Wibble's CCR odyssey

Is there anything specifically incorrect in what I posted?

 

Effect of shutting the O2 valve...

Was doing more drills over the weekend targetting the response to the "3 H's" (hypoxia, hyperoxia, hypercapnia) -- thanks @Vanny

The diluent flush on a Revo is through the mouthpiece as there's no dump in the lungs (excepting an over pressure valve). Inject diluent and use "leaky lips" to let the gas pass out of the mouthpiece. Injected gas (dil & O2) enters the exhale lung, crosses the scrubbers over the 5 cells into the inhale lung then exits through the mouthpiece. The O2 levels rapidly show a decline using this technique. Need to dump some gas from the wing to counteract the increase in lung volume.

(A dil flush was run through on MOD1, but I hadn't re-run it since then. This will become part of the standard set of drills to run on every dive)

Shutdowns. There's an issue with shutting down the O2 valve on a Revo which is the "leaky valve" will run down the first stage pressure leaving the first stage unpressurised and thus vulnerable to loostening and being unable to repressurise after turning back on. This is the same problem for all rebreathers with a stuck solenoid.

I did turn the O2 off; I did depressurise the O2 system; it did erupt in a cloud of bubbles when turned back on. Thankfully, it re-connected after turning off and on twice more. It was impossible for me to reach the valve seal whilst wearing the unit. Solutions to this -- aside from bailing out -- would be to run the unit on diluent only (MOD1) or connect an off-board rich gas (MOD2).

I concluded that in a bailout scenario the O2 should be left switched on to avoid the risk of depressurising the O2 first stage and leaving it vulnerable to flooding (and needing a service).

 

Thanks for that lovely news

 

Cylinder won't have water ingress as it wasn't empty.

Regs are designed to cope with water ingress otherwise they'd be exceedingly unreliable. For example, standard rich decompression stages are kept off, although pressurised, but this can easily empty if the second stage is purged or any leaks.

As for a rebreather, it'll be a small amount of water that will eventually dry out due to the passing gas. The residue and any sediment passing though the gubbins is the bigger challenge, especially in the microscopic leaky valve orifice. Have to say that one would expect that one of the core requirements of a rebreather is to cope with this.

I frequently check that the orifice is running correctly during the build process: should take 15 seconds to drain from 150 to 100 bar. If longer, it's blocked. The 15 seconds was measured when the orifice was known to work. Still don't know the IP nor flow rate, just that it matches my quiescent metabolic rate (which I monitor when on the bottom phase of a dive -- the PPO2 would increase).


Suit inflate cylinders are very prone to flooding as you don't monitor the pressure and especially on a small tin it's easy to empty the tin; do that at depth and water ingress is inevitable. Worse than that, they'll fail a suit inflate as it can't be shot blasted (according to a testing station that failed one of mine).

 

So why then is the SOP to feather valves, especially O2 valves? Could also question, god forbid, that the benefit of the A-clamp is the seal's kept under pressure when the gas is turned off.

Do we see O2 fires caused underwater on rebreathers due to contaminate ingress? (Have seen photos of an Inspo that had been destroyed by an O2 fire; don't know the detail of this incident)

 

Was thinking about a pollutant getting into the first stage from the water, then being ignited when it's re-pressurised. Does assume that the valve re-seals after turning off and on again.

As an aside, the AP cylinders have a nice slow-start feature which one hopes mitigates adiabatic fires.

 

You often get bioluminescence in the Thames Estuary and other places. At night you can sometimes see the glowing wake of the boat. Peeing over the side creates a light show; even peeing into the toilet bowl (raw water flush). Maybe it's an age thing.

 

Big reels (keep on turning)

Have bought another Kent Tooling 100m narrow composite reel. Another because I bought one three years ago and sold it after two dives, on OC it was horrible. It turns out that on CCR the big reel is brilliant. A real asset to the dive, not only because it simplifies the ascent, but because it moderates the deco stops. OK, maybe having three more years of diving experience helps too.

The reel with attached SMB sits nicely on the side of the bottom of the unit. Because of the two stage cylinders (am diving with two to normalise their use in preparation for deeper diving), the reel sits nicely out of the way of any wreckage and is easily accessible when I need to bag up.

For now I’m still using my 2m Halcyon SMB and inflating it in the same way as I’ve done for hundreds of dives, from my drysuit inflate hose (backups are available on the bailout and deco stages). So reach for the reel, set it to freewheel, remove the SMB and hold the inflator nozzle. With the other hand disconnect the drysuit hose and place on the inflator, look around and up, ensure nothing’s caught, then push it home to inflate. Up goes the bag and the reel spins. Occasionally slow it down or lock the ratchet to get moving (will work on that later).

Now the ascent is easy. A largish diameter reel means fewer turns and the ratchet means it won’t unwind if dropped. Simply stop winding to dump from the wing or inject oxygen, dumping from the suit shoulder dump or clearing the counterlungs through the nose (or loose lips) doesn’t need a spare hand.

Then arrive gently at the first stop. Tweak the PPO2, set the lung volume, and relax..... all’s fine and easy.

It turns out that weight of the reel really helps to moderate the buoyancy at the deco stop. Instead of constantly worrying about a slight upward or downward movement requiring a gas injection or dump, the reel gives you some weight if you need it to arrest a slight upward movement and the SMB line does the opposite if descending. This then means your buoyancy can be much more neutral. A revelation and suddenly everything is so much easier.

Even reaching the surface is easier as the reel doesn’t need any special clipping off, although clipping the boltsnap to the SMB means the reel can’t unwind if knocked.


On the same dive was delighted that I no longer need to worry about gas. Discovered that the 40m wreck was 48m. No problems as I’d selected the "below 30m" gas and bailout; 18/45 with 70%. Lovely dive. Vis of 4 to 6m and just under an hour on the bottom with the main worry of the 2h runtime for the skipper (and all the other divers waiting). A fiver for the trimix very well spent.

Just loving life as a CCR diver.

Pity the weather’s cancelling more dives than are running.

 

Yesterday's dive was a little more spicy. Skipper made a great call to use the brief window between Wednesday's storm and Storm Alex. Was quite comfortable steaming out to the wreck, but the wind increased and changed direction whilst we were down.

Did a couple of hours with an hour of deco. Again, getting to like the reel and now seeing the benefit of an auto-inflating SMB (yet to be decided upon).

As usual, the decompression stops are lovely and calm. The only clue to the weather above being the reel jumping up and down. After putting up with this for a while, I finally relented and let out a couple of metres of line and dropped the reel to bounce up and down below me -- this demonstrating the sheer quality of the Kent Tooling mechanism. Used a double-ender held in the hand and wrapped around the line to prevent glove chaffing. If the buoyancy needed any fine tweaking I could just pinch the line to get a little bit of weight or lift to avoid injecting/dumping gas in the wing.

Nice dive. Vis not brilliant, but enough to enjoy the dive.

The steam back to port was a bit spicy though. All of us keeping our drysuits on and sitting on the floor sheltering behind the (off-limits) cabin.

Kudos to the skipper for such a great weather call.