From jdotson Wed Jul 10 17:58 CDT 1996
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Date: Wed, 10 Jul 1996 17:58:38 -0500
From: jdotson (Jessie Dotson)
Message-Id: 199607102258.RAA07557@belmont.astro.nwu.edu.
To: novak@clark.phys.nwu.edu
Subject: size of charcoal pumps
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Content-Length: 2172
Status: O


Giles,

Here's the current story on the necessary amount of charcoal and
the subsequent pump size required.

Amount of required charcoal:
  I tried to identify how many grams of charcoal are necessary for
  1 L STP of He3.  On the high end is an estimate based on Steve's
  recollection of what he put in his dewar (7g of charcoal per L STP
  of He3).  On the low end are estimates based on adsorbtion isotherms
  I found in the literature - an average for several types of carbon
  at the relevent temperatures and pressures is 2g of charcoal
  per L STP of He3.

  Assuming we want 35 L STP (approx 1.5 moles), we'll need somewhere
  between 70 and 245 g of charcoal!

Required size of pump.
  I calculated an "apparent" charcoal density by using the entire pump
  volume as the volume in question.  Based on pump dimensions and 
  charcoal weights from both Steve and Dave P. - these pumps have an
  apparent charcoal density of 0.38 g/cm^3.

  If I fix the pump length at 5", the pump diameter needs to be 
  between 1.8" and 3.2" depending on how much charcoal you decide you
  want from above.

Where does this leave us (in spite of uncertainties greater than factors
of 3)?  The adsorption isotherms should suggest the *minimum* amount
of charcoal necessary.  It's my understanding that these values would
completely "fill up" the charcoal.  The value from Steve (of 7g/L)
is probably quite conservative.  It's even a little more conservative
than what is suggested by Lounsamaa who gives a charcoal weight
which is "equivalent to more than 10 cm3 of liquid without
detriment to the pumps" - which implies 6.6g/L as an upper limit.

I think we should be quite safe picking a value in the middle. 
Especially because I think the 35 L STP is conservative.
Even with a worse case load of 200 uW (we should be able to do
better - possibly even a factor of 2) 35L should provide 55 hrs
hold time.

Do you think the above is sufficient information to make a guess
and go - or would you prefer I keep pushing for info?
[btw - even the smallest implied pump is slightly larger than the
1.25" diameter Bob P suggested - so this will involve some
changes for Jeff.]

Jessie