3441 Industrial Instruments 1 Chapter 4 Thermal Sensors Dr. Bassam Kahhaleh Princess Sumaya Univ. Electronic Engineering Dept.

34413441Industrial Instruments 1Industrial Instruments 1

Chapter 4Chapter 4

Thermal SensorsThermal Sensors

Dr. Bassam KahhalehDr. Bassam Kahhaleh

Princess Sumaya Univ.Princess Sumaya Univ.Electronic Engineering Dept.Electronic Engineering Dept.

Princess Sumaya UniversityPrincess Sumaya University 3441 - Industrial Instruments 13441 - Industrial Instruments 1 22 / 29 / 29


ObjectiveObjective

Understand how thermal sensors work and Understand how thermal sensors work and how to interface them.how to interface them.



Metal ResistanceMetal Resistance

Energy bands for solids:Energy bands for solids:Energy gap: required energy for the electron to become freeEnergy gap: required energy for the electron to become free

Metals

W

Valence

Conduction

Semiconductors

W

Valence

Conduction

Insulators

W

Valence

Conduction



Metal ResistanceMetal Resistance

−100 0 100 200 300 400 500 600

Temperature (°C)

R(T

)R

(25°

C)

1

2

3

NickelPlatinum

A

lR

)25(

)(

)25(

)(

T

R

TR



Metal ResistanceMetal ResistanceLinear ApproximationLinear Approximation

Quadratic ApproximationQuadratic Approximation

]1[)()( 00 TTRTR

12

12

00 )(

1

TT

RR

TR

T1 T2T

R1

R2

])(1[)()( 2210 TTTRTR



Resistance-Temperature Detectors (RTD)Resistance-Temperature Detectors (RTD) SensitivitySensitivity

0: 0.004/°C ~ 0.005 /°C

Response TimeResponse Time0.5 ~ 5 seconds

ConstructionConstructionWire

Signal ConditioningSignal ConditioningBridge with lead compensation

Dissipation Constant (Self-heating)Dissipation Constant (Self-heating)DP

PT



RTDRTDExample

0= 0.005/°C

R = 500 Ω at 20°C

PD = 30 mW /°C

R1 = R2 = 500 Ω

VS = 10 V

RTD is at 0°C

Find R3

R 1 R 2

R 3

VSD

RTD

a b

c

450)]200(005.01[500)(TR

CT

WP

AI

8.1030.0

054.0

054.0450*)011.0(

011.0450500

10

2

5.454)]208.1(005.01[5003R



ThermistorsThermistors

−20 0 20 40 60 80 100

Temperature (°C)

Res

ista

nce

(K

Ω)

10

20

30



ThermistorsThermistors SensitivitySensitivity

~ 10% /°C


ConstructionConstructionDiscs, beads, rods … etc

Signal ConditioningSignal ConditioningDivider circuit, Bridge

Dissipation Constant (Self-heating)Dissipation Constant (Self-heating)



ThermistorsThermistorsExample

R = 3.5 KΩ at 20°C

S = - 10% /°C

PD = 5 mW /°C

VO = 5 at 20°C

Self-heating error?

KR 50.31

CT

mWK

P

42.15

1.7

1.75.3

)5( 2

KC

KCKRTH 3

5.3*%1042.15.3

R 1

R TH

10 V

V D



ThermocouplesThermocouples

T2 T1

Seebeck Effect

T2T1EMFHeat Flow

I

Peltier Effect

)( 12 TTE




TM

TR

TR

+

VTC

−




TypeType MaterialMaterial Normal RangeNormal Range

J Iron-constantan -190°C to 760°C

T Copper-constantan -200°C to 371°C

K Chromel-alumel -190°C to 1260°C

E Chromel-constantan -100°C to 1260°C

S 90%platinum, 10%rhodium-platinum 0°C to 1482°C

R 87%platimum, 13%rhodium-platinum 0°C to 1482°C




−200 0 200 400 600 800 1000 1200

Temperature (°C)

VT

C (

mV

)

10

30

50

Type E

Type J

Type R




−20 0 20 40 60 80 Temperature (°C)

VT

C (

mV

)

1

2

4 0 °C Ref.

3 20 °C Ref.

0

−1



ThermocouplesThermocouples SensitivitySensitivity

Type J: 0.05mV /°C

Type R: 0.006mV /°C


ConstructionConstructionWelded junction

Signal ConditioningSignal ConditioningHigh-gain differential amplifier, with high CMRR

Reference Compensation (cold junction comp.)Reference Compensation (cold junction comp.)


VC


ThermocouplesThermocouples Cold junction Compensation Cold junction Compensation

K +

TemperatureSensor

SignalConditioning

T

TrefVout

KVTC



Bimetal StripsBimetal Strips Thermal ExpansionThermal Expansion

Tll 10

γ1

γ2 < γ1

T0

T > T0



Gas ThermometersGas ThermometersConstant Volume:Constant Volume:

Liquid-Expansion ThermometersLiquid-Expansion Thermometers

2

2

1

1

T

p

T

p

TTVTV 1)()( 0



Solid-State Temperature SensorsSolid-State Temperature Sensors~ 12 mV / K~ 12 mV / K

PD = 5 mW / °C

At 293 K:

VT = 3.516 V

I = (5 – 3.516) / 510 = 0.0029 A

P = 3.516 * 0.0029 = 10.2 mW

ΔT = 10.2 / 5 = 2.04 °C

Increase R

510 Ω

T

5 V

V T



Solid-State Temperature SensorsSolid-State Temperature SensorsExample

J-type TC: ~ 50 μV / °C

SS sensor: 8 mV / °C

VO = 2 V @ 200 °C

VTC (200°C) = 10.78 mV

TC Gain = 8 mV / 50 μV

= 160 Total Gain = 2000 / 10.78

= 185.5



Solid-State Temperature SensorsSolid-State Temperature SensorsExample

V out

11.59 K

−

+

10 K

11.59 K

10 K

320 K

−

+

2 K

320 K

2 K

T

Tref

SS

−8 mV / °C


Thermal SensorsThermal SensorsExample

Turn-on alarm when T = 10 ± 0.5 °C

Turn-off alarm when T ≤ 8 °C

RTH = 10 KΩ @ 10 °C

= 11 KΩ @ 8 °C

Keep self-heating within ± 0.5 °C. Use ± 0.25 °C P = (5 mW / °C)*(0.25 °C) = 1.25 mW @ 10 °C, ITH = 0.354 mA, VTH = 3.5 V

Using voltage divider with 5 V supply: R = (5 – 3.5) / 0.354 = 4.28 KΩ @ 10 °C, VD = 1.5 V

@ 8 °C, VD = 1.41 V Hysteresis = 0.09 V



+

−

9 K

500 K

Vref = 1.5 V

Vout

R TH

4.28 K

5 V

V D

2.327 K

1 K

5 V



Measured T = 50 °C ~ 80 °C (error ≤ ± 1 °C)

VO = 0 ~ 2 V

RTD: R(65°C) = 150 Ω

(65°C) = 0.004 / °C

PD = 30 mW / °C

R(50°C) = 150 [ 1 + 0.004 (50 – 65) ] = 141 Ω R(80°C) = 150 [ 1 + 0.004 (80 – 65) ] = 159 Ω Keep self-heating within ± 1 °C P = (30 mW / °C)*(1 °C) = 30 mW @ 80 °C, IRTD = 13.7 mA, VRTD = 2.17 V



10 K

−

+

10 K

10 K

10 K

220 220

141 RTD

5 V

V out

10 K

+

−

138 K

@ 50 °C: RRTD = 141 Ω, ΔV = 0, Vout = 0

@ 80 °C: RRTD = 159 Ω, ΔV = 0.1447 V, Vout = 0.1447 * 13.8 = 2 V



Measured T = 500 ~ 600 °F (260 °C ~ 315.6 °C)

VO = 0 ~ 5 V

J-type TC: Tref = 25 °C

VTC(260 °C) = 12.84 mV

VTC(315.6 °C) = 15.9 mV

Vout = m VTC + V0

0 = m(0.01284) + V0

5 = m(0.01590) + V0

m = 1634, V0 = − 21, Use Gain = 100 * 16.34



V out

−

+

10 K

163.4 K

10 K

100 K

−

+

1 K

100 K

1 K

T

Tref = 25 °C

2.88 K

1 K

5 V

Vref = 1.289 V


End of Chapter 4End of Chapter 4


3441 Industrial Instruments 1 Chapter 4 Thermal Sensors Dr. Bassam Kahhaleh Princess Sumaya Univ. Electronic Engineering Dept.

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