The instrument uses digital signal processing techniques to determine the temperature and pres-
sure corrected CO
2
concentration based on the optical bench signals using a ratiometric com-
putation. The data are passed through a double rectangular hyperbola (CO
2
) or 3rd order
polynomial (H
2
O) that performs linearization of the detector signal to a mole fraction in air
given in µmol CO
2
per mole of air (ppm), and mmol H
2
O per mole of air (mmol/mol).
Data output is provided in a digital format through an RS-232 interface that supports connection
to an external computer. The instrument comes with Windows
®
-compatible software for instru-
ment configuration, control, data collection and display. Analog signals are available through a
terminal block for collection by a data logger or similar means.
Optical Bench System
The LI-840A CO
2
/H
2
O Gas Analyzer optical path is a thermostatically controlled IR detection
system. The optical bench operation is based upon a broad band IR source and a pyroelectric
detector. The source is mounted in a parabolic reflector to collimate the light and increase
energy throughput down the optical path to the detector. The reflector and optical path are gold
plated to further increase energy transmission. The detectors are pyroelectric devices that operate
based on thermal energy received. The narrow band optical filters allow only the wavebands of
interest to illuminate the detector, allowing for the determination of CO
2
and H
2
O con-
centrations in the presence of other infrared absorbing gases.
Section 4
4-2 Section 4. Theory of Operation
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