Fibre Optic Sensor

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ABSTRACT.

Over many decades the demand for temperature sensors and

controllers has shown that temperature is the principal processvariables of serious concern to the process industries, that is, those

industries that handle and convert gases, liquids, and bulk solids into

products and by-products. Chemical, petroleum, petrochemical,

polymer, plastic, and large segments of metallurgical and food

processors are examples. Most of these measurement tasks can be

carried out using conventional electric temperature sensors, but with

limitations. Particularly under harsh conditions, fibre optic

temperature sensors show their advantages over conventionalinstrumentation. Two common principles of fibre optic temperature

measurement are: blackbody radiation physics and infrared detection.

So far there application is still limited to niche markets but with

decreasing system prices fibre-optic temperature sensing has great

potential for further growth.

INTRODUCTION.

Many material properties show strong temperature dependence. In

order to utilize or compensate temperature effects, its measurement is

required. Examples of such temperature dependencies are dew point,

density, electrical conductivity, refractive index, rigidity and

diffusion. Temperature measurement also plays an important role in

health monitoring of electric circuits or civil structures. Most

measurement tasks in industrial applications and research can be

carried out using conventional electric temperature sensors such as

thermocouples, junction temperature sensors, resistance temperature

detectors or thermistors. But conventional temperature sensors have

their limitations especially if-

large distances have to be covered as is the case of manydistributed measurements,


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large numbers of sensors have to be integrated in order tomonitor many system states or even temperature fields or

gradients,

electromagnetic interference decreases the signal to noise ratiosignificantly,

explosive environments prohibit the application of electricdevices, light-weight structures and monitoring equipment with

low mass impact are desired.

Particularly under these conditions fibre optic temperature sensors are

able to show their full potential. But depending on the actual

application, different types of fibre optic temperature sensors can be

used. The most common fibre optic temperature sensors are:

Fibre optic assembly consisting of single fibre or several fibresbundled together utilizing blackbody radiation physics.

Fibre optic assembly consisting of single fibre or several fibresbundled together gathering infrared radiation from hot target and

transmits it to a infrared detector.

Fibre optic sensors are currently used in process industries, that is,those industries that handle and convert gases, liquids, and bulk solids

into products and by-products. Chemical, petroleum, petrochemical,

polymer, plastic, and large segments of metallurgical and food

processors are examples.

1. TEMPERATURE SENSINGTHROUGH BLACKBODY

RADIATION PHYSICS.

The group of sensors known as fibre optic thermometers generally

refer to those devices measuring higher temperatures wherein

blackbody radiation physics are utilized. Lower temperature Targets

say from 173 K to 673 K can be measured by activating various


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sensing materials such as phosphors, semiconductors or liquid crystals

with fibre optic links offering the environmental and remoteness

advantages listed previously.

1.1WORKING PRINCIPLE.This type of sensor works on blackbody radiation physics, according

to it-

The thermal radiation from a black body is energy converted

electrodynamically from the body's pool of internal thermal energy at

any temperature greater than absolute zero. It is called blackbodyradiation and has a frequency distribution with a characteristic

frequency of maximum radiative power that shifts to higher

frequencies with increasing temperature. As the temperature increases

past a few hundred degrees Celsius, black bodies start to emit visible

wavelengths, appearing red, orange, yellow, white, and blue with

increasing temperature. When an object is visually white, it is

emitting a substantial fraction as ultraviolet radiation.

This principle is governed by Planks law i.e.

where

I(,T) is the energy per unit time (or the power) radiated per unit

area of emitting surface in the normal direction per unit solid

angle per unit frequency by a black body at temperature T;

h is the Planck constant;

c is the speed of light in a vacuum;

kis the Boltzmann constant; is the frequency of the electromagnetic radiation; and
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Tis the temperature of the body in kelvins.

(Fig 1.1) As the temperature decreases, the peak of the blackbody radiation curve

moves to lower intensities and longer wavelengths.
http://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Kelvinhttp://en.wikipedia.org/wiki/Temperature


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(Fig 1.2) The color (chromaticity) of blackbody radiation depends on the temperature of theblack body; the locus of such colors, shown here in CIE 1931x,yspace, is known as

the Planckian locus.

A fibre-optic temperature measuring system involves a sensing head

containing a luminescing phosphor attached at the tip of an optical

fibre. A pulsed light source from the instrument package excites the

phosphor to luminescence and the decay rate of the luminescence is

dependent on the temperature. These methods work well for non-

glowing, but hot surfaces below about 673K.

1.2CONSTRUCTION.A sapphire probe has the sensing end coated by a refractory metal

forming a blackbody cavity. The thin, sapphire rod thermally insulatesand connects to an optical fibre. An optical interference filter and


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photodetector determines the wavelength and hence temperature. The

system that combines optical and electronic multiplexing and can

have as many as 160 individual pickup fibres arranged in up to 10

rows. The fibres transfer the radiation through a lens onto a

photodiode array for detection.

(Fig 1.3) Construction of typical IR Fibre Optic Temperature Sensors.


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(Fig 1.4) Construction of Fibre-optic Probe

Fibre optics for temperature measurements as well as for

communications depends on minimizing losses in the light or infrared

radiation being transmitted. A basic of light conduction is a centralglass fibre which has been carefully produced to have nearly zero

absorption losses at the wavelengths of interest. A cladding material

with a much lower index of refraction reflects all non-axial light rays

back into the central fibre core so that most of the conducted radiation

actually bounces down the length of the cable. Various metal, Teflon

or plastics are used for outer protective jackets. The difference in

refractive indices of the core and cladding also identify an acceptance


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cone angle for radiation to enter the fibre and be transmitted.

However, lenses are often used to better couple the fibre with a target

surface. For relatively short run temperature sensing, losses in the

fibre optic link are generally negligible. Losses in connectors, splicesand couplers predominate and deserve appropriate engineering

attention. Along with the fibre optic cable, a temperature measuring

system will include an array of components such as probes, sensors or

receivers, terminals, lenses, couplers, connectors, etc. Supplemental

items like blackbody calibrators and backlighter units which

illuminate actual field of view are often needed to ensure reliable

operation.

(Fig 1.5) Multipoint Pick-Up Assembly


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A quite useful moving web or roller temperature monitoring system

which will measure temperatures from 393 K to 453 K across webs

up to 4 meters (13 ft.) wide (Fig 1.4). The system combines opticaland electronic multiplexing and can have as many as 160 individual

pickup fibres arranged in up to 10 rows. The fibres transfer the

radiation through a lens onto a photodiode array.

COMPONENT OPTIONS.A basic of light conduction (Fig 1.5) is a central glass fibre which has

been carefully produced to have nearly zero absorption losses at the

wavelengths of interest. A cladding material with a much lower index

of refraction reflects all non-axial light radiation actually bounces

down the length of the cable. Various metal, Teflon or plastics are

used for outer protective jackets.

The difference in refractive indices of the core and cladding also

identify an acceptance cone angle for radiation to enter the fibre and

be transmitted. However, lenses are often used to better couple the

fibre with a target surface.


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(Fig 1.6) Fibre Optic Cable Construction

For relatively short run temperature sensing, losses in the fibre optic

link are generally negligible. Losses in connectors, splices and

couplers predominate and deserve appropriate engineering attention.

Along with the fibre optic cable, a temperature measuring system will

include an array of components such as probes, sensors or receivers,terminals, lenses, couplers, connectors, etc. Supplemental items like

blackbody calibrators and backlighter units which illuminate actual

field of view are often needed to ensure reliable operation.


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APPLICATIONS, ADVANTAGES AND

BENEFITS.

Fibre optic thermometers have proven invaluable in measuring

temperatures in basic metals and glass productions as well as in the

initial hot forming processes for such materials. Boiler burner flames

and tube temperatures as well as critical turbine areas are typical

applications in power generation operations. Rolling lines in steel and

other fabricated metal plants also pose harsh conditions which are

well handled by fibre optics

Typical applications include furnaces of all sorts, sintering operations,

ovens and kilns. Automated welding, brazing and annealing

equipment often generate large electrical fields which can disturb

conventional sensors.

High temperature processing operations in cement, refractory and

chemical industries often use fibre optic temperature sensing. At

somewhat lesser temperatures, plastics processing, paper making and

food processing operations are making more use of the technology.

Fibre optics are also used in fusion, sputtering, and crystal growth

processes

in the semiconductor industry.

Fibre optic glasses can be doped to serve directly as radiation emitters

at hot spots so that the fibre optics serve as both the sensor and the

media. Such an approach is used for distributed temperaturemonitoring in nuclear reactors. A similar approach can be used for

fire detection around turbines or jet engines. Internal hot spotreflecting circuitry has been incorporated to determine the location of

the hot area.

Fibre optics offer some inherent advantages for measurements in

industrial and/or harsh environments:


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Unaffected by electromagnetic interference (EMI) from largemotors, transformers, welders and the like;

Unaffected by radio frequency interference (RFI) from wirelesscommunications and lightning activity;

Can be positioned in hard-to-reach or view places; Can be focused to measure small or precise locations; Does not or will not carry electrical current (ideal for explosive

hazard locations);

Fibre cables can be run in existing conduit, cable trays or bestrapped onto beams, pipes or conduit (easily installed for

expansions or retrofits); and, Certain cables can handle ambient temperatures to over 573K-

higher with air or water purging.


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REFERENCES.

Handbook of Temperature Measurement & Control, OmegaPress, 1997.

Fibre Optic PLC Links, Kenneth Ball, Programmable Controls,Nov/Dec 1988.

Fibre Optic Sensors, Eric Udd, John Wiley & Sons, 1991. Handbook of Intelligent Sensors for Industrial Automation,

Nello Zuech,

Addison-Wesley Publishing Company, 1992.

Infrared Optical Fibres, M.G. Drexhage and C.T. Moynihan,Scientific

American, November 1988.

Measurements for Competitiveness in Electronics, NISTElectronics and

Electrical Engineering Laboratory, 1993.

Multichannel Fibre-Optic Temperature Monitor, L. Jeffers,Babcock &

Wilcox Report; B&W R&D Division; Alliance, Ohio. Optical Fibre Sensors: Systems and Applications, Vol 2, B.

Culshaw

and J. Dakin, Artech House; 1989.

Process Measurement and Analysis; Instrument EngineersHandbook,

Third Edition, B. Liptak, Chilton Book Company, 1995.

Radiation Thermometers/Pyrometers, C. Warren,Measurements& Control, February, 1995.

Sensors and Control Systems in Manufacturing, S. Soloman,McGraw-Hill, 1994.


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Fibre Optic Sensor

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