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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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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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