BA02191C SS2100 TDLAS H
2
S Gas Analyzer
Endress+Hauser 1-5
Sample conditioning system
A sample conditioning system (SCS) is included with the analyzer. The SCS has
been specifically designed to deliver an optimum sample stream that is
representative of the process systems stream at the time of sampling. Most
SS2100s are configured for use at extractive natural gas sampling stations.
Determining firmware version
When the analyzer is powered on for the first time, the firmware version will
display on the system LCD for approximately seven seconds. Refer to
“Powering Up the Analyzer” in the Description of Device Parameters for this
analyzer for operational instructions. The firmware version for each analyzer is
also listed on the analyzer calibration certificate.
How the Analyzers Work
The SS2100 employs SpectraSensors tunable diode laser absorption
spectroscopy (TDLAS) to detect the presence of trace substances in process
gases. Absorption spectroscopy is a widely used technique for sensitive trace
species detection. 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 on page 1–6.
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.
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)
I I
0
exp lN–=
N
1–
l
--------------
I
I
0
-------------
ln=
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