Test of FBG sensors for monitoring high pressure pipes

Fibre Bragg Grating (FBG) sensors are increasingly being used on a wide range of civil, industrial and aerospace structures. The sensors are created inside optical fibres (usually standard telecommunication fibres); the optical fibres technology allows to install the sensors on structures working in harsh environments, since the materials are almost insensitive to corrosion, the monitoring system can be positioned far away from the sensors without sensible signal losses, and there is no risk of electric discharge. FBG sensors can be used to create strain gages, thermometers or accelerometers, depending on the coating on the grating, on the way the grating is fixed to the structure, and on the presence of a specifically designed interface that can act as a transducer. This paper describes a test of several different FBG sensors to monitor an high pressure pipe that feeds the hydraulic actuators of a 6 degrees-of-freedom shaking table at the ENEA Casaccia research centre. A bare FBG sensor and a copper coated FBG sensor have been glued on the pipe. A third sensor has been mounted on a special interface to amplify the vibrations; this last sensor can be placed on the steel pipe by a magnetic mounting system, that also allows the its removal. All the sensor are placed parallel to the axis of the pipe. The analysis of the data recorded when the shaking table is operated will allow to determine which kind of sensor is best suited for structural monitoring of high pressure pipelines

Active Fiber Bragg Grating Flow Sensor Powered By In-Fiber Light

Tunable fiber Bragg gratings (FBGs) are key components for optical communications and sensing applications. Current tuning mechanisms include on-fiber electric heating, the piezo-electric effect, and mechanical stretching and bending. Unfortunately, all of these tuning mechanisms rely on external electrical power supplied by non-optical means. Additional electrical cabling increases manufacturing cost and the risks of failure associated with additional on-fiber electrical contacts and fragile packaging, which are susceptible to electromagnetic interference. These limitations make current fiber components no longer suitable for use in hostile environments, such as extreme temperature, corrosive, and humid environments.The research herein presents a tunable fiber Bragg grating device without sophisticated packaging and external electrical wiring. Shown for the first time, the resonance wavelength, spectrum width, and chirp can be directly controlled by in-fiber light as well as spectral responses of metal-coated fiber Bragg gratings. In-fiber diode laser light at 910-nm was leaked from the fiber and absorbed by the surrounding metallic coating to raise the grating's temperature and to change the background refractive index distribution of the gratings. Wide tunability of the resonance wavelength and spectral width was demonstrated in both uniform and linear chirped gratings.Applications of in-fiber light-powered active grating sensors are demonstrated for dual function temperature and flow sensors based on self-heated optical hot wire anemometry. A grating flow sensor has been experimentally evaluated for different grating lengths and input laser powers. The grating flow sensor demonstrated a minimum measurable flow velocity for nitrogen gas flow of 0.35-m/s at atmosphere pressure, which is comparable to or better than most MEMS-based flow sensors. Optical fiber is not used only for optical signal delivery, but also as a multi-function cable that can deliver optical power for on-fiber self-heating. This one-fiber solution provides a new dimension to designing multifunctional fiber sensors without compromising their intrinsic advantages, which include immunity to electromagnetic fields, low cost, long lifetime, and the capability to function in harsh environments

The viscosity of silica fibres

The viscosity of an optical fibre over 1000 to 1150 {\deg}C is studied by inscribing an optical fibre Bragg grating that can withstand temperatures up to 1200 {\deg}C and monitoring fibre elongation under load through the Bragg wavelength shift. This optical interrogation offers high accuracy and reliability compared to direct measurements of elongation, particularly at lower temperatures, thus avoiding significant experimental error. An excellent Arrhenius fit is obtained from which an activation energy for viscous flow of Ea = 450 kJ/mol is extracted; addition of an additional temperature dependent pre-exponential does not change this value. This value is less than that idealised by some literature but consistent with other literature. The log plot of viscosity is overall found to be consistent with that reported in the literature for silica measurements on rod and beams, but substantially higher to past work reported for optical fibres. The discrepancy from an idealised activation energy Ea ~ 700 kJ/mol may be explained by noting the higher fictive temperature of the fibre. On the other hand, past optical fibre results obtained by beam bending with much lower values leave questions regarding the method of viscosity measurement and the time taken for structural equilibration. We note that because regenerated gratings already involve post-annealing to stabilise their operation at higher temperature, the structures are much more relaxed compared to normal fibres. This work highlights the need to stabilize components for operation in harsh environments before their application, despite some mechanical compromise. Given the increasing expectation of all-optical waveguide technologies operating above 1000 {\deg}C, the need to study the behaviour of glass over the long term brings added significance to the basic understanding of glass in this regime.Comment: Submitted to Acta Material

High Sensitivity 'Hot-wire'-based Gas Velocity Sensor for safe monitoring in mining applications

In this paper, building on a design developed from an advanced mathematical model, a practical fiber optic sensor, which is an analog of the familiar 'hot-wire' wind velocity monitor, has been developed, as an intrinsically-safe sensor device for use on coal mining monitoring applications. The device response time, the dynamic measurement range, the sensitivity, the sensor probe surface heat transfer coefficient and the effect of the wind (gas) velocity has been investigated in the laboratory and in situ and results reported, enabling the mathematical model used in the design process to be validated. The underpinning optical fiber-based principle used is the shift in the center wavelength of a Fiber Bragg Grating which is cooled by the gas flowing over it and the device sensitivity found was determined to be ~1500pm per unit m/s wind velocity (in the range of 0 - 0.5 m/s), ~330pm per unit m/s in the range 0.5-2 m/s and ~50pm per unit m/s in the range of 2.0-4.5 m/s. In the 'proof of concept' tests carried out, it was found that the greater the surface dissipation factor of the sensor, the shorter was its response time. Further, the response time was seen to decrease as the wind velocity increased, showing a typical value of 6s, which is suitable for the mining applications envisaged

Fiber optic gas sensor

A gas sensor includes an in-fiber resonant wavelength device provided in a fiber core at a first location. The fiber propagates a sensing light and a power light. A layer of a material is attached to the fiber at the first location. The material is able to absorb the gas at a temperature dependent gas absorption rate. The power light is used to heat the material and increases the gas absorption rate, thereby increasing sensor performance, especially at low temperatures. Further, a method is described of flash heating the gas sensor to absorb more of the gas, allowing the sensor to cool, thereby locking in the gas content of the sensor material, and taking the difference between the starting and ending resonant wavelengths as an indication of the concentration of the gas in the ambient atmosphere

Flow characterisation using fibre Bragg gratings and their potential use in nuclear thermal hydraulics experiments

With the ever-increasing role that nuclear power is playing to meet the aim of net zero carbon emissions, there is an intensified demand for understanding the thermal hydraulic phenomena at the heart of current and future reactor concepts. In response to this demand, the development of high-resolution flow analysis instrumentation is of increased importance. One such under-utilised and under-researched instrumentation technology, in the context of fluid flow analysis, is fibre Bragg grating (FBG)-based sensors. This technology allows for the construction of simple, minimally invasive instruments that are resistant to high temperatures, high pressures and corrosion, while being adaptable to measure a wide range of fluid properties, including temperature, pressure, refractive index, chemical concentration, flow rate and void fraction—even in opaque media. Furthermore, concertinaing FBG arrays have been developed capable of reconstructing 3D images of large phase structures, such as bubbles in slug flow, that interact with the array. Currently a significantly under-explored application, FBG-based instrumentation thus shows great potential for utilisation in experimental thermal hydraulics; expanding the available flow characterisation and imaging technologies. Therefore, this paper will present an overview of current FBG-based flow characterisation technologies, alongside a systematic review of how these techniques have been utilised in nuclear thermal hydraulics experiments. Finally, a discussion will be presented regarding how these techniques can be further developed and used in nuclear research

Review: optical fiber sensors for civil engineering applications

Optical fiber sensor (OFS) technologies have developed rapidly over the last few decades, and various types of OFS have found practical applications in the field of civil engineering. In this paper, which is resulting from the work of the RILEM technical committee “Optical fiber sensors for civil engineering applications”, different kinds of sensing techniques, including change of light intensity, interferometry, fiber Bragg grating, adsorption measurement and distributed sensing, are briefly reviewed to introduce the basic sensing principles. Then, the applications of OFS in highway structures, building structures, geotechnical structures, pipelines as well as cables monitoring are described, with focus on sensor design, installation technique and sensor performance. It is believed that the State-of-the-Art review is helpful to engineers considering the use of OFS in their projects, and can facilitate the wider application of OFS technologies in construction industry

Overview of sensors suitable for active flow control methods

Hlavným cieľom tejto bakalárskej práce bolo vytvorenie prehľadu vyvíjaných a už aplikovaných senzorov pre účely aktívneho riadenia prúdov. Senzory musia splňovať niektoré podmienky, preto výber senzorov bol naviazaný na reálnych výsledkoch testovacích programov, popis ktorých tvorí prvú časť tejto bakalárskej práce. Opis technológie a princíp fungovania senzorov je popísaný v druhej časti tejto práce.The main purpose of this bachelor thesis was to create the overview of the sensors developed for the future active flow control applications and overview the sensors already used in the active flow control applications. The sensors have to fulfil several requirements, so selection for the overview was based on the real flight test programs results, which were described in the first part of the thesis. The sensors technology description and operation principles were included in the second part of the thesis