Raman Spectrum of Water Vapor in Methane

ABSTRACT Monitoring the humidity of natural gas (NG) during transportation through main pipelines is a critical task, as high pressures and low temperatures promote the undesirable formation of hydrates. Raman spectroscopy represents a promising non‐contact method for developing highly accurate humidity measurement instruments. However, the Raman spectrum of water vapor is sensitive to variations in external conditions typical of transported NG. These spectral changes must be taken into account to achieve high measurement accuracy. This paper presents the first investigation of the Raman spectrum of water vapor in gaseous methane, covering both vibrational bands (ν 1 and ν 3 ) and pure rotational lines of water. The methane‐broadening and shift coefficients of water spectral lines were measured. The magnitude of pressure‐broadening for anisotropic water lines in methane was found to be comparable to that induced by atmospheric air. The obtained results can be utilized to improve the accuracy of water vapor concentration retrieval from Raman spectra.