If the temperature profiles of your processes and assets are key indicators, then you’ve most likely been using a variety of measurement methods in the past, such as spatially distributed sensors or thermal cameras.

Yokogawa’s optical fibre temperature sensing systems – in which the data acquisition station (DTSX series) is the central component – are an attractive alternative. They allow you to capture the temperature profiles of your processes and assets directly and continuously, and integrate the measurement technology into your existing automation environment with just one Ethernet interface. This opens up new opportunities and hence new applications like liquid level measurements via vessel surfaces, temperature monitoring in reactors or monitoring for mechanical component failure.

What Is Distributed Temperature Sensing?

Distributed temperature sensing (DTS) measures temperature distribution over the length of an optical fibre cable using the fibre itself as the sensing element. Unlike traditional electrical temperature measurement (thermocouples & RTD), the length of the fibre optic cable is the temperature sensor. Distributed temperature sensing can provide thousands of accurate and precise temperature measurements over a long distance. Compared to traditional electrical temperature measurements, distributed temperature sensing represent a cost effective method for obtaining accurate and high resolution temperature measurement.

How Does It Work?

Yokogawa DTSX3000 measures temperature and distance over the length of an optical fibre using the Raman scatter principle. A pulse of light (laser pulse) launched into an optical fibre is scattered by fibre glass molecules as it propagates down the fibre and exchanges energy with lattice vibrations. As the light pulse scatters down the fibre optic cable, it produces stokes signal (longer wavelength) and anti-stokes (shorter wavelength) signal, of which both signals shifted from the launch of the light source. The intensity ratio of the two signals components depends on the temperature at the position where the Raman scatter is produced. This temperature can thus be determined by measuring the respective intensities of the stokes and anti-stokes signals. Furthermore, part of the scattered light, known as the backscatter, is guided back towards the light source. The position of the temperature reading can thus be determined by measuring the time taken for the backscatter to return to the source.

What Is Raman Scatter Principle?

All light interacts with matter! For example, imagine standing in a pitch black garage with no external light source. Inside this garage is a bright red sports car. Needless to say, you cannot see the sports car or the colour of the sports car itself. However, when you turn on the lights to the garage, you can immediately see the light source reflecting the bright red colour off the car. The light that is bouncing off the red sports car is only bouncing off the “red” spectrum, therefore, your eyes see the sports car as, well, red.

This phenomena is also true when you shoot a pulse of light (laser pulse) off of a molecule, in this case, the fibre glass molecule in the optical fibre cable. When the light source enters the optical fibre cable, most of the light bounces (backscatter) back unchanged (no change in wavelength). However, a small amount of that light shifted/changed. That shift/change from the light source is called Raman Scatter. Since Raman Scatter is thermally influenced by temperature, the intensity depends on temperature. Distributed temperature sensing is capturing the shift/change from the propagating light pulse and measuring the intensities between the two signal components (stokes and anti-stokes).

What Are The Advantages of Using DTS?

  • Cost! When an application requires hundreds or thousands of sensors to be measured, it becomes very expensive to wire each individual sensor back to a data acquisition station. It is much more cost effective and beneficial to acquire accurate and high resolution temperature measurement using fiber optic cable.
  • Long distance! It is difficult to measure temperature over a long distance using traditional electrical measurement sensors. Not only can DTS fiber optic cable be deployed over a long distance but it also provides a high resolution profile of the area as well as accurate and precise temperature measurement over that distance.
  • High electromagnetic noise environment! DTS is isolated from electromagnetic noise because of its optical characteristics. Unlike traditional electrical measurement sensors (thermocouple & RTD) there is no electrical component within the optical fibre, therefore, it is immune to electromagnetic noise.
  • No knowledge of sensor placement! It is not always possible to identify the correct location to deploy temperature sensors ahead of time. Because of the high spatial resolution along with long distance capability of DTS, engineers can deploy multiple optical fibre along the same area to ensure precise and accurate temperature.
  • Industry-leading measurement distance
    The DTSX3000 can measure the temperature distribution along a fibre-optic cable that is up to 50 km in length, more than eight times the distance possible with conventional product (6 km). The DTSX3000 is ideal for measuring the temperature distribution in power lines, high- or low-temperature liquid and gas pipelines and tanks, and other large facilities.
  • Top-level temperature resolution
    The development of shale gas fields requires sensors that are capable of detecting even minute changes in the temperature distribution in a bedrock formation during the hydraulic fracturing process. In just 10 minutes, the DTSX3000 is capable of measuring the distributed temperature along a 6-km fibre-optic cable, and it does this with a top-level temperature resolution of 0.03ºC, 20 times the precision possible with conventional product. Thus, the DTSX3000 can be used in a wide range of applications, from the hydraulic fracturing of bedrock formations to the monitoring of temperature levels inside wells during the gas recovery process.

Major Target Markets

Oil and natural gas, pulp and paper, iron and steel, electric power, non-ferrous metal, and chemical industries.

Applications

  • Monitoring of underground temperatures for unconventional oil and natural gas exploration and production
  • Monitoring of the temperature distribution in multiple wells
  • Fire detection in conveyors
  • Monitoring of the temperature distribution in long-distance power lines
  • Detection of liquid and gas leaks in pipelines and tanks
  • Monitoring of the temperature of the outer walls of high-temperature furnaces used in the iron and steel, chemical, and other industries.

Contact MicroWatt Sales for more information.