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.
|