Groundwater salinity sensing using PI-FBG with improved capillary coating method

Polyimide (PI)-coated fiber Bragg grating (FBG) technology has certain potential for groundwater salinity measurement. However, this technology is immature for the field salinity monitoring due to the limitation of PI coating technology. Here, we fabricated the PI-FBG salinity sensing elements by improved capillary coating method, and encapsulated it with stainless steel tube to improve the robustness. The results show that different capillaries coated PI-FBGs were all linear with salinity ranging from 0 to 8%. Pre-treatment methods of soaking in the coupling agent and reducing the fiber diameter before coating can effectively improve the salinity sensitivity, and this coating method had good repeatability. The measurement results of the PI-FBG sensor in different gravel aquifers were highly consistent with those of the electronic salinometer. Our results show that the capillary coated PI-FBG has excellent salinity sensing performance, which provides a new means for in-situ monitoring of groundwater salinity.