL so I could connect the YBOD OTS-CL to try, but I sold the unit before trying it. If you have the Copeland-adapters (at the scrubber side) I could bring the plugs for you to try in Sharm.

No need for a manifold.

Just add a Swage-Lok QC-4 or QC-6 to the diluent inlet, then use the same connector on any supply tank (ala Halcyon) to plug the diluent into the system.

AM

I agree, no need for a manifold which is why I personally don't have one. Briefly considered adding it on but it presents a risk I don't like.

As for the swagelock connectors, yes, that is workable and a good solution for connecting offboard bottles to the loop, but I am still in preference to a block where i can put in O2 and onboard dil in from one place (I have an extra port already in the head which is currently blanked off).

In the cave environment I like to have access to all gases offboard both open and closed. I would not put a reg on my O2 onboard though.

Phi, do you know when the teflon seats stopped being put in KISS ADV? No doubt you're aware of a recent ADV failure which prevented a KISS user from maintaining oxygen PP in the loop, in a penetration cave dive this would be a pretty major problem (if you're a gas extender) and/or a semi closed fixes everything mentality. Perhaps we take that one offline?

Regards

AnneMarie

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caveseeker7

First off let me say I don't dive caves, nor am I trained to do so.
But if age doesn't creep up on me too soon I'd like to (hence 'seeker, not diver').
So I've been pondering that question on and off.

In the end it depends both on the environment you're diving and your personal requirements of a rebreather.

Most units have been used successfully in caves, from old MK15 to MK-5p and RB80 to units more targeted towards recreational use like the PRISM and Inspiration, including everything in between. As well as some rather good homebuilds.

Obviously if sidemounting is required most of those units won't work too well.
As Stuart mentioned, Leon made a single counterlung/ asymetrical hose prototype for the Meg. With it's alu cylinder shape I'd say it's a prime candidate for streamlined sidemouting. He showed me lung/hose at DEMA in 03 and it looked pretty good (no pics, sorry). Something like the EDO-05 should work well, too.

Another requirement depending on the dive is scrubber capacity, obviously not every cave or dive requires 8 hours or more.

On the personal preference I agree with Cedric, water drains are an important requirement. The MK-5p is still unbeaten in that respect, allowing for a complete loop recovery. The bellows equiped SCRs are nifty with the dump valve purging water on every breath. The PRISM has one at the bottom of each bag, the Meg just one but also located where it's usable. The MK series allows to drain the inhalation side, but it looks like it's not the easiest skill to aquire. The Ouroboros I'm not sure, but believe the exhale lung can be drained.

I also like the idea of plugging off-board gas supplies into the loop, and again the Cis rules. The switchblock may be prone to corrosion, but it supplies access to offboard gases, includes the dil into the OC part of the DSV when switching and allows to isolate the O2 feed for the solenoid. The Ouroboros also has mechanical switches to isolate automatic dil and O2 addition. Isolating gases/gas paths is as important CC as it is OC in my opinion.

Another item I would want to have is a HUD as it's readable during high task loading or low viz. Cis, Meg, PRISM, Vision, HH and some MKs.

Finally, the unit must withstand the rigors of caving. While not everyone nails down the cave at 200 fpm, I would want a solid unit. That means either a unit that's very solidly build or a custom case for it that is. The Meg seems to lead the pack here, although the hoses and O2 inlet on top could do with a cover (depending on tanks they're in between, so there's some protection). Cis seems to have done well in that respect, with the addition hard rear hose protectors, as do the bellows SCRs. The Ouroboros looks fine. MK series probably too, although those cases aren't very strong. The PRISM's cowling is quite solid, but I would consider a solid metal housing anyway.

On some of the KISSes I've seen (probably earlier models) gas connectors were located on the rear side of the unit. That's a big no-no in my book.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

AM

Thank you so much for all the detail there Stefan.

Picking up on your last point, I have to agree I *really* don't like connectors at the back, or indeed any hoses or points of vulnerability. This was also one of my pet hates on the Inspiration, the T-Pieces are particularly vulnerable.

Agreed on the water drains, this IS one good feature of the Inspiration, it has good water tolerance and the placement of the exhaust is pretty much strategically situated for good water flushing.

I am noticing on a lot of sidemount homebuilds the OPV position and water tolerance is quite poor. These are key design issues that I would probably give precedence to in my own homebuild. Time permitting.

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