U-shaped plastic optical fiber sensor for scale deposition in hot spring water

Fiber optic sensors for monitoring scale deposition in geothermal brine and hot spring water should be safe, easily fabricated, and readily disposable. These desired features already have been enhanced in plastic optical fibers (POFs) and U-shaped sensors for other applications. The present work reports a U-shaped POF sensor for CaCO₃ scale deposition. The sensors were easily fabricated by thermally bending the bulk POF without removing the cladding. At the bend, the percentage of total internal reflection between the water and the POF surface is affected by the high refractive index of the CaCO₃ deposit. The optical responses of the U-shaped sensor to CaCO₃ formation were investigated in a mixture of calcium chloride dehydrate and sodium hydrogen carbonate using a white-light source and a spectroscopic detector. The sensor was responsive to CaCO₃ formation on the sensor surface and was especially sensitive at small bending radii. The sensitivity was further enhanced by increasing the number of bends. Finally, the U-shaped POF sensor was applied to the monitoring of CaCO₃ scale deposition in hot spring water sampled at Matsushiro, Japan.