Practical temperature measurements
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Agilent Technologies Classroom Series. Practical Temperature Measurements. 001. Agilent Technologies Classroom Series. Agenda. Background, history Mechanical sensors Electrical sensors Optical Pyrometer RTD Thermistor, IC Thermocouple

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Practical temperature measurements

Agilent Technologies Classroom Series

Practical Temperature Measurements

001


Agenda

Agilent Technologies Classroom Series

Agenda

  • Background, history

  • Mechanical sensors

  • Electrical sensors

    • Optical Pyrometer

    • RTD

    • Thermistor, IC

    • Thermocouple

  • Summary & Examples

  • A1


    What is temperature

    Agilent Technologies Classroom Series

    What is Temperature?

    • A scalar quantity that determines the direction of heat flow between two bodies

    • A statistical measurement

    • A difficult measurement

    • A mostly empirical measurement

    002


    How is heat transferred

    Agilent Technologies Classroom Series

    How is heat transferred?

    • Conduction

    003

    • Metal coffee cup

    • Convection

    • Radiation


    The dewar

    Agilent Technologies Classroom Series

    The Dewar

    004

    • Glass is a poor conductor

    • Gap reduces conduction

    • Metallization reflects radiation

    • Vacuum reduces convection


    Thermal mass

    Sensor

    Sensor

    Agilent Technologies Classroom Series

    Thermal Mass

    005

    • Don't let the measuring device change the temperature of what you're measuring.

    • Response time =

      • f{Thermal mass}

      • f{Measuring device}


    Temperature errors

    Agilent Technologies Classroom Series

    Temperature errors

    • What is YOUR normal temperature?

    • Thermometer accuracy, resolution

    • Contact time

    • Thermal mass of thermometer, tongue

    • Human error in reading

    006

    97.6 98.6 99.6

    36.5 37 37.5


    History of temperature sensors

    96

    12

    1

    0

    Agilent Technologies Classroom Series

    History of temperature sensors

    • 1600 ad

    • 1700 ad

    007

    • Fahrenheit

      • Instrument Maker

      • 12*8=96 points

      • Hg: Repeatable

      • One standard scale

    • Galileo: First temp. sensor

      • pressure-sensitive

      • not repeatable

    • Early thermometers

      • Not repeatable

      • No good way to calibrate


    The 1700 s standardization

    0

    100

    100

    0

    Agilent Technologies Classroom Series

    The 1700's: Standardization

    • 1700 ad

    • 1800 ad

    008

    • Thomson effect

      • Absolute zero

    • Celsius:

    • Common, repeatable calibration reference points

    • "Centigrade" scale


    1821 it was a very good year

    d

    Agilent Technologies Classroom Series

    1821: It was a very good year

    • 1800 ad

    • 1900 ad

    009

    • The Seebeck effect

    • Davy: The RTD

    • Pt 100 @ O deg.C


    The 1900 s electronic sensors

    Agilent Technologies Classroom Series

    The 1900's: Electronic sensors

    • 1900 ad

    • 2000 ad

    010

    • 1 uA/K

    • Thermistor

    • IC sensor

    • IPTS 1990

    • IPTS 1968

    • "Degree Kelvin">> "kelvins"

    • "Centigrade">> " Celsius"


    Temperature scales

    Freezing point H O

    Boiling point H O

    2

    2

    Agilent Technologies Classroom Series

    Temperature scales

    Absolute zero

    011

    • Celsius

    100

    0

    -273.15

    • Kelvin

    0

    273.15

    373.15

    • Fahrenheit

    32

    212

    -459.67

    • Rankine

    671.67

    0

    427.67

    • "Standard" is "better":

      • Reliable reference points

      • Easy to understand


    Ipts 90 more calibration points

    Agilent Technologies Classroom Series

    IPTS '90: More calibration points

    • 1357.77: FP Cu

    • 273.16: TP H2O

    • 234.3156: TP Hg

    012

    • 1337.33: FP Au

    • 1234.93: FP Ag

    Large gap

    • 933.473: FP Al

    • 692.677: FP Zn

    • 83.8058: TP Ar

    • 505.078: FP Sn

    • 54.3584: TP O2

    • 429.7485: FP In

    • 24.5561: TP Ne

    • 20.3: BP H2

    • 17 Liq/vapor H2

    • 302.9146: MP Ga

    • 13.81 TP H2

    • 3 to 5: Vapor He


    Agenda1

    Agilent Technologies Classroom Series

    Agenda

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples

  • A2


    Bimetal thermometer

    200

    100

    300

    0

    400

    Agilent Technologies Classroom Series

    Bimetal thermometer

    • Forces due to thermal expansion

    • Two dissimilar metals, tightly bonded

    013

    • Result

    • Bimetallic thermometer

      • Poor accuracy

      • Hysteresis

    • Thermal expansion causes big problems in other designs:

      • IC bonds

      • Mechanical interference


    Liquid thermometer paints

    100

    0

    Agilent Technologies Classroom Series

    Liquid thermometer; Paints

    014

    • Thermally-sensitive paints

      • Irreversible change

      • Low resolution

      • Useful in hard-to-measure areas

    • Liquid-filled thermometer

      • Accurate over a small range

      • Accuracy & resolution= f(length)

      • Range limited by liquid

      • Fragile

      • Large thermal mass

      • Slow


    Agenda2

    Agilent Technologies Classroom Series

    Agenda

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples

  • A3


    Optical pyrometer

    Agilent Technologies Classroom Series

    Optical Pyrometer

    015

    • Infrared Radiation-sensitive

    • Photodiode or photoresistor

    • Accuracy= f{emissivity}

    • Useful @ very high temperatures

    • Non-contacting

    • Very expensive

    • Not very accurate


    Agenda3

    Agilent Technologies Classroom Series

    Agenda

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples

  • A4


    R esistance t emperature d etector

    Agilent Technologies Classroom Series

    Resistance Temperature Detector

    016

    • Most accurate & stable

    • Good to 800 degrees Celsius

    • Resistance= f{Absolute T}

    • Self-heating a problem

    • Low resistance

    • Nonlinear


    Rtd equation

    Agilent Technologies Classroom Series

    RTD Equation

    • R= 100 Ohms @ O C

    • Callendar-Van Deusen Equation:

    • R=Ro(1+aT) - Ro(ad(.01T)(.01T-1))

      • Ro=100 @ O C

      • a= 0.00385 / - C

      • d= 1.49

    For T>OC:

    for Pt

    R

    300

    200

    100

    Nonlinearity

    T

    • 0 200 400 600 800

    017


    Measuring an rtd 2 wire method

    d

    Agilent Technologies Classroom Series

    Measuring an RTD: 2-wire method

    Rlead

    Rx

    +

    100

    V

    I ref= 5 mA

    d

    Rlead

    Pt

    -

    • R= Iref*(Rx + 2* Rlead)

      • Error= 2 /.385= more than 5 degrees C for 1 ohm Rlead!

    • Self-heating:

      • For 0.5 V signal, I= 5mA; P=.5*.005=2.5 mwatts

      • @ 1 mW/deg C, Error = 2.5 deg C!

    • Moral: Minimize Iref; Use 4-wire method

      • If you must use 2-wire, NULL out the lead resistance

    018


    The 4 wire technique

    d

    d

    Agilent Technologies Classroom Series

    The 4-Wire technique

    Rx

    +

    100

    V

    019

    Rlead=1

    I ref= 5 mA

    -

    • R= Iref * Rx

      • Error not a function of R in source or sense leads

      • No error due to changes in lead R

      • Twice as much wire

      • Twice as many scanner channels

      • Usually slower than 2-wire


    Offset compensation

    d

    Agilent Technologies Classroom Series

    Offset compensation

    Voffset

    +

    020

    100

    V

    I ref (switched)

    -

    • Eliminates thermal voltages

      • Measure V without I applied

      • Measure V I applied

    With

    V

    R=

    I


    Bridge method

    d

    100

    1000

    d

    V

    d

    1000

    d

    Agilent Technologies Classroom Series

    Bridge method

    021

    100

    • High resolution (DMM stays on most sensitive range)

    • Nonlinear output

    • Bridge resistors too close to heat source


    3 wire bridge

    d

    d

    V

    d

    d

    Agilent Technologies Classroom Series

    3-Wire bridge

    100

    1000

    Rlead 1

    022

    3-Wire PRTD

    Sense wire

    1000

    Rlead 2

    100

    • Keeps bridge away from heat source

    • Break DMM lead (dashed line); connect to RTD through 3rd "sense" wire

    • If Rlead 1= Rlead 2, sense wire makes error small

    • Series resistance of sense wire causes no error


    Agenda4

    Agilent Technologies Classroom Series

    Agenda

    A5

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples


  • Electrical sensors thermistor

    d

    d

    d

    d

    d

    Agilent Technologies Classroom Series

    Electrical sensors: Thermistor

    Rlead=1

    +

    V

    5k

    I= 0.1 mA

    • Hi-Z; Sensitive: 5 k @ 25C; R = 4%/deg C

    Rlead=1

    -

    • Limited range

    • 2-Wire method: R= I * (Rthmr + 2*Rlead)

      • Lead R Error= 2 /400= 0.005 degrees C

    • Low thermal mass: High self-heating

    • Very nonlinear

    023


    I c sensor

    d

    V

    Agilent Technologies Classroom Series

    I.C. Sensor

    d

    AD590

    I= 1 uA/K

    • High output

    • Very linear

    • Accurate @ room ambient

    • Limited range

    • Cheap

    +

    100

    -

    5V

    = 1mV/K

    960

    024


    Summary absolute t devices

    I.C.

    AD590

    RTD

    Thermistor

    • High output

    • Fast

    • 2-wire meas.

    • High output

    • Most linear

    • Inexpensive

    • Most accurate

    • Most stable

    • Fairly linear

    • Very nonlinear

    • Limited range

    • Needs I source

    • Self-heating

    • Fragile

    • Expensive

    • Slow

    • Needs I source

    • Self-heating

    • 4-wire meas.

    • Limited variety

    • Limited range

    • Needs V source

    • Self-heating

    Agilent Technologies Classroom Series

    Summary: Absolute T devices

    025


    Agenda5

    Agilent Technologies Classroom Series

    Agenda

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples

  • A6


    Thermocouples the gradient theory

    Ta

    Tx

    V

    Tx

    V= e(T) dT

    Ta

    Agilent Technologies Classroom Series

    Thermocouples The Gradient Theory

    026

    • The WIRE is the sensor, not the junction

    • The Seebeck coefficient (e) is a function of temperature


    Making a thermocouple

    Ta

    Tx

    B

    V

    A

    Ta

    Tx

    V= e dT

    Ta

    Agilent Technologies Classroom Series

    Making a thermocouple

    027

    • Two wires make a thermocouple

    • Voltage output is nonzero if metals are not the same

    Ta

    + e dT

    B

    A

    Tx


    Gradient theory also says

    Ta

    Tx

    A

    V

    A

    Ta

    Tx

    V= e dT

    Ta

    Agilent Technologies Classroom Series

    Gradient theory also says...

    028

    • If wires are the same type, or if there is one wire, and both ends are at the same temperature, output= Zero.

    Ta

    + e dT = 0

    A

    A

    Tx


    Now try to measure it

    a

    Fe

    Tx

    b

    Con

    Fe

    Cu

    Cu

    Fe

    =

    Tx

    Tx

    V

    V

    Con

    Cu

    Con

    Cu

    Agilent Technologies Classroom Series

    Now try to measure it:

    • Theoretically, Vab= f{Tx-Tab}

    • Result: 3 unequal junctions, all at unknown temperatures

    • But, try to measure it with a DMM:

    029


    Solution reference thermocouple

    Fe

    Cu

    Cu

    Fe

    Tx

    Add

    Tx

    Con

    V

    Con

    V

    Tref

    = 0 C

    Tref

    o

    Cu

    Fe

    Cu

    Fe

    Agilent Technologies Classroom Series

    Solution: Reference Thermocouple

    • Problems: a) 3 different thermocouples, b) 3 unknown temperatures

    030

    • Solutions: a) Add an opposing thermocouple b) Use a known reference temp.

    Isothermal block


    The classical method

    Agilent Technologies Classroom Series

    The Classical Method

    Cu

    Fe

    031

    • If both Cu junctions are at same T, the two "batteries" cancel

    • Tref is an ice bath (sometimes an electronic ice bath)

    • All T/C tables are referenced to an ice bath

    • V= f{Tx-Tref}

    Tx

    Con

    V

    Tref

    = 0 C

    o

    Cu

    Fe

    • Question: How can we eliminate the ice bath?


    Eliminating the ice bath

    Fe

    Cu

    Tx

    Con

    V

    Cu

    Fe

    Agilent Technologies Classroom Series

    Eliminating the ice bath

    • Don't force Tref to icepoint, just measure it

    • Compensate for Tref mathematically:V=f{ Tx - Tref }

    • If we know Tref , we can compute Tx.

    032

    Tice

    Tref

    Tice

    Tice


    Eliminating the second t c

    Fe

    Cu

    Tx

    Con

    V

    Cu

    Fe

    Agilent Technologies Classroom Series

    Eliminating the second T/C

    033

    • Extend the isothermal block

    • If isothermal, V1-V2=0

    2

    Fe

    Cu

    Tref

    Tx

    V

    Tref

    Con

    1

    2

    Cu

    1


    The algorithm for one t c

    Fe

    Cu

    Tx

    Tref

    V

    Con

    Cu

    Agilent Technologies Classroom Series

    The Algorithm for one T/C

    • Measure Tref: RTD, IC or thermistor

    • Tref ==> Vref @ O C for Type J(Fe-C)

    • Know V, Know Vref: Compute Vx

    • Solve for using Vx

    o

    Tx

    Compute

    Vx=V+Vref

    Vx

    V

    Vref

    Tx

    Tref

    0

    o

    034


    Linearization

    Agilent Technologies Classroom Series

    Linearization

    V

    035

    Small sectors

    T

    Tx

    Tref

    0

    o

    2

    9

    3

    • Polynomial: T=a +a V +a V +a V +.... a V

    • Nested (faster): T=a +V(a +V(a +V(a +.......)))))))))

    • Small sectors (faster): T=T +bV+cV

    • Lookup table: Fastest, most memory

    0

    1

    2

    3

    9

    0

    1

    2

    3

    2

    0


    Common thermocouples

    mV

    E

    E

    E

    60

    K

    J

    N

    40

    20

    R

    T

    S

    deg C

    0 500 1000 2000

    Agilent Technologies Classroom Series

    Common Thermocouples

    036

    Platinum T/Cs

    Base Metal T/Cs

    • All have Seebeck coefficients in MICROvolts/deg.C


    Common thermocouples1

    Agilent Technologies Classroom Series

    Common Thermocouples

    Seebeck

    Coeff: uV/C

    037

    Type

    Metals

    • Microvolt output is a tough measurement

    • Type "N" is fairly new.. more rugged and higher temp. than type K, but still cheap

    Fe-Con

    Ni-Cr

    Cu-Con

    Pt/Rh-Pt

    Ni/Cr-Con

    Ni/Cr/Si-Ni/Si

    50

    40

    38

    10

    59

    39

    J

    K

    T

    S

    E

    N


    Extension wires

    Agilent Technologies Classroom Series

    Extension Wires

    • Possible problemhere

    038

    Large extension wires

    Small diametermeasurementwires

    • Extension wires are cheaper, more rugged, but not exactly the same characteristic curve as the T/C.

    • Keep extension/TC junction near room temperature

    • Where is most of the signal generated in this circuit?


    Noise dmm glossary

    Normal Mode

    ac NOISE

    HI

    DMM

    Normal Mode

    Input

    LO

    Resistance

    dc SIGNAL

    HI

    DMM

    Input

    Resistance

    LO

    Common Mode

    ac NOISE

    Agilent Technologies Classroom Series

    Noise: DMM Glossary

    039

    • Normal Mode: In series with input

    • Common Mode: Both HI and LOterminals driven equally


    Generating noise

    Electrostatic

    Noise

    Magnetic

    Noise

    HI

    DMM

    Input

    LO

    Resistance

    dc SIGNAL

    HI

    DMM

    Input

    R lead

    Resistance

    LO

    R leak

    Common Mode ac source

    Common Mode Current

    Agilent Technologies Classroom Series

    Generating noise

    040

    Normal Mode

    • Large surface area, high Rlead: Max. static coupling

    • Large loop area: Max. magnetic coupling

    • Large R lead, small R leak: Max.common mode noise


    Eliminating noise

    Electrostatic

    Noise

    Magnetic

    Noise

    HI

    DMM

    Input

    LO

    R

    HI

    DMM

    Input R

    - +

    LO

    R leak

    Common Mode ac source

    Common Mode Current

    Agilent Technologies Classroom Series

    Eliminating noise

    041

    Normal Mode

    dc SIGNAL

    • Filter, shielding, small loop area(Caution: filter slows down the measurement)

    • Make R leak close to


    Magnetic noise

    Agilent Technologies Classroom Series

    Magnetic Noise

    • Magnetic coupling

    042

    DMM

    InputResistance

    Induced I

    • Minimize area

    • Twist leads

    • Move away from strong fields


    Reducing magnetic noise

    Agilent Technologies Classroom Series

    Reducing Magnetic Noise

    • Equal and opposite induced currents

    043

    DMM

    InputResistance

    • Even with twisted pair:

      • Minimize area

      • Move away from strong fields


    Electrostatic noise

    Agilent Technologies Classroom Series

    Electrostatic noise

    AC Noise

    source

    044

    Stray capacitances

    DMM

    InputResistance

    Inoise

    Stray resistances

    • Stray capacitance causes I noise

    • DMM resistance to ground is important


    Reducing electrostatic coupling

    AC Noise source

    HI

    DMM

    InputResistance

    LO

    Rleak

    • Shield shunts stray current

    • For noise coupled to the tip, Rleak is still important

    Agilent Technologies Classroom Series

    Reducing Electrostatic Coupling

    045


    A scanning system for t cs

    I/O

    (HP-IB,

    RS-232)

    Agilent Technologies Classroom Series

    A scanning system for T/Cs

    OHMs

    Conv.

    • One thermistor, multiple T/C channels

    • Noise reduction

    • CPU linearizes T/C

    • DMM must be very high quality

    Isolators

    HI

    uP

    uP

    ToComputer

    ROM

    Lookup

    LO

    Integrating

    A/D

    Floating Circuitry

    Grounded Circuitry

    046


    Errors in the system

    I/O

    (HP-IB,

    RS-232)

    Agilent Technologies Classroom Series

    Errors in the system

    Ref. Block Thermal gradient

    T/C Calibration

    & Wire errors

    Thermal emf

    OHMs

    Conv.

    Ref. Thermistor cal, linearity

    Reference

    Thermistor

    Ohms

    measurement

    Extension wire

    junction error

    Linearization

    algorithm

    Isolators

    HI

    uP

    uP

    ROM

    Lookup

    LO

    Integrating

    A/D

    Floating Circuitry

    Grounded Circuitry

    047

    DMM offset, linearity, thermal emf, noise


    Physical errors

    Agilent Technologies Classroom Series

    Physical errors

    048

    • Shorts, shunt impedance

    • Galvanic action

    • Decalibration

    • Sensor accuracy

    • Thermal contact

    • Thermal shunting


    Physical errors1

    Agilent Technologies Classroom Series

    Physical Errors

    049

    • Hot spot causes shunt Z, meter shows the WRONG temperature

    • Water droplets cause galvanic action; huge offsets

    • Exceeding the T/C's range can cause permanent offset

    • Real T/C's have absolute accuracy of 1 deg C @ 25C: Calibrate often and take care


    Physical error thermal contact

    Agilent Technologies Classroom Series

    Physical error: Thermal contact

    050

    Surface probe

    • Make sure thermal mass is much smallerthan that of object being measured


    Physical errors decalibration

    Agilent Technologies Classroom Series

    Physical errors: Decalibration

    350 C

    051

    300 C

    975 C

    200 C

    1000 C

    100 C

    This section produces theENTIRE signal

    • Don't exceed Tmax of T/C

    • Temp. cycling causes work-hardening,decalibration

    • Replace the GRADIENT section


    Agenda6

    Agilent Technologies Classroom Series

    Agenda

    • Background, history

    • Mechanical sensors

    • Electrical sensors

      • Optical Pyrometer

      • RTD

      • Thermistor, IC

      • Thermocouple

  • Summary & Examples

  • A7


    The basic 4 temperature sensors

    RTD

    I.C.

    AD590

    Thermistor

    • High output

    • Fast

    • 2-wire meas.

    • Most accurate

    • Most stable

    • Fairly linear

    • High output

    • Most linear

    • Cheap

    • Expensive

    • Slow

    • Needs I source

    • Self-heating

    • 4-wire meas.

    • Very nonlinear

    • Limited range

    • Needs I source

    • Self-heating

    • Fragile

    • Limited variety

    • Limited range

    • Needs V source

    • Self-heating

    Absolute temperature sensors

    Agilent Technologies Classroom Series

    The basic 4 temperature sensors

    052

    Thermocouple

    • Wide variety

    • Cheap

    • Wide T. range

    • No self-heating

    • Hard to measure

    • Relative T. only

    • Nonlinear

    • Special connectors


    Summary

    Agilent Technologies Classroom Series

    Summary

    053

    • Innovation by itself is not enough...you must develop standards

    • Temperature is a very difficult, mostly empirical measurement

    • Careful attention to detail is required


    Examples

    Measurement

    Sensor

    • Photochemical process control:

    • Flower petal:

    • Molten glass:

    • Induction furnace:

    • 100 degree Heat aging oven:

    • RTD (most accurate)

    • Thermistor (lowest thermal mass)

    • Optical pyrometer (hi temp, no contact)

    • RTD (if <800C); or T/C (Beware magnetic I noise)

    • Any of the 4 sensors

    Agilent Technologies Classroom Series

    Examples

    054


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