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<DIV><FONT face=Arial color=#0000ff size=2><SPAN class=514342316-06092006>Jack,
you asked whether one can damage a centerpiece at 40K if one sector is empty and
the other is full. The official rating for the standard charcoal epon
centerpieces is 42K, which means Beckman guarantees they won't actually break if
you get a leak and a channel empties at 42K. However, I think 'not broken' and
'not damaged' are different issues. There are circumstances where the center rib
bends but does not break. Afterward that centerpiece may be leak-tight, but
whether the rib is really straight rather than bowed at that point is
questionable, and if it is bowed you are very likely to get convection. So the
bottom line is that in my lab, if I have a full leak at 40K, that centerpiece
goes into the trash (after a good round of therapeutic expletives, of
course).</SPAN></FONT></DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=514342316-06092006></SPAN></FONT> </DIV>
<DIV><FONT face=Arial color=#0000ff size=2><SPAN
class=514342316-06092006>John</SPAN></FONT></DIV>
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<DIV class=OutlookMessageHeader lang=en-us dir=ltr align=left><FONT
face=Tahoma size=2>-----Original Message-----<BR><B>From:</B>
rasmb-bounces@rasmb.bbri.org [mailto:rasmb-bounces@rasmb.bbri.org] <B>On
Behalf Of </B>Jack Kornblatt<BR><B>Sent:</B> Wednesday, September 06, 2006
8:52 AM<BR><B>To:</B> rasmb@server1.bbri.org<BR><B>Subject:</B> [RASMB] more
on systematic differences<BR><BR></FONT></DIV><FONT size=+1><TT>A follow up on
"systematic differences"<BR>There were two questions:<BR>1. If there are
three cells in the AUC during a sedimentation velocity run, one cell yields an
s value that I arbitrarily set at 100, cell 2 yields a value of 99.4 and cell
3 a value of 99.7. What are possible causes of the differences?<BR>2.
The meniscus of cell one if always found at shorter radius than the meniscus
of cell 2. Cell 3 is intermediate between the other two. What are
possible causes?<BR><BR>Thanks to John, Andrew and Tom for their most recent
comments.<BR>One of John's struck me as being especially insightful:
<BR><BR>If you (me, in this case) have found that the three cells do not yield
the same s values in a velocity run, and if you apply correction factors to
two of them so as to get them to agree with the third, how do you know which
is the correct one to use as a the reference value? <BR>The answer: You
do not know which is the correct value. If I understand correctly, to
have knowledge of the s value to a precision of 0.1%, you would have to
have experimentally determined diffusion coefficients (to better than 0.1%
precision) as well as rho and vbar values to much better than 0.1%; mass
spectroscopy now gives us molecular masses to high precision. Correction
factors of less than 1% are useful in that they allow me to judge the
sedimentation in one cell relative to the others. Correction factors
permit one to level the playing field but one doesn't know whether the game
being played is football in San Francisco or cricket in Nairobi.<BR>Like
biologists all over, I had hoped for absolute knowledge when I started working
with the AUC but ........<BR><BR>There seems to be general agreement that the
differences between cells are probably the result of differences in the center
pieces: <BR>I can easily check for leaks. <BR><BR>NB. If I run one
chamber empty, one full at 40 KRPM, am I likely to deform the center
piece?<BR><BR>As for the other ideas as why the cells might differ as much as
they do, they are all useful. If the differences between cells were
larger than it is, it would be worthwhile delving more deeply. The major
unanswered question is why the s values differ. Here you three all seem
to agree that it might result from convection or from a leak. I'll check
the archives for how to detect convection problems and will get back to you
about leaks when I have the data. (Hopefully < 1 year)<BR>thanks
again<BR>jack</TT></FONT> </BLOCKQUOTE></BODY></HTML>