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Agilent Technologies Classroom Series
Practical
Temperature
Measurements
001
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A1
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?
Conduction
Metal coffee cup
Convection
Radiation
003
The Dewar
Glass is a poor conductor
Gap reduces conduction
Metallization reflects radiation
Vacuum reduces convection
004
Thermal Mass
Don't let the measuring
Sensor
device change the
temperature of what you're
measuring.
Response time =
f{Thermal mass}
f{Measuring device}
Sensor
005
Temperature errors
What is YOUR normal temperature?
Thermometer accuracy, resolution
Contact time
Thermal mass of thermometer, tongue
Human error in reading
97.6 98.6 99.6
36.5 37 37.5
006
History of temperature sensors
1600
ad
1700
12
ad
96
Fahrenheit
1
Instrument
0
Galileo: First
temp. sensor
pressure-
sensitive
not repeatable
Early
thermometers
Not
repeatable
No good way
to calibrate
Maker
12*8=96
points
Hg:
Repeatable
One
standard
scale
007
The 1700's: Standardization
1700 ad
1800 ad
0
100
Thomson effect
Absolute zero
100
Celsius:
Common,
repeatable
calibration
reference points
0
"Centigrade"
scale
008
1821: It was a very good year
1800 ad
1900 ad
The Seebeck
effect
Davy: The RTD
Pt 100
d @ O deg.C
009
The 1900's: Electronic sensors
1900 ad
Thermistor
2000 ad
1 uA/K
IC sensor
IPTS 1968 IPTS
1990
"Degree Kelvin">> "kelvins"
"Centigrade">> " Celsius"
010
Temperature scales
Freezing
point H2O
Absolute
zero
Celsius
0
100
Kelvin
273.15
373.15
32
212
-273.15
0
Fahrenheit
-459.67
0
Boiling point
H O2
Rankine
427.67
671.67
"Standard" is "better":
Reliable reference
points
Easy to understand
011
IPTS '90: More calibration points
–
–
273.16: TP
H2O
234.3156: TP
Hg
Large gap
–
–
–
–
–
–
–
–
–
–
–
–
83.8058: TP
Ar
54.3584: TP
O2
24.5561: TP
20.3:
BP
Ne
17 Liq/vapor
H2
13.81
TP
H2
3 to 5: Vapor
H2
He
–
–
1357.77: FP
Cu
1337.33: FP
Au
1234.93: FP
Ag
933.473: FP
Al
692.677: FP
Zn
505.078: FP
Sn
429.7485: FP
In
–
302.9146: MP
Ga
012
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A2
Bimetal thermometer
Two dissimilar Forces due to
thermal
expansion
metals, tightly
bonded
0
Result
100 200 300 Bimetallic thermometer
400
Poor accuracy
Hysteresis
Thermal expansion causes big
problems in other designs:
IC bonds
Mechanical interference
013
Liquid thermometer; Paints
100
0
Thermally-sensitive paints
Liquid-filled thermometer
Irreversible change
Low resolution
Useful in hard-to-measure area
Accurate over a small range
Accuracy & resolution= f(length)
Range limited by liquid
Fragile
Large thermal mass
Slow
014
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A3
Optical Pyrometer
Infrared Radiation-sensitive
Photodiode or photoresistor
Accuracy= f{emissivity}
Useful @ very high temperatures
Non-contacting
Very expensive
Not very accurate
015
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A4
Resistance Temperature Detector
Most accurate & stable
Good to 800 degrees Celsius
Resistance= f{Absolute T}
Self-heating a problem
Low resistance
Nonlinear
016
RTD Equation
R= 100 Ohms @ O C
Callendar-Van Deusen Equation:
For T>OC:
R=Ro(1+aT) - Ro(ad(.01T)(.01T-1))
R
Ro=100 @ O C
a= 0.00385
/ - C for Pt
d= 1.49
300
200
100
0
Nonlinearity
200
400
600
800
T
017
Measuring an RTD: 2-wire method
Rx
100d
Pt
Rlead
+
Rlead
-
V
I ref= 5 mA
R= Iref*(Rx + 2* Rlead)
Error= 2d /.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
Rx
100d
+
Rlead=1d
-
V
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
019
Offset compensation
Voffset
+
100d
-
V
I ref (switched)
Eliminates thermal voltages
Measure V without I applied
Measure V With I applied
R=
V
I
020
Bridge method
100 d
d
1000
V
d
1000
100d
High resolution (DMM
stays on most sensitive
range)
Nonlinear output
Bridge resistors too
close to heat source
021
3-Wire bridge
100d
1000d
Rlead 1
V
1000d
Sense wire
3-Wire
PRTD
Rlead 2
Keeps bridge away from heat source
Break DMM lead (dashed line); connect to
100d
RTD through 3rd "sense" wire
If Rlead 1= Rlead 2, sense wire makes
error small
Series resistance of sense wire causes no
error
022
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A5
Electrical sensors: Thermistor
Rlead=1
5k d
d
Rlead=1d
+
-
Hi-Z; Sensitive: 5 k
V
d
I= 0.1 mA
@ 25C;
R = 4%/deg C
Limited range
2-Wire method: R= I * (Rthmr + 2*Rlead)
Lead R Error= 2
d /400= 0.005 degrees C
Low thermal mass: High self-heating
Very nonlinear
023
I.C. Sensor
I= 1 uA/K
+
5V -
100 d
V = 1mV/K
960d
High output
Very linear
Accurate @
room ambient
Limited range
Cheap
024
Summary: Absolute T devices
RTD
Thermistor
I.C.
Most accurate
Most stable
Fairly linear
High output
Fast
2-wire meas.
High output
Most linear
Inexpensive
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
025
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A6
Thermocouples
The Gradient Theory
Ta
Tx
The WIRE is the
V=
V
sensor, not the
junction
Tx
The Seebeck
e(T) dT
Ta
coefficient (e) is a
function of
temperature
026
Making a thermocouple
Ta
Tx
B
V
Two wires make a
A
Ta
thermocouple
V=
e
Ta
A
Voltage output is
Ta
Tx
dT +
e
B
dT
nonzero if metals are
not the same
Tx
027
Gradient theory also says...
Ta
Tx
A
V
If wires are the
A
Ta
Ta
Tx
e dT +
A
V=
Ta
e
A
dT = 0
same type, or if there
is one wire, and
both ends are at the
same temperature,
output= Zero.
Tx
028
Now try to measure it:
a
Fe
b
Tx Theoretically,
Vab= f{Tx-Tab}
Con
But, try to measure it with a DMM:
Cu
Tx
V
Cu
Cu
Fe
Con
=
Fe
V
Tx
Con
Cu
Result: 3 unequal junctions, all at unknown
temperatures
029
Solution: Reference Thermocouple
Problems: a) 3 different thermocouples,
b) 3 unknown temperatures
Solutions: a) Add an opposing thermocouple
b) Use a known reference temp. Isothermal
block
Fe
Cu
Cu Fe
Tx
Add
Tx
Con
V
V Con
Tref
Tref
= 0 oC
Cu
Fe
Cu Fe
030
The Classical Method
Cu Fe
Tx
V
Con
Tref
o
=0C
Cu Fe
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}
Question: How can we eliminate the ice bath?
031
Eliminating the ice bath
Don't force Tref to icepoint, just
Cu
V
Fe
Con
Cu
Fe
measure it
Compensate for Tref
Tx
mathematically:
V=f{ Tx
- Tref
}
Tice
Tref
Tice
If we know Tref
, we can
Tice
compute Tx.
032
Eliminating the second T/C
Cu
Fe
Tx
V
Extend the isothermal block
If isothermal, V1-V2=0
2
Con
Cu
Fe
Tref
Cu
Fe
1
Tx
V
Tref
2
Con
Cu
1
033
The Algorithm for one T/C
Cu
V
Tref
Cu
Fe
IC or thermistor
Tx Measure Tref: RTD,
o
Tref ==> Vref @ O C for Type J(FeCon C)
Know V, Know Vref: Compute Vx
Solve for Tx using Vx
Compute
Vx=V+Vref
Vx
Vref
0 o Tref
V
Tx
034
V
Linearization
Small sectors
0 o Tref
Tx
T
2
3 V +.... a9 V
Polynomial: T=a
+a
V
+a
V
+a
0 1
2
3
9
Nested (faster): T=a
0 +V(a
1 +V(a
2 +V(a
3
2
+.......)))))))))
0
Small sectors (faster): T=T +bV+cV
Lookup table: Fastest, most memory
035
Common Thermocouples
mV
E
60
Platinum T/Cs
Base Metal T/Cs
K
J N
40
20
T
0
500
RS
1000
2000
deg C
All have Seebeck coefficients in MICROvolts/deg.C
036
Common Thermocouples
Type
J
K
T
S
E
N
Metals
Seebeck
Coeff: uV/C
Fe-Con
Ni-Cr
Cu-Con
Pt/Rh-Pt
Ni/Cr-Con
Ni/Cr/Si-Ni/Si
50
40
38
10
59
39
Microvolt output is a
tough measurement
Type "N" is fairly new..
more rugged and
higher temp. than type
K, but still cheap
037
Extension Wires
Possible problem
here
Large extension wires
Small diameter
measurement
wires
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?
038
Noise: DMM Glossary
Normal Mode
ac NOISE
DMM
Input
Resistance
HI
LO
Normal Mode
dc SIGNAL
Normal Mode: In series with input
Common Mode: Both HI and LO
HI
DMM
Input
Resistance
terminals driven equally
LO
Common Mode
ac NOISE
039
Generating noise
Electrostatic
Noise
DMM
Input
Resistance
Magnetic
Noise
HI
Normal Mode
LO
dc SIGNAL
Large surface area, high Rlead: Max. static
DMM
Input
Resistance
R leak
coupling
HILarge loop area: Max. magnetic coupling
Large R lead, small R leak:
R lead
Max.
LO
common mode noise
Common Mode Current
Common Mode
ac source
040
Eliminating noise
Electrostati
c
Noise
DM
M
Inpu
tR
Magneti
c
Noise
HI
Normal Mode
dc SIGNAL
L
O
Filter, shielding, small loop area
(Caution: filter slows down the
measurement)
HI
DM
M
Input
R
R
leak
Make R leak close to
L
O
-
+
Common Mode
Current
Common
Mode
ac source
041
Magnetic Noise
Magnetic coupling
DMM
Input
Resistance
Induced I
Minimize area
Twist leads
Move away from strong fields
042
Reducing Magnetic Noise
Equal and opposite induced currents
DMM
Input
Resistance
Even with twisted pair:
Minimize area
Move away from strong fields
043
Electrostatic noise
AC Noise
source
Stray capacitances
DMM
Input
Resistance
Inoise
Stray resistances
Stray capacitance causes I noise
DMM resistance to ground is important
044
Reducing Electrostatic
AC Noise source
Coupling
DMM
Input
Resistance
HI
LO
Rleak
Shield shunts stray current
For noise coupled to the tip,
Rleak is still important
045
A scanning system for T/Cs
One thermistor,
multiple T/C channels
Noise reduction
CPU linearizes T/C
DMM must be
very high quality
OHMs
Conv.
Isolators
HI
uP
LO
Integrating
A/D
Floating Circuitry
ROM
Lookup
uP
I/O
(HP-IB,
RS-232)
To
Computer
Grounded Circuitry
046
Errors in the system
Ref. Block Thermal gradient
T/C Calibration
& Wire errors
Thermal emf
Ref. Thermistor cal, linearity
OHMs
Conv.
Reference
Thermistor
Ohms
measurement
Linearization
algorithm
Isolators
HI
uP
LO
Integrating
A/D
Floating Circuitry
ROM
Lookup
DMM offset, linearity, thermal emf, noise
uP
Extension wire
junction error
I/O
(HP-IB,
RS-232)
Grounded Circuitry
047
Physical errors
Shorts, shunt impedance
Galvanic action
Decalibration
Sensor accuracy
Thermal contact
Thermal shunting
048
Physical Errors
Water droplets
cause galvanic
action; huge offsets
Hot spot causes shunt Z, mete
shows the WRONG temperatur
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
049
Physical error: Thermal contact
Surface probe
Make sure thermal mass is much smaller
than that of object being measured
050
Physical errors: Decalibration
350 C
300 C
200 C
100 C
975 C
1000 C
This section
Don't exceed Tmax of T/C
produces the Temp. cycling causes work-hardening,
ENTIRE signal decalibration
Replace the GRADIENT section
051
Agenda
Background, history
Mechanical sensors
Electrical sensors
Optical
Pyrometer
RTD
Thermistor, IC
Thermocouple
Summary & Examples
A7
The basic 4 temperature sensors
RTD Thermistor
Most accurate High output
Fast
Most stable
2-wire meas.
Fairly linear
Expensive
Very nonlinear
Slow
Limited range
Needs I source Needs I source
Self-heating
Self-heating
4-wire meas. Fragile
I.C.
Thermocouple
High output
Most linear
Cheap
Limited variety
Limited range
Needs V
source
Self-heating
Wide variety
Cheap
Wide T. range
No self-heating
Hard to measure
Relative T. only
Nonlinear
Special
connectors
Absolute temperature sensors
052
Summary
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
Photochemical process
Sensor
RTD (most accurate)
control:
Thermistor
Flower petal:
Molten glass:
Induction furnace:
100 degree Heat aging oven:
(lowest thermal mass)
Optical pyrometer
(hi temp, no contact)
RTD (if <800C); or T/C
(Beware magnetic I
noise)
Any of the 4 sensors
054