In-situ laser detection of water vapor based on circular prism array multi-pass cell enhanced near-infrared absorption spectroscopy

In-situ detection of water vapor concentration is a vital procedure in the monitoring framework of energy exploitation and transportation for oil and gas industry. A water vapor sensor system using direct absorption spectroscopy was developed based on a novel circular prism array multi-pass cell (CPA-MPC). The optimal optical path of 12-star-polygon pattern was determined by fuzzy comprehensive evaluation (FCE) method. The polarization disturbance noise in output laser intensity was analyzed based on polarization theory. The CPA-MPC yields an effective optical path length of 173.9 cm within a compact annular-volume of 217 mL. A laboratory test of the CPA-MPC based laser sensor system shows that the relative error of average H2O concentration is 2.87% in ambient air. SNR of the sensor achieves to 608.3, which constitutes a 13-fold improvement compared to opposite-type laser detection. Allan deviation analysis indicates that minimum detection limit of 7.09 ppm is achieved with a 216 s averaging time. In order to verify the application potential of the proposed CPA-MPC based laser sensor system in industrial field, a dynamic in-situ monitoring experiment is performed. And the typical thermodynamic phenomena of liquid water heated evaporation is successfully captured in the alternation between opening state and sealing state.