Spectrasensors SS2100 - Figure 1-1 Schematic of a Typical Laser Diode Absorption Spectrometer

SS2100 H
2
S Analyzer
1–4 4900002218 rev. C 11-6-15
Because the measurement is made in the volume of the gas, the response is
much faster, more accurate and significantly more reliable than traditional
surface-based sensors that are subject to surface contamination.
In its simplest form, a diode laser absorption spectrometer typically consists of
a sample cell with a mirror at one end, and a mirror or window at the opposite
end, through which the laser beam can pass. Refer to Figure 1–1. The laser
beam enters the cell and reflects off the mirror(s) making multiple passes
through the sample gas and eventually exiting the cell where the remaining
beam intensity is measured by a detector. With the SS2100, sample gas flows
continuously through the sample cell ensuring that the sample is always
representative of the flow in the main pipe.
Figure 1–1 Schematic of a typical laser diode absorption
spectrometer
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)
OPTICAL HEAD
DETECTOR
WINDOW
OUTLET
TEC
LASER
INLET
PRESSURE
SENSOR
FAR MIRROR
I
 I
0
exp lN–=