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<P><FONT size=2>Judith,</FONT></P>
<P><FONT size=2>It is normal for the absorbance scan timing to vary from one
scan to the next. For any given cell the number of data points actually recorded
varies from one scan to the next so clearly the total time will
also.</FONT></P>
<P><FONT size=2>I'm not sure though why this run length variability
presents a problem. If you really want the timing to be consistent then you must
set a delay between scans that is slightly longer than the minimum time to scan
all the cells once. That's the trade-off: if you want the fastest scanning
possible, then the rate will vary; if you want consistent timing, you have to
slow things down a bit.<BR><BR>Regarding why the timing varies, certainly one
reason is simply that the rotor rotation, slit motion, and lamp
charging are not synchronized. For each lamp flash the data
acquisition board (DAB) has to wait until the slit is approximately at the
desired radius and the lamp has re-charged and is ready to fire again, and
then the DAB must wait until the correct cell rotates under the
monochromator. The overall time for that sequence then varies from one flash to
the next. Another reason (and actually I think this is probably the main one) is
that the DAB computer also has many other tasks to do (like communicating with
the PC and the computer than runs the centrifuge). At best it can barely
keep up, so sometimes it is simply not available at the soonest point another
flash can be fired. Sometimes too it will not immediately process the message
from the PC to start the next set of scans (all the cells once).<BR><BR>Another
thing that can happen sometimes is that the DAB computer gets so busy it starts
running behind, and then the messages running back and forth between it and the
PC (through the serial cable) pile up. When this happens you can actually see
that the run time being displayed in the run monitor window on the PC is running
minutes behind the time shown on the centrifuge CRT. A few times when I
stopped a run the rotor had completely stopped but the DAB was so far behind the
PC said it was still going at 60K! This seems to happen most often when you are
running at high speed and when you do a bunch of runs without ever turning off
the power to the centrifuge.</FONT></P>
<P><FONT size=2>John<BR>-----Original Message-----<BR>From:
rasmb-bounces@rasmb.bbri.org [</FONT><A
href="mailto:rasmb-bounces@rasmb.bbri.org"><FONT
size=2>mailto:rasmb-bounces@rasmb.bbri.org</FONT></A><FONT size=2>] On Behalf Of
Judith Kornblatt<BR>Sent: Friday, September 25, 2009 1:19 PM<BR>To:
smcbryan<BR>Cc: rasmb@server1.bbri.org<BR>Subject: Re: [RASMB] timing of auc
runs<BR><BR>In reply to the same question from two of you, yes, the Rmin and
Rmax are the same in every run - we never change them.<BR>Judith
Kornblatt<BR><BR>smcbryan wrote:<BR>> For your first two experiments at 32K,
you are collecting data at<BR>> 3.975 minutes per scan and 4.455 minutes per
scan for the 13:15 and<BR>> 14:51 experiments, respectively. Assuming you are
scanning in the Abs<BR>> mode (which the scans per unit time suggests) that
is a significant<BR>> difference between scan time, if you set the radial
increment and scan<BR>> limits (Rmin and Rmax) the same for both runs. Did
you set Rmin and Rmax to the same limits?<BR>> Our machines won't collect
every scan at the set radial increment,<BR>> typically a 0.001cm experiment
will average 1.0015cm radial<BR>> increments. But that value is the same
between different experiments,<BR>> so your differences are unusual.<BR>>
Steve Mcbryant, PhD<BR>> Director, PPC Facility<BR>> Colorado State
University<BR>><BR>><BR>><BR>><BR>> On Fri, 25 Sep 2009 13:38:01
-0400, Judith Kornblatt<BR>> <judithk@alcor.concordia.ca>
wrote:<BR>> <BR>>> How consistent should the length of time
for a run be? I've been<BR>>> doing the same basic run multiple times -
same speed, same number of<BR>>> scans - and the total time for the run,
as displayed on the panel of<BR>>> the AUC, varies considerably. For
example, 200 scans, 32000 rpm, the<BR>>> times varied from 13:15 to 14:51.
More recently, 200 scans at 37000,<BR>>> times varied from about 13 hours
to 14 hours. Is this variation normal?<BR>>> Judith Kornblatt<BR>>>
Chemistry and Biochemistry<BR>>> Concordia University<BR>>> Montreal
Quebec<BR>>><BR>>>
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