Metek 920, Operator'S Manual With Essential Health And Safety Requirements

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5-26 | Model 920 Multi-Gas Analyzer
PMT Level and PMT Balance
PMT Level ( PmtLvl ) and PMT Balance ( PmtBal ) are adjusted only dur-
ing the Auto-Setup . The typical range of operation for both parameters
is 0–10 VDC. After a successful Auto-Setup, normal values for PMT Level
should be 7.2–7.4 VDC while the PMT Balance should be between 4.0–7.0
VDC. 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 – see “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 these parameters 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.
It is possible to achieve an acceptable completion number, yet have an
unacceptable PMT Level and/or PMT Balance signal. Always check both
of these values after an Auto-Setup.
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 4.5–7.8 VDC)
and start another Auto-Setup.
• Was the Auto-Setup done with Zero gas flowing through the sample
system?
• Are there any PMT- or lamp-related alarms? If so, see
“Troubleshooting and Diagnostics” in this chapter for alarm corrective
action.
• Are the Measuring Cell Windows clean?
• Are the PMT signals weak?
• Do the source lamps need replacement (weak lamps)? If not, are the
source lamps aligned?
• If the lamps were replaced, is the correct socket connected to the cor-
rect lamp? Lamp 1 is the closest to the Optical Bench board.

(PmtLvl) RUN

F1 4
(PmtBal) RUN

F1 5