|
Emile,
My testing was for one person. As to the 0.5 lpm O2 rate, in the testing I did, it depended on
the pilot. I am somewhat on the large size at 220 lb/100 kg. For me
0.5 lpm was just about right. When my brother was in the sub, 150 lbs/68 kg, the
O2 rate was to high as you mention. Since my backup medical regulator has
discreet settings of 0, 0.5, 1.0 lpm, to accommodate a rising O2 level, he would
just set the dial setting to 0 lpm when the O2 level reached 22% and then when
the O2 level would drop to 20%, he would turn it back to 0.5 lpm. The
automatic system works better for me as is independent of differences in
physiology.
Good point to pressure drop technique. Even though
I was doing my testing in the shop, the heat generated by a body in the interior
did elevate the temperature which did have an impact on cabin pressure. I
can see how this would be exaggerated with the range of water temperature chance
during a dive.
As to your backup scrubber (rubber hose) on Sg.
Pepper, do you just breath normally through the hose connected to the
scrubber or do you have a valve arrangement so that you are only exhaling into
the scrubber and inhaling cabin air?
Cliff
----- Original Message -----
Sent: Sunday, January 29, 2006 3:33
PM
Subject: RE: [PSUBS-MAILIST] Life Support
Test Experience
Cliff,
Thanks for sharing your test expirience with us.
Was figure of 0.5 liter/ min.oxygen for one person
allright? When testing/ diving with the sgt. Peppers the oxygen level becomes
to high with the (accurate medical) flowmeter set at 0.5. Oxygen
flowmeter set at 0.3 and the percentage oxygen stays
ok.
I think adding oxygen by the "pressure drop"
method is inaccurate. When operating the boat at 4 deg. celcius ,
the pressure rises considerably due to the warming up of the interior during
the dive. The overpressure valve blows off when ascending to 1 meter below
surface.
In Peppers is no space for a emergancy battery or a
double sysytem for operating the scrubber. If something fails, put the hose to
your mouth and exhale direct "lung power" to the scrubber.
Regards, Emile
When I started designing my 1 atm. psub, I found the
experiences builders had shared at psub.org very interesting and helpful in
conceptualizing the best way forward on my design. When starting a design with a blank
piece of paper, there are an infinite number of ways it seems to do
everything. Seeing posts on what
has worked and not was helpful in eliminating some of these design
possibilities. With this in mind,
I will share some of my experience to date on life support systems based on
testing I have done in my shop on my boat. I am not an expert and this is my first
boat. My goal is to meet ABS guidelines. The essential parts being, to provide
life support for 72 hours while maintaining the cabin O2 concentration between
18-23% and the CO2 concentration below 5000 ppm (1/2%). As background, I built a clear acrylic
scrubber that will hold 3.75 lbs of "4-8" mesh Sofnolime, has a blower that
pulls about 0.17A and operates, in an emergency, on two 12 volt 100 amp-hour
Lifeline model GPL-27T AGM deep cycle marine batteries connected in series
(24v bank). While the hatch is
closed, I run the blower continually.
Eighty scf of O2 (139 bar) stored externally in medical cylinders is
available to be metered into the boat either manually with an adjustable valve
that has a dial indicator or automatically. For the automatic system, initially, I
controlled the O2 make-up solenoid my monitoring the cabin pressure. When it dropped 2 mbar below the hatch
closing pressure the PLC would open the solenoid and would close the valve
when the pressure returned to the pressure at the time the hatch was closed.
This system uses a regulated O2 pressure of 25 psi above ambient water
pressure. The system worked but after several weeks of testing, I switched to
controlling this solenoid based on the measured O2 concentration. When the concentration drops below
20%, I turn on the O2 makeup solenoid and turn it off when it reaches 22%. The
control system just seemed to work better controlling of a direct measurement
of O2 concentration. I went
through 5 different scrubber fans before I found one that met both short term
and long term constraints. Early
fans I used were quite and pulled less than 0.1 amp but did not circulate the
cabin air through the scrubber absorbent at high enough flow rate. As a consequence, the CO2 level in the
cabin would rise to 7000-8000 ppm before stabilizing which would trigger a
high CO2 alarm. ABS requires an audible alarm if CO2 concentration rises above
5000 ppm. Some strong blowers I tried would do a great job in keeping the CO2
level below 5000 ppm but at 1-3 amps, would pull down the 24 v emergency bank
in less than the required 72 hours and were very noisy. I ended up finding two small German 24
VDC fans with brushless DC motors that together pull about 0.17 amps and were
quite. To meet the 72 hour
constraint with my batteries, I needed a hotel load less than or equal to
about 1.35 amps. In my case, this
load consists of the blower-fan, PLC, O2 makeup solenoid-valve coming on
periodically, LED interior lights and communications. My choice of CO2 absorbent, Sofonolime
over Sodasorb was totally arbitrary. If someone has any data or
experience that would say one absorbent is better than the other, it would be
nice to see. My experience
to date has been that having a backup O2 makeup system is prudent. On one 8 hour, closed hatch test
in which I was trying to measure the endurance of one the CO2 absorbent
cartridges, the needle value downstream of my O2 makeup valve failed after 1
hour. By looking at the current
draw in the boat and the O2 tank pressure, I was able to determine that the
solenoid was being energized and that there was adequate pressure but that no
O2 was flowing because the O2 level continued to drop. Rather than abort the
test, I switch over to the manual O2 makeup system and set the rate at 0.5 lpm
which enabled me to complete the 8 hour test. At the conclusion of the test, I found
a small bit of Teflon tape had plugged the O2 needle valve on the automatic
system. My conclusion is having
redundant systems in a boat is a good thing.
Cliff
?One test is worth a thousand
expert opinions.? ? Bill Nye
|