From: Julius.Clauwaert <clauwaer@reks.uia.ac.be>
  To  : rasmb@BBRI.ERI.HARVARD.EDU
  Date: Thu, 22 Jun 1995 12:05:18 +0200 (MET DST)

warning xla calculations

To all people who do sedimentation velocity experiments with the
xla working under the optima XL-A Ultracentrifuge operating
software version 1.02 : warning!

We recently came across a limitation in the use of the xla.
We took the habit to check our sedimentation cells at the
beginning of a run for leaks, big aggregates and all other
eventual misfortunes of a sedimentation velocity experiment, by
starting at low speed (eg 4000rpm), taking a scan of all cells
to check if everything was OK; and indeed if everything was going
smoothly (this procedure took about 20 minutes), we then shifted
to the desired speed and started the normal scanning procedure
in order to determine the sedimentation coefficient. 
But it turned out that in these conditions, the system took a
complete wrong time value for calculating the sedimentation
value. This of course resulted in s values which were wrong, and
too large in our case.
Once this is know, you take this into consideration. 
It is of course a pity that it is not possible to set the time
at zero the moment the centrifuge has reached the preset speed
for the sedimentation experiments so that one can start scanning
the sedimentation profiles. Of course this will limit some
applications.
We hereby want to warn the users of the xla that they have to
care about the way they do their velocity experiments. The
calculated S-value is not wright when one changes the rotor speed
during the run and this is caused by the way the software version
1.02 registers the time t. 

J. Clauwaert, T. Aerts
Biophysics Research Group, Department of Biochemistry,
University of Antwerp, Universiteitsplein, 1,
B-2610 Antwerp Belgium.
Tel. 00 32 3 8202326   Fax. 00 32 3 8202248
email clauwaer@reks.uia.ac.be