TRANSACTIONS Volume 1 05

Non-Contact Temperature MeasurementA Technical Reference Series Brought to You by OMEGA

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VOLUME

I N M E A S U R E M E N T A N D C O N T R O L

Fiber optics are essentially lightpipes, and their basic opera-tion may be traced back morethan a century when British

Physicist John Tyndall demonstratedthat light could be carried within astream of water spouting out andcurving downward from a tank. Athin glass rod for optical transmis-sion was the basis of a 1934 patentawarded to Bell Labs for a “LightPipe.” American Optical demonstrat-ed light transmission through shortlengths of flexible glass fibers in the1950s. However, most modernadvances in fiber optics grew out ofCorning Glass developments in glasstechnology and production methodsdisclosed in the early 1970s.

Like many technical developmentssince WWII, fiber optics programswere largely government funded fortheir potential military advantages.Projects primarily supported telecom-munications applications and laserfiber ring gyroscopes for aircraft/mis-sile navigation. Some sensor develop-ments were included in manufactur-ing technology (Mantech) programs aswell as for aircraft, missile and ship-board robust sensor developments.More recently the Dept. of Energy andNIST have also supported variousfiber optic developments.

Commercial telecommunications

has evolved as the fiber optics tech-nology driving force since the mid-’80s. Increased use of fiber opticswell correlates with fiber materialsdevelopments and lower componentcosts. Advances in glass fibers haveled to transmission improvementsamounting to over three orders ofmagnitude since the early CorningGlass efforts. For example, ordinaryplate glass has a visible light attenua-tion coefficient of several thousanddBs per km. Current fiber optic glass-es a kilometer thick would transmitas much light as say a /” plate glasspane. Table 5-1 indicates relative dig-ital data transmission losses for cop-per and fiber.

Fiber AdvantagesImproved glass transmissions haveresulted in undersea cables withrepeaters required about every 40miles—ten times the distancerequired by copper. Bandwidth androbustness have led to cable serviceproviders selecting fiber optics asthe backbone media for regionalmultimedia consumer services. Theworld market for fiber optic compo-nents was in the $4 billion range in1994 and is projected to reach $8 bil-lion in 1998.

Whether used for communications

or infrared temperature measurement,fiber optics offer some inherentadvantages for measurements inindustrial and/or harsh environments:• Unaffected by electromagneticinterference (EMI) from large motors,transformers, welders and the like;• Unaffected by radio frequencyinterference (RFI) from wireless com-munications and lightning activity;

• Can be positioned in hard-to-reachor view places;• Can be focused to measure smallor precise locations;• Does not or will not carry electri-cal current (ideal for explosive haz-ard locations);• Fiber cables can be run in existingconduit, cable trays or be strapped ontobeams, pipes or conduit (easily installedfor expansions or retrofits); and,• Certain cables can handle ambienttemperatures to over 300°C—higherwith air or water purging.

Any sensing via fiber optic linksrequires that the variable cause a

TRANSACTIONS Volume 1 43

Fiber Advantages

Fiber Applications

Component Options

NON-CONTACT TEMPERATURE MEASUREMENTFiber Optic Extensions

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FFiber Optic Extensions

Figure 5-1: Fiber Optic Probe Construction

Optical Fiber Core

Jacket

Cladding

Clear Elastometer

Phosphor in Elastomer

26 gage twisted wire pair

19 gage twisted wire pair

RG 217/u coaxial cable

Optical fiber 0.82 µm wavelength carrier

1.5Mb/s 6.3Mb/s 45Mb/s

24 48 128

10.8 21 56

2.1 4.5 11

3.5 3.5 3.5

Table 5-1: Relative Transmission Losses for Digital Data Losses in dB/km

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modulation of some type to an opti-cal signal—either to a signal pro-duced by the variable or to a signaloriginating in the sensing device.Basically, the modulation takes theform of changes in radiation intensi-ty, phase, wavelength or polarization.For temperature measurements,intensity modulation is by far themost prevalent method used.

The group of sensors known as fiberoptic thermometers generally refer tothose devices measuring higher tem-peratures wherein blackbody radiationphysics are utilized. Lower tempera-ture targets—say from -100°C to400°C—can be measured by activatingvarious sensing materials such asphosphors, semiconductors or liquidcrystals with fiber optic links offeringthe environmental and remotenessadvantages listed previously.

Fiber ApplicationsFiber optic thermometers haveproven invaluable in measuring tem-peratures in basic metals and glassproductions as well as in the initialhot forming processes for suchmaterials. Boiler burner flames andtube temperatures as well as critical

turbine areas are typical applica-tions in power generation opera-tions. Rolling lines in steel and otherfabricated metal plants also poseharsh conditions which are wellhandled by fiber optics.

Typical applications include fur-naces of all sorts, sintering opera-tions, ovens and kilns. Automatedwelding, brazing and annealingequipment often generate large elec-trical fields which can disturb con-ventional sensors.

High temperature processingoperations in cement, refractory andchemical industries often use fiberoptic temperature sensing. At some-what lesser temperatures, plasticsprocessing, paper making and foodprocessing operations are makingmore use of the technology. Fiberoptics are also used in fusion, sput-tering, and crystal growth processesin the semiconductor industry.

Beyond direct radiant energy col-lection or two-color methods, fiberoptic glasses can be doped to servedirectly as radiation emitters at hotspots so that the fiber optics serveas both the sensor and the media.Westinghouse has developed suchan approach for distributed temper-

ature monitoring in nuclear reactors.A similar approach can be used forfire detection around turbines or jetengines. Internal “hot spot” reflect-ing circuitry has been incorporatedto determine the location of thehot area.

An activated temperature measur-ing system involves a sensing headcontaining a luminescing phosphorattached at the tip of an optical fiber(Figure 5-1). A pulsed light sourcefrom the instrument package excitesthe phosphor to luminescence andthe decay rate of the luminescence isdependent on the temperature.These methods work well for non-glowing, but hot surfaces belowabout 400°C.

A sapphire probe developed byAccufiber has the sensing end coatedby a refractory metal forming a black-body cavity. The thin, sapphire rodthermally insulates and connects toan optical fiber as is shown in Figure 5-2. An optical interference filter andphotodetector determines the wave-length and hence temperature.

Babcock & Wilcox has developed aquite useful moving web or rollertemperature monitoring system

Fiber Optic ExtensionsΩ 5

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Figure 5-2: Typical IR Fiber Optic Probe

Lens

Low Temperature Optical FiberSingle Crystal

Sapphire (Al2O3)

Narrowband Filter

AnalyzerOptical

Detector

Coupler

Thin Film Metal Coating

Al2O3 Protective Film

Blackbody Cavity

Figure 5-3: Multipoint Pick-up Assembly

Surface of Target Web

Single Strand Optical FiberGlass Focusing Sphere

Fiber/Lens Holder

Upper Air Purge Plate

Lower Air Purge Plate

Trans- parent

Retainer Plate

Purge Air In

which will measure temperaturesfrom 120°C to 180°C across webs upto 4 meters (13 ft.) wide (Figure 5-3).The system combines optical andelectronic multiplexing and can haveas many as 160 individual pickupfibers arranged in up to 10 rows. Thefibers transfer the radiation througha lens onto a photodiode array.

Component OptionsFiber optics for temperature mea-surements as well as for communi-cations depends on minimizing loss-es in the light or infrared radiationbeing transmitted. Basics of lightconduction (Figure 5-4) is a centralglass fiber which has been carefullyproduced to have nearly zeroabsorption losses at the wave-lengths of interest. A cladding mate-rial with a much lower index ofrefraction reflects all non-axial light

rays back into the central fiber coreso that most of the conducted radi-

ation actually bounces down thelength of the cable. Various metal,Teflon or plastics are used for outerprotective jackets.

The difference in refractive indicesof the core and cladding also identi-fy an acceptance cone angle for radi-ation to enter the fiber and be trans-mitted. However, lenses are oftenused to better couple the fiber witha target surface.

For relatively short run temperaturesensing, losses in the fiber optic linkare generally negligible. Losses in con-nectors, splices and couplers predom-inate and deserve appropriate engi-neering attention. Along with thefiber optic cable, a temperature mea-suring system will include an array ofcomponents such as probes, sensorsor receivers, terminals, lenses, cou-plers, connectors, etc. Supplementalitems like blackbody calibrators andbacklighter units which illuminateactual field of view are often neededto ensure reliable operation. T

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Figure 5-4: Fiber Optic Cable Construction

CladdingCore

Cladding

Core

Very Small Θ0

Plastic TubingCore and Cladding

Jacket (PVC)

Tape or Sleeve Separator

Braided (Strength) MemberLaquer Coated

Optical Fiber

Silicon (RTV) Inner Jacket

Plastic Outer Jacket

Jacket (PVC)

Coated Optical Fibers

Plastic Tube Containers

Tape Separator

Jacketed Strength Member

Jacket (PVC)

Laquered Optical Fibers

Fabric Strength Members

Plastic Tube Containers

Polyurethane Sheath

SINGLE-FIBER CABLES

MULTIFIBER CABLES

References and Further Reading• Handbook of Temperature Measurement & Control, Omega Press, 1997.• “Fiber Optic PLC Links,” Kenneth Ball, Programmable Controls, Nov/Dec 1988.• Fiber Optic Sensors, Eric Udd, John Wiley & Sons, 1991.• Handbook of Intelligent Sensors for Industrial Automation, Nello Zuech,Addison-Wesley Publishing Company, 1992.• “Infrared Optical Fibers”, M.G. Drexhage and C.T. Moynihan, ScientificAmerican, November 1988.• Measurements for Competitiveness in Electronics, NIST Electronics andElectrical Engineering Laboratory, 1993.• “Multichannel Fiber-Optic Temperature Monitor,” L. Jeffers, Babcock &Wilcox Report; B&W R&D Division; Alliance, Ohio.• Optical Fiber Sensors: Systems and Applications, Vol 2, B. Culshaw and J. Dakin, Artech House; 1989.• Process Measurement and Analysis; Instrument Engineers’ Handbook,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.