Ece 371 jb lecture 3
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This document discusses different types of temperature sensors including thermocouples, resistance temperature devices (RTDs), and thermistors. It provides details on how thermocouples and RTDs function to convert temperature into an electrical signal. Thermocouples generate small voltages based on the temperature difference between junctions of two dissimilar metals, while RTDs change resistance predictably with temperature. The document compares advantages and disadvantages of different sensor types and constructions, and describes considerations for accurate temperature measurement such as compensating for lead wire resistance effects.
Ece 371 jb lecture 3
- 1. Temperature SensorsTemperature Sensors ECE 371 JBECE 371 JB Prof. BernhardProf. Bernhard
- 2. A Simple Thermal SystemA Simple Thermal System Heat Source Temperature Controlling Device Sensor Work Load Sensor Input Output
- 3. TypesTypes ThermocouplesThermocouples Resistance temperature devices (RTD)Resistance temperature devices (RTD) ThermistorsThermistors Infrared sensorsInfrared sensors
- 4. ThermocouplesThermocouples Mostly widely used inMostly widely used in industryindustry Range: sub-zero toRange: sub-zero to 40004000oo F(2000F(2000oo C)C) Formed by joining twoFormed by joining two different metal alloy wiresdifferent metal alloy wires (A,B) at point called(A,B) at point called junctionjunction Junction called theJunction called the measuring or “hot” junctionmeasuring or “hot” junction Lead ends attached toLead ends attached to temp indicator or controllertemp indicator or controller Connection point calledConnection point called reference or “cold” junctionreference or “cold” junction Display Device + - A B Reference Junction Measuring Junction
- 5. How does it work?How does it work? Measuring junction is heated, small DC voltageMeasuring junction is heated, small DC voltage (millivolts) generated in thermocouple wires(millivolts) generated in thermocouple wires ThermocoupleThermocouple converts thermal energy intoconverts thermal energy into electrical energyelectrical energy Note: thermocouple only generates a millivoltageNote: thermocouple only generates a millivoltage signal when there issignal when there is temperature differencetemperature difference between “hot” and “cold” junctionsbetween “hot” and “cold” junctions ““cold” junction usually set to 32cold” junction usually set to 32oo F(0F(0oo C)C)
- 6. Thermocouple TypesThermocouple Types Made up of two different metalMade up of two different metal alloy wires.alloy wires. Different alloys result inDifferent alloys result in different temperature rangesdifferent temperature ranges Ex:Ex: StandardStandard TypeType MetalMetal ContentContent (Pos. Leg)(Pos. Leg) MetalMetal ContentContent (Neg. Leg)(Neg. Leg) Temp.Temp. RangeRange BB 70.4% (Pt)70.4% (Pt) 29.6% (Rh)29.6% (Rh) 93.9% (Pt)93.9% (Pt) 6.1% (Rh)6.1% (Rh) 1600-31001600-3100 oo FF 870-1700870-1700 oo CC EE 90% (Ni)90% (Ni) 10% (Cr)10% (Cr) 55% (Cu)55% (Cu) 45% (Ni)45% (Ni) 32-165032-1650 oo FF 870-1700870-1700 oo CC
- 7. Pros/ConsPros/Cons Each thermocouple type has advantages & disadvantagesEach thermocouple type has advantages & disadvantages – Cost:Cost: Rare metals (i.e. noble metals)Rare metals (i.e. noble metals) $$$$$$ – Types B, R, STypes B, R, S Common metals (i.e. base metals)Common metals (i.e. base metals) $$ – Types E, J, K, N, TTypes E, J, K, N, T Rarer metals = high temperature range & better accuracyRarer metals = high temperature range & better accuracy – Temperature RangeTemperature Range – Accuracy a.k.a. toleranceAccuracy a.k.a. tolerance – Life ExpectancyLife Expectancy Operating Temp.Operating Temp. Wire sizeWire size Thermocouple protectionThermocouple protection EnvironmentEnvironment Accuracy requiredAccuracy required TypeType Max.Max. Temp.Temp. TolerancesTolerances BB 31003100oo FF 17001700oo CC (+/-) 0.5%(+/-) 0.5% EE 16501650oo FF 900900oo CC (+/-) 1.7(+/-) 1.7oo C (+/-) 3.06C (+/-) 3.06oo FF or (+/-) 0.5%or (+/-) 0.5% Whichever greaterWhichever greater
- 8. Life ExpectancyLife Expectancy Failed = inaccuracyFailed = inaccuracy - When wires are heated/cooled changes take place onWhen wires are heated/cooled changes take place on molecular levelmolecular level - Physically: molecular structure changesPhysically: molecular structure changes - Chemically: wires react with oxygen or other substances,Chemically: wires react with oxygen or other substances, changing chemical compositionchanging chemical composition - Result: millivolt signal “drifts”Result: millivolt signal “drifts” Time EMF (mV) Tolerance Band - Recalibration: adjust controller to compensate for errors
- 9. Thermocouple ConstructionsThermocouple Constructions 3 General constructions3 General constructions – Insulated WireInsulated Wire – Ceramic-beadedCeramic-beaded – Metal-sheathedMetal-sheathed
- 10. Insulated Wire ThermocouplesInsulated Wire Thermocouples Bare wires wrapped with insulationBare wires wrapped with insulation – InsulationsInsulations Fibrous, woven material made of fiber-glass, mica,Fibrous, woven material made of fiber-glass, mica, or ceramic fiberor ceramic fiber Plastics (Teflon)Plastics (Teflon) Polyimides (Kapton)Polyimides (Kapton) – PurposePurpose Electrically isolate wiresElectrically isolate wires Protects wires from contaminationProtects wires from contamination Easier wire installationEasier wire installation
- 11. Metal - Sheathed ThermocouplesMetal - Sheathed Thermocouples Junction and wires are assembled in smallJunction and wires are assembled in small diameter metal tubesdiameter metal tubes – InsulationInsulation FiberglassFiberglass MgOMgO – PurposePurpose Protects against contaminationProtects against contamination Defends against chemical attackDefends against chemical attack Provides mechanical stabilityProvides mechanical stability
- 12. Metal - Sheathed ThermocouplesMetal - Sheathed Thermocouples Orientation of thermocouple junction during assemblyOrientation of thermocouple junction during assembly – GroundedGrounded Weld junction directly to inside tip of sheathWeld junction directly to inside tip of sheath Ensures rapid heat transfer from sheath to junctionEnsures rapid heat transfer from sheath to junction Protects junction while minimizing heat transfer delays.Protects junction while minimizing heat transfer delays. – UngroundedUngrounded Similar to grounded except junction isolated from metal sheathSimilar to grounded except junction isolated from metal sheath Electrically isolates junction from sheathElectrically isolates junction from sheath Prevents stray voltages from inducing measuring errorPrevents stray voltages from inducing measuring error More shock resistant & better under rapid temperature changesMore shock resistant & better under rapid temperature changes DISADVANTAGE: Slows down heat transfer to junction (2x-3xDISADVANTAGE: Slows down heat transfer to junction (2x-3x slower)slower) – ExposedExposed Junction protrudes from end of sheath, but insulated from itJunction protrudes from end of sheath, but insulated from it Due to direct exposure with heated material, very quick response toDue to direct exposure with heated material, very quick response to temp. changestemp. changes No sheath to slow down heat transferNo sheath to slow down heat transfer DISADVANTAGE: Not protected from mechanical damage &DISADVANTAGE: Not protected from mechanical damage & chemical attackchemical attack
- 13. Resistance Temperature DevicesResistance Temperature Devices (RTD)(RTD) Precision Temperature SensorsPrecision Temperature Sensors – More accurate than thermocouple elementsMore accurate than thermocouple elements – Maintain accuracy over longer period of timeMaintain accuracy over longer period of time – Range up to 1200Range up to 1200oo F (650F (650oo C)C) StylesStyles – Wire-WoundWire-Wound – Thin filmThin film – Kapton InsulatedKapton Insulated
- 14. How do RTDs work?How do RTDs work? RTD’s resistance as temp.RTD’s resistance as temp. – Controller measures resistance value and converts toController measures resistance value and converts to temp. reading, fairly linear relationship.temp. reading, fairly linear relationship. – Unlike thermocouple, no electrical signal generatedUnlike thermocouple, no electrical signal generated – Controller measures resistance by passing currentController measures resistance by passing current through RTDthrough RTD – Use a base resistance value (ex: for Platinum, valueUse a base resistance value (ex: for Platinum, value of 100 ohms at 0of 100 ohms at 0oo C (32C (32oo F)F) Temperature (o C) Resistance (Ohms) RTD Resistance Vs. Temp. (TCR) Curve TCR = Temperature coefficient of resistance
- 15. RTD Vs. ThermocouplesRTD Vs. Thermocouples Advantages of RTDsAdvantages of RTDs – StabilityStability – RepeatabilityRepeatability – AccuracyAccuracy Disadvantages of RTDsDisadvantages of RTDs – Cost: Platinum = $$$, 2x more expensiveCost: Platinum = $$$, 2x more expensive – Temp. Range limitedTemp. Range limited – Response Time slower, 2x-4x times slowerResponse Time slower, 2x-4x times slower Heat must transfer through epoxy or glass coatingHeat must transfer through epoxy or glass coating Entire RTD element must reach uniform temp. beforeEntire RTD element must reach uniform temp. before accurate measurement taken.accurate measurement taken.
- 16. Lead Wire EffectLead Wire Effect Alters reading due to lead wire resistanceAlters reading due to lead wire resistance Two approachesTwo approaches – Determine lead wire resistance and have controllerDetermine lead wire resistance and have controller compensatecompensate – Attach additional lead wire to one end of RTDAttach additional lead wire to one end of RTD – Connect a transmitter, converts resistance to low ampConnect a transmitter, converts resistance to low amp signal and sent to temperature controllersignal and sent to temperature controller 1 2 3 3-wire RTD 1 3 42 4-wire RTD RTD RTD
- 17. Effect of Lead Resistance: PlatinumEffect of Lead Resistance: Platinum Wire RTDWire RTD Most Common: DIN 43760Most Common: DIN 43760 – Standard temp. coefficient (alpha=0.00385)Standard temp. coefficient (alpha=0.00385) For 100 ohm wireFor 100 ohm wire +0.385 ohms/+0.385 ohms/OO C @ 0C @ 0oo CC alpha = average slope from 0alpha = average slope from 0oo C – 100C – 100oo CC – A 10 ohm lead impedance implies 10/3.85 =A 10 ohm lead impedance implies 10/3.85 = 2626oo C error in measurementC error in measurement Lead Lead R=5 R=5 Ω Ω100 RTD R=5 Ω
- 18. How to correct this problem?How to correct this problem? Wheatstone: 3-Wire BridgeWheatstone: 3-Wire Bridge – Wires A & B are perfectly matched in length, respectiveWires A & B are perfectly matched in length, respective impedances effects will cancel out due to being on opposite legsimpedances effects will cancel out due to being on opposite legs – Wire C acts as sense lead & carries no currentWire C acts as sense lead & carries no current A C B DVM RTD – Non-linear relationship between resistance change and bridgeNon-linear relationship between resistance change and bridge output voltage changeoutput voltage change – Additional equation required to convert bridge output voltage toAdditional equation required to convert bridge output voltage to equivalent RTD impedanceequivalent RTD impedance
- 19. 3-Wire Bridge Calculations3-Wire Bridge Calculations If VIf Vss & V& Voo known, Rknown, Rgg can be found.can be found. Unbalanced VUnbalanced Voo of bridge with Rof bridge with R11=R=R22 )2/1( 3 3 s g so V RR R VV − + = If RIf Rgg=R=R33 VVoo=0 & bridge is balanced=0 & bridge is balanced To determine RTo determine Rgg assuming lead resistance isassuming lead resistance is zerozero + − = os os g VV VV RR 2 2 3
- 20. 3-Wire Bridge Calculations3-Wire Bridge Calculations + − + − = os o L os os g VV V R VV VV RR 2 4 2 2 3 RL RL Rg+- Vo V3/2 + - If RIf Rgg located some distance from 3-wire configurationlocated some distance from 3-wire configuration RRLL appears in series with Rappears in series with Rgg & R& R33
- 21. Another ApproachAnother Approach 4-Wire Ohms4-Wire Ohms – DVM is directly proportional to RTD resistanceDVM is directly proportional to RTD resistance 1 conversion1 conversion equation requiredequation required – Insensitive to length of lead wiresInsensitive to length of lead wires – Accuracy better than 3-wireAccuracy better than 3-wire – Disadvantage: One more extension wire required.Disadvantage: One more extension wire required. Current Source 100 W RTDDVM + - i = 0 i = 0 - + Vo RTD=Rg Vs + -
- 22. Resistance to TemperatureResistance to Temperature ConversionConversion RTD more linear than thermocouple, curve-RTD more linear than thermocouple, curve- fitting still requiredfitting still required Callendar-Van Dusen EquationCallendar-Van Dusen Equation −− −−−= 3 100 1 100)100 1 100 TTTT TRR oT βδα RRTT = Resistance at Temperature T= Resistance at Temperature T RRoo = Resistance at T=0= Resistance at T=0oo CC = Temperature coefficient at T=0= Temperature coefficient at T=0oo CC = 1.49 (typical value for 0.00392 platinum)= 1.49 (typical value for 0.00392 platinum) = 0 T>0, 0.11 (typical) T<0= 0 T>0, 0.11 (typical) T<0δ α β
- 23. IdentificationIdentification 2-wire RTD uses same color lead wire for2-wire RTD uses same color lead wire for both leadsboth leads 3-wire has 2 red leads & 1 white lead3-wire has 2 red leads & 1 white lead 4-wire has 2 red leads & 2 white leads4-wire has 2 red leads & 2 white leads Lead-to-leadLead-to-lead MeasurementMeasurement Distance at RoomDistance at Room TemperatureTemperature 1 to 2; 3 to 41 to 2; 3 to 4 Less than 1ohm to aLess than 1ohm to a few ohms max.few ohms max. 1 to 3; 1 to 41 to 3; 1 to 4 2 to 3; 2 to 42 to 3; 2 to 4 107 to 110 ohms107 to 110 ohms 1 3 42 4-wire RTD RTD
- 24. RTD AssemblyRTD Assembly Wire WoundWire Wound – For 500For 500oo F (260F (260oo C), element welded to copper or nickelC), element welded to copper or nickel lead wireslead wires – Sub-assembly placed in closed-end tubeSub-assembly placed in closed-end tube – Powder, cement or thermal grease fills tubePowder, cement or thermal grease fills tube – Epoxy seal seals out moisture & locks RTD/leads toEpoxy seal seals out moisture & locks RTD/leads to tubetube Thin FilmThin Film – For 1200For 1200oo F (650F (650oo C), element fitted into cavity of MgOC), element fitted into cavity of MgO metal-sheathed cablemetal-sheathed cable – Wires in cable welded to RTD elementWires in cable welded to RTD element – Cap filled with MgO and placed on element end &Cap filled with MgO and placed on element end & mountedmounted
- 25. What are Thermistors?What are Thermistors? Semiconductor used as temperature sensorSemiconductor used as temperature sensor Made from mixture of metal oxides pressed to bead or wafer formMade from mixture of metal oxides pressed to bead or wafer form Bead heated under pressure at high temp & encapsulated withBead heated under pressure at high temp & encapsulated with glass/epoxyglass/epoxy RESULT: Distinct non-linear resistance vs. temp. relationshipRESULT: Distinct non-linear resistance vs. temp. relationship Resistance (Ohms) Temperature (o C) Non-linear decrease in resistance as temperature increases.
- 26. So Sensitive…So Sensitive… Very large resistance change = smallVery large resistance change = small temp. changetemp. change 3 – 5% per3 – 5% per oo C (vs. 0.4% perC (vs. 0.4% per oo C for RTDs)C for RTDs) Temp. changes as small as 0.1Temp. changes as small as 0.1oo CC Significantly smaller in sizeSignificantly smaller in size Temp range: -100Temp range: -100oo C – 300C – 300oo C (-120C (-120oo F –F – 570570oo F)F)
- 27. Thermistor StandardsThermistor Standards No Industrial StandardsNo Industrial Standards Base resistance range: 10Base resistance range: 1033 – 10– 1066 ohmsohms – Typically measured at 25Typically measured at 25oo C vs. 0C vs. 0oo C for RTDsC for RTDs TCRs vary widelyTCRs vary widely Thermistor’s accuracy limited to smallThermistor’s accuracy limited to small temp. rangetemp. range
- 28. Thermistor Lead Wire EffectsThermistor Lead Wire Effects Lead wire does add overall resistanceLead wire does add overall resistance NOTE: base resistance of thermistor veryNOTE: base resistance of thermistor very large (>10large (>1033 ohms), added lead wireohms), added lead wire resistance insignificant.resistance insignificant. RESULT: No resistance compensationRESULT: No resistance compensation required!required!
- 29. Infrared SensorsInfrared Sensors Intercepts portion of infrared energy radiated byIntercepts portion of infrared energy radiated by object ( = 8 - 14 microns).object ( = 8 - 14 microns). Waves focused through lens on infraredWaves focused through lens on infrared detector, converting to an electric output signaldetector, converting to an electric output signal λ Heat Source Optics Infrared Detector Non-Contact Temp. Sensor Temp. Indicator
- 30. EmissivityEmissivity Def: The ability of a material to radiate or absorbDef: The ability of a material to radiate or absorb electromagnetic waves. Higher = Better!electromagnetic waves. Higher = Better! – Ex: Given values below & emissivity varies by 0.05, what isEx: Given values below & emissivity varies by 0.05, what is measuring error?measuring error? Ans: IR Sensor A 5.5% (0.05/0.9)Ans: IR Sensor A 5.5% (0.05/0.9) IR Sensor B 10% (0.05/0.5)IR Sensor B 10% (0.05/0.5) IR Sensor A IR Sensor B e = 0.5e = 0.9
- 31. Field of ViewField of View All infrared radiation in this filed of view will beAll infrared radiation in this filed of view will be detected by the sensordetected by the sensor Infrared Sensor 0.75 in (19 mm)0.60 in (15 mm) 1.0 in (25 mm) 1.4 in (36 mm) 2.5 in (64 mm) 4.5 in (114 mm) 25 mm 76 mm 152 mm
- 32. Good vs. Bad RadiationGood vs. Bad Radiation Position 1, IR sensor sees both target object & background objectsPosition 1, IR sensor sees both target object & background objects Position 2, IR sensor only sees target object. True targetPosition 2, IR sensor only sees target object. True target temperature can now be measured.temperature can now be measured. RULE: target size should be at least 1.5 to 2 times the “spot size.”RULE: target size should be at least 1.5 to 2 times the “spot size.” Infrared Sensor 2 1 Correct Target Placement Incorrect Target Placement Background “Noise”
- 33. Scenarios to AvoidScenarios to Avoid Figure 1: Thin film materials & background radiationFigure 1: Thin film materials & background radiation enter sensorenter sensor Figure 2: Polished metals will not function well withFigure 2: Polished metals will not function well with infrared sensing due to the reflecting radiation.infrared sensing due to the reflecting radiation. Infrared Sensor Figure 1 Infrared Sensor Figure 2
- 34. Sensor to Target DistanceSensor to Target Distance To reduce reflected radiant energy, set IR sensor at rightTo reduce reflected radiant energy, set IR sensor at right angle with respect to targetangle with respect to target If space limitation, mount IR up to a maximum of 45If space limitation, mount IR up to a maximum of 45OO <45o Product Sensor
- 35. Operating EnvironmentOperating Environment Smoke, dust vapors absorb or reflect infraredSmoke, dust vapors absorb or reflect infrared radiation before getting to sensor lens.radiation before getting to sensor lens. Causes controller to maintain target at wrongCauses controller to maintain target at wrong temperaturetemperature Infrared Sensor Target Smoke or Vapors
- 36. So which one is better?So which one is better? AdvantagesAdvantages ThermocoupleThermocouple Simple, ruggedSimple, rugged High temp. operationHigh temp. operation Low CostLow Cost No resistance lead wire problemsNo resistance lead wire problems Point temp. sensingPoint temp. sensing Fastest response to temperature changesFastest response to temperature changes RTDRTD Most stable over timeMost stable over time Most accurateMost accurate Most repeatable temp. measurementMost repeatable temp. measurement Very resistant to contamination/corrosion of theVery resistant to contamination/corrosion of the RTD elementRTD element ThermistorThermistor High sensitivity to small temperature changesHigh sensitivity to small temperature changes Temperature measurements become moreTemperature measurements become more stable with usestable with use Copper or nickel extension wires can be usedCopper or nickel extension wires can be used InfraredInfrared No contact with the product requiredNo contact with the product required Response times as fast or faster thanResponse times as fast or faster than thermocouplesthermocouples No corrosion or oxidation to affect sensorNo corrosion or oxidation to affect sensor accuracyaccuracy High repeatabilityHigh repeatability
- 37. So which one is better?So which one is better? DisadvantagesDisadvantages ThermocoupleThermocouple Least stable, least repeatableLeast stable, least repeatable Low sensitivity to small temperatureLow sensitivity to small temperature changeschanges Extension wire must be of the sameExtension wire must be of the same thermocouple typethermocouple type Wire may pick up radiated electrical noiseWire may pick up radiated electrical noise of not shieldedof not shielded Lowest accuracyLowest accuracy RTDRTD High CostHigh Cost Slowest response timeSlowest response time Low sensitivity to small temperatureLow sensitivity to small temperature changeschanges Sensitive to vibrationSensitive to vibration Decalibration if used beyond sensor’sDecalibration if used beyond sensor’s temperature ratingstemperature ratings Somewhat fragileSomewhat fragile
- 38. So which one is better?So which one is better? DisadvantagesDisadvantages ThermistorThermistor Limited temperature rangeLimited temperature range FragileFragile Some initial accuracy “drift”Some initial accuracy “drift” Decalibration if used beyond the sensor’sDecalibration if used beyond the sensor’s temperature ratingtemperature rating Lack of standards for replacementLack of standards for replacement InfraredInfrared High initial costHigh initial cost More complex – support electronicsMore complex – support electronics requiredrequired Emissivity variations affect temperatureEmissivity variations affect temperature measurement accuracymeasurement accuracy Field of view and spot size may restrictField of view and spot size may restrict sensor applicationsensor application Measuring accuracy affected by dust,Measuring accuracy affected by dust, smoke, background radiation etc.smoke, background radiation etc.