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Distributed Temperature Sensing

What is Distributed Sensing?

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 fibre 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 fibre – usually a standard telecoms fibre – 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 fibre, 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 fibre. 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 fibre is looped around a building or in a process area, distances of several kilometres are more common.

Measurement Variables

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 fibre 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 fibre. Readings are typically presented as average readings over a metre length of fibre – 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. For more information please contact a BMP Representative.