Distributed temperature sensing


Distributed temperature sensing systems are optoelectronic devices which measure temperatures by means of optical fibres functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A 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 and some specialised systems can provide even tighter spatial resolutions.

Measuring principle—Raman effect

Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the damping of the light in the quartz glass fibres through scattering, the location of an external physical effect can be determined so that the optical fibre can be employed as a linear sensor.
Optical fibres are made from doped quartz glass. Quartz glass is a form of silicon dioxide with amorphous solid structure. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles and the electrons of the molecule. Light scattering, also known as Raman scattering, occurs in the optical fibre. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation.
The light scattered back from the fibre optic therefore contains three different spectral shares:
The intensity of the so-called anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fibre is derived from the ratio of the anti-Stokes and Stokes light intensities.

Measuring principle—OTDR and OFDR technology

There are two basic principles of measurement for distributed sensing technology, OTDR and OFDR. For Distributed Temperature Sensing often a Code Correlation technology
is employed which carries elements from both principles.
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.
Alternative DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry. The OFDR system provides information on the local characteristic only when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to Fourier transformation. The essential principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical backscatter signal. This is offset by the technically difficult measurement of the Raman scattered light and rather complex signal processing, due to the FFT calculation with higher linearity requirements for the electronic components.
Code Correlation DTS sends on/off sequences of limited length into the fiber. The codes are chosen to have suitable properties, e.g. Binary Golay code. In contrast to OTDR technology, the optical energy is spread over a code rather than packed into a single pulse. Thus a light source with lower peak power compared to OTDR technology can be used, e.g. long life compact semiconductor lasers. The detected backscatter needs to be transformed—similar to OFDR technology—back into a spatial profile, e.g. by cross-correlation. In contrast to OFDR technology, the emission is finite which avoids that weak scattered signals from far are superposed by strong scattered signals from short distance, improving the Shot noise and the signal-to-noise ratio.
Using these techniques it is possible to analyse distances of greater than 30 km from one system and to measure temperature resolutions of less than 0.01°C.

Construction of Sensing Cable & System Integration

The temperature measuring system consists of a controller and a quartz glass fibre as line-shaped temperature sensor.
The fibre optic cable is passive in nature and has no individual sensing points and therefore can be manufactured based on standard telecoms fibres. This offers excellent economies of scale. Because the system designer/integrator does not have to worry about the precise location of each sensing point the cost for designing and installing a sensing system based on distributed fibre optic sensors is greatly reduced from that of traditional sensors. Additionally, because the sensing cable has no moving parts and design lives of 30 years +, the maintenance and operation costs are also considerably less than for conventional sensors. Additional benefits of fibre optic sensing technology are that it is immune to electromagnetic interference, vibration and is safe for use in hazardous zones, thus making these sensors ideal for use in industrial sensing applications.
With regards to the construction of the sensing cable, although it is based on standard fibre optics, care must be taken in the design of the individual sensing cable to ensure that adequate protection is provided for the fibre. This must take into account operating temperature, gaseous environment and mechanical protection.
Most of the available DTS systems have flexible system architectures and are relatively simple to integrate into industrial control systems such as SCADA. In the oil and gas industry an XML based file standard has been developed for transfer of data from DTS instruments. The standard is maintained by Energistics.

Laser Safety and Operation of System

When operating a system based on optical measurements such as optical DTS, laser safety requirements need to be considered for permanent installations. Many systems use low power laser design, e.g. with classification as laser safety class 1M, which can be applied by anyone. Some systems are based on higher power lasers of a 3B rating, which although safe for use by approved laser safety officers, may not be suitable for permanent installations.
The advantage of purely passive optical sensor technology is the lack of electric or electromagnetic interaction. Some DTS systems on the market use a special low power design and are inherently safe in explosive environments, e.g. certified to ATEX directive Zone 0.
For use in fire detection application, regulations usually require certified systems according to relevant standards, such as EN 54-5 or EN 54-22, UL521 or FM, cUL521 and/or other national or local standards.

Temperature estimations by using DTS

Temperature distributions can be used to develop models based on the Proper Orthogonal Decomposition Method or principal component analysis. This allows to reconstruct the temperature distribution by measuring only in a few spatial locations

Applications

Distributed temperature sensing can be deployed successfully in multiple industrial segments:
More recently, DTS has been applied for ecological monitoring as well: