0733-8724 (c) 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JLT.2018.2866130, Journal of Lightwave Technology > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract— We present a very simple and versatile fiber optic sensor for liquid level measurements based on the Archimedes' law of buoyancy. It includes a proper mass suspended in the liquid with one end fixed to a Fiber Bragg Grating (FBG) strain sensor, while the remaining fiber termination is fixed to a rigid support. When the liquid level increases inside the tank, the load exerted by the suspended mass on the optical fiber is reduced as a consequence of the increase of the force acting on the immersed mass according to the Archimedes’ law of buoyancy, leading to a reduction of the strain applied to the FBG. The proposed device is able to perform real time and continuous liquid level measurements, and is characterized by sensing features (such as sensitivity and operating range) that exclusively depend on the geometrical characteristics (e.g. shape, diameter and length) of the suspended mass. Therefore, by a judicious selection of its geometrical features, the proposed device can be customized so as to suit the requirements for the specific application. In addition, the constitutive material of the attached mass can be properly selected in order to confer to the proposed platform the capability to operate in any kinds of liquid. Here, as proof of principle, we exploited an aluminium cylindrical mass having a radius of 8mm and a length of 30cm, and experimentally demonstrated the capability of the realized device to measure the water level inside a beaker over a range of 25cm with a sensitivity of ~27pm/cm, a resolution of ~0.04cm and a repeatability error of ~0.1% of the full scale reading (FSR). We also demonstrated, both theoretically and experimentally, the possibility to largely tune the sensing performances by acting on the cylinder radius. Obtained results pave the way for the exploitation of the proposed platform for accurate liquid level monitoring in large- scale storage tanks, useful both for petrochemical industry and for agricultural/residential applications. Index Terms—Liquid level Sensing, Fibre Bragg grating, optical fibre sensor, Archimedes' law of buoyancy. I. INTRODUCTION iquid level sensing plays a key role in many industrial and everyday life applications including treatment plants, Submitted for review on 23/03/2018. M. Consales, S. Principe, A. Iele, M. Leone, A. Cutolo and A.Cusano are with the OptoelectronicsGroups, Engineering Department, University of Sannio, C.so Garibaldi 107, 82100 Benevento, Italy (a.cusano@unisannio.it). H. Zaraket is with the Laboratoire de Physique et d‟electronique, Lebanese University, Faculty of Sciences (I), Hadeth, Beirut, Lebanon (hzaraket@ul.edu.lb). IhabJomaa is with the Lebanese Agriculture Research Institute (LARI), Tal Amara, 287, Rayak, Zahle, Lebanon (ijomaa@lari.gov.lb). The authors gratefully acknowledge the financial support for this work from the Project "FOS4I - Fibre Optic Sensor System for Irrigation Applications", funded by the UK Lebanon Tech HUB. oil tanks, gasoline stations, fuel reservoirs for transport systems, public water supplies and so on. Among all, liquid level monitoring for agricultural and/or residential use as well as for petroleum and other petrochemical industries involves numerous large-scale storage tanks with significant depths (2- 6m) [1, 2]. Such applications thus typically require sensing systems composed of different sensors able to provide accurate (<0.5% FSR) and continuous liquid level measurements over a measuring range of several meters[1-6]. In addition, liquid level monitoring for petrochemical applications also involves the use of combustible liquids and sensors exposition to flammable, erosive environments[2]. A large number of level sensors has been proposed in the last decades, mainly based on mechanical, electrical, and optical approaches [1, 7-10]. Traditional devices are mostly based on electromechanical techniques whose operation, however, is limited in harsh and hostile environments such as in conductive, explosive, flammable, and erosive liquids[1]. For this reason, liquid level sensors based on optical fiber technology have gained significant interest in these applications, due to their well-established advantages, among which their immunity to electromagnetic interferences, corrosion resistance, light weight and compact structure. Numerous fiber optic sensing configurations have been proposed for liquid level measurements over the years, including fiber optic interferometric structures [11-16], etched or cladding removed fiber-based devices [2, 16-20], fiber Bragg grating (FBG)- [20-26] and long period grating (LPG)- based platforms [27-29]. An extensive review of the use of the various fiber optic techniques reported so far has been recently published by Loizou and Koutroulis [1].However, they demonstrated a limited operating range and only a few of them, most of which are based on the integration of FBGs with complex cantilever structure, are able to potentially match the requirements of petrochemical or agricultural/residential applications [14, 21-23]. Specifically, Wang et al. proposed a large-range continuous liquid level sensor based on a Fabry-Perot interferometer [14]. The device was fabricated by means of the CO 2 laser heating fusion bonding technology and exhibited an excellent resolution of 0.07 cm over a length of 5m. Sohn and Shim [21] proposed a liquid level sensor based on the integration of a FBG in a cantilever rod connected to a float. The proposed device exhibited a linear response to liquid level in the range 0-40cm, with a sensitivity of 0.1dB/cm. Guo et al. realized a liquid level sensor by integrating an FBG onto a complex bending cantilever beam (BCB) structure A Fiber Bragg Grating liquid level sensor based on the Archimedes' law of buoyancy M. Consales, S. Principe, A. Iele, M. Leone, H. Zaraket, I. Jomaa,A. Cutolo, A. Cusano L