Development of a durable fiber-optic oxygen sensor for harsh underground environments

This paper focuses on effects of protection with a silicone resin to develop a fiber-optic oxygen sensor with long-term stability and durability in harsh underground environments. Ruthenium (II) complexes were used as oxygen-sensing compounds. A uniform composite film composed of silicone resin and the Ru complex was prepared with spin coating technique. A comparison of dissolved-oxygen (DO) sensitivity between the composite film and a Ru complex film was made by exposing to hot water (80 °C). The result of the accelerated degradation test showed that sensitivity of the Ru complex film was stable; meanwhile that of the composite film increased with exposure time in a short period. In order to improve stability, the Ru complex film overcoated with silicone resin was prepared. Differences in sensitivity for saturated DO (8.5 ppm) between with and without the silicone resin overcoating on the Ru complex film were investigated by exposing to the hot water and simulated underground water. These results revealed that the sensitivities and response times of the overcoated films were stable and slow, respectively, compared to those of non-overcoated films. Then, optodes were evaluated for effects of the overcoating on sensing properties by exposing to 100 vol.% oxygen gas. The experiment showed that: (1) the response time was significantly influenced by the thickness of the overcoating; and (2) response speed of the overcoated optode was slow by a factor of about 35 compared to that of the non-overcoated. We concluded that the overcoating was effective in the application to mid- and long-term oxygen monitoring in the harsh environments.