A New RAM Normalized 1<inline-formula><tex-math notation="LaTeX">$f$</tex-math></inline-formula>-WMS Technique for the Measurement of Gas Parameters in Harsh Environments and a Comparison With <inline-formula><tex-math notation="LaTeX">${2f\!/\!1f}$</tex-math></inline-formula>

Abstract

A calibration-free first harmonic (1t ) wavelength modulation spectroscopy (1tWMS) technique for gas species parameter measurement is demonstrated. In this technique, the total magnitude of the 1t-WMS signal is normalized by a component of the 1t residual amplitude modulation signal. This method preserves the advantages of the traditional nt/1t - WMS (n ≥ 2) technique, such as the immunity to the non-absorbing systematic losses and the accurate recovery of gas parameters, without the requirement for non-absorbing regions for normalization at high pressure or high modulation index values (m-values). The proposed technique only requires the 1t signal, which has the largest magnitude of all the harmonics signals, and, therefore, fundamentally has a higher sensitivity to the nt/1t technique. Furthermore, since only the 1t-WMS signal is used, the technique is less complex in terms of signal processing and data acquisition. This paper also shows a comparison of the proposed technique and 2t/1t for measuring CO2 in the exhaust of a combuster. The data highlight how nonlinearities in the optical detection system as a function of frequency have a considerable effect on the recovered 2t/1t spectra, causing variation in the recovered gas concentrations. This effect is not seen in the methodology proposed in this paper.

Keywords

• Wavelength modulation spectroscopy
• Calibration-free TDLS
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