The technology of distributed temperature sensing (DTS) is based on the application of Raman scattering from a laser beam light through optical fibers to detect temperature parameters along a fiber optic cable. The thing is that temperature resolution plays a crucial role. Herewith, this feature makes it possible to efficiently use DTS systems in oceanography.
Even though oceanographic applications of distributed temperature sensing are not new but such observations are not often. The reason is the serious challenges of deployment, calibration, and operation in oceanographic environment conditions. Nevertheless, researchers have tested the DTS system to overcome oceanographic configuration, calibration, and data processing difficulties.
It should be noted that they also evaluate temperature errors of DTS for several common scenarios. Difficult conditions influence the whole process, thus, the researchers look for alternative calibration, analysis, and deployment methods for distributed temperature sensing.
Therefore, these errors will be reduced and the successful application of DTS systems will be increased in dynamic ocean conditions. The thing is that DTS technology allows for “continuously sampling at a relatively high temporal and spatial resolution for significant duration over broad spatial scales.”
Despite distributed temperature sensing is widely applied in environmental applications, the oceanographic area remains challenging and still relatively rare. The main purpose of new DTS development is the solution to common problems present in oceanographic deployments.
To be more precise, the researchers use 2 various DTS systems, 3 fiber optic cables, and 24 thermistors. All of them help to test cables and different calibration configurations and perform distributed temperature sensing. Test results enable them to improve future oceanographic deployments. Moreover, they aid to achieve the best possible temperature signal in difficult deployment and operational environments.
DTS technology is a relatively new oceanographic tool. It allows for detecting temperature across wide spatial and temporal scales. Herewith, the application of such a fragile DTS system in remote and dynamically complex conditions remains difficult. Moreover, sometimes it is impossible to perform distributed temperature sensing at all.
Additionally, DTS systems face challenges during the detection of air/sea boundary. The reason is the change of water level, for instance, tides, waves, surge, etc., when the fiber optic cable can be exposed. Finally, the new DTS has succeeded to detect the temperature variance between the air-sea interface.
Members of NASA claim that they plan to test an enhanced fiber optic sensing system that allows performing thousands of measurements along the optical fiber about the thickness of a human hair for application in space. Herewith, such a promising fiber optic technology can control spacecraft systems during missions to the Moon and landings on Mars.
To be more precise, the system based on fiber optic sensors has been designed at NASA’s Armstrong Flight Research Center in California to obtain strain and other measurement data for aircraft. The researchers adapted the fiber optic system for application in space, where its potential uses contain temperature and strain information essential for space flight safety.
It should be noted that four fiber optic sensing systems are planned to test in space during five months, herewith, such tests carried out will demonstrate whether space fiber optic sensors can pass the hard conditions of a rocket launch. The thing is that rockets and spacecraft are considered to be highly complex systems and they have a myriad of various factors to be measured that is why NASA plans to keep the first applications of space fiber optic systems simple.
The new fiber optic technology based on space-rated sensors enables us to measure distributed temperatures on the Low-Earth Orbit Flight Test. The aim of the aeroshell of the fiber optic system is to slow down and protect heavy payloads from the intense heat of atmospheric re-entry. Additionally, the fiber optic sensors monitor temperatures on the backside of the inflatable decelerator, therefore, the researchers “are working on space optical fiber experiment that will travel as a self-contained experiment on a Blue Origin New Shepard rocket through NASA’s Flight Opportunities program.”
The opportunities provided by fiber optic technology also include the decrease of the heat produced by the unit’s electronics and by way of conduction, or moving the heat away from the unit, because of a lack of air in space. The fiber optic system is regarded as self-contained and essentially ready for plug and play application. The thing is that the operating principle of the system is based on fiber optic sensors that can endure severe conditions to measure distributed temperatures in a cryogenic environment that play a crucial role.
NASA is also developing a compact, economically, and hardly fiber optic sensing system version. Thus, the new fiber optic technology based on a temperature-tuned laser is used to overcome the challenges. The researchers continue improving the production techniques of fiber optic sensors and discussing performing a potential test of the sensors at NASA’s Ames Research Center in California to support the study of the new fiber optic technology.
Almost all pipelines suffer from numerous leaks during their operation, therefore, they require systems for fiber optic pipeline leak detection. Despite the fact that there are various techniques for leak detection, distributed temperature sensing systems (DTS) are considered to be an ideal option for the purpose.
Distributed temperature sensing is a technique that has been applied for more than two decades. DTS systems are regarded as the best option when a leak leads to a temperature differential between the ambient air and the escaping liquid or gas. The thing is that “temperature differentials generally occur when the pipeline product is at high pressure, high temperature or low temperature, all relative to ambient, which is characteristic of numerous pipelines.”
The operating principle of DTS is based on fiber optic sensing systems that operate as a sensor and measure temperatures along the entire length of optical fibers. Herewith, the optical fiber is put along the outside of the pipeline within the protective coating. It should be noted that the accurate installation location depends on the relative area(s) of the anticipated temperature differential caused by a leak, and on other reasons such as available mounting space.
To be more precise, DTS systems allow fastly identifying and precisely locating slow leaks at weld points, pipeline fittings, and herewith, sudden leaks. Fiber optic pipeline leak detection system enables detecting the precise location of leaks, often overcoming other distributed sensing technology. The fact is that even a tiny leak leads to a crucial temperature change, one that can be recorded by the DTS system. Most DTS measures temperatures with a precision of a few degrees, more than sufficient for leak detection.
For instance, a modern leading distributed temperature sensing technology allows measuring temperatures at a distance of 6 km, totaling 6000 points of measurement. The fiber optic sensing system’s transceiver measures “temperatures for 6 km both upstream and downstream of its installation point, for a total coverage of 12 km per each transceiver.” It is possible to employ several transceivers with accompanying fiber optic cables to offer coverage for long pipelines, totaling hundreds or thousands of kilometers in distance.
DTS technology acts as a semi-automatic leak detection system, obtaining data information to enable operators to take action before automation and/or safety system activation. It should be mentioned that a semi-automatic system means that the leak detection occurs automatically, resulting in an alarm signal in a continuously staffed control room.
Dams applied for hydropower, irrigation or mining play a crucial role in human life, herewith, they evoke significant human, economic, and environmental consequences when they fail. Nevertheless, distributed fiber optic sensing increases dam safety by offering early alerts of potential problems.
To be more precise, modern distributed sensing systems are considered to have high accuracy for monitoring promoting a continuous understanding of dam conditions, taking dam safety to a higher level. For instance, distributed temperature sensing (DTS technology) uses high spatial resolution temperature data from distributed temperature sensors to record tiny seepage flow changes and to estimate seepage rates in a dam structure.
It should be noted that seepage happens in most embankment and earth dams as the impounded water looks for the path of least resistance through the dam and its foundations. Therefore, excessive seepage presents a threat while high-tech sensing systems enable to detect and analyze subsurface processes and prevent erosion. Distributed fiber optic sensing is a promising technology that can be employed to control critical geophysical parameters, for instance, temperature and strain with a sub-meter resolution over several km.
Additionally, distributed sensing systems provide the benefits of cost-effective high spatial monitoring coverage. The thing is that optical fiber acts as the sensing system along the full length of the fiber optic cable allowing operators to obtain detailed data information along the entire dam. Distributed temperature sensors can catch tiny, localized changes in the seepage flow rates that would otherwise remain unnoticed. “They deliver temperature readings with the accuracy of point sensors with the indisputable benefit of fiber optics: the highest possible spatial coverage. ”
Moreover, the distributed temperature sensing does not need specialized optical fibers resulting in relatively low-cost installation. The thing is that measurements based on DTS systems provide data along the entire dam with high spatial resolution and high-temperature precision. Herewith, distributed temperature sensors have already been used in tailings dams. One of the main elements of the increasing number of permanent tools is the ever-increasing performance of the DTS systems. Modern fiber optic sensing systems achieve the world’s most accurate measurements, with sampling resolutions of 12cm (over 5km) and with temperature resolution as low as 0.01 C.
Finally, seepage detection used distributed temperature sensing is regarded as a crucial technology and has prominently improved the monitoring capabilities of dam operators. The application of optical fiber networks provides additional benefits like the ability of distributed sensing systems develops further.
Temperature is a key safety indicator in any industry. The technology of distributed temperature sensors using optical fiberallows measuring the temperature at any point in the fiber, with an interval of 1 meter, resulting in the detailed temperature dependence of all required areas. The data obtained by this technique makes it possible to develop intelligent warning systems based on it, therefore, replacing outdated point-based monitoring systems.
The operating principle is based on the reflectivity of stimulated Raman scattering of light (Raman effect). A semiconductor laser is also used to determine the location of temperature changes in a fiber optic cable. The fact is that the structure of the optical fiber changes when the temperature changes.
When laser beam light from the laser system enters the area of temperature change, it interacts with the changed structure of theoptical fiber, and in addition to direct light scattering, reflected light appears.
Benefits of temperature sensing systems
The main advantages of fiber optic sensors in comparison with classical analogs are the following:
Very fast response to parameter changes in the environment;
Resistance to chemicals and aggressive environments;
DTS is not affected by electromagnetic disturbances;
The sensitive part of thefiber sensor does not require connection to power lines.
The processing unit measures the propagation speed and power of both direct and reflected light and determines where the temperature changes. For instance, at a wavelength of 1550 nm, a pulsed generation mode is used with a laser power limit of 10 mW.
Types of sensors for temperature measurement
There are several types of optical fibers, each of which meets certain requirements for its properties, depending on the application due to the fact that the properties of the optical fibercan be varied over a wide range.
Physical effects on the optical fiber, such as pressure, deformation, temperature change, affect the properties of the fiber at the point of exposure and it is possible to measure the environmental parameters by measuring the change in the properties of the fiber at a given point.
In general, afiber optic sensorconsists of two concentric layers: fiber core and optical coating. The fiber optic light guide part can be protected by a layer of acrylate, plastic, reinforced sheath, etc., depending on the application of thisfiber cable.
Thus, distributed fiber optic sensors are perfect for industries related to combustible and explosive materials, such as coal, oil and gas production, etc. for use in fire alarm systems of various structures.
Detecting a fire in an industrial environment is not an easy task because of the large number of disturbing factors, many of which can be considered by detectors as carriers of fire signs. In addition, dust deposited on the DTS‘ sensitive elements makes it difficult to operate and it can disable them.
It is also necessary to take into account the possible smoldering of the deposited dust, which can also lead to false alarms. The presence of fumes and aerosols makes it impossible to operate smoke optical-electronic fire detectors. The presence of carbon monoxide will trigger gas fire detectors.
Industrial facilities and production are characterized by large volumes of premises, high ceilings, the presence of long tunnels, collectors, mines, inaccessible areas, and premises with a complex configuration and geometry. And in these conditions, it is certainly possible to protect using traditional fire alarm systems, but this involves the use of a large number of detectors, and therefore they have high costs, including installation and maintenance of alarm systems and automation.
It is difficult to select detectors for explosive zones, especially for use in underground operations and mines. Aggressive media are often present in chemical industries. There are also objects of sea and river transport, characterized by the aggressive salt fog.
Oil and gas application
The use of non-electric sensing devices, the use of fiber optic cable allows the DTSto be applied in enterprises of the oil and gas complex, mines, underground operations, chemical industries (including those with aggressive environments), and metallurgy and energy enterprises.
As for oil companies, the active development of high-viscosity oil fields, which imposes strict requirements on the production equipment, and the severe depletion of most oil and gas fields require mining organizations to conduct prospecting and exploration operations, change production technologies and control the technical condition of wells.
The main task for mining companies to increase the well’s production capacity in real-time is to track information about the processes occurring in wells and fields. Solutions based on standard temperature sensors suggest well logging using point measuring instruments, which leads to the inaccuracy of the data obtained.
The disadvantages of suchsensing devices include the inability to fix the distribution of one of the most important parameters of the well – the temperature profile in real-time, as well as the need for power supply, the impact on the measurement results of external electromagnetic fields, labor and time costs required for the departure of the team and performing various operations, including the immersion of thefiber sensorelement and its movement along the well, data processing, etc.
The fiber optic cable is resistant to mechanical damage. Additionalfiber optic cableprotection is not required during descent and lifting operations, but the protection of the fiber cable from mechanical damage during descent and lifting operations can be provided by the use of protective coatings.
Distributed temperature system provides continuous underground power lines monitoring of temperatures, detecting hot spots, delivering operational status, condition assessment, and power circuit rating data. This helps operators to optimize the transmission and distribution networks, and reduce the cost of operation and capital.
Usually, the DTS systems can detect the temperature to a spatial resolution of 1 m with precision to within ±1°C at a resolution of 0.01°C. Measurement distances of greater than 30 km can be monitored and some specialized systems can provide even tighter spatial resolutions. The advantages of working with Optromix:
Our DTS system has the superior quality, however, its price is one of the lowest in the market;
Optromix is ready to develop DTS systems based on customer’s specifications.
Quantum-enhanced metrology is regarded as a popular area of research for years because of its promising applications, varying from atomic clocks to biological imaging. According to the researches, a non-standard distributed sensing system offers significant advantages compared to traditional fiber optic systems.
These researches help a team of scientists from Denmark to carry out an experiment on distributed fiber optic sensing and the benefits of employing an entangled quantum network to detect an averaged phase shift among numerous distributed sensing nodes. The fiber optic sensing technology uses several methods that enable collecting more accurate measurements in different areas.
The purpose of the new study is based on squeezed light and homodyne detection that is now established distributed fiber optic sensing techniques. The team aims at “measurement of a global property of numerous spatially separated objects and investigate whether probing these objects simultaneously with entangled light led to more accurate results than probing them individually”.
Thus, the application of a quantum network to probe the objects simultaneously allows distributed sensing systems with far higher accuracy than that attainable when examining probes individually. To be more precise, the team measures the phase shifts (set with wave plates to a known value) by the fiber optic system that sends a weak laser beam through and detects the change in the light’s phase quadrature with homodyne detectors.
The benefit of applying distributed fiber optic sensing plays a really important role when it is necessary to measure the property of numerous objects connected in an optical network. Nevertheless, the losses in the network and detectors are required to be kept low in order to successfully raise the accuracy, alternatively, the quantum benefit of distributed sensing disappears.
The researchers succeeded in the experimental demonstration of the benefits connected with employing multi-mode entanglement for distributed fiber optic sensing. The thing is that the benefits have been previously predicted, however, only highly idealized scenarios and experimentally very difficult probe states or detection methods were taken into consideration. The developed fiber optic system demonstrates that these benefits are available even with current noisy sensing technology.
The fiber optic system finds potential applications in various areas of research and technology development. For instance, they provide a high sensitivity of molecular tracking devices, atomic clocks, and optical magnetometry methods. Moreover, the distributed fiber optic sensing gives valuable information about how quantum-enhanced metrology can be reached utilizing readily accessible technologies, for example, squeezed light generation and homodyne detection.
Nowadays distributed fiber optic sensors remain a subject of great interest for constant structural health monitoring of huge structures, for instance, bridges or dams. Herewith, the more sensing points there are, the more effective monitoring the sensing system offers. Thus, newly improved distributed sensors with 1 million sensing points provide prominently faster detection of structural problems that are now accessible.
Fiber optic sensors allow accurately detecting of erosion or cracking before a dam fails. Therefore, earlier detection of a problem by fiber optic sensing systems may lead to its possible prevention from later deformation or provision of sufficient time for evacuation procedures. Fiber sensors are considered to be perfect for controlling infrastructure since they suit for use in harsh environmental conditions and in areas that lack nearby power supply.
The operating principle of distributed sensors is based on “changes in the structure at any of the sensing points along with the optical fiber that causes detectable changes in the light traveling down the fiber”. Despite the high popularity of distributed fiber optic sensors, the main applications of sensors include the detection of leaks in oil pipes and structural health monitoring for landslides along railroads.
It should be noted that the novel fiber optic sensing system enables tracing train and temperature changes from 1 million sensing points over a 10-kilometer optical fiber in less than 20 minutes, while the strain parameter demonstrates deformation or mechanical stress on an object of interest. Herewith, the new distributed sensor is regarded as to be about 4.5 times faster than previous fiber sensors with 1 million sensing points. A larger number of sensing points require fewer optical fiber units for structural health monitoring of an entire structure resulting in cost reduction.
A large density of sensing points makes distributedfiber optic sensors suitable for such applications as avionics and aerospace, where every inch of a plane wing has to be controlled properly. According to researchers, the conventional technique of generating the continuous signal in fiber sensors undertakes distortions in the fiber optic system at higher laser powers. This problem has been solved by using the approach known as Brillouin optical time-domain analysis.
The fiber optic sensor has been already tested and showed the opportunity to measure the temperature of a hot spot to within 3 degrees Celsius from the end of a 10-kilometer long optical fiber. Additionally, it is planned to improve the distributed sensor and make it even faster resulting in the further reduction of the acquisition time. Future enlargement of sensing point density will promote the fiber sensors being used in new fields, for example, biomedical applications.
A new distributed temperature sensor (DTS) system has been developed to perform optimization of the temperature precision with the enhanced temperature sensitivity of backscattered spontaneous Raman scattering. The DTS system is based on the difference in sensitive-temperature compensation.
Distributed temperature sensors apply the dual-demodulation, self-demodulation, and double-end configuration principles. The DTS system has been already tested and demonstrates great results: the temperature precision is considered to be 12.54 °C, 8.53 °C, and 15.00 °C along the 10.8 km under the traditional R-DTS systems, respectively.
It is possible to use the sensing system with difference sensitive-temperature compensation for the dual-demodulation, self-demodulation, and double-end configuration R-DTS, herewith, this fiber optic sensing technology enables to make the temperature precision better than 1 °C for these three demodulation systems.
The operating principle of Raman Distributed Temperature Sensor is based on “specific optical effects along the sensing optical fiber to obtain a spatially distributed temperature profile”. Compared to traditional discrete sensing techniques, R-DTS systems provide unique attributes and capabilities.
It should be noted that spontaneous Raman scattering of distributed temperature sensors uses the energy exchange in the optical fiber, therefore, when the pulsed light quantum and fiber optic material molecule leads to an inelastic collision in optical fiber, this will create an anti-Stokes light.
The thing is that the anti-Stokes light is regarded to be very sensitive to the surrounding temperature, and it allows modulating the environmental temperature using the principle of Raman scattering. Nowadays, such DTS systems find their application in the temperature safety monitoring thanks to the benefits of distributed measurement, long-distance, and high spatial resolution, as well as in transport infrastructure, smart grid and gas pipeline, etc.
It is necessary to pay on the following parameters when you choose distributed temperature sensors with high-performance: temperature precision, temperature resolution, and spatial resolution. DTS systems can be used as an industrial temperature measurement system, for instance, the carrier density in the power cable can be measured by employing a specific temperature. Additionally, distributed temperature sensors allow locating the position of pipeline leakage.
Tests demonstrate that the temperature demodulation system based on distributed temperature sensing offers higher temperature precision and resolution of the self-demodulation than the dual-demodulation system due to the signal-to-noise ratio. Moreover, the double-ended configuration for DTS systems allows avoiding the measurement error based on the change of local external attenuation.
Proper fire detection in the mining area is the number one priority because fire can evoke damages to valuable assets, downtime in operation, and even loss of lives. Herewith, the installation of fire protection devices can be complicated by harsh environmental conditions of the mining area because industrial equipment produces dirt, dust, humidity, and corrosive atmospheres in the production, storage, or transport of goods. Nevertheless, the application of modernfiber optic sensorsbased on distributed temperature sensing or DTS is considered to be a proven and efficient technology for fire detection and temperature measurement.
The fact is DTS technology provides precise temperature measurement along the length of a conveyor belt due to the use of autonomous fiber optic cables, therefore, even very long conveyor routes can be efficiently monitored. Thus, cases of fires and overheating during mining operations can be quickly and easily identified and localized to within one-meter precision by distributed temperature sensors, enabling to activate the safety countermeasures. Also, it should be mentioned that it is required the proper installation of the fiber optic sensor cables and adjustment of the alarm zones for heat detection of industrial rollers.
Nowadays mining industry applies fiber optic linear heat detection systems for these purposes, however, the new distributed sensing technology may significantly improve and increase the protection of assets in combination with the one mentioned above. The principle ofDTS technologyoperation is based on the use of optical fibers as a distributed microphone, moreover, it means that fiber optic sensors allow measuring not only the temperature but also the acoustics. Such additional information is important because it helps to determine potential sources of danger and improves the whole purpose of fire prevention. To be precise, the DTS is highly important in hot roller detection that is the main reason for conveyor fires.
Also, it is difficult to identify the initial smoldering stage with traditional techniques, herewith, the majority of common gas detecting systems do not work properly because of high air currents. An optical fiber-based on distributed temperature sensing technology, in its turn, provides numerous benefits during both normal operation and dangerous situation that include:
Reliable fiber optic cable that is resistant to dirt and dust;
Long-distance (up to 10 km) and up to 4 measurement channels monitor several conveyors with one DTS system;
Accurate localization of fires and hot rollers;
DTS is able to detect fires in spite of unfavorable environmental conditions.
Groundwater flow sensing is highly important when it comes to the extraction of drinking water because it allows for avoiding well clogging and pollution. Nevertheless, fiber Bragg grating sensors with optical fibers are considered to be a promising new technique for monitoring of groundwater flow.
The fact is that FBG sensors enable us to identify even tiny changes in temperature, pressure, and fiber shape taking into account the sensitivities influenced by the packaging. Thus, fiber Bragg grating sensors are able to create a multiplexed sensor for the groundwater flow direction and magnitude.
It should be mentioned that the fiber-optic technology of FBG sensors has numerous application range, for example, in aerospace (load monitoring and shape sensing), in civil engineering (structural health monitoring), and in the oil and gas industry (temperature and pressure monitoring).
Today the technology of fiber Bragg grating sensing is also used for groundwater flow monitoring. FBG sensors allow measuring various physical properties, strain, or temperature variations such as pressure, vibrations, and curvature of the optical fiber, herewith, fiber sensors make instant and precise measurements.
One more technique that can be used for monitoring is distributed temperature sensing, and the intended application influences directly the choice of technology – fiber Bragg grating or DTS sensing. DTS uses conventional single-mode or multimode fibers that find various applications in the telecom industry, that is why DTS fibers are available in large lengths.
Moreover, the FBG sensors are possible to be written in traditional and specialty fiber optics, and consequently, their price increases per sensor number. However, the cost of fiber interrogators that are required for the DTS sensing data collection is five times more expensive than FBG units.
Traditional fiber Bragg grating sensors already possess a sufficient temperature resolution for the measurement of groundwater temperature changes without additional packaging. In addition, this FBG system has been recently tested to demonstrate its benefits for groundwater flow monitoring.
Thus, the experiment has shown that the use of FBG sensors enables to measure the relative temperature with a precision of 0.85 ℃ for the differential measurements. Also, the used fiber Bragg grating system is able to make a multiple-sensor interrogation with the possibility of an expansion into a larger distributed sensing network.
Finally, the data results presented that fiber Bragg grating sensors can be employed as a temperature sensing system in the subsurface environment, but in this case, they did not demonstrate any benefits over the existing fiber sensing technologies. Nevertheless, multiplexed fiber Bragg grating pressure or strain sensors would possibly find their application in groundwater flow monitoring.