6-28 | Model 933 UV Analyzer
If the analyzer is caught in a repeating loop of Auto-Setups, this indi-
cates the software can’t adjust PMT signals to meet the value set for
Signal Max. This is likely due to the lamps nearing their end-of-life.
As a temporary fix, decrease the Signal Max (Optical Bench dialog
box) to complete the Auto-Setup until the lamps are replaced.
An Auto-Setup is not required as part of a preventive maintenance sched-
ule (that is, do not perform an Auto-Setup every month).
PMT Level and PMT Balance
PMT Level and PMT Balance are adjusted only during the Auto-
Setup sequence. The typical range of operation for both parameters is
0–10000 mV. After a successful Auto-Setup, the PMT Level should be
between 7200–7400 mV while the PMT Balance should be between
2000–8000 mV. The analyzer will function with values outside this range,
but it is a sign that a change has likely occurred in the transmission of
light within the Optical Bench (for example: dirty windows, weak lamps,
weak PMTs, faulty PMT Buffer board) – refer to “Troubleshooting and
Diagnostics” in this chapter for alarm conditions and corrective action.
At the end of every Auto-Setup, always check the PMT Level and PMT
Balance. Take note of severe changes in PMT Level and PMT Balance
because they are as important as the absolute value.
The PMT Level signal adjusts the gain of both PMTs simultaneously in
the same direction. PMT Balance adjusts the gain of the two PMTs in
opposite directions by controlling a virtual ground circuit on the Optical
Bench board.
If the PMT signals are outside their normal range of operation after the
Auto-Setup is complete, check for the following:
• Adjust the Lamp Max setting slightly (typical range is 4500–7800 mV)
and start another Auto-Setup.
• Was the Auto-Setup done with Zero gas flowing through the sample
system?
• Are the Measuring Cell Windows clean?
• Are the PMT signals weak?
• Are the PMTs in their original sockets (that is, is the higher gain PMT
in the Measure path)?
• Is the PMT Buffer board faulty?
NOTE
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