1. 3 - Temperature Sensors 1. Thermoresistive sensors
2. Thermoelectric sensors
3. PN junction temperature sensors
Optical and acoustic temperature sensors
Thermo-mechanical sensors and actuators 1

2. A bit of history Temperature measurements and thermometers
thermometers (water expansion, mercury)
first attempts at temperature scales (Boyle)
“standard” temperature scales (Magelotti, Renaldini, Newton) - did not catch
Farenheit scale (180 div.)
Celsius scale
Kelvin scale (based on Carnot’s thermodynamic work)
IPTS - International Practical Temperature Scale 2

3. More history - sensors Temperature sensors are the oldest
Seebeck effect (Thomas Johann Seebeck)
first sensor - a thermocouple - based on the Seebeck effect (Antoine Cesar Becquerel)
Peltier effect (Charles Athanase Peltier).
First peltier cell built in 1960’s
Used for cooling and heating
discovery of temperature dependence of conductivity (Sir Humphrey Davey)
William Siemens builds the first resistive sensor made of platinum 3

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 4

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 5

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) 6

8. Thermoresistive effect
Conductivity depends on temperature
Conductors and semiconductors
Resistance is measured, all other parameters must stay constant. 7

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] 8

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 20C but may be given at other temperatures as necessary.
a - Temperature Coefficient of Resistance (TCR) [C] given at T0 9

11. Example
Copper: 0=5.9x107 S/m, = C at T0=20C. Wire of cross-sectional area: 0.1 mm2, length L=1m,
Change in resistance of 6.61x10 /C and a base resistance of  at 20C
Change of 0.38% per C . 10

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 11

13. Temperature Coefficient of Resistance12

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 (25W and up) 13

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 14

16. Glass encapsulated RTDs15

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 –250C.
Very sensitive to strain
Sensitive to chemical contaminants
Wire length needed is long (high conductivity) 16

18. Construction (cont.) Materials: Nickel and Copper Less expensive
Reduced temperature range (copper only works up to about 300C)
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 17

19. This is unrelated to the course - just a curiosity

20. This is unrelated to the course - just a curiosity - close-up

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. 18

22. Tnin film RTDs - (cont.)
Two types of thin film RTDs from different manufacturers
Dimensions are typical - some are much larger 19

23. Some parameters Temperature range: -250 C to 700 C
Resistances: typically 100W (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 20

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) 21

25. Self heat in RTDs (cont.)
Errors are of the order of 0.01C/mW to 10C/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 22

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 23

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 24

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 25

29. Resistance of silicon resistive sensor26

30. Resistance of silicon resistive sensor - specific device27

31. Silicon resistive sensors
Silicon resistive sensors are somewhat nonlinear and offer sensitivities of the order %/C.
Can operate in a limited range of temperatures like most semiconductors devices based on silicon
Maximum range is between –55C to +150C.
Typical range: - 45C to +85C or 0C to +80C
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. 28

32. Thermistors Thermistor: Thermal resistor
Became available: early 1960’s
Based on oxides of semiconductors
High temperature coefficients
NTC
High resistances (typically) 29

33. Thermistors (cont.) Transfer function:  [] and  [K] are constants
R(T): resistance of the device
T: temperature in K
Relation is nonlinear but:
Only mildly nonlinear (b is small)
Approximate transfer function 30

34. Construction
Beads
Chips
Deposition on substrate 31

35. Epoxy encapsulated bead thermistors32

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 33

37. Thermistors - properties
Self heating errors as in RTDs but:
Usually lower because resistance is higher
Current very low (R high)
Typical values: 0.01C/mW in water to 1C/mW in air
Wide range of resistances up to a few MW
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 0.25C for good thermistors 34

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. 35

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 36

40. Thermoelectric sensors (cont.)
Simple, rugged and inexpensive
Can operate on almost the entire range of temperature from near absolute zero to about 2700C.
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 37

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) 38

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 39

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). 40

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 41

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) 42

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. 43

47. Themocouple - analysis
Conductors a, b homogeneous
Junctions at temperatures T2 and T1
On junctions 1 and 2:
Total emf: 44

48. Thermocouple - analysis
A and B are the absolute Seebeck coefficients given in V/C and are properties of the materials A, B
AB=A-B is the relative Seebeck coefficient of the material combination A and B, given in V/C
The relative Seebeck coefficients are normally used. 45

49. Absolute Seebeck coefficients46

50. Thermocouples - standard types47

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.) 48

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. 49

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. 50

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. 51

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 52

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 53

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. 54

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 55

59. Reference junctions
Reference junctions must be at constant, known temperatures. Examples:
Water-ice bath (0C)
Boiling water (100C)
Any other temperature if measured
A separate, non-thermocouple sensor
The output compensated based on this temperature from Seebeck coefficients 56

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 57

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 58

62. Thermoelectric series table59

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. 60

64. Seebeck coefficient tables61

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 0C in table 3.3 : =50.4 mV/C gives the entry in the J column at 0 C in Table 3.5. 62

66. Seebeck coefficients by type63

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 64

68. Thermoelectric reference table (cont.)65

69. Thermoelectric reference table
Table entry for type E thermocouples.
Second column is the exact representation of the output emf (voltage) in mV 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 ±0.1C.
The latter can be used by the controller to display temperature or take action 66

70. Standard thermocouples - properties67

71. Thermocouple (exposed junction)

72. Thermocouple (flexible, to be cemented to surface)

73. Thermocouple (protected junction)

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 150C.
Some materials - up to 225C
Newer devices - up to about 800C 68

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 69

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 70

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) 71

78. Periodic table - semiconductors

79. Thermopile n thermocouples in series electrically
In parallel thermally
Output is n times the output of a single thermocouple 72

80. Thermopiles (cont.) Used to increase output
Sometimes done with metal thermocouples
Example: pilot flame detector: 750 mV at temperature difference of about 120C. about 100 metal thermocouples. 73

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 74

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 75

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 100C
Maximum operating temperatures of about 225C
Also used as power generators for small remote installations 76

84. Some thermopiles (Peltier TEGs)77

85. Details of the TEG construction78

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 79

87. P-N junction sensor (cont.)
Construction of the sensor 80

88. P-N junction sensor (cont.)
Forward current is temperature dependent
Any semiconductor diode will work
Usually the voltage across the diode is sensed 81

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) 82

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) 83

91. P-N junction - operation parameters
Forward biased with a current source
10-100mA typically (low currents - higher sensitivity)
Maximum range (silicon) –55 to 150C
Accuracy: ±0.1 C typical
Self heating error: 0.5 mW/C
Packaging: as a diode or as a transistor (with additional circuitry) 84

92. The LM35 sensor 85

93. Other temperature sensors
Optical
Acoustical
Thermomechanical sensors
Thermomecahnical actuators 86

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 87

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. 88

96. Acoustical temperature sensor89

97. Acoustical temperature sensor90

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: 91

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 92

100. Thermo-pneumatic sensor
Called a Golay cell
Gas expands in a flexible cell
Motion moves a mirror and deflects light
Extremely sensitive device 93

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. 94

102. Coefficients of linear expansion95

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 96

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 97

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 98

106. Bimetal sensors (cont.)
To extend motion, the bimetal strip is bent into a coil. The dial rotates as the coil expands/contracts 99

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
100

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
101

109. Bimetal coil thermometer

110. Bimetal switch (car flasher)