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Re: [PSUBS-MAILIST] CO2 absorbant efficiency
Alan,
All my references are on the last slide of that presentation and I
believe I got my data point for capacity of the material at the
Molecular Products web site. The slide set presentation was written
back in 2005 and the specific page I referenced is no longer available
but there are plenty of other data sheets for various products on the
molecular products web site. There are different grades of soda lime
which affect the CO2 capacity of the product. By the looks of current
data sheets on the molecular products web site, it appears that I
selected a rather low grade product (100 liters/kg) since I can't find a
product that has that low of a capacity on their web site any longer
(perhaps that's why they removed the product??). It's possible that in
the tests documented in phil's paper they used a higher capacity product
than the 100 liter/kg product I used as a example, or the test subjects
were better breathers than the numbers I provided in my slide set, or
maybe the product is actually more efficient than molecular products is
advertising.
Duration depends not only on CO2 capacity of the material but also on
efficiency of the scrubber, breathing rate, and tidal transfer of your
lungs. The breathing physiology numbers I used in my example probably
came from webmd.com (been too long, I can't recall) as referenced at the
end of the slide set, however practical testing with real numbers is
going to produce different results. So for example, if I change 20
breaths per minute to 18, and change tidal transfer from .5 to .3, and
use a product with a capacity of 160 liters/kg from the molecular
products web site, I come up with 269 minutes duration per pound.
Both documents should be considered a reference for design but not taken
verbatim as consistent performance. When it comes to scrubbers "your
mileage may vary" is a very real statement just as mileage per tank of
car fuel depends upon the driver. I realize you are saying that the
difference between the two documents is large enough that it would
potentially impact design of storage capacity for extra scrubbing
chemicals. For early design purposes the safest bet would be to use the
more conservative approach if possible, or if that results in too large
of a physical space then you might split the difference. Worst case you
end up with more storage of chemicals which potentially translates to a
longer dive duration in event of emergency. I would also recommend that
you first determine exactly which brand and grade of soda lime you can
readily get access to and then secure a data sheet for it. For a
theoretical analysis you can follow the procedure in my slide set to get
an idea of duration based upon the published CO2 capacity of the
chemical you intend to use. However, realize that those numbers
represent 100% efficiency (as my slide notes) which you are unlikely to
achieve. The most important thing to do, is run a REAL practical test
with your O2 and CO2 system in the actual submarine before diving with
it. Cliff Redus has photos on the web site of him carrying out this
kind of test. Then you will have a much better idea of the performance
of your life support and how much spare chemical you need to bring with
you for a specific duration period.
In regards to NASA and CO2 levels, Phil wrote his paper in 1998 while my
slides were written in 2005. NASA likely changed their dependencies and
requirements in that time period. My reference for that was "Spacecraft
Maximum Allowable Concentrations Volume 2, chapter B3" as shown at the
bottom of the slide set.
Hope that helps.
Jon
Alan James wrote:
Jon
Am designing my scrubbers & have come up with a discrepancy between
papers on the psub life support page.
Phils paper on life support states on page 6 that you get 4 hours (240
minutes)
per pound of soda lime.
Your scrubber design page puts it at 200 minutes per kg wich is 90 minutes
per pound (sofnalime). In other words Phils scrubber lasts about 2&1/2
times longer.
This is quite a differance if you are considering taking down enough
absorbant for 3 days.
Also ( nit picking now ) Phils paper states Nasa aborts missions at 3%
CO2 levels, while yours states 2%.
Regards Alan
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