Fully distributed optical fiber sensor for water and humidity monitoring

Abstract

Internal corrosion can occur when aqueous electrolytes are present inside natural gas transmission pipelines. Despite upstream gas dehydration treatments, liquid water can form through condensation of water vapor or may be introduced from plant upsets. With dissolved salts and acidic gases such as CO₂ and H₂S, aqueous electrolytes become very corrosive with increased conductivity and lower pH. Since water provides the electrolytes that initiate and sustain corrosion, detection of water can locate the spots for potential internal corrosion inside the pipelines. In this work, a simple optical fiber-based sensor for fully distributed water monitoring has been demonstrated and studied. The system consists of an unmodified off-the-shelf single mode (SM) optical fiber and an optical backscatter reflectometer (OBR) capable of measuring the spatial profile of strain changes along the fiber. The polymer jacket coating of the SM fiber is hydroscopic and serves as the water sensing layer due to expansion/swelling from water absorption. The swelling induced strain change is interrogated with the OBR to enable fully distributed water monitoring. This strain-based H₂O sensor is sensitive to H₂O molecules regardless of the phase (liquid or vapor) or the surrounding media. Strain changes were measured at different relative humidity levels from 0% to 100% to demonstrate reversibility and linear correlation between humidity and strain. This sensor has the advantages of fully distributed sensing, low cost, simple preparation, easy operation, and good sensitivity.