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7/31/2019 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
http://en.wikipedia.org/wiki/Timehttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Surface_normalhttp://en.wikipedia.org/wiki/Solid_anglehttp://en.wikipedia.org/wiki/Solid_anglehttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Planck_constanthttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Speed_of_lighthttp://en.wikipedia.org/wiki/Planck_constanthttp://en.wikipedia.org/wiki/Frequencyhttp://en.wikipedia.org/wiki/Solid_anglehttp://en.wikipedia.org/wiki/Solid_anglehttp://en.wikipedia.org/wiki/Surface_normalhttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Time7/31/2019 Fibre Optic Sensor
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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/Temperature7/31/2019 Fibre Optic Sensor
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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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