From: Borries Demeler <demeler@bioc02.uthscsa.edu>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Thu, 22 Jun 1995 08:54:11 -0500 (CDT)

warning xla calculations (fwd)

> 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.

Did you observe this low-speed problem at other speeds than 4000 rpm?
We have done similar experiments, but haven't observed this problem.
If you would use a stopwatch and compare the times recorded in the 
scans to the stopwatch, how far would they be off?

> 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.

To set the time to "0" once the centrifuge has attained speed would
not be entirely correct either. The sample already sediments during the 
acceleration period of the XLA, and that sedimentation should be taken
into account. There are a couple of methods that allow you to arrive 
at the theoretical start of the experiment, would it be possible to 
start the XLA directly at the desired speed without the acceleration
period. The XLA data aquisition program records the integral of the 
omega^2*t integral in a value in the header of each datafile. Together
with the time of the scan you can then correct for the theoretical
start of the experiment.

Regards, -Borries Demeler
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* Borries Demeler                                                             *
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