Scott, filtering your bottled oxygen is a waste for 1 atm
consumption, since it is flowing into a space filled with equipment,
one or more human bodies, possibly oil vapours, and who knows what
else. High purity isn't really relevant in that context.
Contamination is much more serious for divers because the gas is
being breathed at elevated pressures, increasing the partial
pressure of whatever contaminants may exist in the gas. This is
also exacerbated by other physiological factors such as oxygen
toxicity damage to the lungs at elevated PPO2. For a 1 atm space,
while still important to keep the air clean, you can get away with
less stringent attention to this. What you want to do is sample and
analyze the O2 to ensure that it is industrial purity oxygen as
advertised - this is simply to avoid using a cylinder which a
previous customer might have carelessly backfilled with something
else, given that the gas supplier doesn't routinely check this when
topping off partially depleted cylinders. Another customer using a
gas setup without appropriate check valves, for example, could lead
to this situation. In your sub, air impurities occur as the result
of human flatus and surface body odours, plastics outgassing,
lubricant vapours and mold / bacterial growth in damp spaces, which
will constitute a higher level of contamination that that present
(typically) in industrial O2. I would be more inclined to add a
Sofnofil stage to my scrubber to remove odours and non-inert gas
components from the recirculating air at 1 atm, than to worry about
removing whatever trace amounts of impurities might be present in
industrial bottled oxygen at high pressure - such traces would be
filtered by the Sofnofil anyway. I might think about filtering if I
were boosting the supply gas to a higher pressure when transfilling
to other bottles, but otherwise it seems like a wasted effort. The
only thing I would do is include a particulate filter, in the
interest of keeping your piping and gas delivery system clean. As
rare as they are, stray bits of grit can cause major headaches.
-Sean
On 2012-05-09 09:33, swaters@waters-ks.com wrote:
Sean,
Really for what I'm doing with my K-350, I'd be ok with
welding grade O2 if I plumbed it in with a carbon filter and
then had a CO2 scrubber running right? For added saftey I
would have a O2 meter, CO2 meter, and a atmospheric pressure
gauge. I talked to my gas provider and he said welding grade
is 99.95% or higher and medical grade is 99.99% or higher. He
said they come outof the same tanks, the only difference is
they test the medical stuff more and they vacuum the cylinders
a little bit different or something. He said medical grade is
about 3 time as much money and you it is considered a
priscription. Let me know what you think.
Thanks,
Scott Waters
-------- Original Message --------
Subject: Re: [PSUBS-MAILIST] O2
From: "Sean T. Stevenson" < cast55@telus.net>
Date: Mon, May 07, 2012 11:22 am
To: personal_submersibles@psubs.org
I am using gases for diving, not for 1 atm life support.
Nevertheless, the concept is the same as a filter stack you
would use with a breathing air compressor, except that it
will be exposed to 100% oxygen and must be designed
accordingly. The only other difference is that instead of
filtering from the compressor outlet, you are filtering from
storage cylinders, so water removal and CO2 removal should
be non-issues. My filter stack is overkill - it was one of
those hobby projects that I ended up going overboard on
design just because I could. It was originally intended for
use with an oiled compressor. When the compressor purchase
fell through, I made the towers smaller so as not to have a
lot of wasted volume, but finished the filter project
nonetheless. Gas grade as delivered from my gas supplier is
a non-issue, as the purity is effectively increased when run
through the filter.
Right after the cylinder valves, I use an in-line
particulate filter (replaceable sintered filter element) to
catch anything physical, such as dirt that might find its
way into the valve orifice when the leased cylinders are
used elsewhere, from entering my system. I think this is a
Swagelok TF series filter. Then to the supply pressure
gauge, check valve, pressure regulator, fill pressure gauge,
filter stack and needle (flow control) valve to the fill
whip (which has the fill line bleeder). Just before the
flow control valve are the (now diminutive) filter towers,
which pass the gas sequentially through:
1) aluminum oxide / potassium permanganate (Molecular
Products Sofnofil)
2) silicon dioxide (silica gel)
3) 13X molecular sieve media: sodium oxide / calcium oxide /
aluminum oxide / silicon oxide (Molecular Products SofnoSiv)
4) activated carbon (Molecular Products Chemsorb 1000)
5) platinum/palladium/tin oxide catalyst (Molecular Products
Softnocat 423)
Since there isn't much in the way of contaminants (not
dealing with compressor oil or atmospheric CO2/CO) when
you're filling from storage cylinders, the filter
replacement schedule is out the window. Rated shelf life of
the 13X media is 12 months, so I refresh everything once a
year. For a filter dedicated to simply cleaning up bottled
gas (which is probably already close to perfect), you're
probably fine with just a physical filter, 13X and carbon,
but even that is wasted effort for sub life support due to
the fact that you're not breathing the gas under pressure,
nor directly from the cylinder(s), and that you will be
processing your cabin air anyway. It makes more sense to
scrub your air at 1 atm, since you don't then have to design
for cylinder pressure, and can incorporate hydrocarbon
scrubbing media into your CO2 scrubber design to continually
remove contaminants that may be introduced downstream of
your HP gas supply. Even an off-the-shelf activated carbon
impregnated filter incorporated into your scrubber is
probably sufficient for this purpose.
-Sean
On 2012-05-07 06:07, swaters@waters-ks.com
wrote:
Sean,
Thanks, that is very good info. What physical and
cheimical filters do you use with your welding O2 in
your life support system to be sure all hydrocarbons
have been filtered out?
Thanks,
Scott Waters
-------- Original Message
--------
Subject: Re: [PSUBS-MAILIST] O2
From: "Sean T. Stevenson" < cast55@telus.net>
Date: Sun, May 06, 2012 9:18 am
To: personal_submersibles@psubs.org
Scott, I have been dealing with issues like this
with gas suppliers for years. The purity issue is
nonsense. The medical grade oxygen is filled from
the same bulk cryo storage as the industrial stuff.
The difference is that medical grade is periodically
sampled and analyzed for USP specs compliance, which
adds a cost overhead, and on the medical grade, the
filling procedure is slightly different - vacuum is
pulled, O2 is flushed at low pressure, vacuum is
pulled again and then the cylinder is filled.
Industrial cylinders either have one or no vacuum
steps before filling. The idea is to avoid any kind
of hydrocarbon contamination, but such risk is
small. I have been using industrial helium and
oxygen (run through my own physical and chemical
filters) for years without issue. If you can do so,
it will probably be cheaper. If not, just explain
your situation to your GP and you can probably get a
prescription. Another option to look for is
so-called aviator's oxygen, which is certified to
have minimal moisture content (prevents freeze-up
issues), but may be cheaper than medical.
swaters@waters-ks.com
wrote:
I was at my local welding and gas supply the
other day and they were asking me how my sub was
doing and we got on the conversation about the
life support system and how it works. They told
me they could not sell me medical grade oxygen
with out a prescription from a doctor which I
thought was kind of weird. They said I might be
able to use oxygen that you use for welding, but
it is not as pure. Does anyone on psubs have any
ideas on what to do?
Also I had asked a question a while back
about the tubing in the K-350. I have decided to
go with stainless steal, but was still wondering
what wall thickness to go with and how do you
connect them (flair fitting, compression
fitting, or something else?)
Thanks,
Scott Waters
|