Hi everyone,
OK, let's try to answer some of all these questions. First, my
daughter is twelve but she's extremely tall for a twelve year old, being
already the same height as her mother. She would still be on the light
side for an adult, but she's definitely more than half an adult. If I
were diving with a crewmember of about my size, my guess (and that is all
it is) initial O2 setting would be 1.25 lpm instead of the 1 lpm used in the
test with my daughter.
I agree the initial concentrations of CO2 are high,
and I think they are due to the time taken up by getting into position
and dogging down the hatch. I was working in the sub up to about ten minutes
before the first test, so there was probably some lingering CO2 from my earlier
presence too. The getting-ready process was probably about 4 minutes,
the last minute or so being with the hatch closed. Just to make sure
the instrument is not reading high, I tested it outside and it reads 0.02% CO2
and 20.2% O2.
In terms of the permissible concentrations of
CO2, the air at the end of the unassisted dive did not yet feel at all
stale. We were at 1.6% CO2. But according to Phil's life support white
paper, the Navy's standard for manned submersibles is to keep
CO2 under 2.5%, while NASA's standard is 1% for indefinite
exposure and 3% for up to one hour. I could not
tell any difference in air quality between the three dives, other than by
looking at the instruments.
Yes, my O2 meter did track the Sub Aspida readings, which is
reassuring for when I ship the latter back to Jon! Problem is, mine has one
percent discrimination while the Sub Aspida goes to a hundredth of a
percent. On the unassisted dive mine went down to 19% and then 18%, at
which point its lower alarm went off, but on the two life support dives mine
didn't have the precision to show anything meaningful at all, and was at 20% the
whole time. Its functional enough to tell you if you're in trouble, with a
backlit display and low-high alarms, but useless for a test of any accuracy. By
the way, with something as accurate as the Sub Aspida you have to be a bit
careful in the measurements. If you aim a breath in its direction, the reading
shoots way up. If you raise it into the dome, it shoots way up. Just walking
from outside into my home, the CO2 increases by 300%.
Regarding the radial fan current versus battery capacity, I've
a new bit of information. Yesterday I noticed that the fan's spec sheet lists
the current draw as 0.22 amps, while the fan has 0.36 amps printed on it.
Given the conflicting figures, I put down the higher one. But Cliff's
email prompted me to go out and measure the current draw to settle the
matter. It turns out the fan pulls only 0.2 amps with the scrubber
full, so I guess the spec was right and the motor sticker wrong. Another detail
here is that Snoopy, being a little K250, is a 12 volt boat with
a single battery bank. The original K250 design has three batteries wired in
parallel. In the recent redesign I was able to expand that with a
fourth battery for a total of 316 amp hours, but I still have the original
single-bank configuration. Of course, without a separate
hotel battery, my reserve for the fan will depend on the battery charge
condition.
If anyone wants the specifics, the radial fan is a Delta
Electronics model BFB0712H. Here are some photos attached of scrubber
and fan.
Final point... I was not surprised by the fan results. What
really did surprise me was the stability of the cabin pressure, in particular in
view of the precision to which the Sub Aspida was measuring it (1 millibar), and
how easy it was to use a system with no feedback loop regulating the flow. I
wonder if that is typical or lucky?
Thanks,
Alec