Distributed temperature sensing
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Distributed temperature sensing systems (DTS) 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.
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[edit] 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 (SiO2) 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 (photons) 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 Rayleigh scattering with the wavelength of the laser source used,
- the Stokes line components with the higher wavelength in which photons are generated, and
- the anti-Stokes line components with a lower wavelength than the Rayleigh scattering, in which photons are destroyed.
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.
[edit] Measuring principle — OTDR vs OFDR technology
There are 2 principles of measurement for distributed sensing technology. OTDR (Optical Time Domain Reflectometry) & OFDR (Optical Frequency Domain Reflectometry).
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 commonly by a solid state laser 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 Reflection (OFDR). 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 benefits of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal, whereby a significantly higher signal to noise ratio is achieved than with pulse technology (OTDR). This technical benefit allows the use of affordable semiconductor laser diodes and electronic assemblies for signal averaging. This is offset by the technically difficult measurement of the Raman scatter light and rather complex signal processing, due to the FFT calculation with higher linearity requirements for the electronic components. The optical frequency domain reflectometry has been initially developed as a high-resolution measurement process for the characterisation of optical wave guides with length dimensions of just a few millimetres.
Using the DTS technique 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.
[edit] Construction of Sensing Cable & System Integration
The temperature measuring system consists of a controller ( laser source, optical module, HF mixer, receiver and micro-processor unit) and a quartz glass fibre as line-shaped temperature sensor. The fibre optic cable (can be 30km+ in length) 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 (the laser power falls below the levels that can cause ignition), 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 (standard cables operate to 85°C but it is possible to measure upto 700°C with the correct design), gaseous environment (hydrogen can cause deterioration of the measurement) 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
[edit] Laser Safety and Operation of System
When operating a system based on optical measurements such as the DTS technology, it is important to make sure that adequate precautions are taken with regards to laser safety. It is important to ensure that for systems that are used for permanent installations that a laser safety class 1M product is achieved. Certain systems are based on higher power lasers of a 3B rating, which although safe for use by approved laser safety officers, are not suitable for permanent installations.
[edit] Applications
Distributed Temperature Sensing can be deployed successfully in multiple industrial segments:
- Fire Detection in Tunnels and Special Hazard Buildings
- Power Cable and Transmission Line Monitoring (Ampacity Optimisation)
- Oil & Gas Exploration – permanent Downhole Monitoring
- Industrial Induction Furnaces Surveillance
- Integrity of LNG Carriers and Terminals
- Leakage Detection at Dikes and Dams
- Temperature Monitoring in Plant & Process Engineering
[edit] See also
[edit] External links
[edit] DTS in fire detection
- Paper published at International Conference on Automatic Fire Detection (AUBE’04); University of Duisburg;
- European Initiative SOLIT - Safety Of Life In Tunnels
