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HS: Is it correct to think that in an
enclosed space, like your pressure hull you will suffer from the negative
effects of excess CO2 before you suffer from a lack of O2? What I am getting at
is, could you upgrade a simple sub with no life support by adding just a
scrubber to extend your usable bottom time or would you only gain a little bit
of extra time before the O2 would need replenishing as
well.
CLR: The CO2 built up rate is some
what faster than the O2 consumption rate. I have on board sensors for O2
and CO2. I did a life support test over the holiday to establish a base
line before turning on the scrubber or O2 make-up. At time zero, when the
hatch was closed, the CO2 level was 300 ppm and the O2 concentration was
20.9%. After 56 minutes, the CO2 had build up to 6200 ppm and the O2
concentration had dropped to 19.0%. As a reference, ABS requires an alarm
notification if CO2 exceeds 5000 ppm (0.5 mole percent) and if O2 drops below
18.0%. There were no noticeable physiological effects at the time the
hatch was opened. Physiological effects from high CO2 (hypercapnia)
such as shortness of breath and headaches don't hit until you exceed 2-3
mole percent of CO2. A
concentration of 10% or greater can cause respiratory paralysis and death within
a few minutes. Low O2 (hypoxia) causes the
pilot to become unconscious when the O2 concentration drops below
12%.
HS: What
sort of equipment is used by those of you who do replenish the O2? It is
sufficient to have an O2 tank, conten gauge, regulator (and gauge?), and a flow
meter (with a separate O2 sensor and meter). Or does the equipment need to be
more sophisticated than this?
CLR: Most handle O2 make-up with a
tank 100% O2 with a regulator. My boat has both primary and backup O2
make-up systems. My tanks are externally mounted to the hull with a scuba
regulator that is setup for 100% O2. For automatic O2 make-up, I use a PLC
actuated solenoid valve on the LP side of the regulator to bleed O2 into the
cabin after the cabin pressure drops 2 mbar below the pressure at the time the
hatch was closed. This O2 makeup continues until the pressure is greater
than or equal to the pressure at the time the cabin hatch was closed. The
PLC locks out O2 makeup if hatch switch is open. The emergency system uses
a medical O2 regulator that has dial settings of 0.0, 0.5, 1.0, 1.5 ... 8.0
lpm. During a recent life support test I conducted, I found that
at 0.5 lpm, the O2 mole-percent stayed constant. At 1 lpm, the O2 level
would rise in the boat. The CO2 sensor I use is from Greystone Energy Systems,
model CDD1A-1-00-0 . The range on the unit can be set from 0-2000 ppm or
0-10000 ppm. I found the 0-10000 ppm setting to work best for me. The
O2 sensor element I used is the Maxtec MAx-250 . This sensor has a
range of 0-100 mole percent O2. These were the most cost effective sensors
I found. The circuit board I used with the O2 sensor and barometric
pressure sensor was designed by Gary
Boucher.
HS: What do you do about ?oxygen
cleaning? Do you have to just concentrate on the O2 supply kit or all of the
equipment within the sub?
CLR: Assuming the 100% O2 is added
at a point in the hull that is gets mixed with the air circulating in the
boat, then you just need to make sure all of the gas components, i,e., tanks,
regulator, tubing, valves, fittings are 100% O2 compatible. The o-rings
need to be rated for O2. Each components needs to be physically cleaned to
remove any hydrocarbon based oils. See Vance Harlow's "Oxygen Hackers's
Companion" on how to do this.
HS: How many of you provide
yourselves with CO2, O2, and internal pressure gauges to monitor the environment
but do not fit additional life
support.
CLR: I for one have onboard sensors
for CO2, O2, cabin barometric pressure, relative humidity and cabin
temperature. What is required by ABS is CO2 and O2 sensors.
The bottom line is bottom
time. The Kitridge boats have been very successful without any
life-support other than short bottom times and resurfacing to open
hatch to ventilated the cabin. There is a lot to be said about
simplicity. If you want multiple hour bottom times or if you want to
stay within ABS rules, you need to provide for O2 makeup, CO2 scrubbing as well
as O2 and CO2 monitoring with
alarms.
Cliff
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