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Richard Pyle <deepreef@bi*.bi*.Ha*.Or*> and Carl Heinzl
<cgh@ma*.ai*.mi*.ed*> (Subject: Re: Hello all) about aqualung habits
needing to be lost when you change to rebreather. For those too busy to wade
through the other matter in the same messages, these points, plus some that I
noticed, seem to be:-
(1) You can no longer use breath control to effect buoyancy changes because
of the counterlung. (I noticed that when I had a short oxygen rebreather (Mark
IV Amphibian UK naval-type set) course around 1970.)
(2) Avoid sawtooth profiles: to maintain neutral buoyancy they require you
to add/dump gas using up your diluent too quickly.
(3) Avoid sudden descent too fast for absorbent addition and mixing to catch
up causing a sudden high ppO2 `spike'.
(4) There is no need to adopt special air-conserving breathing rhythms. (And
the reverse when changing to aqualung if you started on a rebreather, as
likely some will.)
(5) Get a really WATERTIGHT mask so you don't have to waste gas endlessly
clearing it.
(6) Grip the mouthpiece properly watertight hard all the time; don't
casually drop it when on the surface, to talk or to save air etc. This
avoids:-
(a) Shipping water past the mouthpiece (either a lot at once, or a steady
trickle, which gets in the absorbent), which (if it isn't polluted) doesn't
matter with an aqualung as it is blown out with the next breath out.
(b) Getting extra (atmospheric) nitrogen into the circuit.
(7) Lose any habit of breathing out partly through the nose.
(8) For those reasons, I much favour a fullface mask with a rebreather. To
avoid dead space and in case it breaks, fullface masks often have a mouthpiece
or a small mouth-and-nose mask inside. (A strapped-on fullface mask with a
mouthpiece inside can be a remarkably efficient gag (= un-loseable device to
prevent talking)!) Fullface masks seem to be far easier to keep watertight:
details of face shape and even being one day unshaven can be very critical on
watertightness of mouth-&-nose masks. (Some industrial fullface masks are
unsuitable for underwater use: they seal with a wide sealed thin-walled rubber
tube filled with air at atmospheric pressure; this moulds to the face, but at
depth pressure it collapses and so the seal does not work.)
(9) To avoid leaks and casual dropping, many industrial oxygen rebreather
mouthpieces have an extra outer flap that goes outside the lips, and coming
off the flap a strap to fasten behind the neck to strap the mouthpiece in.
(10) You need to learn about partial pressures and depth, much more than
with an aqualung.
(11) Most rebeathers are smaller and closer to the body and much more
streamlined than aqualungs; you will find that with a rebreather you are far
more agile underwater. (The reverse if changing from rebreather to aqualung.)
P.S. (a) Re (4) hereinabove: Has anyone heard of medical respiration testing
being upset because the patient is an aqualung diver and when he is on the
medical breathing testing apparatus he automatically goes into a diving-type
air-use-economizing breathing rhythm?
P.S. (b) Does anyone make a breathing set connection to connect to the hole
in the front of the neck (= tracheostomy) left by laryngectomy?
P.S. (c) For habitual sea divers, I recommend a good l-o-n-g freshwater dive
once in a while, as a way to give your kit a good long soak to get salt out,
especially if your bathroom bath is not big enough or not free for long enough
at a stretch for adequate kit soaking.
P.S. (d) At one diving centre, I encountered one ultimate luxury after cold
sea dives: I could take my kit off under a hot shower!
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