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chapter3.ppt
1. 3 - Temperature Sensors
1. Thermoresistive sensors
2. Thermoelectric sensors
3. PN junction temperature sensors
4. Optical and acoustic temperature sensors
5. Thermo-mechanical sensors and actuators
2. A bit of history
Temperature measurements and
thermometers
1600 - thermometers (water expansion, mercury)
1650 - first attempts at temperature scales (Boyle)
1700 - “standard” temperature scales (Magelotti,
Renaldini, Newton) - did not catch
1708 - Farenheit scale (180 div.)
1742 - Celsius scale
1848 - Kelvin scale (based on Carnot’s
thermodynamic work)
1927 - IPTS - International Practical Temperature
Scale
3. More history - sensors
Temperature sensors are the oldest
1821 - Seebeck effect (Thomas Johann Seebeck)
1826 - first sensor - a thermocouple - based on the
Seebeck effect (Antoine Cesar Becquerel)
1834 - Peltier effect (Charles Athanase Peltier).
First peltier cell built in 1960’s
Used for cooling and heating
1821 - discovery of temperature dependence of
conductivity (Sir Humphrey Davey)
1771 - William Siemens builds the first resistive sensor
made of platinum
4. Temperature sensors -
general
Temperature sensors are deceptively simple
Thermocouples - any two dissimilar materials,
welded together at one end and connected to a
micro-voltmeter
Peltier cell - any thermocouple connected to a dc
source
Resistive sensor - a length of a conductor
connected to an ohmmeter
• More:
Some temperature sensors can act as actuators as well
Can be used to measure other quantities
(electromagnetic radiation, air speed, flow, etc.)
• Some newer sensors are semiconductor based
5. Temperature sensors - types
Thermoelectric sensors
Thermocouples and thermopiles
Peltier cells (used as actuators but can be used as sensors)
Thermoresistive sensors and actuators
Conductor based sensors and actuators (RTDs)
Semiconductor based sensors - thermistors, diodes
Semiconductor junction sensors
Others
Based on secondary effects (speed of sound, phase of light)
Indirect sensing (infrared thermometers - chapter4)
Expansion of metals, bimetals
6. Thermal actuators
A whole class of thermal actuators
Bimetal actuators
Expansion actuators
Thermal displays
Sometimes sensing and actuation is
combined in a single device
7. Thermoresistive sensors
Two basic types:
Resistive Temperature Detector (RTD)
Metal wire
Thin film
Silicon based
Thermistors (Thermal Resistor)
NTC (Negative Temperature Coefficient)
PTC (Positive Temperature Coefficient)
9. Thermoresistive effect (cont.)
Resistance of a length
of wire
Conductivity is:
Resistance as a
function of
temperature:
a - Temperature
Coefficient of
Resistance (TCR) [C]
R = L
S
= 0
1 + a T T0
R T = L
0S
1 + a T T0
10. Thermoresistive effect (cont.)
T is the temperature [C ]
0 is the conductivity of the conductor
at the reference temperature T0.
T0 is usually given at 20C but may be
given at other temperatures as
necessary.
a - Temperature Coefficient of
Resistance (TCR) [C] given at T0
11. Example
Copper: 0=5.9x107 S/m, a=0.0039 C
at T0=20C. Wire of cross-sectional area:
0.1 mm2, length L=1m,
Change in resistance of 6.61x10 /C
and a base resistance of 0.017 at 20C
Change of 0.38% per C .
12. Example (cont.)
Conclusions from this example:
Change in resistance is measurable
Base resistance must be large - long and
or thin conductors or both
Other materials may be used
14. Other considerations
Tension or strain on the wires affect
resistance
Tensioning a conductor, changes its length
and cross-sectional area (constant volume)
has exactly the same effect on resistance as a
change in temperature.
increase in strain on the conductor increases the
resistance of the conductor (strain gauge)
Resistance should be relatively large (25
and up)
15. Construction - wire RTD
A spool of wire (length)
Similar to heating elements
Uniform wire
Chemically and dimensionally stable in the sensing range
Made thin (<0.1mm) for high resistance
Spool is supported by a glass (pyrex) or mica support
Similar to the way the heating element in a hair drier is
supported
Keeps strain at a minimum and allows thermal expansion
Smaller sensors may not have an internal support.
Enclosed in a glass, ceramic or metal enclosure
Length is from a few cm, to about 50cm
17. Construction (cont.)
Materials:
Platinum - used for precision applications
Chemically stable at high temperatures
Resists oxidation
Can be made into thin wires of high chemical purity
Resists corrosion
Can withstand severe environmental conditions.
Useful to about 800 C and down to below –250C.
Very sensitive to strain
Sensitive to chemical contaminants
Wire length needed is long (high conductivity)
18. Construction (cont.)
Materials:
Nickel and Copper
Less expensive
Reduced temperature range (copper only works up to
about 300C)
Can be made into thin wires of high chemical purity
Wire length needed is long (high conductivity)
Copper is not suitable for corrosive environments
(unless properly protected)
At higher temperatures evaporation increases
resistance
21. Thin Film RTDs
Thin film sensors: produced by depositing a
thin layer of a suitable material (platinum or
its alloys) on a thermally stable, electrically
non-conducting, thermally conducting
ceramic
Etched to form a long strip (in a meander
fashion).
Eq. (3.1) applies but much higher resistance
sensors are possible.
Small and relatively inexpensive
Often the choice in modern sensors
especially when the very high precision of
Platinum wire sensors is not needed.
22. Tnin film RTDs - (cont.)
Two types of thin film RTDs from different
manufacturers
Dimensions are typical - some are much
larger
23. Some parameters
Temperature range: -250 C to 700 C
Resistances: typically 100 (higher available)
Sizes: from a few mm to a few cm
Compatibility: glass, ceramic encapsulation
Available in ready made probes
Accuracy: ±0.01 C to ±0.05 C
Calibration: usually not necessary beyond
manufacturing
24. Self heat in RTDs
RTDs are subject to errors due to rise in their
temperature produced by the heat generated
in them by the current used to measure their
resistance
Wire wound or thin film
Power dissipated: Pd=I2R ( I is the current
(RMS) and R the resistance of the sensor)
Self heat depends on size and environment
Given as temperature rise per unit power
(C/mW)
Or: power needed to raise temperature (mW/ C)
25. Self heat in RTDs (cont.)
Errors are of the order of 0.01C/mW to
10C/mW (100mW/C to 0.1mW/C)
Given in air and in water
In water values are lower (opposite if mW/C used)
Self heat depends on size and environment
Lower in large elements, higher in small elements
Important to lower the current as much as possible
26. Response time in RTDs
Response time
Provided as part of data sheet
Given in air or in water or both, moving or stagnant
Given as 90%, 50% (or other) of steady state
Generally slow
Wire RTDs are slower
Typical values
0.5 sec in water to 100 sec in moving air
27. Silicon Resistive Sensors
Conduction in semiconductors
Valence electrons
Bound to atoms in outer layers (most electrons in pure
semiconductors)
Can be removed through heat (band gap energy)
When removed they become conducting electrons (conduction
band)
A pair is always released - electron and hole
Conductivity of semiconductors is
temperature dependent
Conductivity increases with temperature
Limited to a relatively small temperature range
28. Silicon Resistive Sensors (cont.)
Pure silicon:
NTC device - negative temperature
coefficient
Resistance decreases with temperature
Resistance in pure silicon is extremely high
Need to add impurities to increase carrier density
N type silicon - add arsenic (As) or antimony (Sb)
Behavior changes:
Resistance increases up to a given temperature
(PTC)
Resistance decreases after that (NTC)
PTC up to about 200 C
31. Silicon resistive sensors
Silicon resistive sensors are somewhat
nonlinear and offer sensitivities of the order
0.5-0.7 %/C.
Can operate in a limited range of
temperatures like most semiconductors
devices based on silicon
Maximum range is between –55C to
+150C.
Typical range: - 45C to +85C or 0C to
+80C
Resistance: typically 1k at 25 C.
Can be calibrated in any temperature scale
Made as a small chip with two electrodes and
encapsulated in epoxy, metal cans etc.
32. Thermistors
Thermistor: Thermal resistor
Became available: early 1960’s
Based on oxides of semiconductors
High temperature coefficients
NTC
High resistances (typically)
33. Thermistors (cont.)
Transfer function:
a [] and [K] are constants
R(T): resistance of the device
T: temperature in K
Relation is nonlinear but:
Only mildly nonlinear ( is small)
Approximate transfer function
R T = ae/T
36. Thermistors - properties
Most are NTC devices
Some are PTC devices
PTC are made from special materials
Not as common
Advantageous when runaway
temperatures are possible
37. Thermistors - properties
Self heating errors as in RTDs but:
Usually lower because resistance is higher
Current very low (R high)
Typical values: 0.01C/mW in water to 1C/mW in air
Wide range of resistances up to a few M
Can be used in self heating mode
To raise its temperature to a fixed value
As a reference temperature in measuring flow
Repeatability and accuracy:
0.1% or 0.25C for good thermistors
38. Thermistors - properties
Temperature range:
50 C to about 600 C
Ratings and properties vary along the range
Linearity
Very linear for narrow range applications
Slightly nonlinear for wide temperature ranges
Available in a wide range of sizes, shapes and also
as probes of various dimensions and shapes
Some inexpensive thermistors have poor
repeatability - these must be calibrated before use.
39. Thermoelectric sensors
Among the oldest sensors (over 150 years)
Some of the most useful and most common
Passive sensors: they generate electrical
emfs (voltages) directly
Measure the voltage directly.
Very small voltages - difficult to measure
Often must be amplified before interfacing
Can be influenced by noise
40. Thermoelectric sensors (cont.)
Simple, rugged and inexpensive
Can operate on almost the entire range of
temperature from near absolute zero to about
2700C.
No other sensor technology can match even
a fraction of this range.
Can be produced by anyone with minimum
skill
Can be made at the sensing site if necessary
41. Thermoelectric sensors (cont.)
Only one fundamental device: the
thermocouple
There are variations in construction/materials
Metal thermocouples
Thermopiles - multiple thermocouples in series
Semiconductor thermocouples and thermopiles
Peltier cells - special semiconductor thermopiles
used as actuators (to heat or cool)
42. Thermoelectric effect
The Seebeck effect (1821)
Seebeck effect is the sum of two other effects
The Peltier effect
The Thomson effect
The Peltier effect: heat generated or absorbed at the
junction of two dissimilar materials when an emf
exists across the junction due to the current produced
by this emf in the junction.
By connecting an external emf across the junction
By the emf generated by the junction itself.
A current must flow through the junction.
Applications in cooling and heating
Discovered in 1834
43. Thermoelectric effect (cont.)
The Thomson effect (1892): a current
carrying wire if unevenly heated along
its length will either absorb or radiate
heat depending on the direction of
current in the wire (from hot to cold or
from cold to hot).
Discovered in 1892 by William Thomson
(Lord Kelvin).
44. Thermoelectric effect (cont.)
The Seebeck effect: an emf produced
across the junction of two dissimilar
conducting materials connected together.
The sum of the Peltier and the Thomson
effects
The first to be discovered and used (1821)
The basis of all thermoelectric sensors
Peltier effect is used in Thermoelectric
Generators (TEG) devices
45. The Seebeck effect
If both ends of the two conductors are
connected and a temperature difference is
maintained between the two junctions, a
thermoelectric current will flow through the
closed circuit (generation mode)
46. The Seebeck effect
If the circuit is opened an emf will appear
across the open circuit (sensing mode). It is
this emf that is measured in a thermocouple
sensor.
47. Themocouple - analysis
Conductors a, b
homogeneous
Junctions at
temperatures T2 and
T1
On junctions 1 and
2:
Total emf:
emfA = aA T2 T1 emfB = aB T2 T1
emfT = emfA emfB = aA aB T2 T1 = aAB T2 T1
48. Thermocouple - analysis
aA and aB are the absolute Seebeck
coefficients given in V/C and are
properties of the materials A, B
aAB=aAaB is the relative Seebeck
coefficient of the material combination A
and B, given in V/C
The relative Seebeck coefficients are
normally used.
50. Thermocouples - standard
types
Table 3.4. Thermocouples (standard types and others) and some of their properties
Materials Sensitivity
[V/C]
at 25C.
Standard
Type
designation
Temperature
range [C]
Notes
Copper/Constantan 40.9 T 270 to 600 Cu/60%Cu40%Ni
Iron/Constantan 51.7 J 270 to
1000
Fe/60%Cu40%Ni
Chromel/Alumel 40.6 K 270 to
1300
90%Ni10%Cr/55%Cu45%Ni
Chromel/Constantan 60.9 E 200 to
1000
90%Ni10%Cr/60%Cu40%Ni
Platinum(10%)/Rhodium-Platinum 6.0 S to 1450 Pt/90%Pt10%Rh
Platinum(13%)/Rhodium-Platinum 6.0 R to 1600 Pt/87%Pt13%Rh
Silver/Paladium 10 200 to 600
Constantan/Tungsten 42.1 0 to 800
Silicon/Aluminum 446 40 to 150
Carbon/SiliconCarbide 170 0 to 2000
Note: sensitivity is the relative Seebeck coefficient.
51. Seebeck coefficients - notes:
Seebeck coefficients are rather small –
From a few microvolts to a few millivolts per
degree Centigrade.
Output can be measured directly
Output is often amplified before interfacing to
processors
Induced emfs due to external sources cause noise
Thermocouples can be used as thermometers
More often however the signal will be used to take
some action (turn on or off a furnace, detect pilot
flame before turning on the gas, etc.)
52. Thermoelectric laws:
Three laws govern operation of
thermocouples:
Law 1. A thermoelectric current cannot
be established in a homogeneous circuit
by heat alone.
This law establishes the need for junctions
of dissimilar materials since a single
conductor is not sufficient.
53. Thermoelectric laws:
Law 2. The algebraic sum of the thermoelectric
forces in a circuit composed of any number
and combination of dissimilar materials is
zero if all junctions are at uniform
temperatures.
Additional materials may be connected in the
thermoelectric circuit without affecting the output
of the circuit as long as any junctions added to the
circuit are kept at the same temperature.
voltages are additive so that multiple junctions
may be connected in series to increase the output.
54. Thermoelectric laws:
Law 3. If two junctions at temperatures T1
and T2 produce Seebeck voltageV2 and
temperatures T2 and T3 produce voltage V1,
then temperatures T1 and T3 produce
V3=V1+V2.
This law establishes methods of calibration of
thermocouples.
55. Thermocouples: connection
Based on the thermoelectric laws:
Usually connected in pairs
One junction for sensing
One junction for reference
Reference temperature can be lower or higher than sensing
temperature
56. Thermocouples (cont.)
Any connection in the circuit between
dissimilar materials adds an emf due to that
junction.
Any pair of junctions at identical temperatures
may be added without changing the output.
Junctions 3 and 4 are identical (one between
material b and c and one between material c and
b and their temperature is the same. No net emf
due to this pair
Junctions (5) and (6) also produce zero field
57. Thermocouples (cont.)
• Each connection adds two junctions.
• The strategy in sensing is:
For any junction that is not sensed or is not a reference
junction:
• Either each pair of junctions between dissimilar materials
are held at the same temperature (any temperature) or:
• Junctions must be between identical materials.
• Also: use unbroken wires leading from the sensor to the
reference junction or to the measuring instrument.
• If splicing is necessary to extend the length, identical wires
must be used to avoid additional emfs.
58. Connection without reference
The connection to a voltmeter creates two junctions
Both are kept at temperature T1
Net emf due to these junctions is zero
Net emf sensed is that due to junction (2)
This is commonly the method used
59. Reference junctions
Reference junctions must be at
constant, known temperatures.
Examples:
Water-ice bath (0C)
Boiling water (100C)
Any other temperature if measured
A separate, non-thermocouple sensor
The output compensated based on this
temperature from Seebeck coefficients
60. Thermocouples - practical
considerations
Choice of materials for thermocouples.
Materials affect:
The output emf,
Temperature range
Resistance of the thermocouple.
Selection of materials is done with the aid of
three tables:
Thermoelectric series table
Seebeck coefficients of standard types
Thermoelectric reference table
61. Thermoelectric series tables
Each material in the table is
thermoelectrically negative with respect
to all materials above it and positive
with respect to all materials below it.
The farther from each other a pair is,
the larger the emf output that will be
produced.
Tables are arranged by temperature
ranges
62. Thermoelectric series table
Table 3.5 The thermoelectric series:selectedelementsand alloys at selectedtemperatures
100C 500C 900C
Antimony Chromel Chromel
Chromel Copper Silver
Iron Silver Gold
Nichrome Gold Iron
Copper Iron 90%Pt-10Rh
Silver 90%Pt-10Rh Platinum
90%Pt-10Rh Platinum Cobalt
Platinum Cobalt Alumel
Cobalt Alumel Nickel
Alumel Nickel Constantan
Nickel Constantan
Constantan
63. Seebeck coefficients tables
Seebeck coefficients of materials with
reference to Platinum 67
Given for various thermocouple types
The first material in each type (E, J, K,
R, S and T) is positive, the second
negative.
65. Seebeck coefficients tables
The Seebeck emf with reference to Platinum
is given for the base elements of
thermocouples with respect to Platinum 67.
Example, J type thermocouples use Iron and
Constantan.
Column JP lists the Seebeck emf for Iron with respect to
Platinum
Column JN lists the emfs for Constantan.
Adding the two together gives the corresponding value for
the J type thermocouple in Table 3.5. JP and JN values at
0C in table 3.3 : 17.9+32.5=50.4 V/C gives the entry in
the J column at 0 C in Table 3.5.
67. Thermoelectric reference table
List the transfer function of each type of
thermocouple as an nth order polynomial, in a
range of temperatures.
Ensure accurate representation of the
thermocouple’s output and can be used by
the controller to accurately represent the
temperature sensed by the thermocouple.
An example of how these tables represent
the transfer function is shown next
69. Thermoelectric reference table
Table entry for type E thermocouples.
Second column is the exact representation of the
output emf (voltage) in V as a 9th order polynomial.
The third column shows the inverse relation and
gives the temperature based on the emf of the
thermocouple within a specified error – in this case
±0.1C.
The latter can be used by the controller to display
temperature or take action
74. Semiconductor thermocouples
Semiconductors have highest Seebeck
coefficients
Typical values are about 1mV/C
Junctions between n or p type
semiconductors with a metal (aluminum) are
most common
Smaller temperature ranges (usually –55 C
to about 150C.
Some materials - up to 225C
Newer devices - up to about 800C
75. Semiconductor
thermocouples: operation
Pure semiconductor: electrons in valence/covalence
bonds
Few electrons are available for conduction
Adding heat moves them across the energy gap into
the conduction band
To increase number of electrons - need to dope the
material
76. Semiconductor
thermocouples: operation
Doping
Add impurities - various materials
Increases availability of electrons (n-type)
or holes (p-type)
Increases the Seebeck coefficient
Silicon has 4 valent electrons
Add impurity with 5 electrons to create n
type silicon
Add impurity with 3 electrons to create p
type silicon
77. Semiconductor
thermocouples: operation
P type silicon junction (on aluminum)
Aluminum is deposited on an intrinsic layer of
silicon
The silicon is doped with materials from the IIIrd
group in the periodic table
materials such as Boron (B), Aluminum (Al),
Galium (Ga), Indium (In) and Thalium (Tl)
N type silicon junction (on aluminum)
The silicon is doped with materials from the Vth
group in the periodic table
materials such as Phosphorus (P), Arsenic (As),
Antimony (Sb) and Bismuth (Bi)
79. Thermopile
n thermocouples in
series electrically
In parallel thermally
Output is n times the
output of a single
thermocouple
80. Thermopiles (cont.)
Used to increase output
Sometimes done with metal
thermocouples
Example: pilot flame detector: 750 mV
at temperature difference of about
120C. about 100 metal thermocouples.
81. Semiconductor thermopiles
Each thermocouple has higher output
than metal based devices
A few thermocouples in series can
produce relatively high voltage
Used to produce thermoelectric
generators.
Outputs upwards of 15V are available
Known as Peltier cells
82. Peltier cells
Made of crystalline semiconductor materials
such as bismuth telluride (Bi2Te3) (n-p
junctions)
Peltier Cells are often used for cooling and
heating in dual purpose refrigerators,
Can also be used as sensors and can have
output voltages of a few volts (any voltage
can be achieved)
Also used as power generators for small
remote installations
83. Peltier cells (cont.)
Junctions are sandwiched between two
ceramic plates
Standard sizes are 15, 31, 63, 127 and 255
junctions
May be connected in series or parallel,
electrically and/or thermally.
Maximum temperature difference of about
100C
Maximum operating temperatures of about
225C
Also used as power generators for small
remote installations
86. P-N Junction temperature
sensors
A junction between a p and an n-doped
semiconductor
Usually silicon (also germanium,
galium-arsenide, etc.)
This is a simple diode
Forward biased
88. P-N junction sensor (cont.)
Forward current is temperature dependent
Any semiconductor diode will work
Usually the voltage across the diode is sensed
89. P-N junction sensor (cont.)
Forward current through
diode
Voltage across diode
I0 - saturation current
Eg - band gap energy
q - charge of electron
k - Boltzman’s constant
C - a temp. independent
constant
T - temperature (K)
I = I0eqV/2kT
Vf =
Eg
q 2kT
q ln C
I
90. P-N junction sensor (cont.)
If C and I are constant, Vf is linear with
temperature
Diode is an NTC device
Sensitivity: 1-10mV/C (current dependent)
91. P-N junction - operation
parameters
Forward biased with a current source
10-100A typically (low currents -
higher sensitivity)
Maximum range (silicon) –55 to 150C
Accuracy: ±0.1 C typical
Self heating error: 0.5 mW/C
Packaging: as a diode or as a transistor
(with additional circuitry)
94. Optical temperature sensors
Noncontact
Conversion of optical radiation into heat
Most useful in infrared temperature sensing
Relies on quantum effects - discussed in the
following chapter
Other sensors rely on phase difference in
propagation
Light propagates through a silicon optical fiber
Index of refraction is temperature sensitive
Phase of detected light is a measure of temperature
95. Acoustical temperature sensor
Speed of sound is
temperature dependent
Measure the time it
takes to travel through
the heated medium
Most sensors use
ultrasonic sensors for
this purpose.
vs = 331.5 T
273.15
m
s
98. Thermo-mechanical sensors
Changes of physical properties due to temperature
Length
Volume
Pressure, etc.
Expansion of gasses and fluids (thermometers)
Expansion of conductors (thermometers,
thermostats)
Many have a direct reading (graduation, dials)
Some activate switches directly (thermostats)
Examples:
99. Gas expansion temperature
sensor
Rise in temperature expands the gas
Diaphragm pushes on a “sensor” (strain
gauge, potentiometer) or even a switch
The sensor’s output is graduated in
temperature
100. Thermo-pneumatic sensor
Called a Golay cell
Gas expands in a flexible cell
Motion moves a mirror and deflects light
Extremely sensitive device
101. Thermal expansion of metals
All metals expand
with temperature
Volume stays
constant - length
changes
Each metal has a
coefficient of linear
expansion a.
a is usually given at
T1, temperatures in
C.
l2 = l1 1 + a T2 T1 m
103. Thermal expansion of metals
Coefficients of linear expansion are
small
They are however measurable
Can be used to directly operate a lever
to indicate temperature
Can be used to rotate a shaft
In most cases the bimetal configuration
is used
Serve as sensors and as actuators
104. Example: direct dial indication
Metal bar expands as temperature increases
Dial arrow moves to the left as temperature rises
Very small motion
The dial can be replaced to a pressure sensor or a
strain gauge
105. Bimetal sensors
Two metal strips welded together
Each metal strip has different coefficient of
expansion
As they expand, the two strips bend. This
motion can then be used to:
move a dial
actuate a sensor (pressure sensor for example)
rotate a potentiometer
close a switch
106. Bimetal sensors (cont.)
To extend motion, the bimetal strip is bent
into a coil. The dial rotates as the coil
expands/contracts
107. Bimetal sensors (cont.)
Displacement for the
bar bimetal:
r - radius of
curvature
T2 - sensed
temperature
T1 - reference
temperature
(horizontal position)
t - thickness of
bimetal bar
r = 2t
3 au al T2 T1
d = r 1 cos 180L
r
m
108. Bimetal switch (example)
Typical uses: flashers in cars, thermostats)
Operation
Left side is fixed
Right side moves down when heated
Cooling reverses the operation