Fiber-optic Liquid Level Sensor Research Articles

A liquid level sensor based on spiral macro-bending plastic optical fiber

Liquid level sensing refers to the monitoring of liquid level height information under working or static conditions. Compared with traditional liquid level monitoring methods, optical fiber liquid level sensor has advantages of immunity to electromagnetic interference, excellent electronic insulation and remote operation. The light propagates in the fiber through total internal reflection. When the fiber is bent, a large number of cladding modes will be generated. These cladding modes will have Fresnel reflection at the interface between the bent cladding and environment. When the high refractive index medium is in contact with or close to the fiber cladding, part of the cladding mode light will leak into the external medium, resulting in light energy loss. Therefore, according to the basic working principle of optical fiber macro-bending loss, a new type of spiral plastic optical fiber (POF) liquid level sensor was designed. The POF with a diameter of 0.5 mm was used as the sensing optical fiber, and then it was spirally wound and fixed on a self-designed square cylinder. The liquid level was monitored by the designed sensor, and the effects of environment and liquid temperature on the test process were analyzed. The results show: (i) the measuring range of the liquid level sensing system is within 80 mm and it has good repeatability; (ii) the resolution and sensitivity are 1.03 mm and 0.088 dB/mm, respectively; (iii) the influence of temperature on the sensor is less than 0.06 dB/℃.

Temperature compensated fiber-optic liquid level sensor by micro-sphere based in-line Michelson interferometer

In this paper, a novel micro-sphere based in-line Michelson interferometer is proposed and completed experimentally through arc-discharge technique. The light field distributions are theoretically investigated to gain optimal core offset and diameter of micro-sphere. Multiple fabricated samples are compared and the comprehensive tests are then performed in terms of liquid level and temperature. The experimental results show that obvious blue shift of fringes is demonstrated with the rise of liquid level and the average sensitivity reaches 62.6 pm/mm with high linearity in the range of 0–40 mm. Moreover, owing to highly-consistent temperature response (∼99.5%), the crosstalk caused by ambient temperature is eliminated by differential method and the measurement error is compressed within 1.33%. Due to the merits of compactness, low cost, ease of fabrication and operation, our sensor is very potential in the liquid-level related engineering applications.

High-precision fiber optic liquid level sensor based on fast Fourier amplitude demodulation in a specific range of spectrum

This study presents a fiber optic liquid level sensor (FOLLS) by acquiring information from the amplitude of spectral fast Fourier transform (FFT) in a specified narrow wavelength range. The sensing light path is simply formed by embedding a section of Panda-type polarization-maintaining fiber (PMF) into the sensing arm of a conventional Michelson interferometer (MI) structure. The output composite spectrum consists of two parts: the fine interference fringes due to MI optical path difference (OPD) and the envelope fringes stemming from the PMF’s birefringence. By applying liquid level variations on the sensing structure, shifts in envelope wavelength indicate a sensitivity of −0.193 nm/cm, and a liquid level resolution of 1 mm can be easily acquired. Furthermore, by demodulating the amplitude after spectral FFT in a specific narrow wavelength range, and intensity sensitivity of 1.906 dB/cm is achieved, leading to a greatly improved overall resolution of 0.03 mm in liquid level measurement. Besides, the sensing element is sealed and packaged by two sheets of silicone rubber, which can be thrown into liquid when in use, thus easing the sensor installation. Moreover, due to the proposed demodulation method, the sensor is insensitive to the light source power jitter, reference arm phase shift and optical spectral analyzer wavelength shift. Therefore, the merits of the proposed sensor include high precision, high stability, ease of installation, which are beneficial in pore water pressure and liquid level monitoring.

High resolution and large sensing range liquid level measurement using phase-sensitive optic distributed sensor.

Liquid level sensor with large sensing range and high-resolution is essential for the application of industry monitoring. In this work, a distributed optical fiber liquid level sensor is proposed and demonstrated based on phase-sensitive optical time domain reflectometry (φ-OTDR). In the basic of the thermal optic effect, the temperature change will induce the fluctuation of the effective refractive indexes of the fiber core, as well as the fluctuation of the optical path of the light transmitting in the fiber. Therefore, the φ-OTDR can detect the liquid level with a large measurement range by interrogating the phase information along the fiber due to the temperature difference between the liquid and air. Further, the scattering enhanced optical fiber (SEOF) is used as the sensing fiber to improve the signal to noise ratio (SNR) of the phase signal. Moreover, a high sensitivity liquid level sensing head by wrapping the SEOF on a heat conductive cylinder is designed and optimized to improve the sensing resolution. In the experiment, the proposed distributed liquid level sensor presents a high sensitivity of 73.4 rad/mm, corresponding to a competitive liquid level resolution of 142μm based on the noise floor of 10.4 rad within 160 s. The field test validates a large sensing range of 20 cm which is limited by the cylinder length, while a potential sensing range could reach 320 m with the sensing fiber of 40 km, proving a dynamic range of 127.1 dB. The proposed liquid level sensor with large dynamic range and high sensing resolution can benefit potential application in smart industry platforms and biomedicine monitoring.

Low Refractive-Index and Temperature Crosstalk Fiber-Optic Liquid Level Sensor Based on Cascaded Quasi-Microfiber and Double-Cladding Fiber Structure

In this paper, a novel in-fiber modal interferometer for liquid level sensing is proposed and experimentally demonstrated based on the cascaded quasi-microfiber and double-cladding fiber (QM-DCF) structure. The QM is utilized to monitor the change of liquid level with intensity modulation and the part of DCF is always immersed into the liquid and serves to sense the variation of ambient temperature. Then the comprehensive tests are performed and the experimental results show that, due to the massive loss of evanescent field in the area of taper waist, a competitive liquid-level response up to -0.972 dB/mm with high linearity is gained and the calculated detection limit reaches ~ 10μm. Moreover, this cascaded structure is almost immune to the varied refractive index (RI) of liquid especially in the range of 1.33-1.357 RIU and its temperature sensitivity is 0.164 nm/°C but with a fluctuation of <; 0.1 dB. Thus, high intensity-sensitive liquid-level measurement is achieved with near-zero crosstalk in terms of RI and temperature. Besides our sensor with the merits of practicality, low-cost, ease of fabrication is very promising and potential for simultaneous high-discrimination measurement of multi-parameter.

Temperature-independent Optical Fiber Liquid Level Sensor Based on an In-fiber Mach–Zehnder Interferometer Formed by Cascaded Polarization-maintaining Long-period Fiber Gratings

Temperature-independent Optical Fiber Liquid Level Sensor Based on an In-fiber Mach–Zehnder Interferometer Formed by Cascaded Polarization-maintaining Long-period Fiber Gratings

Hydrostatic Fiber Optic Liquid Level Sensor with position sensitive detector

This article presents the rationale and methodology for developing an intrinsically safe device, namely, a hydrostatic fiber optic sensor with a position-sensitive detector for monitoring the level of oil products in large-capacity tanks at oil depots and during pumping in a raw material warehouses. This device suitable for continuous monitoring of the liquid level, based on the measurement of a hydrostatic column of liquid with automatic offset of changes in the density of the liquid. Offset is carried out by means of a displacer (a fully submerged float), inside which a housing with a position-sensitive detector (PSD) is integrated. Theoretical validation of the bellows suspension usage for a displacer is given. During filling a container with a liquid whose level is measured, liquid bellows, the movement of which is recorded by an optical triangulation sensor using the reflected infrared ray incident on the bottom of the bellows. The principle of the triangulation sensor operation is based on the geometric properties of the triangles. The pulses of infrared radiation come through a fiber optic cable. In order to measure the movement of the surface (the bottom of the bellows) by measuring the movement of the reflected beam, a position-sensitive detector is used, which is located in a remote controller. In this device for the intrinsic safety problem solution, optical inputs of a fiber optic flat cable are located in the active zone of the sensor, which is connected to the optical inputs of a position-sensitive detector, operated on the principles of photoelectric effect. The light spot moving along the sensitive zone and converted by the detector into a one-dimensional signal proportional to the distance to the object. hydrostatically applies pressure over the entire effective area of the measuring

Ultra-high sensitivity liquid level sensor based on CO2 laser local refractive index curved modulation effect

On the basis of the curved modulation of refractive index (RI) on the spliced points, a highly sensitive fiber-optic liquid level sensor is fabricated by embedding a piece of double-hole elliptical-core eccentric fiber in single-mode fibers. Curved RI modulation enhances the high-order cladding modes coupling efficiency, which increases the light-material contact. The liquid level sensing characteristics of the structure show a ultra-high sensitivity of up to 0.712 nm/mm in the range of 0–24 mm with good linearity. The liquids with different RI are also demonstrated to have a sensitivity of 6.071 (nm/mm)/RIU. Meanwhile, the sensor exhibits strong stability and low temperature crosstalk.

Optical fiber liquid level sensor realized by cascading two spherical-shape structures

An optical fiber sensor for liquid level measurement of NaCl soultion based on Mach-Zehnder interferometer (MZI) is proposed and demonstrated. The MZI consists of two spherical-shape structures. The simulations of the light propagation of the spherical-shape structure with different diameters are performed, and it shows that the spherical-shape structure with diameter values of 200 μm can couple 50% energy of the core-mode into the cladding mode, and it is helpful to obtain best interference contrast. The liquid level sensitivity is -1.172 nm/cm for pure water, and -1.401 nm/cm for NaCl solution(n = 1.382). The sensor has advantages of easy fabrication, good robustness, and low hysteresis.

Liquid-level sensing based on a hollow core Bragg fiber.

A novel optical fiber liquid level sensor based on a hollow core Bragg fiber (HCBF) was proposed and demonstrated. The HCBF was first designed and successfully fabricated with periodic transmission band in the spectrum and a transmission loss of ~3.48 dB/cm. An inline optical fiber liquid-level sensor was fabricated by simply sandwiching a piece of HCBF between two single mode fibers. The sensing performance was experimentally tested. A linear liquid-level sensitivity of ~1.1 dB/mm, and fast response time less than 3s was obtained by the intensity demodulation measurement. The temperature and refractive index cross-sensitivities were also investigated. The experimental results indicate that our proposed structure has tiny temperature and RI dependence, which makes it a promising liquid level sensing platform for different liquids.

The influence of curvature configuration on the characteristic of alcohol gel insertion jacket of polymer optical fiber liquid level sensor

This study aimed to determine the effect of curvature configuration to sensitivities and linearities of Polymer Optical Fiber (POF) water level sensor. POF type SH-4001-1.3 has been used in this study. The jacket of POF of 20 cm was removed. Transparent piped inserted by alcohol gel has been used to replace the jacket. This is head of a sensor. The head of a sensor is curved with variations of the specified path length, peel length, the width of curvature, the height of curvature and waveform. Configuration A (20 cm, 34 cm, 6 cm, 2 cm, 1 wave), configuration B (20 cm, 34 cm, 8 cm, 2 cm, 1 wave), configuration C (20 cm, 34 cm, 9 cm, 2 cm, ½ wave), configuration D (20 cm, 34 cm, 10 cm, 2 cm, ½ wave). The head of a sensor inserted into the water tank. The light source inserted to one end POF is a He-Ne laser light with a power of 5 mW and a wavelength of 632.8 nm. Power output at the other end received by the Optical Power Meter (OPM). The curvature configuration the head sensor of POF affects the output. Configuration A has good sensitivity, however good linearity given by configuration.

Ultrahigh-sensitive multimode interference-based fiber optic liquid-level sensor realized using illuminating zero-order Bessel–Gauss beam

A proposal toward the enhancement in the sensitivity of a multimode interference-based fiber optic liquid-level sensor is explored analytically using a zero-order Bessel–Gauss (BG) beam as the input source. The sensor head consists of a suitable length of no-core fiber (NCF) sandwiched between two specialty high-order mode fibers. The coupling efficiency of various order modes inside the sensor structure is assessed using guided-mode propagation analysis and the performance of the proposed sensor has been benchmarked against the conventional sensor using a Gaussian beam. Furthermore, the study has been corroborated using a finite-difference beam propagation method in Lumerical’s Mode Solutions software to investigate the propagation of the zero-order BG beam inside the sensor structure. Based on the simulation outcomes, the proposed scheme yields a maximum absolute sensitivity of up to 3.551 dB / mm and a sensing resolution of 2.816 × 10 − 3 mm through the choice of an appropriate length of NCF at an operating wavelength of 1.55 μm. Owing to this superior sensing performance, the reported sensing technology expedites an avenue to devise a high-performance fiber optic-level sensor that finds profound implication in different physical, biological, and chemical sensing purposes.

High sensitivity optical fiber liquid level sensor based on a compact MMF-HCF-FBG structure

An ultra-high sensitivity fiber liquid level sensor based on wavelength demodulation is proposed and demonstrated. The sensor is composed of a segment of multimode fiber and a large aperture hollow-core fiber assisted by a fiber Bragg grating (FBG). Interference occurs due to core mismatching and different modes with different effective refractive indices. The experimental results show that the liquid level sensitivity of the sensor is 1.145 nm mm−1, and the linearity is up to 0.996. The dynamic temperature compensation of the sensor can be achieved by cascading an FBG. Considering the high sensitivity and compact structure of the sensor, it can be used for real-time intelligent monitoring of tiny changes in liquid level.

An Optical Fiber Liquid Level Sensor Based on Side Coupling Induction Technology

An optical fiber liquid level sensor based on two twisted polymer optical fibers twining around a racetrack column is demonstrated in this study. The side-coupling power of the passive fiber is modulated by the refractive index (RI) of the environment medium and decreases while the liquid level increases. The variation patterns of the side-coupling power in the bent section and the straight section form a step attenuation, which can improve the measurement range with a superior sensitivity and distinguish liquids with different RIs. Furthermore, the cost-effective sensor also shows good reversibility and low temperature-dependent properties.

Liquid Level Measurement Based on FBG-Embedded Diaphragms With Temperature Compensation

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.

Quasi-Distributed Optical Fiber Sensor for Liquid-Level Measurement

A novel fiber-optic liquid level sensor based on a cross-correlation optical time-domain Fresnel reflection technology is proposed and demonstrated. By processing a single-mode fiber (SMF) to create multiple gaps for generating Fresnel reflections, liquids surrounding the SMF modify Fresnel reflection intensity from the multiple gaps and the liquid level can be read by using a cross-correlation technique. We show the sensor for liquid-level remote sensing and its simultaneously measuring level of different liquids located in remote different places. Meanwhile, the sensor is sensitive to refractive index of liquids based on the cross-correlation peak change. The sensor will have many potential applications, because it is intrinsically safe, antielectromagnetic interference, and highly functional with a simple fabrication procedure.

Continuous Liquid-Level Sensor Based on a Long-Period Grating and Microwave Photonics Filtering Techniques

A fiber optic liquid-level sensor based on a long period grating (LPG) is proposed and experimentally validated. The principle of operation is based on a technique used to analyze microwave photonics filters. A 4-cm-long LPG cascaded with a high-reflectivity fiber Bragg grating is employed to achieve a continuous liquid-level sensor. The measurements have been performed using a modulator and a photo-detector with a modest bandwidth of less than 500 MHz, showing a sensitivity of −12.71 dB/cm and a standard deviation of 0.52 dB. One of the significant advantages of such sensing structure is that it is based on low-bandwidth radio frequency and off-the-shelf photonic components. In addition, the simple proposed scheme presents good repeatable performance and proves to be intrinsically robust against environmental changes, stable, and easy to reconfigure.

Optical fiber liquid level sensor based on macro-bending coupling

A novel optical liquid level sensor based on macro-bending coupling of fiber is proposed. Two plastic fibers are twisted and twined on a polyamide stick to construct a liquid level sensor. The results show that the output light power has a linear change with the variation of the liquid level. The higher sensitivity is acquired for the higher refractive index liquid and the smaller radius of the macro-bending of fibers. At the same time, the temperature effect to the sensor has also been researched and discussed.

Liquid level gauge based in plastic optical fiber

This work reports de development and testing of a low cost plastic optical fiber liquid level sensor for operation within highly flammable or other hazardous environments, such in oil piping or fuel tanks. The sensor consists on a vertical fiber, with engraved grooves, that will be sequentially fulfill when the liquid rises, resulting in an increase of the transmitted optical power. A variation of 2.60% in the transmitted optical power intensity was obtained per measured point. Moreover, the sensor sensitivity and resolution can be tailored accordingly to the application.

Light Path Model of Fiber Optic Liquid Level Sensor Considering Residual Liquid Film on the Wall

The working principle of the refractive-type fiber optic liquid level sensor is analyzed in detail based on the light refraction principle. The optic path models are developed in consideration of common simplification and the residual liquid film on the glass tube wall. The calculating formulae for the model are derived, constraint conditions are obtained, influencing factors are discussed, and the scopes and skills of application are analyzed through instance simulations. The research results are useful in directing the correct usage of the fiber optic liquid level sensor, especially in special cases, such as those involving viscous liquid in the glass tube monitoring.