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Temperature sensor_Ajinkya kamble_pptxxx
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
Food industry
Monitor temperature-time cycles to ensure high food
quality
Automotive industry
Combustion and exhaust temperature
Solar Energy conversion
Accurate temperature measurement to achieve optimal
heat flow
Energy efficiency in the home and industry
Measurement of temperature
Hospital infant incubator
Temperature must be kept in the proper range
4. Chap 0 4
P-N Junction Thermometers
Principle of Diode
Thermometer
Forward Biased Current
Voltage
Where T is in K
Voltage vs. Temperature
Useful range
40 ~ 400K
[exp( ) 1]
2
s
qV
I I
kT
4.6
(ln ln )
g
E kT
V M I
q q
5. Chap 0 5
Diode thermometer with known
characteristics
Motorola MTS 105
Calibrate Diode to obtain
accurate output
Constant Current source
must be very stable
1C accuracy
0.002K with precision
GaAs Diode
Calibration Procedures
Determine VBE at
extremes (-40C and
150C)
Plot line using VBE(-40C) and
VBE(150C)
Given VBE(Tx), Tx can be found
using curve from step 2 or
equation
Diode are more sensitive and
linear than others
Wide range
Less repeatable
Affected by Magnetic (>
2.25 0.003( 600) /
c BE
T V mV C
[ ( ) (25 )]/ 25
x BE x BE c
T V T V C T
6. Chap 0 6
Diode thermometer with Unknown
characteristics
Diode must be
calibrated over the
desired range
Using Table or Curves
Changing temperature
(e.g., 0 ~ 50C)
Recording Vi vs. Ti
After calibrating, Tx
can be determined
using measure Vx and
Vi vs. Ti curve
Interpolation
techniques are
needed
Using Equation
Linear regression
T = a + bV
• Where a and b are
constant
• Can be determined
using
– Ti = a + bVi
Tx can be determined
using measured Vx and
Equation
Manufacturer provides
Table or Curve
Equation
7. Chap 0 7
Transistor as a Temperature Sensor
Base-Emitter voltage of a
transistor varies directly
with temperature at a
constant collector current
Thermometer using MTS 105
R1 determine collector
current. Must be stable
R2 is adjusted until Vo=0 for a
display in C
Accuracy of 0.01C
Range of –50 to 125C
8. Chap 0 8
BASIC Program for Transistor
Thermometers
Calibrating and using the transistor thermometer
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
For prototype board developed in Chapters 3, 4, and 5
10. Chap 0 10
IC Temperature Sensors
Temperature sensing
circuit with output voltage
proportional to absolute
temperature
If IC1/IC2 is constant
VR1 is proportional to
temperature
LX5700 from NS
Range: -55 ~ 125C
Sensitivity: 10mV/C
Time Constant
50 sec (Still Air)
< 1 sec (Stirred Oil Bath)
Output: 2.98V at 298K
Accuracy: 3.8K
Linearity: < 1K
Not satisfactory in many
applications
Poor Accuracy
1
1 2 1 2
2
ln( )
C
C BE BE
C
I
kT
R I V V
q I
11. Chap 0 11
IC Sensor: LM135, LM235, LM335
Operate as two terminal Zenor
Breakdown Voltage
proportional to absolute
temperature of +10mV/K
When calibrated at 25C
LM135 < 1.5C Error
LM335 < 2C Error
Range: -55 ~ 150C
Output voltage:
T0 is reference temperature
Thermal Response time of LM335
Flowing Air
Still Air Stirred Oil Bath
0 0 0
0
( ) ( )
T
v T v T
T
12. Chap 0 12
IC Sensor: LM134-3, LM234-3, LM134-6, LM234-6
IC temperature sensors with
current output
Three terminal adjustable
current source
Range
-55 ~ 125C : LM134-3, 6
-25 ~ 100C : LM234-3, 6
Operate over wide voltage
1 ~ 40V
Accuracy
3C : LM134-3, LM234-3
6C : LM134-6, LM234-6
Not for precision
temperature measurement
Output current
T: temperature in K
i0 is programmable
By adjusting R
1 A ~ 10mA
0
(227 / )
V K T
i
R
13. Chap 0 13
IC Sensor: AD590
Two terminal IC
temperature sensor
Better accuracy and
linearity than LM135
Current output depends
on absolute temperature
Insensitive to the voltage
across it
Used with long lead
wires
VT: voltage across R
If R=358,
Error
< 0.3C : AD590J
< 0.05C: AD590M
Time Constant
60 sec (Still Air)
1.4 sec (Stirred Oil)
Operating Range
-55 ~ 150C
High Output Impedance
> 10M
Excellent rejection of
supplying voltage drift
and ripple
1
2
ln 179 ( )
T
I
kT
V T V
q I
1 /
T
I
A K
T
14. Chap 0 14
Thermocouples
Thermocouple
is a two-wire
device
Composed of
dissimilar
metals or
alloys with one
end welded
together
Types of thermocouples
15. Chap 0 15
Type of thermocouple junctions
Exposed junction
Extending beyond the
protective metallic
sheath
Fast Response
Static or flowing non-
corrosive gas
Ungrounded junction
Insulated by MgO powder
Suitable for corrosive
environment
Grounded junction
High pressure application
For static or flowing
corrosive gas and liquid
16. Chap 0 16
Seebeck Effect: Principles of Thermocouples
Two dissimilar metal or
alloy wires A and B joined
together at the end to form
a circuit
If temperature are different
(T2 > T1), a current will flow
in circuit
Seebeck thermal emf
Emf(electromotive force)
producing above current
Magnitude of thermal emf
can be measure using
Voltmeter or Ammeter
17. Chap 0 17
Principles of Thermocouples
Broken at center
Open loop circuit
voltage EAB
Temperature
difference (T2 - T1)
Composition of two
metal
Example
T2 = 0C , T1 = 1C
T-type
Copper + Constantan
EAB = 39V
S-type
Platinum + Platinum-
10% rhodium
EAB = 5V
18. Chap 0 18
Thermoelectric Laws
Law of interior temperatures
Emf is not affected by T3 and
T4
Law of intermediate metals
Emf is not affected by Metal
X if J1 and J2 are at the
same temperature
Can solder or attach lead
wire
Law of intermediate
temperature
Can use reference table even
if reference junction is not
0C
Law of Additive Emf
Can create nonstandard
thermocouple combinations
and still use the reference
tables
19. Chap 0 19
Picking Up the Thermovoltage
Directly connect voltmeter
to thermocouple
Can not read thermal emf
New thermal junction
• J2 : No emf
– Copper – copper
• J3 : emf V3
– Copper -
constantan
The output of voltmeter
Proportional to voltage
difference between V1
and V3
To find T1, we must know
T3
Put J3 in ice bath
• 0C
• V = function of T1
20. Chap 0 20
Cold junction compensation
In practice, No need to put
J3 to ice bath
Add Voltage to cancel V2
to zero
Then output is directly
proportional to V1
If TA increase
VA increase with VA
IA of AD590 also
increase
AD590
• IC temperature sensor
AD580
• 2.5V stable voltage
reference
Most of IA flows through
RA
Produce - VA which
cancels VA in cold
junction
The output voltage Eo
Eo VT
21. Chap 0 21
Simpler approach using AD595
Built in capacity
for
Cold junction
compensation
Fault detection
Output voltage
10mV/C
22. Chap 0 22
Conversion of Thermal Voltage to
Temperature
Temperature Vs. Voltage
relationship
Slightly Nonlinear
To achieve accuracy
Entire range must be
calibrated
Manufacturer provide table
and curve
Lookup table in computer
Interpolation needed
Large memory consumption
Curve Fitting
Power series polynomial
Better accuracy as n
increases
2
0 1 2
n
n
T A AV A V A V
23. Chap 0 23
Calibration of Thermometers
To ensure temperature
accuracy
Noise Reduction of
thermocouples
Output voltage is order
of V
Sensitive to interface
Analog active filter
and Guarding
techniques are
needed
Thermal Time constant
Depends on particular
mounting
arrangements
Heat transfer
Surrounding medium
Generally, the smaller
the sensor, the faster it
will respond
Generally
Thermocouple: 550ms
• Exposed butt-welded
25m diameter: 3ms
• Time constant
increase with diameter
of wire and sheath
Diode, Transistor: 10s
24. Chap 0 24
Resistive Temperature Sensors
Resistance of materials are changed with
temperature
Conductive materials
Metals
R increases as T increases
Called RTD
• Resistance Temperature Detector
Semiconductors
R decreases as T increases
Called Thermistors
25. Chap 0 25
Resistive Thermometers
Nickel, Copper and Platinum is
most commonly used
Resistance Vs. Temperature
curve
Not linear
Ro : R at 0C
Simplified Equation
Limited range (0 ~ 100C)
Platinum is most widely
Copper
Low resistive
need long wire
Nickel
Low cost
Resistance Vs.
Temperature
2
0 1 2
(1 )
n
T n
R R a T a T a T
0 1
(1 )
T
R R a T
26. Chap 0 26
Platinum Thermometers
SPRT
Standard Platinum
Resistance
Thermometer
Range
13.81K ~ 903.89K
Some are designed to
1050C
Callendar-Van Dusen
Equation
-183 ~ 630C Range
Typically
0
3
[ (0.01 1)(0.01 )
(0.01 1)(0.01 ) ]
T
R R T T T
T T
1
0
0.00392
1.49
0( 0),0.11( 0)
100
C
T T
R
27. Chap 0 27
Current source and Amp for RTD
2mA current source
Causes voltage drop
in RTD
Amp gain = 10
To fit DAS
Tendency
Increase current
source to obtain a
higher output voltage
It causes Self-heating
in platinum RTD
29. Chap 0 29
Thermistors
Comparison of NTC and
PTC thermistor and
platinum resistance
thermometer
NTC
Negative Temperature
Coefficient
High sensitivity
Highly nonlinear
PTC
Positive Temperature
Coefficient
Inserting Barium and
Titanate mixtures
Called switching
thermistors
Switching temperature
(Curie Pont)
• -20 ~ 125C
30. Chap 0 30
Empirical Correction
Basic Characteristics
Where
Ro : R at known To
• Usually 298.15K
: Material constant for
thermistor in K
• Determined from R
obtained at 0 and 50C
• 1500 ~ 6000K range
– Typically, 4000K
Few ~ 10M range
Steinhart-Hart Equation
Where
A, B, C are found by
solving three equations
with known R and T
Accuracy < 0.01C
More narrow range
-40C < T1, T2, T3 <150C,
|T2-T1|<50C, |T3-T2|<50C
0
0
1 1
exp[ ( )]
T
R R
T T
3
1
ln (ln )
A B R C R
T
1
ln
B
C
T R A
31. Chap 0 31
Terminology
Temperature Coefficient of
Resistance
Typically, -4.4%/C at 27C
Self Heating
Power (I2R) dissipated in
thermistor
To avoid self heating, the
exciting current should be
very low
Voltage current
characteristics
For small current Ohm’s law
is hold
No self heating
With higher current
Self heating
More current to flow due to
decreased resistance
Heat sink is useful
2
1
(%/ ) 100
T
T
dR
C
R dT T
32. Chap 0 32
Applications
Thermistor
Pneumography
Used to obtain
breathing rate
by detecting the
temperature
difference between
inspired cool air and
expired warm air
Temperature measurement
Simple circuit
Battery + Thermistor +
Microammeter
More sensitive circuit
Differential circuit
0.0005C change can be
indicated
33. Chap 0 33
Applications
Temperature compensation
To compensate for ambient
temperature change effects
on copper coils in meters,
generators and motors
PTC of copper and NTC of
thermistor produce relatively
constant coil resistance for
changing ambient
temperature
Liquid Level Measurement
R of thermistor in air
Decreases as heat up
Enough current to close
relay
R of thermistor in liquid
Increases as cooling
The relay will open
34. Chap 0 34
Applications
Altimeter
Called Hyposometer
Sea level ~ 37500m
With precision of better
than 1%
Heat until liquid boils
Measure R of thermistor
R depends on pressure
Pressure depends on
Altitude
Power measurement
R in bridge: 200
Thermistor: 2K
Thermistor heat up until 200
Balance in bridge
Calculate DC power
Applying High Frequency
power
R of thermistor more
decreases
Reduce DC power until bridge
balance again
Calculate DC power
The difference of two DC
power is HF power
35. Chap 0 35
Linearization
Using parallel resistors
Choosing Rp
Tm: Midscale temp.
Rt,m: R at Tm
More linear, Less
Sensitive:
Using Series resistors
Choosing Gs
More Linear, Less
Sensitivity
,
2
2
m
P t m
m
T
R R
T
2
,
( / )
( / ) 1
m
P
t m P
T
R R
,
2
1
2
m
P t m
S m
T
G G
R T
2
,
( / )
( / ) 1
m
P
t m P
T
G R
36. Chap 0 36
Linearization
Implementation of series
linearization using OP
Amps
Minimal deviation of
linearity
0.15C for 0 ~40C
Temperature to Frequency
Conversion
Hysteresis-based
oscillator
Frequency of oscillation
nonlinearly depends on
temperature
CPU counts frequency of
oscillator's output
37. Chap 0 37
Temperature to Frequency Conversion
Look up Table
Stores temperature
values
Address of Look up
table
Frequency Value
Practical for Small
ranges
Large memory for large
ranges
Hard to recalibration for
each new sensors
Complicate circuit
Fast response
m-degree resolution
0 ~ 100C range
Accuracy < 0.15C
38. Chap 0 38
Interfacing to the IBM PC
Regulator: 7805
Very stable 5V from 12V
FET OP Amps: RCA CA3140
YSI(Yellow Springs Instrument
Co.) series 400 thermometer
Time Constant: 800ms
Maximal operating
Temperature: 150C
0.15 ~ 5.6V output for 100 ~
0C
See Fig 7.41, For
BASIC program
Calibrating and using a
YSI series 400
thermistor
Uses Tecmar Lab
Master Data Acquisition
Board
Homework #7-1
Analyzed the Basic
Program
39. Chap 0 39
Other temperature measurement
techniques
Ultrasonic Thin-wire Thermometer
Velocity of sound depends on
temperature
High temperature
2000 ~ 3000C
Maximal error: 30C
Quartz-Crystal Thermometer
Resonant frequency of quartz-crystal
oscillator is linearly related to
temperature
Accuracy: 0.04C
Range: -80 ~ 250C
Johnson Noise Thermometer
Noise voltage power density
spectrum is function of temperature
Accuracy: 20C
Range: 400 ~ 1770K
Nuclear Quadrupole Resonance
Thermometer
Nuclear quadrupole resonance
absorption frequency
decreases with increasing
temperature
Accuracy : 1mK
Range : 90 ~ 398K
Eddy Current Thermometer
Non Contacting temperature
measurement
HF magnetic filed on steel
Eddy current New magnetic
field Detecting coil
The magnitude of eddy current
depends on temperature and
distance
Accuracy : < 3C
Range: 25 ~ 300C