Section 3
3-2
Theory of Operation
The
sample cells
are gold-plated to enhance IR reflection and resist
tarnishing over time. One set of sample cells is used for both H
2
O and CO
2
measurements by using a
dichroic beam splitter
to provide radiation to two
separate detectors.
A 150 nm bandpass
optical
filter
is used to tune the CO
2
detector to the 4.26
micron absorption band for CO
2
, and a 50 nm bandpass optical filter tunes
the H
2
O detector to the 2.59 micron absorption band. Both filters provide
excellent rejection of IR radiation outside the desired band, allowing the
analyzer to reject the response of other IR absorbing gases. The filters are
mounted directly on the detectors for thermal stability.
Each
detector
is a lead selenide solid state device that is insensitive to
vibration. The detectors are cooled and regulated to -5
°
C by thermo-
electric coolers, and electronic circuits continuously monitor and maintain a
constant detector sensitivity. The result of this detector circuitry is a detector
system that is very stable.
Infrared radiation from the source is focused through the gas cells and onto
the detector by a
lens
at each end of the optical bench. Focusing the
radiation maximizes the amount of radiation that reaches the detector in
order to provide maximum signal sensitivity.
All of these features provide a CO
2
noise level that is typically 0.3 ppm
peak-to-peak (at 350 ppm) when using 1 second signal averaging, and 1 ppm
peak-to-peak with 0.1 second signal averaging; the H
2
O noise level is
approximately 0.002 kPa at 2.0 kPa when using 1 second signal averaging,
and 0.006 kPa when using 0.1 second signal averaging. Signal averaging
times are selectable (in software) between 1 and 30 seconds and are used to
achieve even lower noise levels. For example, with 4 seconds of signal
averaging, the noise levels typically decrease 50%.
3.2 Calculating Gas Concentration - General
LI-COR gas analyzers use a lead selenide detector that operates
approximately as a linear quantum counter; that is, over much of its range
the detector signal output
ν
is proportional to the number of photons
reaching the detector. The output voltage V that is used to compute CO
2
mole fraction is proportional to the difference between the signals generated
by the detector when it sees the sample cell (
ν
s
) and when it sees the
reference cell (
ν
r
).
