Alan,
Some issues to contemplate with using NITROX with an
ambient PSUB:
1.
Without constant exhaust to the water, the issues
become much more complicated.
2.
If you use a scrubber you now need to have an oxygen
supply and a NITROX supply, the O2 to make up consumed oxygen and
the NITROX to maintain your bubble volume in the PSUB with increases in
depth.
a.
If you use a constant flow of oxygen, optimally this
should be equivalent to your rate of consumption of O2 and is
usually accomplished with an orifice. The problem is that the size of
the orifice is very small and can be clogged easily. Rebreathers that
in the past have used this method have been plagued with clogged orifices
and the Navy had numerous warnings concerning this issue in their course
?Principles of Navy Diving?.
b.
If you were to just add NITROX as the ambient
atmosphere is scrubbed the partial pressure of nitrogen would continue to
creep up.
3.
If you just consume the O2 and let its
partial pressure drop you are using the equivalent of a constantly changing
O2 concentration in the NITROX instead of having a fixed mix and
there are no decompression tables for this case.
4.
If you are to depend on just a flow of NITROX
without the addition of make-up oxygen, your optimized to the equivalent of
discharging every fifth breath to the sea. The deeper you go, the more
breaths you can go before venting to the sea.
5.
Careful appraisal of the above issues would suggest
that oxygen levels should be constantly monitored if you are planning long
dives in your ambient NOTROX sub. Again falling back on rebreathers, 3
independent O2 monitors are employed and constantly
compared. A further concern with oxygen monitoring cells is that
extreme humidity can be an issue if not carefully placed. A very
experienced technical diver and the lead engineer for a Navy rebreather
project was pulled from the bottom of a pool during a test dead (no heart
beat) but was fortunately revived?the oxygen sensors provided faulty
readings due to high humidity. Needless to say there was a revision in
the design of the rebreather.
As Jon noted, many items that are not normally thought
of as combustible, will ignite and burn vigorously under increased partial
pressures of oxygen. If you deal with all of these issues, it would
have probably been easier to build a dry sub. All of these issues have
previously been explored in depth previously in the archives.
R/Jay
Resepectfully,
Jay
K. Jeffries
Andros Is.,
Bahamas
Save the whales, collect the
whole set.
-----Original Message-----
From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Alan
James
Sent: Tuesday, August 11, 2009 7:13 PM
To:
personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Ambient
running on nitrox
Thanks for your responses.
I've seen Phils white paper, but that system has a
bellows at 1atm that responds to pressure changes in the cabin to release
additional O2. It won't work in an ambient.
I'll look into the fire risk a bit more. O2 doesn't
combust but I'm not sure wether the flash point of a fuel will be lower with
the higher % O2 or how much more ferocious a fire would be. I have a friend
who is a dangerous goods expert that accompanies fire fighters to industrial
fires as an adviser.
I'll pick his brains & update you on
that.
Re the physiology; you just stick to the Nitrox no
decompression dive tables. In general you get double the bottom time &
your limits round 100ft with no decompression limit above
40ft.
This idea was originally suggested to me by a pretty
clued up guy who runs a dive shop & instructs in rebreather & mixed
gasses.
Alan