Endress+Hauser SS2000XP, Руководство по эксплуатации

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BA02183C SS500XP/SS2000XP Gas Analyzer
Hauser 1
–
7
Note that contamination of the mirrors results solely in lower overall signal.
However, by tuning the laser off-resonance as well as on-resonance and
normalizing the data, the technique self calibrates every scan resulting in
measurements that are unaffected by mirror contamination.
Wavelength Modulation Spectroscopy (WMS) signal detection
Hauser takes the fundamental absorption spectroscopy concept a
step further by using a sophisticated signal detection technique called
wavelength modulation spectroscopy (WMS). When employing WMS, the laser
drive current is modulated with a kHz sine wave as the laser is rapidly tuned.
A lock-in amplifier is then used to detect the harmonic component of the signal
that is at twice the modulation frequency (2 f ), as shown in Figure 1–4. This
phase-sensitive detection enables the filtering of low-frequency noise caused
by turbulence in the sample gas, temperature and/or pressure fluctuations,
low-frequency noise in the laser beam or thermal noise in the detector.
With the resulting low-noise signal and use of fast post-processing algorithms,
reliable parts per million (ppm) detection levels are possible (depending on
target and background species) at real-time response rates (on the order of 1
second).
All Hauser TDL gas analyzers employ the same design and hardware
platform. Measuring different trace gases in various mixed hydrocarbon
background streams is accomplished by selecting a different optimum diode
Normalized
2 f Signal
Wavelength [a.u.]
S
ig
na
l [
a.
u.
]
Figure 1–4 Typical normalized 2
f
signal; species
concentration is proportional to the peak height