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1. 1
Interfacing Temperature Sensors
P-N Junction Thermometers
IC Temperature Sensors
Thermocouples
Calibration of Thermometers
Resistive Temperature Sensors
Other Temperature Measurement Techniques
2. Chap 0 2
Applications of temperature sensing
īŽ Food industry
īĩ Monitor temperature-time cycles to ensure high food
quality
īŽ Automotive industry
īĩ Combustion and exhaust temperature
īŽ Solar Energy conversion
īĩ Accurate temperature measurement to achieve optimal
heat flow
īŽ Energy efficiency in the home and industry
īĩ Measurement of temperature
īŽ Hospital infant incubator
īĩ Temperature must be kept in the proper range
4. Chap 0 4
P-N Junction Thermometers
īŽ Principle of Diode
Thermometer
īĩ Forward Biased Current
īĩ Voltage
ī Where T is in ī°K
īŽ Voltage vs. Temperature
īĩ Useful range
ī 40 ~ 400ī°K
[exp( ) 1]
2
s
qV
I I
kT
īŊ ī
4.6
(ln ln )
g
E kT
V M I
q q
īŊ ī ī
5. Chap 0 5
Diode thermometer with known
characteristics
īŽ Motorola MTS 105
īĩ Calibrate Diode to obtain
accurate output
īĩ Constant Current source
must be very stable
īĩ īą1ī°C accuracy
ī īą0.002ī°K with precision
GaAs Diode
īŽ Calibration Procedures
īĩ Determine VBE at
extremes (-40ī°C and
150ī°C)
īĩ Plot line using VBE(-40ī°C) and
VBE(150ī°C)
īĩ
īĩ Given VBE(Tx), Tx can be found
using curve from step 2 or
equation
īŽ Diode are more sensitive and
linear than others
īĩ Wide range
īĩ Less repeatable
īĩ Affected by Magnetic (>
2.25 0.003( 600) /
c BE
T V mV C
īŊ ī īĢ ī
[ ( ) (25 )]/ 25
x BE x BE c
T V T V C T
īŊ ī īĢ
6. Chap 0 6
Diode thermometer with Unknown
characteristics
īŽ Diode must be
calibrated over the
desired range
īĩ Using Table or Curves
ī Changing temperature
(e.g., 0 ~ 50ī°C)
ī Recording Vi vs. Ti
ī After calibrating, Tx
can be determined
using measure Vx and
Vi vs. Ti curve
ī Interpolation
techniques are
needed
īĩ Using Equation
ī Linear regression
ī T = a + bV
âĸ Where a and b are
constant
âĸ Can be determined
using
â Ti = a + bVi
ī Tx can be determined
using measured Vx and
Equation
īŽ Manufacturer provides
īĩ Table or Curve
īĩ Equation
7. Chap 0 7
Transistor as a Temperature Sensor
īŽ Base-Emitter voltage of a
transistor varies directly
with temperature at a
constant collector current
īŽ Thermometer using MTS 105
īĩ R1 determine collector
current. Must be stable
īĩ R2 is adjusted until Vo=0 for a
display in ī°C
īĩ Accuracy of īą0.01ī°C
īĩ Range of â50 to 125ī°C
8. Chap 0 8
BASIC Program for Transistor
Thermometers
īŽ Calibrating and using the transistor thermometer
īŽ For Tecmar Lab Master data acquisition Board
Initialize ADC
Select Channel 0
Start Conversion
Check EOC
9. Chap 0 9
C program for Calibrating and Using
the Transistor Thermometer
īŽ For prototype board developed in Chapters 3, 4, and 5
10. Chap 0 10
IC Temperature Sensors
īŽ Temperature sensing
circuit with output voltage
proportional to absolute
temperature
īĩ
īĩ If IC1/IC2 is constant
ī VR1 is proportional to
temperature
īŽ LX5700 from NS
īĩ Range: -55 ~ 125ī°C
īĩ Sensitivity: 10mV/ī°C
īĩ Time Constant
ī 50 sec (Still Air)
ī < 1 sec (Stirred Oil Bath)
īĩ Output: 2.98V at 298K
īĩ Accuracy: īą3.8K
īĩ Linearity: < īą1K
īĩ Not satisfactory in many
applications
ī Poor Accuracy
1
1 2 1 2
2
ln( )
C
C BE BE
C
I
kT
R I V V
q I
īŊ ī īŊ
11. Chap 0 11
IC Sensor: LM135, LM235, LM335
īŽ Operate as two terminal Zenor
īĩ Breakdown Voltage
proportional to absolute
temperature of +10mV/K
īĩ When calibrated at 25ī°C
ī LM135 < 1.5ī°C Error
ī LM335 < 2ī°C Error
īĩ Range: -55 ~ 150ī°C
īĩ Output voltage:
ī T0 is reference temperature
īŽ Thermal Response time of LM335
Flowing Air
Still Air Stirred Oil Bath
0 0 0
0
( ) ( )
T
v T v T
T
īŊ
12. Chap 0 12
IC Sensor: LM134-3, LM234-3, LM134-6, LM234-6
īŽ IC temperature sensors with
current output
īĩ Three terminal adjustable
current source
īŽ Range
īĩ -55 ~ 125ī°C : LM134-3, 6
īĩ -25 ~ 100ī°C : LM234-3, 6
īŽ Operate over wide voltage
īĩ 1 ~ 40V
īŽ Accuracy
īĩ īą3ī°C : LM134-3, LM234-3
īĩ īą6ī°C : LM134-6, LM234-6
īĩ Not for precision
temperature measurement
īŽ Output current
īĩ
ī T: temperature in K
īĩ i0 is programmable
ī By adjusting R
ī 1 īA ~ 10mA
0
(227 / )
V K T
i
R
ī
īŊ
13. Chap 0 13
IC Sensor: AD590
īŽ Two terminal IC
temperature sensor
īĩ Better accuracy and
linearity than LM135
īĩ Current output depends
on absolute temperature
ī Insensitive to the voltage
across it
ī Used with long lead
wires
īŽ VT: voltage across R
īŽ If R=358ī,
īŽ Error
īĩ < 0.3ī°C : AD590J
īĩ < īą0.05ī°C: AD590M
īŽ Time Constant
īĩ 60 sec (Still Air)
īĩ 1.4 sec (Stirred Oil)
īŽ Operating Range
īĩ -55 ~ 150ī°C
īŽ High Output Impedance
īĩ > 10Mī
īĩ Excellent rejection of
supplying voltage drift
and ripple
1
2
ln 179 ( )
T
I
kT
V T V
q I
īŊ īŊ
1 /
T
I
A K
T
ī
īŊ
14. Chap 0 14
Thermocouples
īŽ Thermocouple
is a two-wire
device
īĩ Composed of
dissimilar
metals or
alloys with one
end welded
together
īŽ Types of thermocouples
15. Chap 0 15
Type of thermocouple junctions
īŽ Exposed junction
īĩ Extending beyond the
protective metallic
sheath
īĩ Fast Response
ī Static or flowing non-
corrosive gas
īŽ Ungrounded junction
īĩ Insulated by MgO powder
īĩ Suitable for corrosive
environment
īŽ Grounded junction
īĩ High pressure application
ī For static or flowing
corrosive gas and liquid
16. Chap 0 16
Seebeck Effect: Principles of Thermocouples
īŽ Two dissimilar metal or
alloy wires A and B joined
together at the end to form
a circuit
īŽ If temperature are different
(T2 > T1), a current will flow
in circuit
īŽ Seebeck thermal emf
īĩ Emf(electromotive force)
producing above current
īŽ Magnitude of thermal emf
can be measure using
Voltmeter or Ammeter
17. Chap 0 17
Principles of Thermocouples
īŽ Broken at center
īĩ Open loop circuit
voltage EAB
ī Temperature
difference (T2 - T1)
ī Composition of two
metal
īŽ Example
īĩ T2 = 0ī°C , T1 = 1ī°C
īĩ T-type
ī Copper + Constantan
ī EAB = 39īV
īĩ S-type
ī Platinum + Platinum-
10% rhodium
ī EAB = 5īV
18. Chap 0 18
Thermoelectric Laws
īŽ Law of interior temperatures
īĩ Emf is not affected by T3 and
T4
īŽ Law of intermediate metals
īĩ Emf is not affected by Metal
X if J1 and J2 are at the
same temperature
īĩ Can solder or attach lead
wire
īŽ Law of intermediate
temperature
īĩ Can use reference table even
if reference junction is not
0ī°C
īŽ Law of Additive Emf
īĩ Can create nonstandard
thermocouple combinations
and still use the reference
tables
19. Chap 0 19
Picking Up the Thermovoltage
īŽ Directly connect voltmeter
to thermocouple
īĩ Can not read thermal emf
ī New thermal junction
âĸ J2 : No emf
â Copper â copper
âĸ J3 : emf V3
â Copper -
constantan
īŽ The output of voltmeter
īĩ Proportional to voltage
difference between V1
and V3
īĩ To find T1, we must know
T3
ī Put J3 in ice bath
âĸ 0ī°C
âĸ īV = function of T1
20. Chap 0 20
Cold junction compensation
īŽ In practice, No need to put
J3 to ice bath
īĩ Add Voltage to cancel V2
to zero
īĩ Then output is directly
proportional to V1
īŽ If TA increase
īĩ VA increase with īVA
īĩ IA of AD590 also
increase
ī AD590
âĸ IC temperature sensor
ī AD580
âĸ 2.5V stable voltage
reference
īĩ Most of IA flows through
RA
ī Produce - īVA which
cancels īVA in cold
junction
īŽ The output voltage Eo
ī Eo īģ VT
21. Chap 0 21
Simpler approach using AD595
īŽ Built in capacity
for
īĩ Cold junction
compensation
īĩ Fault detection
īŽ Output voltage
īĩ 10mV/ī°C
22. Chap 0 22
Conversion of Thermal Voltage to
Temperature
īŽ Temperature Vs. Voltage
relationship
īĩ Slightly Nonlinear
īĩ To achieve accuracy
ī Entire range must be
calibrated
īŽ Manufacturer provide table
and curve
īĩ Lookup table in computer
ī Interpolation needed
ī Large memory consumption
īĩ Curve Fitting
ī Power series polynomial
ī Better accuracy as n
increases
2
0 1 2
n
n
T A AV A V A V
īŊ īĢ īĢ īĢ īĢ
23. Chap 0 23
Calibration of Thermometers
īŽ To ensure temperature
accuracy
īŽ Noise Reduction of
thermocouples
īĩ Output voltage is order
of īV
ī Sensitive to interface
ī Analog active filter
and Guarding
techniques are
needed
īŽ Thermal Time constant
īĩ Depends on particular
mounting
arrangements
ī Heat transfer
ī Surrounding medium
īĩ Generally, the smaller
the sensor, the faster it
will respond
īĩ Generally
ī Thermocouple: 550ms
âĸ Exposed butt-welded
25īm diameter: 3ms
âĸ Time constant
increase with diameter
of wire and sheath
ī Diode, Transistor: 10s
24. Chap 0 24
Resistive Temperature Sensors
īŽ Resistance of materials are changed with
temperature
īĩ Conductive materials
ī Metals
ī R increases as T increases
ī Called RTD
âĸ Resistance Temperature Detector
īĩ Semiconductors
ī R decreases as T increases
ī Called Thermistors
25. Chap 0 25
Resistive Thermometers
īŽ Nickel, Copper and Platinum is
most commonly used
īŽ Resistance Vs. Temperature
curve
īĩ Not linear
ī Ro : R at 0ī°C
īĩ Simplified Equation
ī
ī Limited range (0 ~ 100ī°C)
īŽ Platinum is most widely
īĩ Copper
ī Low resistive
ī ī need long wire
īĩ Nickel
ī Low cost
īŽ Resistance Vs.
Temperature
2
0 1 2
(1 )
n
T n
R R a T a T a T
īŊ īĢ īĢ īĢ īĢ
0 1
(1 )
T
R R a T
īŊ īĢ
26. Chap 0 26
Platinum Thermometers
īŽ SPRT
īĩ Standard Platinum
Resistance
Thermometer
īŽ Range
īĩ 13.81K ~ 903.89K
īĩ Some are designed to
1050ī°C
īŽ Callendar-Van Dusen
Equation
īĩ -183 ~ 630ī°C Range
ī Typically
0
3
[ (0.01 1)(0.01 )
(0.01 1)(0.01 ) ]
T
R R T T T
T T
īĄ ī¤
īĸ
īŊ ī ī
ī ī
1
0
0.00392
1.49
0( 0),0.11( 0)
100
C
T T
R
īĄ
ī¤
īĸ
ī
īŊ
īŊ
īŊ īŗ īŧ
īŊ ī
27. Chap 0 27
Current source and Amp for RTD
īŽ 2mA current source
īĩ Causes voltage drop
in RTD
īŽ Amp gain = 10
īĩ To fit DAS
īŽ Tendency
īĩ Increase current
source to obtain a
higher output voltage
īĩ It causes Self-heating
in platinum RTD
29. Chap 0 29
Thermistors
īŽ Comparison of NTC and
PTC thermistor and
platinum resistance
thermometer
īŽ NTC
īĩ Negative Temperature
Coefficient
īĩ High sensitivity
īĩ Highly nonlinear
īŽ PTC
īĩ Positive Temperature
Coefficient
īĩ Inserting Barium and
Titanate mixtures
īĩ Called switching
thermistors
ī Switching temperature
(Curie Pont)
âĸ -20 ~ 125ī°C
30. Chap 0 30
Empirical Correction
īŽ Basic Characteristics
īĩ Where
ī Ro : R at known To
âĸ Usually 298.15K
ī īĸ : Material constant for
thermistor in K
âĸ Determined from R
obtained at 0 and 50ī°C
âĸ 1500 ~ 6000K range
â Typically, 4000K
īĩ Few ī ~ 10Mī range
īŽ Steinhart-Hart Equation
īĩ Where
ī A, B, C are found by
solving three equations
with known R and T
īĩ Accuracy < 0.01ī°C
īŽ More narrow range
īĩ -40ī°C < T1, T2, T3 <150ī°C,
|T2-T1|<50ī°C, |T3-T2|<50ī°C
0
0
1 1
exp[ ( )]
T
R R
T T
īĸ
īŊ ī 3
1
ln (ln )
A B R C R
T
īŊ īĢ īĢ
1
ln
B
C
T R A
īŊ ī
ī
31. Chap 0 31
Terminology
īŽ Temperature Coefficient of
Resistance
īĩ Typically, -4.4%/ī°C at 27ī°C
īŽ Self Heating
īĩ Power (I2R) dissipated in
thermistor
ī To avoid self heating, the
exciting current should be
very low
īŽ Voltage current
characteristics
īĩ For small current Ohmâs law
is hold
ī No self heating
īĩ With higher current
ī Self heating
ī More current to flow due to
decreased resistance
īĩ Heat sink is useful
2
1
(%/ ) 100
T
T
dR
C
R dT T
īĸ
īĄ īŊ īŊ ī
32. Chap 0 32
Applications
īŽ Thermistor
Pneumography
īĩ Used to obtain
breathing rate
ī by detecting the
temperature
difference between
inspired cool air and
expired warm air
īŽ Temperature measurement
īĩ Simple circuit
ī Battery + Thermistor +
Microammeter
īĩ More sensitive circuit
īĩ Differential circuit
ī 0.0005ī°C change can be
indicated
33. Chap 0 33
Applications
īŽ Temperature compensation
īĩ To compensate for ambient
temperature change effects
on copper coils in meters,
generators and motors
īĩ PTC of copper and NTC of
thermistor produce relatively
constant coil resistance for
changing ambient
temperature
īŽ Liquid Level Measurement
īĩ R of thermistor in air
ī Decreases as heat up
ī Enough current to close
relay
īĩ R of thermistor in liquid
ī Increases as cooling
ī The relay will open
34. Chap 0 34
Applications
īŽ Altimeter
īĩ Called Hyposometer
īĩ Sea level ~ 37500m
īĩ With precision of better
than 1%
īĩ Heat until liquid boils
ī Measure R of thermistor
ī R depends on pressure
ī Pressure depends on
Altitude
īŽ Power measurement
īĩ R in bridge: 200ī
īĩ Thermistor: 2Kī
īĩ Thermistor heat up until 200ī
ī Balance in bridge
ī Calculate DC power
īĩ Applying High Frequency
power
ī R of thermistor more
decreases
ī Reduce DC power until bridge
balance again
ī Calculate DC power
īĩ The difference of two DC
power is HF power
35. Chap 0 35
Linearization
īŽ Using parallel resistors
īĩ Choosing Rp
ī Tm: Midscale temp.
ī Rt,m: R at Tm
īĩ More linear, Less
Sensitive:
īŽ Using Series resistors
īĩ Choosing Gs
īĩ More Linear, Less
Sensitivity
,
2
2
m
P t m
m
T
R R
T
īĸ
īĸ
ī
īŊ
īĢ
2
,
( / )
( / ) 1
m
P
t m P
T
R R
īĸ
īĄ īŊ
īĢ
,
2
1
2
m
P t m
S m
T
G G
R T
īĸ
īĸ
ī
īŊ īŊ
īĢ
2
,
( / )
( / ) 1
m
P
t m P
T
G R
īĸ
īĄ
ī
īŊ
īĢ
36. Chap 0 36
Linearization
īŽ Implementation of series
linearization using OP
Amps
īĩ Minimal deviation of
linearity
ī 0.15ī°C for 0 ~40ī°C
īŽ Temperature to Frequency
Conversion
īĩ Hysteresis-based
oscillator
īĩ Frequency of oscillation
nonlinearly depends on
temperature
īĩ CPU counts frequency of
oscillator's output
37. Chap 0 37
Temperature to Frequency Conversion
īŽ Look up Table
īĩ Stores temperature
values
īĩ Address of Look up
table
ī Frequency Value
īĩ Practical for Small
ranges
īĩ Large memory for large
ranges
īĩ Hard to recalibration for
each new sensors
īŽ Complicate circuit
īĩ Fast response
īĩ m-degree resolution
īĩ 0 ~ 100ī°C range
īĩ Accuracy < 0.15ī°C
38. Chap 0 38
Interfacing to the IBM PC
īŽ Regulator: 7805
īĩ Very stable 5V from 12V
īŽ FET OP Amps: RCA CA3140
īŽ YSI(Yellow Springs Instrument
Co.) series 400 thermometer
īĩ Time Constant: 800ms
īĩ Maximal operating
Temperature: 150ī°C
ī 0.15 ~ 5.6V output for 100 ~
0ī°C
īŽ See Fig 7.41, For
BASIC program
īĩ Calibrating and using a
YSI series 400
thermistor
īĩ Uses Tecmar Lab
Master Data Acquisition
Board
īŽ Homework #7-1
īĩ Analyzed the Basic
Program
39. Chap 0 39
Other temperature measurement
techniques
īŽ Ultrasonic Thin-wire Thermometer
īĩ Velocity of sound depends on
temperature
īĩ High temperature
ī 2000 ~ 3000ī°C
ī Maximal error: 30ī°C
īŽ Quartz-Crystal Thermometer
īĩ Resonant frequency of quartz-crystal
oscillator is linearly related to
temperature
īĩ Accuracy: īą0.04ī°C
īĩ Range: -80 ~ 250ī°C
īŽ Johnson Noise Thermometer
īĩ Noise voltage power density
spectrum is function of temperature
īĩ Accuracy: īą20ī°C
īĩ Range: 400 ~ 1770K
īŽ Nuclear Quadrupole Resonance
Thermometer
īĩ Nuclear quadrupole resonance
absorption frequency
decreases with increasing
temperature
īĩ Accuracy : īą1mK
īĩ Range : 90 ~ 398K
īŽ Eddy Current Thermometer
īĩ Non Contacting temperature
measurement
īĩ HF magnetic filed on steel ī
Eddy current ī New magnetic
field ī Detecting coil
īĩ The magnitude of eddy current
depends on temperature and
distance
īĩ Accuracy : < īą3ī°C
īĩ Range: 25 ~ 300ī°C