There are many optical fiber-based sensors for liquid level
measurement. For example, Nath et al. [5] presented a simple
intensity-modulated fiber optic sensor, which is based on frustrated
total internal reflection effect caused by refractive index
change of a medium surrounding an optical fiber. Khaliq et al.
[6] proposed and demonstrated a liquid-level sensor based
on the refractive-index sensitivity of long-period fiber-optic
gratings. Sheng et al. [7] developed a temperature-independent
differential pressure sensor based on two FBGs. The results
showed that the sensor was capable of providing simultaneous
measurements of both the temperature and the differential pressure,
and was suitable for applications involving liquid level,
liquid density or specific gravity measurement. However, in the
sensors proposed above, the liquid level can only be obtained
if the specific gravity of the liquid is known in advance. In
practice, this specific gravity is not always available, and thus
the proposed sensors have only limited applicability.
In the present study, a novel dual-optical-fiber-sensor system
comprising a fiber Bragg grating (FBG) level sensor and a
Fabry-Pérot (FP) pressure sensor is proposed. The sensitivities
of the two sensors are evaluated experimentally, and analytical
formulae are then proposed for obtaining the unknown level
and specific gravity of a liquid simultaneously. In addition,
a calibration method is proposed in Section 4 using actual
measurement values for enhancing the accuracy of the measurement
results. Finally, the performance of the proposed
dual-optical-fiber-sensor system is evaluated experimentally.
There are many optical fiber-based sensors for liquid levelmeasurement. For example, Nath et al. [5] presented a simpleintensity-modulated fiber optic sensor, which is based on frustratedtotal internal reflection effect caused by refractive indexchange of a medium surrounding an optical fiber. Khaliq et al.[6] proposed and demonstrated a liquid-level sensor basedon the refractive-index sensitivity of long-period fiber-opticgratings. Sheng et al. [7] developed a temperature-independentdifferential pressure sensor based on two FBGs. The resultsshowed that the sensor was capable of providing simultaneousmeasurements of both the temperature and the differential pressure,and was suitable for applications involving liquid level,liquid density or specific gravity measurement. However, in thesensors proposed above, the liquid level can only be obtainedif the specific gravity of the liquid is known in advance. Inpractice, this specific gravity is not always available, and thusthe proposed sensors have only limited applicability.In the present study, a novel dual-optical-fiber-sensor systemcomprising a fiber Bragg grating (FBG) level sensor and aFabry-Pérot (FP) pressure sensor is proposed. The sensitivitiesof the two sensors are evaluated experimentally, and analyticalformulae are then proposed for obtaining the unknown leveland specific gravity of a liquid simultaneously. In addition,a calibration method is proposed in Section 4 using actualmeasurement values for enhancing the accuracy of the measurementresults. Finally, the performance of the proposeddual-optical-fiber-sensor system is evaluated experimentally.
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