Detection of gas dissolved in water

The detection and concentration measurement of dissolved gases in a water medium are essential in natural gas deposit searches on a seafloor, environmental monitoring and providing safety within industrial facilities. Current methods being used suffer from complexity and slow response times. Optical methods are very promising and offer higher accuracy, possibility of real time measurement and remote operation [1]. Among many existing spectroscopic methods only a few can be applied for practical use, because a majority of them are being used only as laboratory methods. The most appropriate methods for dissolved gas sensing are laser modulation spectroscopy and Raman spectroscopy. Raman spectroscopy is a well used technique for water measurements which uses Raman scattering spectra for detection and concentration estimation of measured matter, but it requires utilization of expensive components, such are powerful laser sources and sensitive photodetectors [2,3]. Modulation laser spectroscopy is used merely for atmospheric measurements and needs more exploration to be used in different areas such as measurements in water. Utilization of laser spectroscopy method for dissolved gas sensing is completed according to conditions in which gas is measured e.g. very high pressure and low temperature what can theoretically make some influence on absorption lines width and their position in spectrum. Implementation of this method for deep-water measurement requires few utterly important fundamental and practical tasks to be solved. Firstly, water has very high absorption and scattering in near infrared region. According to this, optimal interaction between laser radiation and studied substance should be provided to ensure that gas absorption on discrete lines is higher than continuous absorption of water. Secondly, spectroscopic measurements make sense only when actual absorption lines frequencies of dissolved gasses are exactly known. Absorption frequencies of gasses such as me- - thane in normal conditions may differ for gasses dissolved in water on a great depth. Also there is an issue relating to how the laser radiation would be brought deep under water where spectroscopic measurements are performed, considering high attenuation of NIR radiation in water.