DISTRIBUTED TEMPERATURE SENSING
In a Distributed Temperature Sensing (DTS) system a fibre optic cable measures temperature at thousands of points over long distances or across large surfaces. These systems are fibre optic based optoelectronic instruments which measure temperature using optical fibres functioning as linear sensors. Instead of installing countless numbers of conventional sensors, DTS systems use a single strand of optical fibre as a sensor. The unique feature of a distributed temperature sensing system is that it provides a continuous (or distributed) temperature profile along the length of the sensing cable and not at discrete sensing points which must be pre-determined. High accuracy of temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1°C at a resolution of 0.01°C. Measurement distances of greater than 30 km can be monitored.
The DTS Technology is also known as Raman OTDR or Raman OFDR (Optical Frequency Domain Reflectometry). The Raman effect is named after the Indian physicist Sir Chandrasekhara Venkata Raman (1888-1970), who discovered that when light traverses a transparent material, some of the deflected light changes in wavelength. This ground-breaking work in the field of light scattering earned him the Nobel Prize for Physics.
Some other DTS technologies also use the Brillouin backscatter (B-OTDR or B-OTDA), which carries strain and temperature information. Such systems are also called DTSS (Distributed Temperature and Strain Sensing). The challenge with these systems is to isolate the fibre from strain to get accurate temperature information.
HOW DOES DTS WORK?
Distributed Temperature Sensing utilizes the Raman effect to measure temperature. An optical laser pulse is sent through the fibre and interacts with glass molecules at every location. The thermal vibrations of the glass atoms change the direction and energy of a few photons. This causes backward scattering towards the transmitting end, where the information is analyzed.
Nearly all the scattered light has a wavelength identical to the incident light, called Rayleigh scattering. However, a small amount of the scattered light has a different wavelength – this is called the Raman effect. The Raman effect is influenced by the temperature, when the temperature changes somewhere along with the optical fibre, the Raman scatter changes.
The intensity of the Raman scattering is a measure of the temperature along with the fibre. Changes in amplitude (Raman) or frequency (Brillouin) at the anti-Stokes frequencies are temperature dependent, whereas those at the Stokes frequencies are practically independent of temperature. The temperature is determined by measuring the difference between the Stokes and anti-Stokes intensity. This is done by a directional coupler in the laser source. The location of the temperature change is determined by measuring the time it takes for the scatter to return to the source enabling you to pinpoint a very exact position of the change. Therefore, the optical fibre cable operates like a sensor.
The Raman signal is less than 1 nanowatt and becomes weaker with the distance, for the desired temperature accuracy and measurement speed smart trick like coding the light pulses are applied. To resolve every meter of the fibre the pulse is very short and the signal is sampled each time the light travels 1 meter at 100 million times per second. This principle is integrated into the device that reliably protects different infrastructures 24×7.
This all sounds complicated – and it is. However, rather than the single temperature snap-shot of a conventional temperature sensor, the DTS system provides a very precise, full temperature profile at the speed of light…
Measuring principle—OTDR and OFDR technology
There are two basic principles of measurement for distributed sensing technology, OTDR (Optical Time Domain Reflectometry) and OFDR (Optical Frequency Domain Reflectometry). For Distributed Temperature Sensing often a Code Correlation technology is employed which carries elements from both principles.
OTDR is the most straightforward localization technique in which a short light pulse is sent into the fibre, the backscattered signals over time are recorded and then the time is converted to the distance.