Distributed sensor technologyOptical fibre in a new light?

Question 1

• Which of these can be used to measure temperature to 0.007°c?

Digital thermometer Mercury in glassthermometer

50/125 Fibre

A B C

Question 2

• Which of these can be used to record sound?

Microphone 50/125 Fibre

A B C

Loudspeaker

Question 3

• Which of these can be used to measure strain?

Piezo film sensor Pressure gauge 50/125 Fibre

A B C

Agenda

• How it works ­ the physics

• Case Study : Strain sensing

• Case Study : Audio sensing

• Case Study : Temperature sensing

• Questions & Answers

Distributed Sensor Technology

The fibre is the sensor

• Optical losses determine the maximum distance we can transmit data over a standardised fibre link

• Material attenuationAbsorption by atoms, at specific wavelengths

• Waveguide attenuation Bends in the fibre ­ macrobends, microbends

• Fibre coupling lossesSplices, connectors etc…

• Scattering processes : Collectively called ‘noise’ in a comms link. Rayleigh, Brillouin, Raman

LordRayleigh1842-1919

SirRaman1888-1970

MrBrillouin1889-1969

Rayleigh Scattering

• Technique used by traditional OTDR’s

• The elastic scattering of light by particles much smaller than the wavelength of the light. The wavelength of the backscattered photon is unchanged.

• Rayleigh scattering of sunlight in the atmosphere is the reason for the blue colour of the sky and the yellow tone of the sun itself.

• The sun appears redder near the horizon as the light passes through more air which scatters almost all of the blue light.

• Blue light is scattered more than red.

• Rayleigh scattering is an important limitation in a fibre optic comms links.

�Brillouin

RamanRayleigh

Stokes Anti-stokes

Brillouin Scattering

• Occurs when light interacts with optical density

variations and changes its energy (frequency) and

path

• Any compression of the glass will change the index

of refraction

• Even tiny vibration transmitted to the fibre affects

the density and can be detected

• Used for strain and audio measuring

�Brillouin

Raman

Rayleigh

Stokes Anti-stokes

Raman Scattering

• A small fraction of scattered light (approximately 1

in 10 million photons) is scattered by an excitation.

The backscattered photons have a different

frequency from the source photon.

• There are two Raman signal elements the stokesand anti-stokes reflections.

• The stokes reflection is weakly temperature

sensitive and is not very useful for temperature

sensing.

• The intensity of backscattered light at anti­stokes

wavelengths increases with temperature.

• Anti­stokes Raman Scattering is therefore used for

temperature sensing.

�Brillouin

Raman

Rayleigh

Stokes Anti-stokes

Measuring distance

1/100,000 seconds = 1m2/100,000 seconds = 2m3/100,000 seconds = 3m

1m 2m 3m

Simple principle – How OTDR’s work today

Raman Scattering – Anti­Stokes for temperature

• Raman scattering is the interaction of a photon with molecular vibrations(Only 1 in 10,000,000 photons)

• Stokes ­ Red Shift• Molecule absorbs some of the

photons energy

• Reflected photon has lower energy (red shift)

• Anti­Stokes ­ Blue Shift• Molecule loses energy

• Reflected photon has higher energy (blue shift)

• Temperature is determined from the backscattered Stokes / Anti­Stokes power ratio

Stokes ­ molecule absorbs energy

Anti­stokes ­ molecule loses energy

• Measures using standard 50/125 zip cord

• 4 x sensing fibres each up to

4 km in length (MM 50/125 fibre)

• 2.0m length resolution

• 8000 points per unit

• 0.1°c to 0.01°c accuracy0.1°c = 15 Seconds

0.01°c = 3000 Seconds

Temperature accuracy & resolution

Length

Tem

per

atu

re

Advantages of fibre as a temperature sensor

• Simultaneously measures temperature and position over

long distances

• Low cost – the sensor is made from standard 50/125

optical fibre zip cord ­ very cost effective

• Immune to shock/vibration and electromagnetic

interference

• No electronics, wireless, batteries or moving parts in

monitoring zone. Totally passive, minimal maintenance.

• Inherent high reliability (fibre has a design life of 30+ years)

• High temperature range ­200°c to +500°c

• Extremely small for access in legacy areas with restricted

space

• Easily installed in without any downtime or interruption of

service

Case Study

Acoustic Measurement

Acoustic Measurement Applications

• Any acoustic event along the entire fibre

length creates sound waves in the ground

which cause micro­changes in the shape of

the fibre which disturbs the backscatter. An

acoustic signal is then rebuilt from the

change in the backscatter data.

• A powerful processing architecture for

analysing acoustic activity has been built,

drawing upon decades of military sonar

research.

• Protecting borders, pipelines, railways from

unauthorised activity.

• Provides a dependable means of monitoring

traffic flow ­ hard shoulder shared usage

Case Study

Temperature Sensing

Wind Farm Power Cables

Wind Farm Power Line Monitoring

• In UK power distribution must be owned by a different

company to power generation.

• Multimode comms fibre in power cable is used to

measure the temperature of the cable core.

• Cables run 60km from offshore substation to land.

• Using Dynamic Cable Rating software the cable

utilisation can be doubled.

• Occasional peak loads for short periods can be

accommodated based on thermal performance of sub­

sea cable.

• Turbines no longer need to be turned off when

generated capacity exceeds cable capacity.

Case Study

Temperature Sensing

Large Data Centre

Data Centres ­ Energy Consumption is Critical

• Carbon Reduction Commitment

Mandatory legislation

Voluntary schemes

• BS EN 16001

• The Green Grid PUE

• EU Code of Conduct on Data

Centre Energy Efficiency

Data Centres ­ Security of Supply

• UK Electricity consumption is increasing

• UK generation capacity is declining

• Nuclear capacity has not been replaced

• Coal replaced by gas powered stations

• Since 2004 UK is a net importer of gas

• Electric vehicles will increase electricity

demand still further

1 Electric Car = 3 Households

Raising Temperature = Saving Money

• Most UK data centres currently run between 18­20°c

• In 2008 ASHRAE TC 9.9 second edition recommend :

• These temperatures allow almost 100% use of free

cooling (no chillers are required)

• For every 1°c degree the temperature is increased

savings of around 5% per year can be achieved.

Class Allowable Recommended

Class 1 32°c 27°c

Class 2 35°c 27°c

Large Data Centre

• The client had difficulty monitoring

temperatures in real­time

• Problems with hot­spots

• Overcooling in some areas

• Wanted to increase supply air

temperature but feared breaching SLAs

• Required high definition temperature

measurement across the data centre

• Using award winning technology

• Innovative use of fibre optic cable to

measure temperature

Monitoring Alternatives

• Air temperature monitoring

• High Level – Fibre optic zip cord attached to high level 

catenary support cables.

• Run down hot and cold isles above racks

• Run around equipment in racks

• Sub­floor to detect supply air temperatures

• Leak Detection

• On floor slab to detect water leaks from cooling systems

• Bus bar monitoring

• Along electrical bus bars to measure temperature 

particularly around joints. Detect early signs of hot spots.

• High Definition and real time view of data centre energy consumption

• Real time display of critical hotspots

• Monitoring ­ Current and historical analysis of temperature data

• Notification – User defined zones and 

alarms (average, min, max)

• Control ­ DC­View will trigger alarms and 

send SMS / SNMP / Email / ModBus to 

the data centre manager

• Verification – automate audit of energy 

reduction with high definition 

benchmarking

Visualisation Software

Data Centre Floor Plan

Case Study Results

• Our clients need to safely increase temperatures 

from 18°c to 27° over the next 5 years

• For every 1°c degree the temperature is increased 

cost savings of around 5% per year can be achieved 

• You need high definition monitoring tools to 

manage the energy being used

• Verify the effectiveness of all improvements

• Full historical record of thermal performance

• Visualise energy consumption in real time for staff, 

suppliers and clients

• By monitoring in high definition the client can 

minimise the risks as temperatures increase

• The result; Energy reduced and money saved!

Summary

• There are an incredible number of 

applications for fibre optics beyond their use 

as a simple comms links.

Acoustic Monitoring

PowerMonitoring

Data CentreMonitoring

Fibre OpticChristmas Tree

Contact details