RE: [PSUBS-MAILIST] CO2 absorbant efficiency

["Hugh Fulton" <hc.fulton@gmail.com>]

Jon, shouldn't there be only 3 variables and nothing to do with breaths per
minute.   ABS figure of Production CO2 per person	0.115 lbs/hr, The
manufacturers figure of 140 - 160 litres CO2/kg (whichever is correct) and
the efficiency. Then you have the requirement for 72 hrs and dive time. Hugh



-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
Sent: Thursday, 10 June 2010 2:46 p.m.
To: personal_submersibles@psubs.org
Subject: 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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