Honeywell ACM 150 Manual To Installation, Operation, And Maintenance page 111

ACM 150
Figure 66. ZPD trends
Background, S/N and drift
S/N is the signal-to-noise ratio. It is calculated automatically during every
'Background'.
S/N determines the lower detection sensitivity of the FTIR analyzer, the higher the
value the better. If the S/N degrades below preset limits, the ACM 150 monitor
generates an incident message. A degraded S/N indicates the need for service
before it reaches unacceptable levels.
The upper graph in
daily S/N and how it drifts over a 15-day period. Ideally, this will be a flat line,
however, you should expect a downward trend over time as the S/N degrades
between service intervals.
The lower graph, labeled "Background Factory Drift" is also a measure of the S/N
and its drift over time, however, it is calculated using the "factory" background
spectrum (stored in memory) compared to the current background instead of just the
current background spectrum taken every 2 hours.
Figure 67, labeled "Background S/N Ratio," shows the average
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