Distributed temperature sensing
Encyclopedia
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.
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 (photon
s) 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.
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 (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 principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal. 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.
Using the Raman OTDR/OFDR 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.
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 up to 700°C with the correct design), gaseous environment (hydrogen
can cause deterioration of the measurement though "hydrogen darkening
" - aka attenuation - of the silica glass compounds) 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 (WITSML) has been developed for transfer of data from DTS instruments. The standard is maintained by Energistics.
. 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.
More recently, DTS has been applied for ecological monitoring as well:
Measuring principle — Raman effect
Physical measurement dimensions, such as temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
or pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and tensile forces, can affect glass fibres and locally change the characteristics of light transmission in the fibre. As a result of the damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
of the light in the quartz glass fibres through scattering
Scattering
Scattering is a general physical process where some forms of radiation, such as light, sound, or moving particles, are forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which they pass. In conventional use, this also includes deviation of...
, 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
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
(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 (photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s) and the electrons of the molecule. Light scattering, also known as Raman scattering
Raman scattering
Raman scattering or the Raman effect is the inelastic scattering of a photon. It was discovered by Sir Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids, and by Grigory Landsberg and Leonid Mandelstam in crystals....
, 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 scatteringRayleigh scatteringRayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
with the wavelength of the laser source used, - the Stokes lineStokes lineIn complex analysis a Stokes line, named after Sir George Gabriel Stokes, is a line in the complex plane which 'turns on' different kinds of behaviour when one 'passes over' this line – although, somewhat confusingly, this definition is sometimes used for anti-Stokes lines...
components from photons shifted to longer wavelength (lower frequency), and - the anti-Stokes line components with photons shifted to shorter wavelength (higher frequency) than the Rayleigh scattering.
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 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
Rayleigh scattering
Rayleigh scattering, named after the British physicist Lord Rayleigh, is the elastic scattering of light or other electromagnetic radiation by particles much smaller than the wavelength of the light. The particles may be individual atoms or molecules. It can occur when light travels through...
backscattering signals. The principle for OTDR is quite simple and is very similar to the time of flight measurement used for radar
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
. 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 (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 principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical back scatter signal. 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.
Using the Raman OTDR/OFDR 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.
Construction of Sensing Cable & System Integration
The temperature measuring system consists of a controller ( laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
source, optical module, HF
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
mixer, receiver and micro-processor unit) and a quartz glass fibre as line-shaped temperature sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
.
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
Electromagnetic interference
Electromagnetic interference is disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade or limit the effective performance of the circuit...
, vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
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
Operating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
(standard cables operate to 85°C but it is possible to measure up to 700°C with the correct design), gaseous environment (hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
can cause deterioration of the measurement though "hydrogen darkening
Hydrogen darkening
Hydrogen darkening is a physical degradation of the optical properties of glass. Free hydrogen atoms are able to bind to the SiO2 silica glass compound forming hydroxyl - a chemical compound that interferes with the passage of light through the glass....
" - aka attenuation - of the silica glass compounds) 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
SCADA
SCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...
. In the Oil and Gas industry an XML based file standard (WITSML) 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 the DTS technology, it is important to make sure that adequate precautions are taken with regards to laser safetyLaser safety
Laser safety is safe design, use and implementation of lasers to minimize the risk of laser accidents, especially those involving eye injuries. Since even relatively small amounts of laser light can lead to permanent eye injuries, the sale and usage of lasers is typically subject to government...
. 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.
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 locationsApplications
Distributed Temperature Sensing can be deployed successfully in multiple industrial segments:- Oil & Gas Exploration – Permanent Downhole Monitoring, Coil Tubing Optical enabled deployed intervention systems, Slickline Optical Cable Deployed intervention systems.
- Power Cable and Transmission Line Monitoring (AmpacityAmpacityAmpacity is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration.Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device can continuously carry while remaining...
Optimisation) - Fire Detection in Tunnels and Special Hazard Buildings
- Industrial Induction Furnaces Surveillance
- Integrity of LNG Carriers and Terminals
- Leakage Detection at Dikes and Dams
- Temperature Monitoring in Plant & Process Engineering, including Transmission Pipelines
- Storage Tanks and Vessels
More recently, DTS has been applied for ecological monitoring as well:
- Stream temperature
- Groundwater source detection
- Temperature profiles in a mine shaft and over lakes and glaciers
- Deep Rainforest ambient temperature at various foliage densities
- Temperature profiles in an underground mine, Australia.
External links
- The Fiber Optic Association
- Technical Talks on Principal of Technology
- Fiber-Optic Distributed Temperature Sensing Technology Demonstration and Evaluation Project
- SEAFOM, a nonprofit organization promoting the standards and growth in use of fiber optic-based sensing in subsea applications
- DTS applications in environmental monitoring
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
DTS in power cable monitoring
- Case study: Permanent Temperature Monitoring of a 220kV XLPE cable at the Olympic City 2008, Beijing
DTS in ecological monitoring
- Tyler, S.W., J.S. Selker, M.B. Hausner, C.E. Hatch, T. Torgersen and S. Schladow. 2009. Environmental temperature sensing using Raman spectra DTS fiber optic methods. Water Resources Res doi: 10.1029/2008WR007052.
- Selker, J.S., N. van de Giesen, M. Westhoff, W. Luxemburg, and M. Parlange. Fiber Optics Opens Window on Stream Dynamics. Geophysical Research Letters, DOI:10.1029/2006GLO27979, 2006
- Selker, J.S., L. Thévenaz, H. Huwald, A. Mallet, W. Luxemburg, N. van de Giesen, M. Stejskal, J. Zeman, and M. Westhoff, and M.B. Parlange.Distributed Fiber Optic Temperature Sensing for Hydrologic Systems. Water Resources Research, 42, W12202, doi:10.1029/2006WR005326, 2006
- Nick Tufillaro, John Dorighi, Mike Collier and John Selker, Measuring stream dyamics with fiber optics, Agilent Measurement Journal, 3, 2007
- Tyler, S.W., S. Burak, J. McNamara, A. Lamontagne, J. Selker and J. Dozier. 2008. Spatially distributed temperatures at the base of two mountain snowpacks measured with fiber-optic sensors. Journal of Glaciology. 54(187):673-679
DTS in pipeline leak detection
- Gas Pipeline Leak Detection Test Case Study (pdf)
- Gas Pipeline Leak and Ground Movement Detection Case Study