Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments

Abstract

We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.

In situ H2O and temperature detection close to burning biomass pellets using calibration-free wavelength modulation spectroscopy

“…Recently, several approaches for calibration-free WMS (CF-WMS) have been suggested [19][20][21][22][23][24][25], usually based on normalizing nf-WMS with 1f-WMS signals to account for instrumentation factors and a wavelength dependent laser intensity. CF-WMS systems were successfully employed for temperature and concentration measurements in shock-tubes [26], engines [27], scram-jet combustors in the presence of inhomogeneously distributed temperature and species concentration [28], and in coal-fired high pressure combustion facilities [29].…”

Remote ambient methane monitoring using fiber-optically coupled optical sensors

“…The optimum laser input signal frequencies and amplitudes for the WMS technique are often stated when the 2f signal is maximized [29][30][31][32]. However, for this system, the optimum (9)…”

“…The theoretical signals used to generate the lookup table were generated following the procedures found in [28,29], as further explained here. The pressure, P (Pa), and temperature, T (K), range covered by the table encompasses what is normally observed in the laboratory, and the overall resolution of the table is balanced to offset the interpolation time of a single value.…”

Cantilever-enhanced photoacoustic detection of hydrogen sulfide (H2S) using NIR telecom laser sources near 1.6 µm

“…This generally adds to the bulkiness of the systems, and hence, a small size is hardly achieved with H 2 S TDLAS sensors. The path length can be reduced by using wavelength modulation or balanced detection approaches, which furthermore improve the sensitivity of TDLAS sensors [17][18][19]. Although the detection of H 2 S concentrations of several ppb has been demonstrated by employing integrated cavity output spectroscopy approach (ICOS) [20], such sensitivities are difficult to obtain in field measurements as the robustness of this kind of systems remains limited [21,22].…”