Optical fiber sensing for sub-millimeter intrinsically-safe liquid level monitoring

Abstract

The popularization and fast growth of the optical fiber sensing technology has stimulated in different fields WHERE measurements of diverse physical and chemical parameters are required. Among these parameters, liquid level sensing plays an essential role in industry applications such as chemical processing, fuel storage, transportation systems, oil tanks/reservoirs, and wastewater treatment plants. In order to measure this parameter different sensing techniques based on acoustical, mechanical, electrical and electromagnetical technologies have been already proposed. Nevertheless, they suffer from intrinsic safety concerns in harsh environments, especially with corrosive, and explosive or flammable atmospheres. Fiber optic based liquid level sensors (FOLLS) 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 doctoral Thesis presents two specific optical fiber sensor technologies to measure liquid level. Both the MachZehnder and FabryPerot interferometers are researched. The Thesis also focus on uniform fiber Bragg grating (FBG). Since these technologies have different operation principle, the liquid level measurement was based on refractive index changes for the MachZehnder sensor and based on hydrostatic pressure in the case of both the FPI and FBG sensors. Furthermore, analysis of temperature crosssensitivity is performed with the aim to improve the pressurebased sensors performance. Despite the FBGs provide high accurate measurements, the interrogation systems are the most important drawback for their large commercial application, due to their high cost. Therefore, a new and lower cost interrogation technique based on FPI microcavities was proposed as a final contribution.