A new RAM normalized 1f-WMS technique for the measurement of gas parameters in harsh environments and a comparison with 2f/1f

Upadhyay, Abhishek and Lengden, Michael and Wilson, David and Humphries, Gordon Samuel and Crayford, Andrew P. and Pugh, Daniel G. and Johnson, Mark P. and Stewart, George and Johnstone, Walter (2018) A new RAM normalized 1f-WMS technique for the measurement of gas parameters in harsh environments and a comparison with 2f/1f. IEEE Photonics Journal, 10 (6). 6804611. ISSN 1943-0655

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    Abstract

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