Endress+Hauser SpectraSensors SS2100r, Руководство по оборудованию

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SS2100r H
2
S Analyzer
1
–
6 4900002189 rev. D 3-7-18
Differential TDLAS
Similar to TDLAS, this SpectraSensors technology involves subtracting two
spectrums from one another. A “dry” spectrum, a response from the sample
when the analyte of interest has been completely removed, is subtracted from
the “wet” spectrum, a response from the sample when the analyte is present.
The remainder is a spectrum of the pure analyte. This technology is used for
very low or trace measurements and is also useful when the background matrix
changes over time. Refer to http://www.spectrasensors.com/spectral-
subtraction-in-tdlas-analysis/ for more information.
Wavelength Modulation Spectroscopy (WMS) Signal Detection
SpectraSensors 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.
Figure 1–4 Typical normalized
2
f signal; species
concentration is proportional to the peak height
With the resulting low-noise signal and use of fast post-processing algorithms,
reliable parts per million (ppm) or parts per billion (ppb) detection levels are
possible (depending on target and background species) at real-time response
rates (on the order of 1 second).
Normalized
2
f
Signal
Wavelength [a.u.]
Signal [a.u.]