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Temperature SensorsTemperature Sensors
ECE 371 JBECE 371 JB
Prof. BernhardProf. Bernhard
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A Simple Thermal SystemA Simple Thermal System
Heat Source
TemperatureControlling
Device
Sensor
Work Load
Sensor Input
Output
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TypesTypes
ThermocouplesThermocouples
Resistance temperature devices (RTD)Resistance temperature devices (RTD)
ThermistorsThermistors
Infrared sensorsInfrared sensors
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ThermocouplesThermocouples
Mostly widely used in Mostly widely used in industryindustryRange: sub-zero to Range: sub-zero to 40004000ooF(2000F(2000ooC)C)Formed by joining two Formed by joining two different metal alloy wires different metal alloy wires (A,B) at point called (A,B) at point called junctionjunctionJunction called the Junction called the measuring or “hot” junctionmeasuring or “hot” junctionLead ends attached to Lead ends attached to temp indicator or controllertemp indicator or controllerConnection point called Connection point called reference or “cold” junctionreference or “cold” junction
Display Device
+
-
A
B
ReferenceJunction
Measuring Junction
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How does it work?How does it work?
Measuring junction is heated, small DC voltage Measuring junction is heated, small DC voltage (millivolts) generated in thermocouple wires(millivolts) generated in thermocouple wires
Thermocouple Thermocouple converts thermal energy into converts thermal energy into electrical energyelectrical energy
Note: thermocouple only generates a millivoltage Note: thermocouple only generates a millivoltage signal when there is signal 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 32ooF(0F(0ooC)C)
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Thermocouple TypesThermocouple Types
Made up of two different metal Made up of two different metal alloy wires.alloy wires.
Different alloys result in Different alloys result in different temperature rangesdifferent temperature rangesEx: Ex: Standard Standard
TypeTypeMetal Metal
Content Content (Pos. Leg)(Pos. Leg)
Metal Metal 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)
1600-3100 1600-3100 ooFF
870-1700 870-1700 ooCC
EE 90% (Ni)90% (Ni)
10% (Cr)10% (Cr)
55% (Cu)55% (Cu)
45% (Ni)45% (Ni)
32-1650 32-1650 ooFF
870-1700 870-1700 ooCC
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Pros/ConsPros/ConsEach 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, S Types 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. tolerance Accuracy a.k.a. tolerance – Life ExpectancyLife Expectancy
Operating Temp.Operating Temp.Wire sizeWire sizeThermocouple protectionThermocouple protectionEnvironmentEnvironmentAccuracy requiredAccuracy required
TypeType Max. Max. Temp.Temp.
TolerancesTolerances
BB 31003100ooFF
17001700ooCC
(+/-) 0.5%(+/-) 0.5%
EE 16501650ooF F 900900ooCC
(+/-) 1.7(+/-) 1.7ooC (+/-) 3.06C (+/-) 3.06ooF F or (+/-) 0.5%or (+/-) 0.5%
Whichever greaterWhichever greater
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Life ExpectancyLife ExpectancyFailed = inaccuracyFailed = inaccuracy- When wires are heated/cooled changes take place on When 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
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Thermocouple ConstructionsThermocouple Constructions
3 General constructions3 General constructions– Insulated WireInsulated Wire– Ceramic-beadedCeramic-beaded– Metal-sheathedMetal-sheathed
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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 fiberPlastics (Teflon)Plastics (Teflon)Polyimides (Kapton)Polyimides (Kapton)
– PurposePurposeElectrically isolate wiresElectrically isolate wiresProtects wires from contamination Protects wires from contamination Easier wire installationEasier wire installation
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Metal - Sheathed ThermocouplesMetal - Sheathed Thermocouples
Junction and wires are assembled in small Junction and wires are assembled in small diameter metal tubesdiameter metal tubes– InsulationInsulation
FiberglassFiberglass
MgOMgO
– PurposePurposeProtects against contaminationProtects against contamination
Defends against chemical attackDefends against chemical attack
Provides mechanical stabilityProvides mechanical stability
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Metal - Sheathed ThermocouplesMetal - Sheathed ThermocouplesOrientation of thermocouple junction during assemblyOrientation of thermocouple junction during assembly
– GroundedGroundedWeld junction directly to inside tip of sheathWeld junction directly to inside tip of sheathEnsures rapid heat transfer from sheath to junctionEnsures rapid heat transfer from sheath to junctionProtects junction while minimizing heat transfer delays. Protects junction while minimizing heat transfer delays.
– UngroundedUngroundedSimilar to grounded except junction isolated from metal sheathSimilar to grounded except junction isolated from metal sheathElectrically isolates junction from sheathElectrically isolates junction from sheathPrevents stray voltages from inducing measuring errorPrevents stray voltages from inducing measuring errorMore shock resistant & better under rapid temperature changesMore shock resistant & better under rapid temperature changesDISADVANTAGE: Slows down heat transfer to junction (2x-3x DISADVANTAGE: Slows down heat transfer to junction (2x-3x slower)slower)
– ExposedExposedJunction protrudes from end of sheath, but insulated from itJunction protrudes from end of sheath, but insulated from itDue to direct exposure with heated material, very quick response to Due to direct exposure with heated material, very quick response to temp. changestemp. changesNo sheath to slow down heat transferNo sheath to slow down heat transferDISADVANTAGE: Not protected from mechanical damage & DISADVANTAGE: Not protected from mechanical damage & chemical attackchemical attack
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Resistance Temperature Devices Resistance Temperature Devices (RTD)(RTD)
Precision Temperature SensorsPrecision Temperature Sensors– More accurate than thermocouple elements More accurate than thermocouple elements – Maintain accuracy over longer period of timeMaintain accuracy over longer period of time– Range up to 1200Range up to 1200ooF (650F (650ooC)C)
StylesStyles– Wire-WoundWire-Wound– Thin filmThin film– Kapton InsulatedKapton Insulated
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How do RTDs work?How do RTDs work?
RTD’s resistance as temp. RTD’s resistance as temp. – Controller measures resistance value and converts to Controller 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 current Controller measures resistance by passing current
through RTDthrough RTD– Use a base resistance value (ex: for Platinum, value Use a base resistance value (ex: for Platinum, value
of 100 ohms at 0of 100 ohms at 0ooC (32C (32ooF)F)
Temperature (oC)
Resistance(Ohms)
RTD Resistance Vs. Temp. (TCR) Curve
TCR = Temperature coefficient of resistance
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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 slower Response Time slower, 2x-4x times slower
Heat must transfer through epoxy or glass coatingHeat must transfer through epoxy or glass coatingEntire RTD element must reach uniform temp. before Entire RTD element must reach uniform temp. before accurate measurement taken.accurate measurement taken.
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Lead Wire EffectLead Wire EffectAlters reading due to lead wire resistanceAlters reading due to lead wire resistance
Two approachesTwo approaches
– Determine lead wire resistance and have controller Determine lead wire resistance and have controller compensatecompensate
– Attach additional lead wire to one end of RTD Attach additional lead wire to one end of RTD
– Connect a transmitter, converts resistance to low amp Connect 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
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Effect of Lead Resistance: Platinum Effect 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 wire For 100 ohm wire +0.385 ohms/ +0.385 ohms/OOC @ 0C @ 0ooCCalpha = average slope from 0alpha = average slope from 0ooC – 100C – 100ooC C
– A 10 ohm lead impedance implies 10/3.85 = A 10 ohm lead impedance implies 10/3.85 = 2626ooC error in measurementC error in measurement
Lead
Lead
R=5
R=5
100 RTD
R=5
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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, respective Wires 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 bridge Non-linear relationship between resistance change and bridge output voltage changeoutput voltage change
– Additional equation required to convert bridge output voltage to Additional equation required to convert bridge output voltage to equivalent RTD impedanceequivalent RTD impedance
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3-Wire Bridge Calculations3-Wire Bridge Calculations
If VIf Vss & V & Vo o known, R known, Rgg can be found. can be found.
Unbalanced VUnbalanced Voo of bridge with R of bridge with R11=R=R22
)2/1(3
3s
gso V
RR
RVV
If RIf Rgg=R=R3 3 V Voo=0 & bridge is balanced=0 & bridge is balanced
To determine RTo determine Rgg assuming lead resistance is assuming lead resistance is
zerozero
os
osg VV
VVRR
2
23
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3-Wire Bridge Calculations3-Wire Bridge Calculations
os
oL
os
osg VV
VR
VV
VVRR
2
4
2
23
RL
RL
Rg+-
Vo
V3/2+
-
If RIf Rgg located some distance from 3-wire configuration located some distance from 3-wire configuration RRLL appears in series with R appears in series with Rgg & R & R33
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Another ApproachAnother Approach4-Wire Ohms4-Wire Ohms– DVM is directly proportional to RTD resistance DVM is directly proportional to RTD resistance 1 conversion 1 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.
CurrentSource
100 W RTDDVM
+
-
i = 0
i = 0
- +Vo
RTD=Rg
Vs
+-
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Resistance to Temperature Resistance to Temperature ConversionConversion
RTD more linear than thermocouple, curve-RTD more linear than thermocouple, curve-fitting still requiredfitting still requiredCallendar-Van Dusen EquationCallendar-Van Dusen Equation
3
1001
100)1001
100
TTTTTRR oT
RRTT = Resistance at Temperature T = Resistance at Temperature T
RRo o = Resistance at T=0= Resistance at T=0ooCC
= Temperature coefficient at T=0= Temperature coefficient at T=0ooC C
= 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<0
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IdentificationIdentification2-wire RTD uses same color lead wire for 2-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-lead Lead-to-lead MeasurementMeasurement
Distance at Room Distance at Room TemperatureTemperature
1 to 2; 3 to 41 to 2; 3 to 4 Less than 1ohm to a Less 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
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RTD AssemblyRTD Assembly
Wire WoundWire Wound– For 500For 500ooF (260F (260ooC), element welded to copper or nickel C), 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 to tubeEpoxy seal seals out moisture & locks RTD/leads to tube
Thin FilmThin Film– For 1200For 1200ooF (650F (650ooC), element fitted into cavity of MgO C), 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
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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 with Bead 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 (oC)
Non-linear decrease in resistanceas temperature increases.
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So Sensitive…So Sensitive…
Very large resistance change = small Very large resistance change = small temp. change temp. change
3 – 5% per 3 – 5% per ooC (vs. 0.4% per C (vs. 0.4% per ooC for RTDs)C for RTDs)
Temp. changes as small as 0.1Temp. changes as small as 0.1ooCC
Significantly smaller in sizeSignificantly smaller in size
Temp range: -100Temp range: -100ooC – 300C – 300ooC (-120C (-120ooF – F – 570570ooF)F)
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Thermistor StandardsThermistor Standards
No Industrial StandardsNo Industrial Standards
Base resistance range: 10Base resistance range: 1033 – 10 – 1066 ohms ohms– Typically measured at 25Typically measured at 25ooC vs. 0C vs. 0ooC for RTDsC for RTDs
TCRs vary widelyTCRs vary widely
Thermistor’s accuracy limited to small Thermistor’s accuracy limited to small temp. rangetemp. range
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Thermistor Lead Wire EffectsThermistor Lead Wire Effects
Lead wire does add overall resistance Lead wire does add overall resistance
NOTE: base resistance of thermistor very NOTE: base resistance of thermistor very large (>10large (>1033 ohms), added lead wire ohms), added lead wire resistance insignificant.resistance insignificant.
RESULT: No resistance compensation RESULT: No resistance compensation required!required!
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Infrared SensorsInfrared Sensors
Intercepts portion of infrared energy radiated by Intercepts portion of infrared energy radiated by object ( = 8 - 14 microns).object ( = 8 - 14 microns).
Waves focused through lens on infrared Waves focused through lens on infrared detector, converting to an electric output signal detector, converting to an electric output signal
Heat Source
Optics
Infrared Detector
Non-Contact Temp. SensorTemp. Indicator
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EmissivityEmissivity
Def: The ability of a material to radiate or absorb Def: 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 is Ex: 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
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Field of ViewField of View
All infrared radiation in this filed of view will be All 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
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Good vs. Bad RadiationGood vs. Bad RadiationPosition 1, IR sensor sees both target object & background objectsPosition 1, IR sensor sees both target object & background objectsPosition 2, IR sensor only sees target object. True target Position 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
CorrectTarget
Placement
IncorrectTarget
Placement
Background“Noise”
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Scenarios to AvoidScenarios to AvoidFigure 1: Thin film materials & background radiation Figure 1: Thin film materials & background radiation enter sensorenter sensorFigure 2: Polished metals will not function well with Figure 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
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Sensor to Target DistanceSensor to Target Distance
To reduce reflected radiant energy, set IR sensor at right To reduce reflected radiant energy, set IR sensor at right angle with respect to targetangle with respect to targetIf space limitation, mount IR up to a maximum of 45If space limitation, mount IR up to a maximum of 45OO
<45o
Product
Sensor
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Operating EnvironmentOperating Environment
Smoke, dust vapors absorb or reflect infrared Smoke, dust vapors absorb or reflect infrared radiation before getting to sensor lens.radiation before getting to sensor lens.Causes controller to maintain target at wrong Causes controller to maintain target at wrong temperaturetemperature
Infrared Sensor
Target
Smoke or Vapors
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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
RTDRTDMost stable over timeMost stable over time
Most accurateMost accurate
Most repeatable temp. measurementMost repeatable temp. measurement
Very resistant to contamination/corrosion of the Very resistant to contamination/corrosion of the RTD element RTD element
ThermistorThermistorHigh sensitivity to small temperature changesHigh sensitivity to small temperature changes
Temperature measurements become more Temperature 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 than Response times as fast or faster than thermocouplesthermocouples
No corrosion or oxidation to affect sensor No corrosion or oxidation to affect sensor accuracyaccuracy
High repeatabilityHigh repeatability
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So which one is better? So which one is better? DisadvantagesDisadvantages
ThermocoupleThermocouple
Least stable, least repeatableLeast stable, least repeatable
Low sensitivity to small temperature Low sensitivity to small temperature changeschanges
Extension wire must be of the same Extension wire must be of the same thermocouple typethermocouple type
Wire may pick up radiated electrical noise Wire 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 temperature Low sensitivity to small temperature changeschanges
Sensitive to vibration Sensitive to vibration
Decalibration if used beyond sensor’s Decalibration if used beyond sensor’s temperature ratingstemperature ratings
Somewhat fragileSomewhat fragile
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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’s Decalibration 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 electronics More complex – support electronics requiredrequired
Emissivity variations affect temperature Emissivity variations affect temperature measurement accuracymeasurement accuracy
Field of view and spot size may restrict Field 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.