Hi Greg,
I had always wondered about which fan direction was better
through the scrubber, and found Sean Stevenson had once made a good case
for drawing into the cylinder instead of blowing out. I paste from his
email:
The
scrubber is a radial flow design, with the flow direction from the outer
diameter to the inner one. This makes most efficient use of the scrubber
material, as the flow area is greatest at the outer diameter when the gas
contains more CO2, and reduces as you approach the inner tube, as CO2 is
removed. Also, the inlet flow area of the scrubber (outer diameter x pi x length) is huge in comparison to the minimum
scrubber inlet diameter, making the velocity (assuming even pressure
distribution) almost nil through the absorbent.
Another thing I like about drawing air out is that the scrubbed air
comes out in a specific direction. The scrubber is way back against the aft end
cap, so I can point the fan outlet forward and get a slight ventilation effect.
If I reversed it, the flow out of the scrubber would not be directional.
Just two cents... Alec From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Greg Sent: Monday, August 08, 2011 11:45 AM To: personal_submersibles@psubs.org Subject: RE: [PSUBS-MAILIST] Snoopy life support test Hi
Alec, First, great job on the
test! With regard to fan
amperage, the photos look as if your radial fan was set up to draw air through
the media as opposed to blowing into- is that
correct? Generally speaking, if
a fan draws a partial vacuum, amperage will drop (because the load is going
down). Also, efficiency of the
media should go up with an increase in pressure. It would be very interesting to
conduct the same test with the fan “blowing” into the media
instead. Greg From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Smyth, Alec 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,
The contents of this e-mail are
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[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace The CO2 readings are definitely
high, especially in the test without life support. I know that when I used
the monitor out in the open the CO2 percentage was between .01 and .03 so I
don't expect a calibration error is the cause of the high readings unless
something happened to the device during shipment. That should be easy
enough for Alec to test however by simply going to an open area and checking the
CO2 reading. Alec, thanks for posting the
life support test for Snoopy. I quite enjoyed seeing actual test
data. Your results of axial vs. radial fans mirrored my results and my
conclusions were the same; i.e., radial fans are the way to go. Have you
done any test or calcs to see how long a 0.36 amp current load will take to burn
your backup batteries? My guess is that at this current, you would not
make 72 hours. IF they don you may have to find a radial fan that draws
less current. |