This non-invasive gas monitor for exhaust gas monitoring must has high reliability and
requires little maintenance. Monitor for in-situ measurements using tunable diode laser absorption
spectroscopy (TDLAS) in the near infrared, can meet these requirements. TDLAS has evolved over the
past decade from a laboratory especially to an accepted, robust and reliable technology for trace gas
sensing. With the features of tunability and narrow linewidth of the distributed feedback (DFB) diode
laser and by precisely tuning the laser output wavelength to a single isolated absorption line of the gas,
TDLAS technique can be utilized to measure gas concentration with high sensitivity. Typical
applications for monitoring of H2S, NH3, HC1 and HF are described here together by wavelength
modulation spectroscopy with second-harmonic(WMS-2F) detection. This paper will illustrate the
problems related to on-line applications, in particular, the overfall effects, automatic light intensity
correction, temperature correction, which impacted on absorption coefficient and give details of how
effect of automatic correction is necessary. The system mainly includes optics and electronics, optical
system mainly composed of fiber, fiber coupler and beam expander, the electron part has been placed in
safe analysis room not together with the optical part. Laser merely passes through one-meter-long pipes
by the fiber coupling technology, so the system itself has anti-explosion. The results of the system are
also presented in the end, the system's response time is only 0.5s, and can be achieved below
1×10−5 the detection limit at the volume fraction, it can entirely replace the traditional methods of
detection exhaust gas in the process industry.
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