 Construction
started with what is arguably the hardest part of building a rebreather,
the scrubber cannister. The scrubber took a lot of thinking even though
an axial scrubber is theoretically very simple. It is just a cannister
with a couple of end caps, but without getting the parts custom machined
it is actually quite a difficult thing to build. If it wasn't then people
like AUL would be out of business.
Everyone suggested PVC pipe. It is
cheap, easy to get hold of and can be solvent welded to give a very strong
joint. There are also a good range of fittings available. Which is great
apart from the the "removable" thing, solvent welds are not renowned for
their removability. Marley do end caps for PVC pipe but I had my doubts
about being able to drill through and seal the cap as it has a double wall
construction. Sealing through one layer of plastic was going to be hard
enough, two layers would be twice as hard.
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 I
decided I'd make the end cap idea plan B. Plan A was to take a Marley 110mm
pipe, chop a small bit of pipe off, weld a cap to it and use it for the
top. Marley pipe has a socket with a rubber seal in one end so I took the
end with the socket, chopped it to about 400mm and welded another cap on
the bottom. This leaves a bottom section with a socket and seal in the
end into which the top section can slide and form a watertight seal.
The end caps I used had 50mm outlets already pre-formed into them. After I welded these onto the pipe I welded 50-32mm reducing bushes on to the outlets with 45deg 32mm elbows to take flexible pipe to connect to the rest of the loop. The whole unit gives around 300mm x 110mm dia of scrubber capacity with about 100mm of clear space at the top for O2 sensors and wiring. It is too small. Just a little too small. Ideally I should use the next size of pipe up, 160mm. But B&Q don't stock it and it looked very expensive from other sources. With split counter-lungs the current scrubber falls just on the wrong side of it but it will be enough that I can finish prototyping the rebreather and change it for a larger scrubber later on. The only two worries that I have is that the work of breathing might be too much and the dwell time of the exhale gas in the scrubbe will not be enough. To be honest, I think it might work for work of breathing although there isn't much I can do about dwell time. The unit was constructed and I planned
to test it for watertightness on the 25th at Capernwray Dive Centre where
my friend, Andy Hayhurst of Dales
Divers in Sedbergh, was doing Inspiration try dives. I'm not giving
a link to Capernwray, mainly because it is shit. £7.50 to get in
and be hassled by anal idiots for diving solo. In the end I didn't dive
for logistics reasons: timing my dive to tag my getting in the water with
another group and timing my exit to be with someone as well. It was also
cold and getting late, and the thought of lugging a pair of 20's for a
10min dive was not appealing.
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 In
the end, I settled on testing the cannister in the bath. The exhale and
inhale pipes were blocked with condoms, Durex Elite (unused) and cable
ties. Worryingly, the condoms got a million leaks with only a little bit
of rough handling. Anyhow, I let the cannister soak for an hour and apart
from the condoms, everything appeared to be watertight. When I've finished
the rest of the loop I'll take it to Dorothea where the entry is free,
the diving is excellent and there are no moaning faced gits whingeing about
dangerous practices.
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| Also this week I have ordered the neoprene for the counterlungs from RS Components, some PVC pressure pipe parts to make the connections from the counterlungs to the loop and for making the DSV from Wise Water Solutions, 38mm flexible corrugated hose from Water Gardening Direct, some skin connectors from Seamark-Nunn to make the connection to the counterlungs and one-way respirator mask exhalation valves from Severn Safety which will go into the DSV. I have some check valves from the BQ-50 that I cannibalised some drysuit dumps for but these are a bit too small. If the Severn Safety valves don't work out then I'll by some cages and valves from Vintage Scuba Supply. |