I- Introduction

II- Operation Principle and Sensor Design

III- Experimental Setup

IV- Results

V- Conclusions

References

Abstract

Liquid level sensing is nowadays a relevant issue in a broad range of applications, forcing the sensors performance and cost to be evaluated in parallel. This work proposes a fiber opticbased liquid level sensor system using a Fabry-Pérot interferometer (FPI) embedded into a polyurethane resin diaphragm. The FPI is based on microcavities generated upon catastrophic fuse effect, enabling the fiber recycling and sensors fabrication in a cost-effective way, compared to traditional methods. To enable the simultaneous temperature control, a Fiber Bragg Grating was used as thermal reference sensor to compensate the temperature cross-sensitivity. The sensor prototype was tested in a field application, using two different configurations, an open chamber configuration, where the diaphragm is in contact with the atmosphere, and a closed chamber configuration, revealing sensitivities of 4.4±۰٫۱ pm/mm and 1.57 ± ۰٫۰۴, respectively. These sensitivity values are within the figures of merit for diaphragm-based sensors recently reported.

INTRODUCTION

NOWADAYS, the monitoring of the liquid-air interface is required to infer level in a wide variety of applications such as, for instance, fuel supply systems, oil reservoirs, wastewater treatment plants [1]. Other relevant applications can be found in medical treatments, chemical processing and pharmaceutical development, demanding high resolution on liquid level detection [2]. Traditionally, liquid level sensing is mainly based on mechanic and electrical techniques, such as capacitance and radio frequency admittance [3], float type devices as magnetic floating gauge [4], ultrasonic and radar based sensing [3]. Despite that electrical-based sensors are widely employed, their applicability is limited when the sensing medium is conductive, potentially explosive and erosive [5], [6]. Optical fiber-based sensors have been considered an attractive prospect for liquid level monitoring, due to its unique advantages, such as electromagnetic immunity, electrical insulation, compatibility with remote sensing and sensor multiplexing. Moreover, these advantages render optical fiber appropriate for long term, reliable level measurements in special conditions [7]–[۹]. Distinct operation principles are behind optical-based sensing, namely refractive index variation. These sensors measure the liquid level based on the immersed sensor length which lead to a specific effective surrounding refractive index and several implementation examples can be found: etched Fiber Bragg Grating (FBG) [6], etched chirped FBG [2] or long period grating (LPG) [10]. An additional technology is based on excessively tilted FBG that generates a fast and slow-axis due to its birefringence [11]. However, due to its length (۱۰۲ m), these sensors are restricted to a reduced range liquid measurement.