Introduction
101-92-4 A .ver 97020-00094
–7
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 th
e 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 o
verlap with
ba
ckground 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, HCl, 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,
sy
ngas 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. Use of ultra-high
Figure 1–4 Typical normalized 2f signal where the
species concentration is proportional to the peak
height.
UK Distibution by IMA Ltd Tel: +44 (0)1943 878877 Email: info@ima.co.uk Web: www.ima.co.uk
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