©1997,2001 by M. Kostic

Ch.8: TemperatureCh.8: Temperature MeasurementMeasurement• Temperature:Temperature:

measure of hotness or coldnessindicating the direction in which heat (energy) will spontaneously flow, i.e., from a hotter body (one at a higher temperature) to a colder one (one at a lower temperature).

• Temperature is not the equivalent of the system Temperature is not the equivalent of the system energyenergy

• Historical backgroundHistorical background

©1997,2001 by M. Kostic

Temperature Measurement Temperature Measurement DevicesDevices• thermocouples,thermocouples,

• resistive temperature devices (RTDs and resistive temperature devices (RTDs and thermistors), thermistors),

• infrared radiators, infrared radiators,

• bimetallic devices, bimetallic devices,

• liquid expansion devices, and liquid expansion devices, and

• change-of-state devices. change-of-state devices.

©1997,2001 by M. Kostic

8.2: Temperature Standards8.2: Temperature Standards

• Temperature scale Temperature scale (ITS-90)• Definition of the degreeDefinition of the degree

Kelvin K, defined as the 1/273.16 fraction of the thermodynamic temperature of the

triple point of water (TPW).

• Fixed (reference) pointsFixed (reference) points

• Interpolation between the fixed pointsInterpolation between the fixed points

©1997,2001 by M. Kostic

8.3: Thermometry Based on 8.3: Thermometry Based on Thermal ExpansionThermal Expansion• Liquid-in-glass thermometersLiquid-in-glass thermometers

• Bimetallic thermometersBimetallic thermometers

©1997,2001 by M. Kostic

8.4:8.4: Electrical Resistance ThermometryElectrical Resistance Thermometry

• RTD:RTD:Resistance Resistance Temperature Temperature DetectorsDetectors

• RTD RTD measurement measurement circuits circuits (Bridge (Bridge circuits)circuits)

• ThermistorsThermistors

©1997,2001 by M. Kostic

8.5: 8.5: Thermoelectric Temperature Thermoelectric Temperature MeasurementsMeasurements• SeebeckSeebeck effect: in dissimilar metals open circuit effect: in dissimilar metals open circuit

emf proportional to emf proportional to T T

• PeltierPeltier (refrigeration) effect: (refrigeration) effect: T proportional T proportional to current I (reversible conversion of energy)to current I (reversible conversion of energy)

• ThomsonThomson (later Kelvin) effect: effect due to T (later Kelvin) effect: effect due to T gradients along a single conductor under current gradients along a single conductor under current II

For I=0 only Seebeck effect!For I=0 only Seebeck effect!

©1997,2001 by M. Kostic

Fundamental Thermocouple LawsFundamental Thermocouple Laws

• Law of homogeneous material Law of homogeneous material (no emf at (no emf at T)T)

• Law of intermediate material Law of intermediate material (no emf at (no emf at uniform T for dissimilar metals)uniform T for dissimilar metals)

• Law of successive or intermediate Law of successive or intermediate

temperature: temperature: emfemf1-31-3=emf=emf1-21-2+emf+emf2-32-3

©1997,2001 by M. Kostic

Measurement with ThermocouplesMeasurement with Thermocouples• TC measurement circuits with(out) reference TC measurement circuits with(out) reference

junctions junctions

©1997,2001 by M. Kostic

Thermocouple Standards (“+” vs. “-”)Thermocouple Standards (“+” vs. “-”)

E:E: Chromel vs. Chromel vs. ConstantanConstantan

J:J: Iron vs. Iron vs. ConstantanConstantan

K:K: Chromel vs. Chromel vs. AlumelAlumel

S:S: Platinum/10% Platinum/10% rhodium vs. rhodium vs. PlatinumPlatinum

T:T: Copper vs. Copper vs. ConstantanConstantan

©1997,2001 by M. Kostic

Thermocouple emf (Thermocouple emf (TablesTables))

Emf = E = Emf = E = ccii T Tii

i=0,1,2,…8 (or 14)i=0,1,2,…8 (or 14)

see Text TABLE 8.7 on p.313 see Text TABLE 8.7 on p.313

or:or:

Reference Functions for Thermocouple Types J and T: Table 8.7 (MathCAD)

©1997,2001 by M. Kostic

Multiple-Multiple-JunctionJunctionTC CircuitsTC Circuits

• Thermopiles Thermopiles (in series)(in series)

• Parallel Parallel arrangementsarrangements

©1997,2001 by M. Kostic

Multiple-Junction TC CircuitsMultiple-Junction TC CircuitsThermopiles (in series)Thermopiles (in series)

©1997,2001 by M. Kostic

For the given data (see the next slide), For the given data (see the next slide), determine the determine the required number of required number of thermopile junctions Nthermopile junctions N for the T-type for the T-type thermocouple thermocouple to yield an uncertainty in to yield an uncertainty in heat loss Q of ±5%,heat loss Q of ±5%, assuming the assuming the uncertainty in all other variables, but uncertainty in all other variables, but T, T, may be neglected.may be neglected.

Problem 8.29:Problem 8.29:

©1997,2001 by M. Kostic

8.3%5

5/95.0

/

/

/

/

;)(

Qu

Tu

Qu

QuN

T

u

Q

uu

T

Qu

QR

T

QR

Q

TQTQ Find N

Problem 8.29:Problem 8.29: Table 8.7 (MathCAD)

%5

04.0

5

25.0

4.0

15

..

2

Q

u

mVu

CT

mL

k

mA

L

TkAQ

CTtypeT

QR

emf

o

CmW

c

c

o

Given

CmVu

uTicuT

emfu

TcTfemf

o

CmVt

Ti

iTemf

levelavgCTi

i

o

o

95.0)04.0()042.0(

1

)()(

)(

1

.40@

Find ut

N=4

N

N

QR=QN