Análise de sistema de medição de nível de líquido com duas FBGs embutidas em diafragma

Abstract

Currently, there is a growing demand for monitoring liquid levels using fiber optic sensors in several areas of the industry due to the various advantages presented by optical fibers and the importance of this monitoring in industries [1, 2]. This master's thesis presents a fiber optic liquid level sensor system based on a pair of fiber Bragg gratings (FBGs) embedded in a circular silicone rubber (polydimethylsiloxane - PDMS) diaphragm and a nitrile rubber diaphragm. The measurement principles of these sensors are presented. The silicone rubber diaphragm sensor is about 2.2 mm thick and 45 mm in diameter. To analyze the linearity and sensitivity of the sensor, the diaphragm was subjected to compression tests as well as liquid level loading and unloading. Flexion, compression and liquid level rise tests showed that the insertion of two FBGs (adjusted coefficient of determination (Ra2) equal to 0.995 for flexion, 0.951 for compression and 0.997 for liquid level) in the diaphragm resulted in a system with greater linearity than with individual FBGs. However, the water level decrease test showed that the system containing both FBGs (Ra2 equal to 0.999) showed greater linearity when compared to FBG2 (0.973) and linearity equal to that of FBG1 (0.999). Temperature characterization was also performed and presented a sensitivity to temperature variation of 11.40 pm/°C for FBG1 and 10.03 pm/°C for FBG2. Temperature sensitivity was improved for both FBGs when compared to uncoated FBGs with typical values of 9.75 pm/°C. Therefore, the proposed FBG-based sensor system is capable of simultaneous force and temperature measurement in a compact diaphragm-embedded system. The nitrile diaphragm sensor is constructed of 3 layers of rubber with two FBGs inserted into the diaphragm. Four configurations were constructed, where the thickness of the central layer was varied. To analyze the linearity and sensitivity of the sensor, the diaphragm was submitted to bending tests, where in the 4 configurations the linearity of the system with two FBGs is better than the use of only one FBG, with the exception of configuration 3, in which the FBG1 presented higher linearity and lower FBG2. Finally, configuration 4 presented a system with greater linearity with two FBGs, since the R2 of the response of both FBGs (0.952) is greater than that of FBG1 (0.948) and FBG2 (0.818) alone. Temperature characterization for all configurations resulted in a value lower than the sensitivity normally found in uncoated FBGs (9.75 pm/°C).