Fully distributed optical fiber sensor for humidity monitoring at high temperatures

The presence of water can provide aqueous electrolytes for corrosion to occur inside the pipelines. The capability of monitoring water vapor condensation enables in-situ monitoring of internal corrosion in natural gas transmission pipelines. Previously, a fully distributed optical fiber sensor for water and humidity monitoring has been demonstrated, consisting of an unmodified off-the-shelf single mode (SM) optical fiber connected to an optical backscatter reflectometer. The intrinsic polymer jacket of the SM fiber is hygroscopic and can serve as a water sensing layer due to expansion/swelling from water absorption. In this work, strain changes were measured and calibrated in jacketed and unjacketed sections at different relative humidity levels (RH, 0% to 100%) and different temperatures (T, 21 to 50°C). In the jacketed section, the sensitivity to humidity decreased from 1.2 to 0.6 με/%RH and then diminished as T increased from 21 to 50 °C, which could be due to the intrinsic absorption property of polymer at higher T or the wet gas flow at room temperature being absorbed in the polymer jacket. The unjacketed section demonstrated a minimal sensitivity to humidity (<0.2 με/%RH) at 21-50 °C and a relatively consistent sensitivity to temperature.