Fiber-optic Liquid Level Sensor Research Articles

Fiber Optic Liquid Level Sensor Based on Integration of Lever Principle and Optical Interferometry

We present a fiber-optic liquid level sensor that is conducted by a combination of optical interferometry and lever principle. The sensing unit is a Mach-Zehnder interferometer (MZI), which is formed by sandwiching a piece of photonic crystal fiber (PCF) between two single-mode fibers (SMFs). The measuring equipment is composed of a rotatable lever and a fixed link. The rotatable lever includes two different length arms, i.e., L1 and L2. Both ends of the MZI are glued on the tip of the L2 arm and the fixed link using ethoxyline, respectively. A hanging stick, which is dipped into a liquid tank, is directly mounted on the other end of the rotatable lever. The buoyancy will increase as the stick depth of immersion into the liquid increases. The tension the MZI subjected will increase according to the proportion of L1/L2 on account of the lever principle. The sensitivity of the sensor could be regulated with different ratios of lever arms. In our experiment, a maximum sensitivity of 111.27 pm/mm was obtained with a 1 : 7.8 ratio of two lever arms L1/L2. The demonstrated liquid level sensor has the advantages of simple structure, easy fabrication, low cost, and high sensitivity.

Highly Sensitive Tapered Fiber Mach–Zehnder Interferometer for Liquid Level Sensing

A low cost, simple, and in-line liquid level sensor based on a dual-tapered fiber Mach-Zehnder interferometer (DTFMZI) is proposed. In this letter, the configuration, operation principle, experimental results, and discussion for the proposed fiber-optic liquid level sensor are investigated. Experimental results show that the proposed DTFMZI sensor is highly sensitive and linear in the response; a good sensitivity of ~2.8 nm/cm within the whole measurement range of 10 cm is achieved in this letter.

Novel light-leaking optical fiber liquid-level sensor for aircraft fuel gauging

We present a novel light leaking optical fiber liquid level sensor for aircraft fuel measuring systems that is based on the side-emitting optical fiber. In operation, the fiber is bent as a spiral and inserted into the liquid, then the conducting light in the fiber will be scattered and transformed to be cladding mode and radiation mode. The radiation mode power in the scattering light is determined by the ambient refractive index. Since more power is lost in liquids than in the air, the output power of the sensor will monotonically decrease while the liquid level increases. The fiber is bent as a spiral to increase the sensor’s sensitivity. A laboratory prototype with a range of 1 m, a radius of 14 mm, and a screw pitch of 15 mm has been built and successfully demonstrated in water level measurement. The sensor’s response to liquid level is presented and shown to be consistent with the theory.

A Thin-Core Fiber Modal Interferometer for Liquid-Level Sensing

A fiber-optic liquid-level sensor is proposed and experimentally demonstrated. It is a Mach—Zehnder interferometer (MZI) composed of a thin-core single-mode fiber (TCSMF) without coating sandwiched between two single-mode fibers (SMFs). The transmission spectrum properties of the MZI modified by liquid around the TCSMF are used for detecting the liquid level. It is found that the sensor exhibits an excellent linear relationship between the level and the shift of interference dip wavelength and its achieved sensitivities are 0.160 nm/mm and 0.288 nm/mm for liquid with an RI of 1.3330 and 1.3696, respectively. Employing the spectrum differential integration (SDI) method to analyze transmission spectra can increase the detecting resolution of the liquid level. Due to its advantages of an extremely easy fabrication process, high sensitivity and a large sensing range, the sensor is an ideal candidate for continuous liquid level sensing.

Fiber-optic liquid level sensor based on coupling optical path length variation

The concept for a new and simple fiber-optic liquid level sensor is presented and experimental results are shown to demonstrate the principle. The sensing principle is based on light intensity modulation when rising and falling mode of liquid level causes coupling optical path distance variation between two optical fibers. Near continuous mode of liquid level variation could be monitored with resolution as low as 1 mm can be measured in the length scale of 25 cm.

Comparative study of f iber optic liquid level sensors based on long-period f iber gratings with dif ferent doping concentrations

The performances of two liquid level sensors based on long-period fiber gratings are studied. The long-period gratings (LPGs) have similar characteristics (length and period), but are fabricated with two photosensitive B-Ge co-doped fibers with different dopant concentrations. We investigate the temperature sensitivities of LPGs and exploit their refractive index sensitivity to implement liquid level measurement. By controlling fiber parameters, such as the dopant concentrations, the measurement sensitivity of a LPG-based fiber optic liquid level sensor can be improved.

Fiber-Optic Liquid Level Sensor

A fiber-optic liquid level sensor based on multimode interference (MMI) effects is proposed and demonstrated. We show that MMI and self-image effects can be effectively applied for multiplexed liquid level sensing, because the natural response as a band-pass filter for each sensor is clearly distinct from each other, in the case for several sensors working at the same time. Using a standard 105/125 step-index multimode fiber (MMF) a simple discrete level sensor was fabricated, that can also discriminate the refractive index (RI) of the liquid during the level measurement. The MMI liquid level sensors are not only inexpensive, but their fabrication is simple.

A Side-Coupled Optical-Fiber Liquid Level Sensor

A side-coupled optical-fiber liquid level sensor is proposed to realize intrinsically safe measurement to liquid level in flammable environments. The sensor consists of two parallel side-polished fibers is based on the different coupling coefficient between the fibers in different media according to the Fresnel equation. One of the fibers is for emitting and the other is for receiving, and a reflector is attached to the end of the fibers to enhance the power of light. The power of the light from the top of receiving fiber is measured by the signal processing circuit which is mainly organized by an instrumentation amplifier and a correlator. This sensor is fabricated in the laboratory and the results of the experiments show good performance. It is demonstrated that the sensor is safe and sensitive, but there is still a lot of work to do before its commercial usage.

PMMA (polymethyl methacrylate) fiber optic probe as a noncontact liquid level sensor

AbstractThis article reports the principle of operation, design aspects, experimentation, and performance of a noncontact fiber optic liquid level sensor. The sensor is based on the phenomenon of reflective concept. The device consists of fiber optic transmitter, fiber optic probe, floating reflector, photodiode detector, and digital multimeter. The fiber optic probe consists of two 60‐cm‐long PMMA (polymethyl methacrylate) fibers of diameter 1 mm, numerical aperture 0.5, core refractive index 1.492, and cladding refractive index 1.402. The fiber optic sensor is a promising alternative to other well‐established methods for the measurement of liquid level due to its simplicity of design, high precision, long‐term stability, linearity, high degree of sensitivity, dynamic range, noncontact sensing, and low cost of the fabrication make it suitable for applications in precise level control in analytical and process chemistry, biochemistry, bioanalytics, and on‐line measurement or inspection of liquid level. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 2114–2118, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25415

A Novel Fiber-Optic Liquid Level Sensor with Adjustable Light Path Based on Principle of Light Refraction

Considering the difficulty to precisely indicate the specified height for transparent or semitransparent thin liquid column, a novel fiber-optic liquid level sensor is reported which remarkably enhances the sensitivity by regulating the external light path based on the principle of light refraction. The structure and working principle of this sensor is introduced. A light path model is developed on the condition that the incident light ray obliquely penetrates the medium in the liquid level indicator. The formulae of deflection angle are deduced. Factors affecting the precision of the sensor are analyzed by means of an example. Finally the feasibility of this novel design is verified by experiments.

Liquid-level monitoring sensor systems using fiber Bragg grating embedded in cantilever

A fiber-optic liquid-level sensor based on the bending of a fiber Bragg grating (FBG) is proposed. The FBG embedded in a cantilever rod such that the elongation and contraction of the Bragg grating indicate a liquid-level change. The shift in the Bragg wavelength has a good linear response to the bending of the cantilever. An optical bandpass filter is used to determine the Bragg wavelength by converting the wavelength shift to an intensity demodulation. The relative power measured by a photo-detector is also linearly proportional to the liquid-level variation. The sensitivity is approximately 0.1 dB/cm. To eliminate the temperature dependence of the FBG sensor, we suggest that the tuning of the optical bandpass filter should take into account the temperature variation. The operating principle is described in detail.

대형선박의 액체 탱크용 수위 모니터링 센서 시스템 연구

Abstract ： A fiber-optic liquid level sensor based on bending cantilever b eam has been proposed. A fiber Bragg grating (FBG) embedded in the cantileve r beam is used to sensing elements. The basic concept is elongation and constrict ion of the FBG corresponding to the liquid level variation. The best FBG posit ion on the cantilever for obtaining the high sensitivity was 4 cm from the fixing point. When the liquid level moves up and down vertically, the Bragg wavelength is linearly shifted. But, the wavelength sensitivity of the FBG installed on the upper side of cantilever was four times better than that of the FBG equipped in the lateral side due to the difference of unit strain applied to the FBG. Intensity demodulation using th e low-cost edge filter is used to interrogate the Bragg wavelength through converting the wavelength signals into the optical intensity ones. Experiment results show that the electrical output is exponentially proportional to the liquid level. But, it should be overcome for applying to the ships.

Extrinsic Fabry-Perot cavity optical fiber liquid-level sensor

An optical fiber liquid-level sensor based on an extrinsic Fabry-Perot (FP) cavity is proposed and demonstrated. The FP cavity consists of the end of the single-mode optical fiber and the elastic silicon layer. Liquid pressures act on the mechanical construction to change the cavity length, resulting in differential phase shifts that may be observed as variations of the output signal intensity. Self-compensated steps have been taken to obtain high accuracy and long-term stability in realistic circumstances. Experimental results indicate that accuracy of 2 mm over a full scale of 3.5 m (water) is obtained under ambient temperature 10-38 degrees C. The sensor can be used to measure liquid levels continuously and accurately in explosive and flammable environments.

Multi-sensor system using plastic optical fibers for intrinsically safe level measurements

A system for measuring liquid level in multiple tanks using optical fiber technology has been developed. Oil field service industry or any sector requiring liquid level measurements in flammable atmospheres can be benefited from this intrinsically safe technology. The device used a single lens for the emitting and receiving fiber and it is based on amplitude variations as a function of the liquid distance and not in time of flight or phase detection. Being the first fiber-optic liquid level sensor with those characteristics for long ranges (>200 cm). A simple model to describe their behavior has been derived and tested on two prototypes. A Monte-Carlo method is used to fit the experimental data and obtain the model parameters. High accuracy between experimental data and fitted curve is obtained. The prototypes have a good linearity, better than 1.5% FS (full scale). Sensor heads are made of plastic optical fibers (POF) that are easy to handle, flexible and economical. They are excited by 650 nm lasers, housed in ST-connectors to obtain compact and rough prototypes. Optical multiplexing is used to increase the measuring safety area. Frequency division multiplexing is used to address each sensor head. A discussion about the influence of tilts and aberrations is also included.

Fiber-optic liquid-level sensor

An intensity-based fiber-optic liquid-level sensor for point measurement is described. The sensing principle is based on the total internal reflection of light, which is disturbed by contact with a liquid. The main problems with this kind of level sensor are discussed. The performance of the sensor has been studied using four different shapes of the tips of the sensing elements: conical, rounded conical, second-order polynomial and third-order polynomial.

Differential absorption fiber-optic liquid level sensor

An optical fiber liquid level sensor based on differential optical absorption is described. The sensor uses a two-wavelength ratiometric approach to cancel out errors arising from variations in fuel characteristics, tank vibrations, etc. The sensor is mechanically simple and uses inexpensive LED sources and multimode optical fiber. The sensor displays ±2-mm resolution over an 18-cm range with immunity to 5° surface tilt variations and immunity to optical fiber connector and bending loss variations up to 1 dB.

A digital fiber-optic liquid level sensor

A fiber-optic liquid level sensor is presented which directly outputs digitally encoded liquid level information. The sensor operates on selective coupling at the liquid surface between a source waveguide and an array of digitally masked receiving waveguides. The receiving waveguides carry optical "high/low" signals to a remote detector and discriminator circuit. The sensor is capable of liquid level resolution of ± 2 mm over a range of up to several meters. A 40-cm-long prototype sensor has demonstrated an optical signal-to-noise ratio of 10-20 dB for several different transparent and semi-transparent liquids. The digital nature of the sensor provides good isolation from drift and the effect of tilt, source intensity fluctuations, and variations in liquid characteristics. Optical fibers are the only connections to the liquid tank, providing complete electrical isolation. The associated electronics are relatively simple and inexpensive.