Development of a Novel Settlement Monitoring System Using Fiber-Optic Liquid-Level Transducers With Automatic Temperature Compensation

A novel monitoring system using innovative fiberoptical liquid-level transducers is proposed for continuously monitoring the settlement of geotechnical infrastructures. Each fiber-optical liquid-level transducer has two fiber Bragg gratings (FBGs) embedded in an elongated cantilever beam such that the temperature effect of the FBGs is automatically compensated and the Bragg's wavelength shift indicates a liquid-level change. The operating principles of an individual transducer and a settlement monitoring system consisting of multiple transducers are described in detail. Four fiber-optical liquid-level transducers were fabricated in the laboratory. The measurement range was ±75.61 mm, and the designed sensitivity was 0.089 με/mm and was then updated through a calibration test. A settlement monitoring system was formed by connecting the four transducers with a water tank as an automatic level controller. A demonstration test of the system showed that its relative measurement error is within 4%, confirming the feasibility of the system for in situ applications.