Abstract
The validity of two new approaches to tunable diode laser spectroscopy (TDLS) in
the near-IR, namely the residual amplitude modulation approach and the phasor
decomposition method, is investigated for application in industrial process monitoring
where the operating temperatures and pressures are high and subject to significant
change. Both techniques allow the recovery of absolute absorption profile line shapes
and are completely calibration free, making them very attractive for online deployment
in stand alone instrumentation in harsh environments where the calibration factors in
conventional TDLS methods are subject to significant cumulative errors and drift.
Currently established TDLS techniques, and indeed conventional gas composition analysis
techniques, suffer from significant limitations when applied under these conditions, and
there is a clear need for the development of a suitable alternative. The primary focus
in this work is the analysis of water vapor in solid oxide fuel cell diagnostics where
the operating temperatures range from 700$^{\circ}{\hbox{C}}$ to
950$^{\circ}{\hbox{C}}$, the gas pressures are subject to change and the recovered
signal levels are low. The 1391.7 nm overtone water vapor transition is interrogated
over the above temperature range of interest at concentrations of 6%–50%, while the
1650.96 nm methane transition is also analyzed over a range of gas pressures at a fixed
concentration of 1%. Excellent agreement between the experimentally recovered absorption
line shapes and simulations based on parameters from the HITRAN (2004) database is
observed; further evidence for the efficacy of the techniques is demonstrated through
the accuracy of the gas concentration measurements which were achieved by curve-fitting
absorption line shape simulations to the experimental data.
© 2009 IEEE
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