2-Pack Analyzer System
1–6 4900002256 rev. B 4-13-21
system, sample gas flows continuously through the sample cell ensuring that
the sample is always representative of the flow in the main pipe.
Due to their inherent structure, the molecules in the sample gas each have
characteristic natural frequencies (or resonances). When the output of the
laser is tuned to one of those natural frequencies, the molecules with that
particular resonance will absorb energy from the incident beam. That is, as the
beam of incident intensity, I
0
(), passes through the sample, attenuation occurs
via absorption by the trace gas with absorption cross section (). According
to the Beer-Lambert absorption law, the intensity remaining, I(), as measured
by the detector at the end of the beam path of length / (cell length x number
of passes), is given by
,(1)
where N represents the species concentration. Thus, the ratio of the absorption
measured when the laser is tuned on-resonance versus off-resonance is
directly proportional to the number of molecules of that particular species in
the beam path, or
.(2)
TEC
LASER
INLET
0.8 M MEASUREMENT CELL
OPTICAL
HEAD
DETECTOR
WINDOW
OUTLET
FAR MIRROR
TEC
LASER
INLET
PRESSURE
SENSOR
8/28-M MEASUREMENT CELL
OPTICAL
HEAD
DETECTOR
WINDOW
OUTLET
FAR MIRROR
PRESSURE
SENSOR
Figure 1–1 Schematic of a typical laser
diode absorption spectrometer
I I
0
exp lN–=
N
1–
l
--------------
I
I
0
-------------
ln=
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