Optical Fiber Sensing for Sub-Millimeter Liquid-Level Monitoring: A Review

Fiber optic-based liquid-level sensors (FOLLSs) can work in harsh environments with inherent advantageous features that only optical fiber offers, such as intrinsic safety, resistance to chemical corrosion, immunity to electromagnetic interference, electric isolation, small size, lightweight sensing heads, high accuracy and resolution, easy multiplexing, and capability for extremely remote monitoring without the need of electrical power at the measuring point. In this context, this paper presents three specific optical fiber sensor technologies that show considerable potential for liquid-level monitoring: the Mach-Zehnder and Fabry-Perot interferometers and the fiber Bragg gratings (FBGs). Mathematical models and operation principle are presented for each sensor technology, and parameters, such as level range, level sensitivity, and temperature cross sensitivity, are analyzed. Finally, a critical and comparative analysis among the sensors is performed, where it is reported a tradeoff between sensitivity and level range.