Section 4. Theory of Operation
System Overview
The LI-840A CO
2
/H
2
O Gas Analyzer is an absolute, non-dispersive, infrared (NDIR) gas analyzer
based upon a single path, dual wavelength, infrared detection system. The CO
2
and H
2
O meas-
urements are a function of the absorption of IR energy as it travels through the optical path.
Concentration measurements are based on the difference ratio in the IR absorption between a
reference and sample signal. Reference and sample channels measure infrared gas absorption in
a single path through the use of narrow band optical filters with appropriately selected bands.
The CO
2
sample channel uses an optical filter centered at 4.26 micrometers, corresponding to
an absorption band for CO
2
. The reference channel for CO
2
has an optical filter centered at 3.95
micrometers, which has no absorption due to CO
2
. The H
2
O sample channel uses an optical fil-
ter centered at 2.595 micrometers, corresponding to an absorption band of H
2
O. The reference
channel for H
2
O has an optical filter centered at 2.35 micrometers, which is a non-absorbing
region for H
2
O.
Section 4. Theory of Operation 4-1
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