Practical Temperature Measurements - Unicampelnatan/ee641/Temperatura.pdf · Temperature...
Transcript of Practical Temperature Measurements - Unicampelnatan/ee641/Temperatura.pdf · Temperature...
Practical
Hewlett-Packard Classroom Series
PracticalTemperature
Measurements
001
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A1
� Thermocouple�Summary & Examples
What is Temperature?
�A scalar quantity that determines the direction of heat flow between two bodies
Hewlett-Packard Classroom Series
direction of heat flow between two bodies�A statistical measurement�A difficult measurement�A mostly empirical measurement
002
How is heat transferred?
�Conduction� Metal coffee cup
Hewlett-Packard Classroom Series
�Convection
�Radiation
003
The Dewar
�Glass is a poor conductor
Hewlett-Packard Classroom Series
�Glass is a poor conductor�Gap reduces conduction�Metallization reflects radiation�Vacuum reduces convection
004
Thermal Mass
�Don't let the measuring
Hewlett-Packard Classroom Series
Sensor
�Don't let the measuring device change the temperature of what you're measuring.
�Response time = � f{Thermal mass}� f{Measuring device}
005
Sensor
� f{Measuring device}
Temperature errors
�What is YOUR normal temperature?�Thermometer accuracy, resolution�Contact time
Hewlett-Packard Classroom Series
�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
�1600 ad
�1700 ad12 96
Hewlett-Packard Classroom Series
ad ad
�Galileo: First temp. sensor � Early
12
10
96�Fahrenheit
� Instrument Maker
� 12*8=96 points
� Hg:
007
temp. sensor� pressure-
sensitive� not repeatable
� Early thermometers� Not
repeatable� No good way
to calibrate
� Hg: Repeatable
� One standard scale
The 1700's: Standardization
�1700 ad �1800 ad
0 100
Hewlett-Packard Classroom Series
�Celsius:�Common,
�Thomson effect� Absolute zero
0
100 0
100
008
�Common, repeatable calibration reference points
�"Centigrade" scale
1821: It was a very good year
�1800 ad �1900 ad
Hewlett-Packard Classroom Series
�The Seebeck effect
�Pt 100 @ O deg.C
�Davy: The RTD
�
009
�Pt 100 @ O deg.C�
The 1900's: Electronic sensors
�1900 ad �2000 ad
Hewlett-Packard Classroom Series
�Thermistor�1 uA/K
�IC sensor
�IPTS 1968 �IPTS 1990
010
�IPTS 1968
�"Degree Kelvin">> "kelvins"�"Centigrade">> " Celsius"
�IPTS 1990
Temperature scales
-273.15
Absolute zero
�Celsius 0 100
Freezing point H O2
Boiling point H O 2
Hewlett-Packard Classroom Series
-273.15
0
-459.67
0
Celsius
�Kelvin
�Fahrenheit
�Rankine
0
273.15
32
427.67
100
373.15
212
671.67
011
0 427.67 671.67
�"Standard" is "better": � Reliable reference
points� Easy to understand
IPTS '90: More calibration points
– 273.16: TP H2O
1234.93: FP – 1337.33: FP
Au– 234.3156: TP
Hg
– 1357.77: FP Cu
Hewlett-Packard Classroom Series
– 83.8058: TP Ar54.3584: TP
Large gap
– 1234.93: FP AgAu
– 692.677: FP Zn
Hg
– 505.078: FP
– 933.473: FP Al
012
Ar– 54.3584: TP
O2– 24.5561: TP Ne– 20.3: BP H2– 17 Liq/vapor H2 – 13.81 TP H2
– 429.7485: FP In
– 302.9146: MP Ga
– 505.078: FP Sn
– 3 to 5: Vapor He
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A2
� Thermocouple�Summary & Examples
Bimetal thermometer
�Two dissimilar metals, tightly
�Forces due to thermal �Result
Hewlett-Packard Classroom Series
metals, tightly bonded
thermal expansion
�Result
�Bimetallic thermometer� Poor accuracy� Hysteresis
�Thermal expansion causes big
0100 300
200
400
013
�Thermal expansion causes big problems in other designs:� IC bonds� Mechanical interference
Liquid thermometer; Paints100
Hewlett-Packard Classroom Series
0
�Liquid-filled thermometer� Accurate over a small range
�Thermally-sensitive paints� Irreversible change� Low resolution� Useful in hard-to-measure areas
014
� Accurate over a small range� Accuracy & resolution= f(length)� Range limited by liquid� Fragile� Large thermal mass� Slow
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A3
� Thermistor, IC� Thermocouple
�Summary & Examples
Optical Pyrometer
Hewlett-Packard Classroom Series
�Infrared Radiation-sensitive�Photodiode or photoresistor�Accuracy= f{emissivity}�Useful @ very high temperatures�Non-contacting
015
�Non-contacting�Very expensive�Not very accurate
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A4
� Thermistor, IC� Thermocouple
�Summary & Examples
Resistance Temperature Detector
Most accurate & stable
Hewlett-Packard Classroom Series
�Most accurate & stable�Good to 800 degrees Celsius�Resistance= f{Absolute T}�Self-heating a problem�Low resistance�Nonlinear
016
�Nonlinear
RTD Equation
�R= 100 Ohms @ O C�Callendar-Van Deusen Equation:
Hewlett-Packard Classroom Series
�R=Ro(1+aT) - Ro(ad(.01T)(.01T-1))� Ro=100 @ O C� a= 0.00385 / - C � d= 1.49
Callendar-Van Deusen Equation:
R
300
For T>OC:
for Pt
017
�0 200 400 600 800T
300200100
Nonlinearity
Measuring an RTD: 2 -wire method
100Rlead
V-
+I ref= 5 mAPt
�Rx
Rlead
Hewlett-Packard Classroom Series
��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!
-
018
� @ 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
The 4-Wire technique
100Rlead=1 V
-
+I ref= 5 mA�
�
Rx
Hewlett-Packard Classroom Series
� R= Iref * Rx� Error not a function of R in source or sense
leads� No error due to changes in lead R
-�
019
� Twice as much wire� Twice as many scanner channels� Usually slower than 2-wire
Offset compensation
100 V+
I ref (switched)
Voffset
�
Hewlett-Packard Classroom Series
�Eliminates thermal voltages� Measure V without I applied� Measure V I appliedWith
V-
I ref (switched)�
020
� Measure V I applied
R=V
I
With
Bridge method
100�1000�
Hewlett-Packard Classroom Series
V
�High resolution (DMM stays on most sensitive range)
�Nonlinear output�Bridge resistors too
100�1000�
021
�Bridge resistors too close to heat source
3-Wire bridge
V
1000 100
Rlead 1
3-Wire
Hewlett-Packard Classroom Series
��
V
100
1000
�Keeps bridge away from heat source�Break DMM lead (dashed line); connect to RTD through 3rd "sense" wire
Rlead 2Sense wire
3-Wire PRTD�
�
022
RTD through 3rd "sense" wire�If Rlead 1= Rlead 2, sense wire makes error small
�Series resistance of sense wire causes no error
Agenda
�Background, history
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A5
� Thermistor, IC� Thermocouple
�Summary & Examples
Electrical sensors: Thermistor
5k V-
+I= 0.1 mARlead=1� �
Rlead=1�
Hewlett-Packard Classroom Series
�Hi-Z; Sensitive: 5 k @ 25C; R = 4%/deg C
-
�2-Wire method: R= I * (Rthmr + 2*Rlead)
�
�
�Limited range
023
� Lead R Error= 2 /400= 0.005 degrees C�Low thermal mass: High self-heating�Very nonlinear
�
I.C. Sensor
I= 1 uA/K
Hewlett-Packard Classroom Series
�
+-
V
I= 1 uA/K
5V �100
960= 1mV/K
�High output�Very linear�Accurate @ room ambient
�Limited range�
024
�Limited range�Cheap
Summary: Absolute T devices
RTD Thermistor I.C.
Hewlett-Packard Classroom Series
�Expensive
RTD
�Most accurate �Most stable�Fairly linear
Thermistor
�High output�Fast�2-wire meas.
�Very nonlinear
I.C.
�High output�Most linear�Inexpensive
�Limited variety
025
�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
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
Thermocouple
A6
� Thermocouple�Summary & Examples
ThermocouplesThe Gradient Theory
TxTa
Hewlett-Packard Classroom Series
Tx
V
Tx
�The WIRE is the sensor, not the junction
�The Seebeck coefficient (e) is a
026
V= e(T) dT
Ta
coefficient (e) is a function of temperature
Making a thermocouple
Tx
Ta
B
Hewlett-Packard Classroom Series
�Two wires make a thermocouple
�Voltage output is nonzero if metals are
Tx Ta
TxV
TaA
B
027
nonzero if metals are not the sameV= e dT
Ta
+ e dT
Tx
A B
Gradient theory also says...
Tx
Ta
A
Hewlett-Packard Classroom Series
�If wires are the same type, or if there is one wire, and both ends are at the same temperature,
Tx Ta
TxV
TaA
A
028
same temperature, output= Zero.V= e dT
Ta
Tx
+ e dT = 0
Tx
A A
Now try to measure it:
�Theoretically,Vab= f{Tx-Tab}
Con
aTx
Fe
b
Hewlett-Packard Classroom Series
�But, try to measure it with a DMM:
Tx
Fe
V
Cu
=
Conb
Fe
Tx
Cu
V
�Result: 3 unequal junctions, all at unknown temperatures
029
ConCu=
Cu Con
Solution: Reference Thermocouple�Problems: a) 3 different thermocouples,
b) 3 unknown temperatures�Solutions: a) Add an opposing thermocouple
Hewlett-Packard Classroom Series
�Solutions: a) Add an opposing thermocoupleb) Use a known reference temp.
Cu
V Con
Fe
Tx
Isothermal block
Cu
V Con
Fe
Tx Add
030
V
Cu Fe
Tref= 0 C
Con
o
V
Cu Fe
Tref
The Classical Method
Cu Fe
Tx�If both Cu junctions are at
Hewlett-Packard Classroom Series
V
Cu Fe
Tref= 0 C
Con
Tx
o
�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
031
�All T/C tables are referenced to an ice bath
�V= f{Tx-Tref}
�Question: How can we eliminate the ice bath?
Eliminating the ice bath
Cu Fe�Don't force Tref to icepoint, just measure it
Hewlett-Packard Classroom Series
Tref
Cu
V
Cu Fe
Con
Fe
Tx
measure it�Compensate for Tref mathematically:V=f{ Tx - Tref }
�If we know Tref , we can
TiceTice
032
Cu Fe �If we know Tref , we can
compute Tx.Tice
Eliminating the second T/C
�Extend the isothermal blockCu Fe
Hewlett-Packard Classroom Series
�Extend the isothermal block�If isothermal, V1-V2=0
2V
Cu Fe
Con
Tx
1
Cu
V Con
Fe
Tx
2Tref
Tref
033
1V
Cu
Con2
1
Tref
The Algorithm for one T/C
�Measure Tref: RTD, IC or thermistor�Tref ==> Vref @ O C for Type
Cu Fe
Tx
Trefo
Hewlett-Packard Classroom Series
�Tref ==> Vref @ O C for Type J(Fe-C)
�Know V, Know Vref: Compute Vx�Solve for using Vx
TxV
Cu
ConTref
VxCompute
034
0 Tref
VxVref
Tx
ComputeVx=V+Vref V
o
LinearizationV
Small sectors
Hewlett-Packard Classroom Series
�Polynomial: T=a +a V +a V +a V +.... a V�Nested (faster): T=a +V(a +V(a +V(a
2
1 2 30 1 2 3
39
9
0
0 Tref Txo T
Small sectors
035
�Nested (faster): T=a +V(a +V(a +V(a +.......)))))))))
�Small sectors (faster): T=T +bV+cV �Lookup table: Fastest, most memory
1 2 32
00
Common ThermocouplesmV
60
EE
K
E
Platinum T/Cs
Hewlett-Packard Classroom Series
0 500 1000 2000 deg C
20
40
60
R
NKJ
ST
Platinum T/CsBase Metal T/Cs
036
0 500 1000 2000
�All have Seebeck coefficients in MICROvolts/deg.C
Common Thermocouples
SeebeckCoeff: uV/CType Metals
Hewlett-Packard Classroom Series
JKTSEN
Fe-ConNi-CrCu-ConPt/Rh-PtNi/Cr-ConNi/Cr/Si-Ni/Si
504038105939
�Microvolt output is a tough measurement
�Type "N" is fairly new.. more rugged and higher temp. than type K, but still cheap
037
N Ni/Cr/Si-Ni/Si 39 K, but still cheap
Extension Wires �Possible problemhere
Hewlett-Packard Classroom Series
Large extension wiresSmall diametermeasurementwires
�Extension wires are cheaper, more rugged,
038
�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
DMM
Normal Modeac NOISE
HI
Hewlett-Packard Classroom Series
DMMInputResistance
Normal Modedc SIGNAL
DMM
HI
HI
LO
�Normal Mode: In series with input�Common Mode: Both HI and LOterminals driven equally
039
DMMInputResistance
Common Modeac NOISE
LO
terminals driven equally
Generating noise
DMMHI
ElectrostaticNoise
MagneticNoise
Hewlett-Packard Classroom Series
Normal Mode
�Large surface area, high Rlead: Max. static coupling
�Large loop area: Max. magnetic coupling
DMMInputResistance dc SIGNAL
DMM
HI
HI
LO
�Large R lead, small R leak:
040
DMMInputResistance LO
Common Modeac source
R lead
R leak
Common Mode Current
�Large R lead, small R leak: Max.common mode noise
Eliminating noise
DMHI
ElectrostaticNoise
MagneticNoise
Hewlett-Packard Classroom Series
Normal Modedc SIGNAL
�Filter, shielding, small loop area(Caution: filter slows down the measurement)
�Make R leak close to
DMMInputR
DMM
HI
LO
041
�Make R leak close to MInput R L
OCommon Modeac source
R leak
Common Mode Current
- +
Magnetic Noise
�Magnetic coupling
DMMInduced I
Hewlett-Packard Classroom Series
DMMInputResistance
Induced I
042
�Minimize area�Twist leads�Move away from strong fields
Reducing Magnetic Noise
�Equal and opposite induced currents
DMM
Hewlett-Packard Classroom Series
DMMInputResistance
�Even with twisted pair:� Minimize area� Move away from strong fields
043
� Move away from strong fields
Electrostatic noiseAC Noisesource
Stray capacitances
Hewlett-Packard Classroom Series
DMMInputResistance
Stray resistances
Stray capacitances
Inoise
044
�Stray capacitance causes I noise�DMM resistance to ground is important
Reducing Electrostatic Coupling AC Noise source
Hewlett-Packard Classroom Series
DMMInputResistance
Shield shunts stray current
HI
LO
Rleak
045
�Shield shunts stray current�For noise coupled to the tip, Rleak is still important
Rleak
A scanning system for T/Cs
�One thermistor,
Hewlett-Packard Classroom Series
OHMsConv.
�One thermistor,multiple T/C channels
�Noise reduction�CPU linearizes T/C�DMM must be
very high quality
Conv.
046
HI
LO
Floating Circuitry Grounded Circuitry
Isolators
uP uPI/O
(HP-IB,RS-232) To
ComputerROMLookup
IntegratingA/D
Errors in the system
Thermal emf
Ref. Thermistor cal, linearity
T/C Calibration& Wire errors
Ref. Block Thermal gradient
Hewlett-Packard Classroom Series
OHMsConv.
Linearization algorithm
ReferenceThermistorOhmsmeasurement
Ref. Thermistor cal, linearity
Extension wirejunction error
Conv.
047
HI
LO
Floating Circuitry Grounded Circuitry
Isolators
uP uPI/O
(HP-IB,RS-232)
ROMLookup
IntegratingA/D
DMM offset, linearity, thermal emf, noise
Physical errors
�Shorts, shunt impedance �Sensor accuracy
Hewlett-Packard Classroom Series
Shorts, shunt impedance�Galvanic action�Decalibration
Sensor accuracy�Thermal contact�Thermal shunting
048
Physical Errors
Hewlett-Packard Classroom Series
�Water droplets cause galvanic action; huge offsets
�Hot spot causes shunt Z, meter shows the WRONG temperature
049
�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
Hewlett-Packard Classroom Series
Surface probe
�Make sure thermal mass is much smallerthan that of object being measured
050
than that of object being measured
Physical errors: Decalibration
300 C350 C
975 C
Hewlett-Packard Classroom Series
1000 C200 C300 C 975 C
100 C
This section Don't exceed Tmax of T/C
051
This section produces theENTIRE signal
�Don't exceed Tmax of T/C�Temp. cycling causes work-hardening,decalibration
�Replace the GRADIENT section
Agenda
�Background, history�Mechanical sensors
Hewlett-Packard Classroom Series
�Mechanical sensors�Electrical sensors
� Optical Pyrometer
� RTD� Thermistor, IC
A7
� Thermistor, IC� Thermocouple
�Summary & Examples
The basic 4 temperature sensors
ThermocoupleRTD Thermistor I.C.
Hewlett-Packard Classroom Series
Thermocouple
�Wide variety�Cheap�Wide T. range�No self-heating
�Hard to measure�Relative T. only
�Expensive�Slow�Needs I source
�Most accurate �Most stable�Fairly linear
�High output�Fast�2-wire meas.
�Very nonlinear�Limited range�Needs I source
�High output�Most linear�Cheap
�Limited variety�Limited range�Needs V
052
�Relative T. only�Nonlinear�Special
connectors
�Needs I source�Self-heating�4-wire meas.
�Needs I source�Self-heating�Fragile
�Needs V source
�Self-heating
Absolute temperature sensors
Summary
�Innovation by itself is not enough...
Hewlett-Packard Classroom Series
�Innovation by itself is not enough...you must develop standards
�Temperature is a very difficult, mostly empirical measurement
�Careful attention to detail is required
053
Examples
Measurement Sensor
Hewlett-Packard Classroom Series
�Photochemical process control:
�Flower petal:
�Molten glass:
�RTD (most accurate)
�Thermistor(lowest thermal mass)
�Optical pyrometer (hi temp, no contact)
�RTD (if <800C); or T/C
054
�Induction furnace:
�100 degree Heat aging oven:
�RTD (if <800C); or T/C(Beware magnetic I noise)
�Any of the 4 sensors