Fiber Optic Strain Sensors Technology and Applications

Nowadays, different types of fiber optic strain sensors have attracted attention from all over the world. Fiber Bragg Grating (FBG) has become the most widespread technique, directly applicable to bridges, concrete, and dams for strain measurement.

To create the actual strain sensor, the optical fiber is inscribed during production with a Fiber Bragg Grating (FBG). This is basically a pattern of material interferences, which reflects the light differently from the rest of the fiber. For better understanding, visualize the fiber as a cylindrical length of transparent material, with a number of thin slices in it. When the light from the laser hits this pattern, certain wavelengths are reflected, while others pass through.

The material interferences are placed at certain intervals. When the fiber is stretched or compressed and is therefore subjected to positive or negative strain—these intervals change. When the fiber is stretched, it lengthens and the spaces get bigger and vice versa. Not only does the reflected light take a little longer or shorter to travel back when the Fiber Bragg Grating is under strain, but the wavelength that is reflected also changes. In scientific terms, the Fiber Bragg Grating has a certain refractive index. The refractive index of a material describes how much light is bent or refracted when passing through the material. When the grating changes shape due to strain, its refractive index changes as well.

For measurements, the optical fiber needs to be connected to a so-called interrogator; it continuously sends out light in different wavelengths, one at a time, thus covering a wide spectrum.  In order to ensure the safety of personal and public property, the precise and real-time monitoring of strain becomes more and more important in all kinds of engineering applications, such as chemical plants, gas stations, power stations, bridges, tunnels, oil pipelines, etc.. In general, these application environments full of poisonous gas, intense radiation, and elevated temperature are dangerous to human health, so safe and efficient remote monitoring of strain is of great significance. Compared with conventional electrical sensing methods, an optical fiber strain sensor is more suitable for present applications because of its compact size, high sensitivity, multiplexing capability, immunity to electromagnetic interference, high-temperature tolerance, and resistance to harsh environments.

Fiber optic strain sensors are welded directly to the surface of the metal structure (pipes, beams, etc.), and it has a protective silicone cover. Fiber optic strain sensors are durable and stable, widely used for civil engineering constructions, particularly they reinforce concrete structures exceptionally well.

If you would like to purchase Optromix FBG Strain Sensors, please contact us: info@optromix.com  or +1 617 558 98 58

Fiber Optic Sensors for Vibration Monitoring

Vibration is a common phenomenon in nature and vibration monitoring technology is of significant importance in scientific measurements and engineering applications. Accurate measurement and monitoring of vibration are crucial for the detection of the abnormal events and pre-warning of infrastructure damage. Traditional vibration sensors suffer from electromagnetic (EM) interference, which presents the difficulty for applications in harsh environments. In addition, the short monitoring distance and high maintenance cost mean they do not meet the actual needs of modern engineering measurements.

Optical fibers have attracted a significant amount of research attention in a wide range of applications during the last several decades due to the outstanding advantages of lightweight, flexible length, high accuracy, signal transmission security, easy installation, corrosion resistance, and immunity to EM interference. These characteristics render them attractive for use in harsh environments where the application of traditional sensors is severely limited. The high sensitivity to changes in external physical quantities, such as temperature, strain, and vibration, makes optical fibers suitable for sensing purposes. Up to now, fiber-optic vibration sensors mainly consist of the point, quasi-distributed, and distributed sensors. Several schemes of point sensors including fiber Bragg grating (FBG), Fabry–Perot, self-mixing, and Doppler vibrometry are deployed for vibration measurement. Among them, FBG vibration sensors have become a fast-developing scientific research field owing to intrinsic advantages such as low noise, good embeddability, and the ability to be easily multiplexed to construct a distributed sensor array. Based on the FBG sensing principle, many investigations are applied to the measurement of vibration. Distributed fiber optic vibration sensing technology is able to provide fully distributed vibration information along with the entire fiber link, and thus external vibration signals from an arbitrary point can be detected and located. Compared with point and quasi-distributed vibration sensors, which can only be used individually on a small scale and often have poor concealment, distributed fiber-optic vibration sensors inherit the advantages of general fiber sensors and offer clear advantages such as lightweight, large-scale monitoring, good concealment, excellent flexibility, geometric versatility of optical fibers, quick response, system simplicity, immunity to EM interference, high sensitivity, accurate location, etc. Distributed fiber-optic vibration sensors mainly include interferometric sensors and backscattering-based sensors. Various interferometric sensors have attracted a significant amount of research attention and are widely investigated.

Continue reading

Types of gas flow measurement

Measurement of gas flow is a key point in the field of gas application. In recent decades, thermal mass flow (TMF) meters are widely used in measuring the mass of gas. The TMF meters have many advantages such as wide applicable fields it can be applied to many kinds of pipelines and different types of gasses), wide measurement range, and high measurement accuracy and repeatability.

Thermal mass flow meters generally use combinations of heated elements and temperature sensors to measure the difference between static and flowing heat transfer to a fluid and infer its flow with a knowledge of the fluid’s specific heat and density. The fluid temperature is also measured and compensated for. If the density and specific heat characteristics of the fluid are constant, the meter can provide a direct mass flow readout, and does not need any additional pressure temperature compensation over their specified range.

Today, thermal mass flowmeters are used to measure the flow of gasses in a growing range of applications, such as chemical reactions or thermal transfer applications that are difficult for other flow metering technologies. This is because thermal mass flow meters monitor variations in one or more of the thermal characteristics (temperature, thermal conductivity, and/or specific heat) of gaseous media to define the mass flow rate.

Continue reading

Fiber Optic Sensors for Borehole seismic technology system

Seismic techniques are the main techniques for the characterization of subsurface structures and stratigraphy. Borehole technology system provides the highest resolution characterization and most precise monitoring results because it generates a higher signal to noise ratio and higher frequency data than surface seismic techniques. A new generation of fiber optic borehole sensor systems has been developed based on all fiber optic data transmission and fiber optic sensor technologies. The new fiber sensors are much more sensitive and are able to record much larger bandwidth data with better vector fidelity than is possible with current seismic sensor technologies. The new sensors also can operate in most hostile environments found in boreholes such as pressure and temperature conditions. This improvement in data quality and density will generate better images and more precise monitoring results, which will allow a much improved high-resolution interpretation and ultimately better oil and gas production.

Since the borehole seismic system does not require electric power for either the optical sensors or the hydraulically operated deployment system, the entire system is intrinsically safe. The fiber-optic seismic sensor system measures the strain of the fiber between two Fiber Bragg gratings surrounding the mandrel using an interferometric measurement technique comparing the phase angle between two spaced reflections from the same light pulse traveling in the fiber. It is using a time-division multiplexing technique to transmit the dynamic fiber strain information to the interrogators. This allows the measurement of extremely small strains in the fiber. The fiber optic seismic sensor is self-standing to electric and electromagnetic interference in the borehole since the system does not require any electronics at the fiber optic sensor end. This design also makes the fiber optic seismic sensor extremely robust and able to operate in extreme environments such as temperatures up to 300 C.  All the sensors are calibrated so the optical output amplitude into absolute acceleration can be mapped. The sensors have also proved to be about 100 times more sensitive than the regular coil geophones that are used in borehole seismic systems today.

Optromix Company manufactures a wide range of sensors, that are able to feel the slightest deformation of the structures.

FBG monitoring systems for hydraulic structures

Any sophisticated construction requires constant tracking of its technical condition to detect inconsistencies between the projected calculations and the actual working parameters and to evaluate its compliance to the established standard. Currently, the most used sensors to monitor hydraulic structures are stringer transducers that perform very well under various climate conditions, especially in the areas of deep-frozen soil. However, the increasing sophistication of modern constructions is more demanding in terms of monitoring standards. Often it is no longer sufficient to use regular sensors. Hence, a new type of monitoring with fiber optic devices was created.

The length of these constructions and its complex structure demand special fiber optic devices monitor its safety. One of the most complicated constructions to place the fiber optic solutions is a hydro-electric power station. FBG sensors can be placed in the structures where regular stringer transducers fail, for example, in long shafts and tunnels. On top of that, fiber optic solutions are completely fire-safe, resistant to electromagnetic fields and radiation, and other interferences.

Tunnels in hydro-electric power stations can be equipped with the following fiber optic devices: FBG sensors for deformation, temperature, displacement, and pressure sensors. The deformation sensor is measuring the condition of the steel reinforcing, and it embedded on its surface. The sensor is covered with the isolating material to avoid the impact of the concrete. An additional temperature sensor is installed in the same location to track temperature changes that affect deformation. A displacement sensor is used to measure the joint opening between steel compression and a ram. The pressure sensor is used to measure the water pressure in the tunnel.

Optromix offers a variety of fiber optic solutions, and we will write a tailored FBG based on your specific needs.