This paper proposes a fiber optic liquid level sensor system based on a silica fiber Bragg grating embedded into an epoxy resin diaphragm coupled to a temperature reference sensor, used to compensate the temperature cross-sensitivity for improving the liquid level measurement accuracy. The proposed system was tested in an industrial water tank with heating and recirculation. The results demonstrated a temperature cross-sensitivity reduction, enhancing the liquid level measurement thermal stability by a factor of nine, when compared with some single head sensor configurations reported in literature. Our system presents high linearity ( $R>0.999$ ), superior sensitivity (2.8 pm/mm), and much lower temperature related error (1.04 mm/°C), when compared with the other diaphragm-based sensors recently reported in the literature.

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 review paper presents three specific optical fiber sensor technologies which show considerable potential for liquid level monitoring: Both 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.

The 3-D printing process is a flexible, automated, customizable, and low-cost fabrication process. Fiber Bragg grating (FBG) sensors can be embedded into 3-D printed elements to obtain customized and low-cost sensing elements. However, the material infill is an important printing parameter that may affect the FBG response on both temperature and strain. This paper investigates the material infill influence on FBG-embedded 3-D printed sensing pads by both numerical and experimental approaches. The obtained sensing pads are able to withstand a force of 1 kN with sensitivities of about 0.7 pm/N and linearity higher than 98%. Numerical results show that the fiber has an influence on the strain distribution of the 3-D printed structure. Experimental results show that the infill’s thermal expansion-induced temperature sensitivity of the embedded FBG reported in the literature can be reduced by simply setting the correct infill density. The results obtained enable the application of 3-D printed sensing pads for structural health monitoring, soft robotics application, and force detectors for teleoperation in harsh environments.

Fiber Bragg grating (FBG) based sensors have been applied to measure several parameters, such as pressure, vibration, liquid level, humidity, the concentration of chemical compounds, among others. An approach to measure parameters like liquid level, pressure and vibration are to embed the FBG on a diaphragm, which is generally made of a polymeric material. Nevertheless, the mechanical properties of polymers depend on temperature variation. For this reason, a polymer diaphragm can enhance the cross-sensitivity between the strain and temperature on an FBG sensor. In order to overcome this limitation, this paper presents a compensation technique for the temperature effects on an oblong polymer diaphragm-based FBG pressure sensor. The presented technique is based on the analytical model of the sensor, which takes into account the variation of the diaphragm properties with temperature obtained through a dynamic mechanical analysis of the diaphragm material. Results show that the developed technique reduces the sensor cross-sensitivity to about 1.74 Pa/°C. Furthermore, the presented technique is compared with the direct difference between the FBG strain and temperature responses presented in reference works. The comparison shows a better performance of the technique presented in this paper with respect to the cross-sensitivity and the root mean squared error.