Measurements all the way down the line
Instead of making measurements at discrete, pre-determined points Distributed Temperature Sensing (DTS) makes continuous measurements over the full length of the optical fibre. As a result DTS is capable of detecting changes in temperature smaller than 0.01°C without prior knowledge of where that event might occur. Distributed sensing is also real time – so you get continuous monitoring at all points along the cable at all times.
Fast, frequent and accurate measurements of physical factors such as temperature, pressure or strain play a key role when it comes to ensuring the smooth operation of processes in many domestic, commercial and industrial settings. For example, most factories and process plants rely on temperature and pressure measurements to operate.
Conventional vs distributed temperature sensing
Conventional types of temperature control system are usually based on the use of point measurements: data gathered from individual sensors and gauges that measure single values at specific locations. This can limit the speed, accuracy and resolution of monitoring in many applications. Distributed sensing, a technology that relies on analysis of light pulses reflected down optical fibres, offers a better and more efficient way to monitor changes in temperature and pressure. By using an optical fibre as the sensor, distributed sensing makes it possible to take real-time readings of temperature and strain every metre, along the fibre which can be up to 60km long.
Distributed Temperature Sensor (DTS) – Physics of Measurement
The Sentinel DTS is able to take temperature measurements every 1-5m along a fibre optic cable with a coverage of up to 60km per unit. The Distributed Temperature Sensor illuminates the glass core of the optical fibre with a laser pulse of 10 nanosecond duration (this corresponds to a 1m pulse.) As the optical pulse propagates down the fibre, it undergoes scattering even in the absence of impurities and structural defects. Part of this scattered radiation is known as Raman scattering. Because this vibrational energy is a well-defined function of temperature, the ratio of the signals is also. It is this ratio, in conjunction with the time of flight of an optical pulse, which is used to determine the temperature of the fibre at a given point.
How does it work?
Distributed sensing takes advantage of the fact that the reflection characteristics of laser light traveling down an optical fiber vary with the temperature and strain along its length. A distributed sensing system is made up of two basic components:
- The sensor. This consists of an optical fiber – usually a standard telecoms fiber – which is normally housed inside a protective sheath to form a cable. The cable is then carefully placed to make the required measurements.
- The detector system. This includes a laser which fires light pulses down the optical fiber, and a detector which measures the reflections from each light pulse. By analysing these reflections it is possible to determine temperature and strain at all points along the fiber. With the help of more powerful lasers and more sensitive detection systems, measurements can be made using cables up to 60km long. But in a typical installation, where the fiber is looped around a building or in a process area, distances of several kilometres are more common.
The measurements themselves depend on four variables, or parameters. These include:
- Distance, or range: the distance over which the measurements will be made
- Speed: the time required for each measurement
- Temperature resolution: the size of temperature changes that will be detected
- Spatial resolution: the smallest distance over which a change in temperature can be detected.
The trade-off between these variables determines the performance of the measuring system, and the choice of parameters usually depends on the nature of the application. Although distributed sensing systems are capable of recording a measurement every second, increasing the time intervals between measurements to minutes or hours makes it possible to achieve finer resolution results. The length of the optical fiber sensor also affects the resolution. To get the best results in each particular application, it is important to take into account the type and resolution of measurements required when deciding on the detection set-up.
The system can detect temperature changes as small as 0.01°C, but readings can take minutes to hours depending on the length of the fiber. Readings are typically presented as average readings over a metre length of fiber – e.g. the Sentinel DTS-SR obtains a 0.3°C resolution at 5km in 10 seconds with a 1m spatial resolution. Coarser resolutions – say every 10m – can be made more quickly.
Distributed Temperature Sensing has numerous advantages including the following:
- Fully distributed data greatly increases information and reduces uncertainty
- Sensor is made from standard optical fibre (cost effective)
- Immune to shock/vibration and electromagnetic interference
- No electronics or moving parts in monitoring zones
- Inherently high reliability (sensor design life of >30 years)
- High temperature performance of up to 650°C
- Extremely small for access into space restricted areas (e.g. ducts, oil wells, pipes)
Technology Principle of Distributed Temperature Sensor
The key to understanding Distributed Temperature Sensors is that the optical fibre itself is the sensor and the instrumentation provides measurements at every 1m along the length of the fibre. This means that once the cable is installed the operator is able to receive temperature information at all points along the installation – leaving no areas unmonitored and no room for uncertainty.
For more information on the actual physics behind distributed temperature sensors – please visit the FAQ – What is Distributed Sensing section
Cost Advantages of Distributed Temperature Sensor
The Sensornet Distributed Temperature Sensor uses standard telecoms fibre to make the measurement. There are no special sensors (e.g. Bragg Gratings) embedded in the cable making distributed temperature sensing a very cost effective solution when a large number of measurement points are required.
In addition to the low material cost per sensor, distributed temperature sensing technology also has low system design costs (you do not have to plan the exact location of each sensor), low installation costs and also low maintenance costs (distributed sensing cable has no moving parts and a design life of 30 years). Thus the total cost of ownership of a distributed temperature sensing system is very low.
Typical Applications for Distributed Temperature Sensor
Distributed Temperature Sensing technology shows real advantages over conventional temperature sensing technology when a temperature profile of the installation is required or when a large number of sensing points is crucial. Therefore this technology lends itself to long length applications (pipelines, tunnels, power cables), applications where only small sensors can access (oil wells) and safety critical applications where it is important to have all points monitored (refineries, LNG plants, electrochemical processes). Please refer to our case studies for application examples of distributed temperature sensors.
Safety Advantages of Distributed Temperature Sensor
The Sensornet Distributed Temperature Sensor is ideal for use in hazardous zones and environments where safety is of critical importance. The sensors operate using low optical powers (1mW mean power output) and are not capable of causing ignition thus making them ideal for areas where only intrinsically safe equipment is allowed (e.g. gaseous environments). The Distributed Temperature Ssensor is suitable to monitor Zone 0 Hazardous areas according to the European Commission report no. EUR 16011 EN (1994). Additionally, because optical fibres are not susceptible to electromagnetic interference they are ideal for use in power stations and for monitoring of distribution lines.
The Sensornet Distributed Temperature Sensor has a 1M laser safety classification – meaning that an onsite laser safety supervisor is not required at site of installation.
What is the Longest Range of a Distributed Temperature Sensor?
The Sentinel range of Distributed Temperature Sensors are capable of measuring ranges of up to 60km in distance with a single DTS unit. These means that for a 10km distributed temperature sensing unit you can have up to 10,000 individual measuring points. This ensures full coverage of your assets over extremely long distances.
For more information about our range of Distributed Temperature Sensing products – please visit our product specification page.
What is the Temperature Resolution of a Distributed Temperature Sensor?
The Sentinel range of distributed temperature sensors are capable of measuring temperatures as fine as 0.01°C. For more information of the measurement times need to achieve this resolution – please see the specification sheets for our Distributed Temperature Sensors.
Sensornet Range of Distributed Temperature Sensors
Sensornet offers a range of distributed temperature sensors – ranging from 4km in range to 60km and from a sampling resolution of 50cm. For more information about our range of products – please visit the product specification page
What are the Pressure and Temperature Limitations?
Sensornet have fibres capable of long-term operation at over 650ºC – so there are no practical temperature limits in applications such as oil wells. As optical fibre is simply glass, the practical pressure limits come from the mechanical components e.g. pressure rating of wellhead pass-through, rather than the fibre itself.
What Other Applications can you use DTS for?
Distributed Temperature Sensing is very versatile and potential applications are enormous. Sensornet focuses on four main market segments – Upstream Oil & Gas, Downstream Process, Power, and Hydro. The following list gives you some ideas of other potential applications for distributed sensors:
- Structural monitoring – anything from bridges to aeroplane wings – monitor and understand the movements to achieve more cost-effective designs.
- Linear Heat Detection
- Building Temperature Control
Loop Measurements Vs. Single Ended Measurements
With OTDR technology it is possible to perform a measurement in a uni-directional mode (also known as single ended) or in a loop configuration (double ended or combined measurements). For the loop measurement the sensing fibre is measured from both ends and then the geometric mean of the two traces can be combined to create a single measurement.
Advantages of Loop Measurements & when to use
Although, the single ended measurement configuration with OTDR technology is a very robust measurement in certain scenarios temperature anomalies can occur if the sensing fibre and related accessories are not configured within specifications. This can occur if splices are out of specification, if there are damaged fibre or if connectors are used in the line (not recommended). With the loop configuration, these anomalies are corrected for.
Additionally, you have the added security with a loop configuration that if the fibre breaks at one point you still have a continuous uni-directional measurement from either end and monitoring can continue undisturbed until the fibre is repaired.
Loop measurements are particularly useful when you have situations where
- There are a lot of splices or connectors present (e.g. power cable when fibre is installed within the structure of the power cable)
- There is potential for damaged fibre (e.g. in a hot oil well where there is high levels of hydrogen)
- Where there is a need for high accuracy or temperature resolution
Cons of Double Ended Measurements
Although generally a higher quality measurement, there is 1 main drawback to double ended measurements. This drawback is that the measurement range is halved. For example if you are monitoring a 5km power cable, a system with a range of 10km measurement range will be required to provide this measurement. The range of DTS systems is generally quoted for uni-directional (single ended measurements).
What Is The Difference Between Multimode And Singlemode Optical Fibers?
There are two main different types of optical fiber used – singlemode and multimode fiber (and there are many variants of these).
The light travelling along a fiber can be considered as rays of light that bounce off the interface between the core and the cladding of the fiber. Multimode fiber has a large diameter core which allows the rays of light to travel along several different pathways through the fiber, bouncing at different angles between the core and cladding.
These different pathways are referred to as spatial modes, and hence the description as a multimode fiber. Singlemode fiber optic cable has a small core which only allows one path for the rays of light to travel through the fiber, hence the description as a singlemode fiber. Multimode fiber typically comes with two different core sizes: 50 micron or 62.5 micron. Singlemode cable features a 9-micron glass core.
Why is multimode fiber used for Temperature sensing?
Since multimode fibre has a larger core (and a larger numerical aperture) you are able to launch more power into the fiber than for single mode fiber. The larger core size also results in a higher threshold for nonlinearities, which can cause errors in the measurement of temperature of the fiber. Distributed temperature sensing measures the backscattered light along the fiber. However, the amount of backscattered light is a very tiny proportion of the initial laser pulse and so in order to get enough backscattered signal to make a temperature measurement it is very important to have as much power as possible in the initial laser pulse. Given its larger core size and higher threshold for nonlinearities, multimode fibre provides a large improvement in backscatter signal power over singlemode fiber.
What about for longer sensing distances?
Sensornet has evaluated the option of using singlemode fiber for the longer distances (as some other DTS systems do), but we have found that multimode fiber provides far better temperature resolution since more power can be launched into the multimode fiber than the singlemode fiber.
The optical attenuation incurred in the multimode fibers are negligibly lower at longer wavelengths, however due to the fact that more optical power is being received this easily outweighs any reduction in fiber losses.
Why is singlemode fiber used in telecoms over longer distances?
Singlemode fiber is used extensively in telecoms because of the data rates that are involved. Transmission of data using multimode fiber results in something called modal dispersion, arising from the multiple modes. This weakens the transmitted data, limiting the distances over which it can be transmitted. This effect gets worse as you use faster data rates. The DTS only uses a very broad pulse in comparison with telecoms systems, so modal dispersion has a negligible effect on the DTS performance. Singlemode has to be used for telecoms systems where data can be transmitted at multiple Gigabit/s rates over distances of 100’s of kilometres. Multimode fiber optic cable can be used for more general fiber applications where the distances are much shorter, such as LAN’s.
Why is singlemode fiber used for distributed strain sensing?
Singlemode fiber is used for distributed strain sensing due to the nature of the optical components required. Almost all of the components are standard telecommunications components, designed to operate around a wavelength of 1550nm using singlemode fiber. It is very difficult to use multimode fiber as the sensing fibre of the instrument is based on singlemode components, since a lot of light is lost trying to squeeze the light from the large multimode core into the much smaller singlemode core. The backscatter signal used in strain sensing is more powerful than that used for the temperature sensing, hence the relative lower power of using singlemode fiber can be tolerated.
In both the sensing and telecommunications industries two types of technologies are used for distributed sensing and for checking the quality/health of the fibre optic. These two technologies are known as OTDR (Optical Time Domain Reflectometry) and OFDR (Optical Frequency Domain Reflectometry).
Within the telecommunications industry OTDR is the main technology utilised. The same trend is following within the distributed sensing industry and the majority of DTS (Distributed Temperature Sensors) including Sensornet are OTDR based.
The reason OTDR has come to prominence both for telecoms equipment and within distributed temperature sensing is that OTDR offers a number of technical benefits.
Robustness of Measurement
For distributed temperature measurements the OTDR is a robust measurement and is less affected by potential anomalies within the fibre and accessories (e.g. bends, connectors, reflections).
The Sensornet Distributed Temperature Sensing technology based on OTDR currently measures along sensing cables of distances greater than 45km.
OTDR based Distributed Temperature Sensors are able to resolve temperature changes down to 0.01°C.
OTDR is a more versatile technology for distributed sensing and it is possible to measure both distributed temperature and distributed strain.
The majority of the OTDR distributed temperature sensing systems are classified as 1M units. (The laser safety classification of all DTS units must be clearly marked on the outside). The implication is that this is safe under all reasonable working conditions and the majority of OTDR DTS systems are safe for use in hazardous zone ratings and comply with EU regulations EUR16011EN (1994).
OTDR Principle of Measurement
OTDR was developed more than 20 years ago and has become the industry standard for telecom loss measurements which detects the – compared to Raman signal very dominant – Rayleigh backscattering signals. The principle for OTDR is quite simple and is very similar to the time of flight measurement used for radar. Essentially a narrow laser pulse generated either by semiconductor or solid state lasers is sent into the fibre and the backscattered light is analysed. From the time it takes the backscattered light to return to the detection unit it is possible to locate the location of the temperature event.
Sensornet therefore highly recommends OTDR technology due its all-round technical superiority.
Monitor With Integrity
Customer safety is of foremost importance to Sensornet. This is why we only sell DTS systems with a Class 1M rating*. This means that the DTS laser is safe under all reasonable circumstances, provided that you do not view the end of the fiber with optical aids. The 1M laser safety rating means that an onsite laser safety supervisor is not required at site of installation.
Lasers vary greatly in output power and other parameters. They are categorised by the relevant safety standard (i.e. BS EN-60825, IEC 60825-2) into four classes of increasing risk level:
- Class 1 (e.g. CD writer, laser printer) – safe under all foreseeable conditions
- Class 1M (e.g. Sensornet DTS) – do not use optical instruments
- Class 2 (e.g. Laser pointers)
- Class 2M
- Class 3R (e.g. CD laser diode) – damaging to the eye
- Class 3B – up to 500mW
- Class 4 (e.g. laser welding machines) – damaging to eye and skin
Sensornet DTS Safety Features
The DTS has numerous laser safety features to make the instrument as user friendly and safe as can be. Sensornet carries out training courses to ensure that your DTS operators are completely familiar with safety precautions. We incorporate several main safety features into our DTS units:
- Laser key switch
- User remote interlock feature
- Shuttered fiber connector
Laser Safety Precautions
Please remember that the light emitted from the DTS laser is INVISIBLE. Sensornet highlights the following main laser safety precautions to be put into practice whenever using any DTS instrument:
- Never look directly into the laser emission aperture
- Never look directly into the end of a fiber cable
- Always ensure laser is OFF before disconnecting the optical path
- Keep fiber and cable ends clean
- Never attempt to open the cover of a DTS unit
General Fiber Optic Safety
Sensornet recommends the following safety practises to be taken into account at all times:
- Optical fibers are very sharp and can get embedded in the skin
- Dispose of all fragments properly – use sharps box
- Pick up with tape or tweezers – not fingers
- Wear smooth clothing
- Sharps containers – dispose of properly, and do not overfill
- Beware where others are working and consider safety of others
- Keep food & drink away
*The Sentinel DTS has been independently classified to EN 60825-1 (2001-03) as a Class 1M laser product.
Examples of typical laser safety warning labels:
The Sensornet team offers a full solution and will design the entire engineering solution for you. This includes the fiber optic cable required and its installation, the sensing equipment and the monitoring and reporting software. Sensornet has the personnel and equipment to perform fiber optic installations across a number of industries and in the past has carried out installations ranging from inside oil-wells to along the length of pipelines to inside dams.
Each specific installation requires specialised knowledge and equipment as well as dedicated fiber optic cable designs. Sensornet has built this knowledge over a number of years and together with installation partners is able to tackle the most challenging of installations.
The Sensornet team will manage your entire project right through to its handover. We operate to the highest standards of quality; our solution is, after all, about increased safety and security. We are ISO 9001 accredited and meet all Health & Safety Executive requirements.
Types of cables that we offer
In the Sensornet distributed detection system, the fiber is the sensor. Sensornet has developed rigorous quality assurance systems to verify the effectiveness of the fibers that we package into our sensing cables. Depending on the environment of the specific installation the cable will require different levels of protection – varying from the bare fibers which are not much thicker than a hair to very rugged cables where the fiber is encased in a combination of steel tubes and polymers, with design lives of up to 30 years.
Care to be taken when installing fiber optics
Because they are used in a variety of harsh environments, including oil rigs, dams, power stations, refineries and other industrial settings, fiber optic sensors need protection from both mechanical and chemical damage. Mechanical hazards can range from dropped tools to damage from traffic.
Fiber optic sensors offer many advantages over conventional sensors. They are very small and flexible, intrinsically safe, and not susceptible to electromagnetic interference. They are also very strong – an optical fiber can support a weight of 5kg without breaking. But in spite of their strength, the fibers are susceptible to damage and scratches, and micro-cracks can weaken the fiber and lead to errors in measurement data. Although these errors can generally be compensated for, better results will be achieved if the fiber is correctly installed and adequately protected from its environment.
Chemical damage can result from exposure to process chemicals or from exposure to hydrogen, which can diffuse into the fiber and cause darkening. Hydrogen darkening occurs even more rapidly in environments, such as steam injection wells, where the fiber is also exposed to high temperatures and pressures. The darkening reduces the amount of light that can pass through the fiber. This, in turn, reduces the amount of backscattering signal. This not only affects the resolution of temperature measurements but, because the Stokes and anti-Stokes reflections each respond differently, it may also reduce their accuracy, if inappropriate calibration techniques are used.
To avoid these pitfalls, the fibers need to be protected. The fibers first line of protection is a primary coating. This can vary from 10 to 400 microns thick and is usually applied to the surface of the fiber during manufacture. The coating prevents the development of microcracks on the fiber surface resulting from mechanical damage or exposure to the atmosphere. A variety of different materials can be used as coatings. Although the fiber itself can withstand temperatures up to 1200°C, the coating materials generally cannot, and once the coating breaks down it no longer offers any protection to the fiber surface. Therefore the choice of coating material depends on the operating temperatures the fiber will be subjected to.
Although the primary coating provides a good initial layer of protection, in order to survive in harsher environments the fiber optic is generally inserted into a cable, which is specially designed to withstand the hazards of the environment in which the fiber optic will be used. Cable designs can range from thin cables just 1mm thick, to thicker cables rugged enough to ensure a fiber is not damaged if the cable is run over by a tank.
All splices also need to be housed in appropriate splice boxes – these protective environments ensure that the splice is correctly shielded against damage.
Where the fiber is exposed to hydrogen, it is necessary to provide a hermetic layer to prevent hydrogen from entering the core of the fiber. One way to do this is to deposit a carbon or metal coating onto the surface of the fiber. Another is to manufacture a tube around the fiber, and then fill the tube with a hydrogen scavenging gel. These methods normally provide adequate protection against hydrogen darkening, even at elevated temperatures.
However, these precautions are in vain if the cable is damaged during installation. A common installation method involves inserting a conduit into the installation, and then pumping the primary coated fiber into the conduit, a practice which often damages the fiber and reduces the performance of the sensor. To avoid this, Sensornet uses a technique where a well-protected, cabled fiber is injected, rather than pumped, along the conduit.
Want to find out more?
Sensornet Ltd is the technology leader in distributed sensing. To find out more about what our technologies can do for please contact one of our sales representatives.
Uncertain Times Call For Absolute Certainty.
Tougher targets, stricter safety and environmental regulations and increasingly complex environments mean it’s never been more important to know precisely what’s happening to your critical infrastructure. But often there’s a large space between what you think you know about your assets – and what you ought to know.
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