Temperature SensorsTemperature Sensors

ECE 371 JBECE 371 JB

Prof. BernhardProf. Bernhard

A Simple Thermal SystemA Simple Thermal System

Heat Source

TemperatureControlling

Device

Sensor

Work Load

Sensor Input

Output

TypesTypes

ThermocouplesThermocouples

Resistance temperature devices (RTD)Resistance temperature devices (RTD)

ThermistorsThermistors

Infrared sensorsInfrared sensors

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

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)

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

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

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

Thermocouple ConstructionsThermocouple Constructions

3 General constructions3 General constructions– Insulated WireInsulated Wire– Ceramic-beadedCeramic-beaded– Metal-sheathedMetal-sheathed

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

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

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

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

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

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.

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

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

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

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

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

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

+-

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

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

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

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.

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)

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

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!

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

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

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

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”

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

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

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

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

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

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.