Alec, very interesting, thanks for sharing! Hopefully your O2 meter generally tracked with Jon's?
On Aug 6, 2011 4:27 PM, "Smyth, Alec" <
Alec.Smyth@compuware.com> wrote:
> 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.
>
>
> Cheers,
>
> Alec
>
> <<Snoopy life support test.xls>>
>
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