I finally did life support
testing today and thought I'd share the resulting data.
Snoopy has had life support for years, yet I never used it
because I found my dives were too short to require anything
other than surface-and-replenish. So far, I have always
dived with the O2 tank full but shut off at the hull stop,
and a sealed supply of Sofnolime aboard but not loaded in
the scrubber. However, Snoopy has now been modified to carry
two people, so I felt it was time to revisit the life
support and start using it. Also, space limitations led me
to make a new scrubber as I could no longer fit the old one
with the extra person aboard. A K250 can be a two person,
but only if the two people are skinny, and there isn't an
inch to spare.
Snoopy carries HP oxygen
outboard, which is supplied to a O2 clean first stage, and
then through the hull to a pediatric medical regulator with
a range of 0-4 lpm. The scrubber is a hollow cylinder
containing 9 lbs of Sofnolime. I carry scrubber reloads in
sealed containers, to keep the scrubber dimensions small and
to avoid using unnecessarily large quantities of Sofnolime
for what are typically short dives. A key objective of the
test was to choose an appropriate scrubber fan, and I had
two candidates on hand to test. One was an 11.3 CFM radial
fan and the other a 27.6 CFM axial computer fan. The
computer fan has the advantage of being quieter, 29.2 dBa
versus 38 dBa, and also draws only 0.16 amps compared to
0.36 amps for the radial fan. However, axial fans develop
very low pressures, so I was skeptical that the computer fan
would draw enough air through the Sofnolime despite its
comparatively higher CFM. For instrumentation, I have an O2
meter and a converted skydiver altimeter for indicating
cabin pressure variations. For the test, I also had a Sub
Aspida kindly lent by Jon. It indicates percent CO2, percent
O2, and cabin pressure, and is a far, far more precise
instrument than mine.
I did three twenty minute garage
"dives" with my daughter. There didn't seem to be any need
to go longer, since twenty minutes were more than sufficient
for the life support system to reach a steady state. The
first dive was with no life support at all, to establish a
baseline. The second and third dives tested the system using
the different fan choices. Finally, I did a five minute
vacuum test to ensure the reliability of the pressure
readings.
A scrubber decreases cabin
pressure, while the O2 feed increases it. In a more advanced
life support system, the O2 supply would be automatically
controlled as a function of either O2 concentration or cabin
pressure, but since my simple system has no such automation,
I was expecting it would require many manual adjustments to
the O2 bleed rate. Happily I was wrong, or else very lucky,
because my initial guess of 1 lpm seems to have turned out
exactly right. During both life support dives, I simply took
note of the readings and never made a single adjustment.
After a 1 mbar change in the first minute, the radial fan
yielded zero pressure variation for the rest of the dive,
while with the axial fan I got a negligible 2 millibar
buildup toward the end. That's 0.03 psi. It was much easier
than I expected, I literally did nothing.
As you can see from the attached
results, the radial fan was much more effective. The
computer fan stabilized the CO2 at just under 0.7%, which is
probably acceptable, but the radial fan stabilized it at
0.2%, which happened to be the concentration at the start of
the dive. You will notice the radial fan data has lower
variability. I attribute that to the fact the radial fan
caused a slight draft in the cabin, thereby ventilating the
instrument better. In conclusion, I am going with the radial
fan because there is a clear performance advantage, and the
noise is still quite moderate.