From: Don K Mc Rorie <dkmcrorie@ccgate.dp.beckman.com>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Thu, 19 May 1994 09:09:26 -0800 (pst)

PMT settings in the XLA

John Philo is essentially correct in how the gain is set for the 
photomultiplier tube in the XLA. But, after speaking with Barney, I 
felt some details needed to be explained so unexpected problems will 
not arise due to the current function.

With a step scan, the gain is adjusted between steps but not between 
measurements in the reference and sample sectors. With a continuous 
scan, the gain is set once using the 6.5cm position as a reference 
point. So depending on the light intensities at this position an 
absorbance higher in the reference sector could cause a setting where 
the PMT would become saturated in parts of the scan causing errors in 
data measurement The best way to avoid this problem is to insure the 
reference sector always has a lower absorbance at the 6.5cm position.

We are currently examining the best solutions to allow accurate 
differential scans to be taken.

Don