From: Olin Spivey <OSPIVEY@bmb-fs1.biochem.okstate.edu>
  To  : RASMB@BBRI.HARVARD.EDU
  Date: Mon, 11 Nov 1996 11:09:09 -0500

Questions

Dear colleagues,

     I am trying to save a few thousand dollars by using some of our model
E centerpieces and housings in our new XL-A.  However, most of the
assemblies leak.  Our centerpieces are primarily epon-Al.  I had heard that
others have had this problem, which they cured by grinding/polishing the
centerpiece faces.  I would appreciate any instructions on how this can be
done.  I have a limited experience with grinding and polishing optical
surfaces and expect my physics friends here could also help me.  What I
would appreciate knowing is: 1) what grades of carborundum or similar agent
are used, what surface(s) are the centerpieces ground on. 2) What tests
(prior to testing for leaks in the ultracentrifuge) may be done for
flatness.  For example is a simple test such as Newton rings between the
centerpiece and window available to judge adequate flatness?  3) In
addition, I would appreciate knowing whether the suggestions in the Beckman
An-60 Ti rotor manual are wise.  They suggest successive steps of:
lubricating the screw-ring, which we always do, torquing the cell to 140
in-pounds or adding a slight film of silicon vacuum grease on the
centerpieces.  They also suggest using no more than 5 microliters larger
volume in the reference sector than the sample sector  I suspect this is
good advice, but I would like to avoid the additional torquing and grease
if it were practical to reflatten the centerpiece ends and I am unsure how
practical or easy it is to match the volumes to within 5 microliters.  4)
What is the best test for the absence of leaking?  I would be tempted to
use 10 microliters more in the reference sector and spin BSA (pure monomer)
at 60,000 rpm and look for movement of the two menisci (meniscuses?) and
correct s values.

     6) Finally, I would appreciate your opinions on how important it is to
use blackened cell housings and screw rings.  I don't see how anything but
the screw ring surfaces could act as light scattering sources in the cell,
or is the black important in temperature equilibration?  During
installation and tests using our shiny aluminum holders and rings our
instrument barely met the lower limit of stray light specifications.
Blackening the screw ring with a magic marker didn't give any noticeable
improvement.  I was planning on having these housings and or screw rings
black anodized, but I was warned that the added surface might prevent the
cells from fitting properly together and into the rotor.  Another option
was to lightly spray them with black enamel.  If anyone has experience in
these procedures, I would be grateful for your advice.

     With much appreciation,
     Olin

H. Olin Spivey                    Phone: (405) 744-6192 or -6189
Dept of Biochem. & Molec. Biol.     Fax: (405) 744-7799
246 NRC
Oklahoma State University
Stillwater, OK 74078-0454        E-Mail: Ospivey@BIOCHEM.OKSTATE.EDU