This report reviews the feasibility of detection of impurities, in particular water vapour, in natural gas, using real-time correlation spectroscopy. The general concept relies on using a gas sample, contained in a reference cell, as a matched optical filter, to preferentially detect similar absorption spectra of the same gas in the measurement cell. All variations of the method have the advantage of excellent selectivity, provided the gases to be detected have narrow line spectra, even when using broadband sources for illumination. They are also suitable for direct-path measurements or for remote detection over optical fibre leads.<br/><br/>The report focuses on two main areas. Firstly, the Stark modulation method has been used to detect water and ammonia vapour. Secondly, we present an alternative correlation spectroscopic arrangement, in which the route of the light path is modulated. This new arrangement has been tested with both water vapour and ammonia. We give a comparison of the characteristics of both methods and discuss their suitability for determining water vapour contamination in natural gas.<br/><br/>Our conclusions are that the switched-path method has some potential of being developed to meet the Shell application for trace measurement of water vapour in high-pressure natural gas
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