Introduction
1–6 H
2
S in Natural Gas
resonance and normalizing the data, the technique self calibrates every scan
resulting in measurements that are unaffected by mirror contamination.
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 (2f), 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
combined with careful calibration to correct for secondary effects caused by
temperature and pressure variations and occasional spectral overlap with
background species, reliable parts per million (ppm) or even 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).
All SpectraSensors TDL gas analyzers employ the same design and hardware
platform. Measuring different trace gases such as H
2
O, C
2
H
2
, H
2
S, NH
3
, CO
2
,
CO and O
2
in various mixed hydrocarbon background streams, including
natural gas (alkanes), ethylene, propylene, refinery fuel gas, hydrogen
reformer gas, syngas and others, is accomplished by simply choosing a
different optimum diode laser wavelength between 700 nm and 3000 nm
which provides the least amount of sensitivity to background stream variations.
Figure 1–2 Typical raw signal from a laser diode
absorption spectrometer with and without mirror
contamination.
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