Download - 3 - Temperature Sensors
3 - Temperature Sensors3 - Temperature Sensors
1. Thermoresistive sensors2. Thermoelectric sensors3. PN junction temperature sensors4. Optical and acoustic temperature sensors5. Thermo-mechanical sensors and actuators
1. Thermoresistive sensors2. Thermoelectric sensors3. PN junction temperature sensors4. Optical and acoustic temperature sensors5. Thermo-mechanical sensors and actuators
A bit of historyA 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
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
More history - sensorsMore 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’sUsed 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
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’sUsed 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
Temperature sensors - general
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 wellCan be used to measure other quantities
(electromagnetic radiation, air speed, flow, etc.)• Some newer sensors are semiconductor based
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 wellCan be used to measure other quantities
(electromagnetic radiation, air speed, flow, etc.)• Some newer sensors are semiconductor based
Temperature sensors - typesTemperature 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
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
Thermal actuatorsThermal actuators
A whole class of thermal actuators Bimetal actuators Expansion actuators Thermal displays Sometimes sensing and actuation is
combined in a single device
A whole class of thermal actuators Bimetal actuators Expansion actuators Thermal displays Sometimes sensing and actuation is
combined in a single device
Thermoresistive sensorsThermoresistive sensors
Two basic types: Resistive Temperature Detector (RTD)
Metal wireThin filmSilicon based
Thermistors (Thermal Resistor)NTC (Negative Temperature Coefficient)PTC (Positive Temperature Coefficient)
Two basic types: Resistive Temperature Detector (RTD)
Metal wireThin filmSilicon based
Thermistors (Thermal Resistor)NTC (Negative Temperature Coefficient)PTC (Positive Temperature Coefficient)
Thermoresistive effectThermoresistive effect
Conductivity depends on temperature
Conductors and semiconductors
Resistance is measured, all other parameters must stay constant.
Conductivity depends on temperature
Conductors and semiconductors
Resistance is measured, all other parameters must stay constant.
R = LσS
Thermoresistive effect (cont.)Thermoresistive effect (cont.)
Resistance of a length of wire
Conductivity is:Resistance as a
function of temperature:
- Temperature Coefficient of Resistance (TCR) [C]
Resistance of a length of wire
Conductivity is:Resistance as a
function of temperature:
- Temperature Coefficient of Resistance (TCR) [C]
R = LσS
σ = σ01 + α T − T0
RT = Lσ0S
1 + α T − T0
Thermoresistive effect (cont.)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.
- Temperature Coefficient of Resistance (TCR) [C] given at T0
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.
- Temperature Coefficient of Resistance (TCR) [C] given at T0
Example Example
Copper: 0=5.9x107 S/m, =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 .
Copper: 0=5.9x107 S/m, =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 .
Example (cont.) 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
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
Temperature Coefficient of Resistance
Temperature Coefficient of Resistance
Material Conductivity [S/m] Temperature Coefficint of Resistance (TCR) C
Copper (Cu) 5.7-5.9 x 107 0.0039 Carbon (C) 3.0 x105 0.0005 Constantan (60%Cu,40%Ni) 2.0 x106 0.00001 Cromium (Cr) 5.6 x 106 0.0059 Germanium (Ge) 2.2 0.05 Gold (Au) 4.1 x 107 0.0034 Iron (Fe) 1.0 x 107 0.0065 Mercury (Hg) 1.0 x 106 0.00089 Nichrome (NiCr) 1.0 x 106 0.0004 Nickel (Ni) 1.15 x 107 0.0069 Platinum (Pl) 9.4 x 106 0.01042 Silicon (Si) (pure) 4.35 x 10-6 0.07 Silver (Ag) 6.1 x 107 0.0016 Titanium (Ti) 1.8 x 106 0.042 Tungsten (W) 1.8 x 107 0.0056 Zinc (Zn) 1.76 x 107 0.0059 Aluminum (Al) 3.6 x 107 0.0043 Note: Instead of conductivity [S/m], some sources list resistivity , measured in ohm.meter = 1/ [ m). 1S/m=1/ m
Material Conductivity [S/m] Temperature Coefficint of Resistance (TCR) C
Copper (Cu) 5.7-5.9 x 107 0.0039 Carbon (C) 3.0 x105 0.0005 Constantan (60%Cu,40%Ni) 2.0 x106 0.00001 Cromium (Cr) 5.6 x 106 0.0059 Germanium (Ge) 2.2 0.05 Gold (Au) 4.1 x 107 0.0034 Iron (Fe) 1.0 x 107 0.0065 Mercury (Hg) 1.0 x 106 0.00089 Nichrome (NiCr) 1.0 x 106 0.0004 Nickel (Ni) 1.15 x 107 0.0069 Platinum (Pl) 9.4 x 106 0.01042 Silicon (Si) (pure) 4.35 x 10-6 0.07 Silver (Ag) 6.1 x 107 0.0016 Titanium (Ti) 1.8 x 106 0.042 Tungsten (W) 1.8 x 107 0.0056 Zinc (Zn) 1.76 x 107 0.0059 Aluminum (Al) 3.6 x 107 0.0043 Note: Instead of conductivity [S/m], some sources list resistivity , measured in ohm.meter = 1/ [ m). 1S/m=1/ m
Other considerationsOther 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)
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)
Construction - wire RTDConstruction - 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
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
Glass encapsulated RTDsGlass encapsulated RTDs
Construction (cont.) 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)
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)
Construction (cont.) 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
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
This is unrelated to the course - just a curiosity
This is unrelated to the course - just a curiosity
This is unrelated to the course - just a curiosity - close-up
This is unrelated to the course - just a curiosity - close-up
Thin Film RTDsThin 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.
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.
Tnin film RTDs - (cont.)Tnin film RTDs - (cont.)
Two types of thin film RTDs from different manufacturers
Dimensions are typical - some are much larger
Two types of thin film RTDs from different manufacturers
Dimensions are typical - some are much larger
Some parametersSome parameters
Temperature range: -250 C to 700 CResistances: typically 100 (higher available)Sizes: from a few mm to a few cmCompatibility: glass, ceramic encapsulationAvailable in ready made probesAccuracy: ±0.01 C to ±0.05 CCalibration: usually not necessary beyond
manufacturing
Temperature range: -250 C to 700 CResistances: typically 100 (higher available)Sizes: from a few mm to a few cmCompatibility: glass, ceramic encapsulationAvailable in ready made probesAccuracy: ±0.01 C to ±0.05 CCalibration: usually not necessary beyond
manufacturing
Self heat in RTDsSelf 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 filmPower dissipated: Pd=I2R ( I is the current
(RMS) and R the resistance of the sensor) Self heat depends on size and environmentGiven as temperature rise per unit power
(C/mW) Or: power needed to raise temperature (mW/ C)
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 filmPower dissipated: Pd=I2R ( I is the current
(RMS) and R the resistance of the sensor) Self heat depends on size and environmentGiven as temperature rise per unit power
(C/mW) Or: power needed to raise temperature (mW/ C)
Self heat in RTDs (cont.)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
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
Response time in RTDsResponse 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
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
Silicon Resistive SensorsSilicon Resistive Sensors
Conduction in semiconductorsValence 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
Conduction in semiconductorsValence 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
Silicon Resistive Sensors (cont.)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
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
Resistance of silicon resistive sensor
Resistance of silicon resistive sensor
Resistance of silicon resistive sensor - specific device
Resistance of silicon resistive sensor - specific device
Silicon resistive sensorsSilicon 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.
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.
ThermistorsThermistors
Thermistor: Thermal resistorBecame available: early 1960’sBased on oxides of semiconductors
High temperature coefficients NTC High resistances (typically)
Thermistor: Thermal resistorBecame available: early 1960’sBased on oxides of semiconductors
High temperature coefficients NTC High resistances (typically)
Thermistors (cont.)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 ( is small) Approximate transfer function
Transfer function:
[] 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
RT = αeβ/T
ConstructionConstruction
Beads ChipsDeposition on substrate
Beads ChipsDeposition on substrate
Epoxy encapsulated bead thermistors
Epoxy encapsulated bead thermistors
Thermistors - propertiesThermistors - properties
Most are NTC devicesSome are PTC devicesPTC are made from special materials
Not as common Advantageous when runaway
temperatures are possible
Most are NTC devicesSome are PTC devicesPTC are made from special materials
Not as common Advantageous when runaway
temperatures are possible
Thermistors - propertiesThermistors - 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
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
Thermistors - propertiesThermistors - 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.
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.
Thermoelectric sensorsThermoelectric sensors
Among the oldest sensors (over 150 years)Some of the most useful and most commonPassive 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
Among the oldest sensors (over 150 years)Some of the most useful and most commonPassive 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
Thermoelectric sensors (cont.)Thermoelectric sensors (cont.)
Simple, rugged and inexpensiveCan 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
Simple, rugged and inexpensiveCan 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
Thermoelectric sensors (cont.)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)
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)
Thermoelectric effectThermoelectric 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
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
Thermoelectric effect (cont.)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).
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).
Thermoelectric effect (cont.)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 sensorsPeltier effect is used in Thermoelectric
Generators (TEG) devices
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 sensorsPeltier effect is used in Thermoelectric
Generators (TEG) devices
The Seebeck effectThe 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)
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)
The Seebeck effectThe 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.
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.
Themocouple - analysisThemocouple - analysis
Conductors a, b homogeneous
Junctions at temperatures T2 and T1
On junctions 1 and 2:
Total emf:
Conductors a, b homogeneous
Junctions at temperatures T2 and T1
On junctions 1 and 2:
Total emf:
emfA
= A
T2
− T
emfB
= B
T2
− T
emfT
= emfA
− emfB
= A
− B
T2
− T
= AB
T2
− T
Thermocouple - analysisThermocouple - analysis
A and B are the absolute Seebeck coefficients given in V/C and are properties of the materials A, B
AB=AB is the relative Seebeck coefficient of the material combination A and B, given in V/C
The relative Seebeck coefficients are normally used.
A and B are the absolute Seebeck coefficients given in V/C and are properties of the materials A, B
AB=AB is the relative Seebeck coefficient of the material combination A and B, given in V/C
The relative Seebeck coefficients are normally used.
Absolute Seebeck coefficientsAbsolute Seebeck coefficients
Table 3.3. Absolute Seebeck coefficients for selected elements (Thermoelectric series) Material [ V/°K] p-Silicon 00 - 000 Antimony ( )Sb 32 Iron (F )e 3.4 Gold (A )u 0. Copper ( )Cu 0 Silver (Ag) 0.2 Aluminum (Al) 3.2 Platinum (Pt) .9 Cobalt (Co) 20. Nickel (Ni) 20.4 Bismut h (Sb) 72.8 n-Silicon 00 to -000
Table 3.3. Absolute Seebeck coefficients for selected elements (Thermoelectric series) Material [ V/°K] p-Silicon 00 - 000 Antimony ( )Sb 32 Iron (F )e 3.4 Gold (A )u 0. Copper ( )Cu 0 Silver (Ag) 0.2 Aluminum (Al) 3.2 Platinum (Pt) .9 Cobalt (Co) 20. Nickel (Ni) 20.4 Bismut h (Sb) 72.8 n-Silicon 00 to -000
Thermocouples - standard types
Thermocouples - standard types
Table 3.4. Thermocouples (standard types and others) and some of their propertiesMaterials Sensitivity
[V/°C]a t2°C.
StandardTypedesignation
Temperaturerange [°C]
Notes
Copper/Constantan 40.9 T −270 600to Cu/60% 40Cu %NiIron/Constantan .7 J −270 to
000Fe/60% 40Cu %Ni
Chrome /l Alumel 40.6 K −270 to300
90%Ni0%Cr/% 4Cu %Ni
Chrome /l Constantan 60.9 E −200 to000
90%Ni0%Cr/60% 40Cu %Ni
Platinum(0%)/Rhodium-Platinum 6.0 S 0 40to /Pt90%Pt0%RhPlatinum(3%)/Rhodium-Platinum 6.0 R 0 600to /Pt87%Pt3%RhSilver/Paladium 0 200 600toConstantan/Tungsten 42. 0 800toSilicon/Aluminum 446 −40 0toCarbon/Sili conCarbide 70 0 2000toNote: sensitivity is the relati veSee beck coefficien .t
Table 3.4. Thermocouples (standard types and others) and some of their propertiesMaterials Sensitivity
[V/°C]a t2°C.
StandardTypedesignation
Temperaturerange [°C]
Notes
Copper/Constantan 40.9 T −270 600to Cu/60% 40Cu %NiIron/Constantan .7 J −270 to
000Fe/60% 40Cu %Ni
Chrome /l Alumel 40.6 K −270 to300
90%Ni0%Cr/% 4Cu %Ni
Chrome /l Constantan 60.9 E −200 to000
90%Ni0%Cr/60% 40Cu %Ni
Platinum(0%)/Rhodium-Platinum 6.0 S 0 40to /Pt90%Pt0%RhPlatinum(3%)/Rhodium-Platinum 6.0 R 0 600to /Pt87%Pt3%RhSilver/Paladium 0 200 600toConstantan/Tungsten 42. 0 800toSilicon/Aluminum 446 −40 0toCarbon/Sili conCarbide 70 0 2000toNote: sensitivity is the relati veSee beck coefficien .t
Seebeck coefficients - notes: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.)
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.)
Thermoelectric laws: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.
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.
Thermoelectric laws: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.
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.
Thermoelectric laws: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.
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.
Thermocouples: connectionThermocouples: 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
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
Thermocouples (cont.)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
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
Thermocouples (cont.)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.
• 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.
Connection without referenceConnection 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
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
Reference junctionsReference 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
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
Thermocouples - practical considerations
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
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
Thermoelectric series tablesThermoelectric 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
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
Thermoelectric series tableThermoelectric series table
Table 3.5 The thermoelectric series: selected elements and alloys at selected temperatures 100°C 00°C 900°C Antimony Chromel Chromel Chromel Copper Silver Iron Silver Gold Nichrome Gold Iron Copper Iron 90%Pt-0Rh Silver 90%Pt-0Rh Platinum 90%Pt-0Rh Platinum Cobalt Platinum Cobalt Alumel Cobalt Alumel Nickel Alumel Nickel Constantan Nickel Constantan Constantan
Table 3.5 The thermoelectric series: selected elements and alloys at selected temperatures 100°C 00°C 900°C Antimony Chromel Chromel Chromel Copper Silver Iron Silver Gold Nichrome Gold Iron Copper Iron 90%Pt-0Rh Silver 90%Pt-0Rh Platinum 90%Pt-0Rh Platinum Cobalt Platinum Cobalt Alumel Cobalt Alumel Nickel Alumel Nickel Constantan Nickel Constantan Constantan
Seebeck coefficients tablesSeebeck 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.
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.
Seebeck coefficient tablesSeebeck coefficient tables
Table 3.6. Seebeck coefficients with respect to Platinum 67 Thermoelement type – Seebeck coefficient [ V/° ]C Tem .p [° ]C JP JN TP ,TN EN K ,P EP KN 0 7.9 32. .9 32.9 2.8 3.6 00 7.2 37.2 9.4 37.4 30. .2 200 4.6 40.9 .9 4.3 32.8 7.2 300 .7 43.7 4.3 43.8 34. 7.3 400 9.7 4.4 6.3 4. 34. 7.7 00 9.6 46.4 46.6 34.3 8.3 600 .7 46.8 46.9 33.7 8.8 700 .4 46.9 46.8 33.0 8.8 800 46.3 32.2 8.8 900 4.3 3.4 8. 000 44.2 30.8 8.2
Table 3.6. Seebeck coefficients with respect to Platinum 67 Thermoelement type – Seebeck coefficient [ V/° ]C Tem .p [° ]C JP JN TP ,TN EN K ,P EP KN 0 7.9 32. .9 32.9 2.8 3.6 00 7.2 37.2 9.4 37.4 30. .2 200 4.6 40.9 .9 4.3 32.8 7.2 300 .7 43.7 4.3 43.8 34. 7.3 400 9.7 4.4 6.3 4. 34. 7.7 00 9.6 46.4 46.6 34.3 8.3 600 .7 46.8 46.9 33.7 8.8 700 .4 46.9 46.8 33.0 8.8 800 46.3 32.2 8.8 900 4.3 3.4 8. 000 44.2 30.8 8.2
Seebeck coefficients tablesSeebeck 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.
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.
Seebeck coefficients by typeSeebeck coefficients by type
Table 3.7. Seebeck coefficients for various types of thermocouples Thermocouple type – Seebeck coefficient [ V/° ]C Tem .p [° ]C E J K R S T -200 2. 2.9 .3 .7 -00 4.2 4. 30. 28.4 0 8.7 0.4 39. .3 .4 38.7 00 67. 4.3 4.4 7. 7.3 46.8 200 74.0 . 40.0 8.8 8. 3. 300 77.9 .4 4.4 9.7 9. 8. 400 80.0 . 42.2 0.4 9.6 6.8 00 80.9 6.0 42.6 0.9 9.9 600 80.7 8. 42. .3 0.2 700 79.8 62.2 4.9 .8 0. 800 78.4 4.0 2.3 0.9 900 76.7 40.0 2.8 .2 000 74.9 38.9 3.2 .
Table 3.7. Seebeck coefficients for various types of thermocouples Thermocouple type – Seebeck coefficient [ V/° ]C Tem .p [° ]C E J K R S T -200 2. 2.9 .3 .7 -00 4.2 4. 30. 28.4 0 8.7 0.4 39. .3 .4 38.7 00 67. 4.3 4.4 7. 7.3 46.8 200 74.0 . 40.0 8.8 8. 3. 300 77.9 .4 4.4 9.7 9. 8. 400 80.0 . 42.2 0.4 9.6 6.8 00 80.9 6.0 42.6 0.9 9.9 600 80.7 8. 42. .3 0.2 700 79.8 62.2 4.9 .8 0. 800 78.4 4.0 2.3 0.9 900 76.7 40.0 2.8 .2 000 74.9 38.9 3.2 .
Thermoelectric reference tableThermoelectric 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
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
Thermoelectric reference table (cont.)
Thermoelectric reference table (cont.)
Table 3.8. Transfer function for type E thermocouples Temperature range [° ]C Exact reference emf (volt ) age
[E m ]v Reference temperature T [° ]C
0 to 400 400 to 000
(+.86987799 0 x x T +4.3094462 x 0-2 x T2 +.722038202 x 0- x T3 -.402066808 0x -7 x T4 +.42922x 0-9 x T -2.48008936 0x -2 x T6 +2.33897249 x 0- xT7 -.9462968 0x -8 x T8 +2.627497 x 0-22 xT9) x0
Within ± 0.°C .70222 0 x x E -2.2097240 0x - x E2 +.480934 x 0-3 x E3 -.7669892 0x - x E4 2.9347907 0x +.33834 x 0 x E -2.6669928 0x -2 x E2 +2.3388779 x 0-4 x E3
Table 3.8. Transfer function for type E thermocouples Temperature range [° ]C Exact reference emf (volt ) age
[E m ]v Reference temperature T [° ]C
0 to 400 400 to 000
(+.86987799 0 x x T +4.3094462 x 0-2 x T2 +.722038202 x 0- x T3 -.402066808 0x -7 x T4 +.42922x 0-9 x T -2.48008936 0x -2 x T6 +2.33897249 x 0- xT7 -.9462968 0x -8 x T8 +2.627497 x 0-22 xT9) x0
Within ± 0.°C .70222 0 x x E -2.2097240 0x - x E2 +.480934 x 0-3 x E3 -.7669892 0x - x E4 2.9347907 0x +.33834 x 0 x E -2.6669928 0x -2 x E2 +2.3388779 x 0-4 x E3
Thermoelectric reference tableThermoelectric 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
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
Standard thermocouples - propertiesStandard thermocouples - properties
Table 3.9. Common thermocouple types and some of their properties. Materials Sensitivity
[ V/° ] C a t2° .C
Standard Type designation
Recommended temperat urerange [° ]C
Notes
Co /pper Constantan 40.9 T 0 to 400 ( 270 - 400)
/60% 40%Cu Cu Ni
Iron/Constantan ,7 J 0 to 760 ( 20 - 200)
F /60% 40%e Cu Ni
Chromel/Alumel 40.6 K 200 to 300 ( 270 - 372)
90%Ni0% /% 4%Cr Cu Ni
Chromel/Constantan 60.9 E 200 to 900 ( 270 - 000)
90%Ni0% /60% 40%Cr Cu Ni
Platinum(0%)/Rhodium-Platinum
6.0 S 0 to 40 ( 0 - 760)
Pt/90%Pt0%Rh
Platinum(3%)/Rhodium-Platinum
6.0 R 0 to 600 ( 0 - 760)
Pt/87%Pt3%Rh
Silv /er Paladium 0 200 to 600 Constantan/Tungsten 42. 0 to 800 Silicon/Aluminum 446 40 to 0 Carbon/Silico nCarbide 70 0 to 2000 Platinum(30%)/Rhodium-Platinum
6.0 B 0 to 820 Pt/70%Pt30%Rh
Nickel/Cromium-silico nalloy N ( 270 - 260) Not :e the temperature ranges shown are recomm .ended Nominal ranges are lower and higher and shown in bracket .s
Table 3.9. Common thermocouple types and some of their properties. Materials Sensitivity
[ V/° ] C a t2° .C
Standard Type designation
Recommended temperat urerange [° ]C
Notes
Co /pper Constantan 40.9 T 0 to 400 ( 270 - 400)
/60% 40%Cu Cu Ni
Iron/Constantan ,7 J 0 to 760 ( 20 - 200)
F /60% 40%e Cu Ni
Chromel/Alumel 40.6 K 200 to 300 ( 270 - 372)
90%Ni0% /% 4%Cr Cu Ni
Chromel/Constantan 60.9 E 200 to 900 ( 270 - 000)
90%Ni0% /60% 40%Cr Cu Ni
Platinum(0%)/Rhodium-Platinum
6.0 S 0 to 40 ( 0 - 760)
Pt/90%Pt0%Rh
Platinum(3%)/Rhodium-Platinum
6.0 R 0 to 600 ( 0 - 760)
Pt/87%Pt3%Rh
Silv /er Paladium 0 200 to 600 Constantan/Tungsten 42. 0 to 800 Silicon/Aluminum 446 40 to 0 Carbon/Silico nCarbide 70 0 to 2000 Platinum(30%)/Rhodium-Platinum
6.0 B 0 to 820 Pt/70%Pt30%Rh
Nickel/Cromium-silico nalloy N ( 270 - 260) Not :e the temperature ranges shown are recomm .ended Nominal ranges are lower and higher and shown in bracket .s
Thermocouple (exposed junction)
Thermocouple (exposed junction)
Thermocouple (flexible, to be cemented to surface)
Thermocouple (flexible, to be cemented to surface)
Thermocouple (protected junction)
Thermocouple (protected junction)
Semiconductor thermocouplesSemiconductor 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 225CNewer devices - up to about 800C
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 225CNewer devices - up to about 800C
Semiconductor thermocouples: operation
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
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
Semiconductor thermocouples: operation
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
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
Semiconductor thermocouples: operation
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)
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)
Periodic table - semiconductorsPeriodic table -
semiconductors
ThermopileThermopile
n thermocouples in series electrically
In parallel thermallyOutput is n times the
output of a single thermocouple
n thermocouples in series electrically
In parallel thermallyOutput is n times the
output of a single thermocouple
Thermopiles (cont.)Thermopiles (cont.)
Used to increase outputSometimes done with metal
thermocouplesExample: pilot flame detector: 750 mV
at temperature difference of about 120C. about 100 metal thermocouples.
Used to increase outputSometimes done with metal
thermocouplesExample: pilot flame detector: 750 mV
at temperature difference of about 120C. about 100 metal thermocouples.
Semiconductor thermopilesSemiconductor 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 availableKnown as Peltier cells
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 availableKnown as Peltier cells
Peltier cellsPeltier 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
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
Peltier cells (cont.)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
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
Some thermopiles (Peltier TEGs)
Some thermopiles (Peltier TEGs)
Details of the TEG construction
Details of the TEG construction
P-N Junction temperature sensors
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 diodeForward biased
A junction between a p and an n-doped semiconductor
Usually silicon (also germanium, galium-arsenide, etc.)
This is a simple diodeForward biased
P-N junction sensor (cont.)P-N junction sensor (cont.)
Construction of the sensorConstruction of the sensor
P-N junction sensor (cont.)P-N junction sensor (cont.)
Forward current is temperature dependent Any semiconductor diode will work Usually the voltage across the diode is sensed
Forward current is temperature dependent Any semiconductor diode will work Usually the voltage across the diode is sensed
P-N junction sensor (cont.)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)
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 = Egq − 2kT
q lnCI
P-N junction sensor (cont.)P-N junction sensor (cont.)
If C and I are constant, Vf is linear with temperature
Diode is an NTC deviceSensitivity: 1-10mV/C (current dependent)
If C and I are constant, Vf is linear with temperature
Diode is an NTC deviceSensitivity: 1-10mV/C (current dependent)
P-N junction - operation parameters
P-N junction - operation parameters
Forward biased with a current source10-100A typically (low currents -
higher sensitivity)Maximum range (silicon) –55 to 150CAccuracy: ±0.1 C typicalSelf heating error: 0.5 mW/CPackaging: as a diode or as a transistor
(with additional circuitry)
Forward biased with a current source10-100A typically (low currents -
higher sensitivity)Maximum range (silicon) –55 to 150CAccuracy: ±0.1 C typicalSelf heating error: 0.5 mW/CPackaging: as a diode or as a transistor
(with additional circuitry)
The LM35 sensorThe LM35 sensor
Other temperature sensorsOther temperature sensors
OpticalAcousticalThermomechanical sensorsThermomecahnical actuators
OpticalAcousticalThermomechanical sensorsThermomecahnical actuators
Optical temperature sensorsOptical 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
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
Acoustical temperature sensorAcoustical 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.
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
vs
= 331.5
T
273.15
m
s
Acoustical temperature sensorAcoustical temperature sensor
Acoustical temperature sensorAcoustical temperature sensor
Thermo-mechanical sensorsThermo-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:
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:
Gas expansion temperature sensor
Gas expansion temperature sensor
Rise in temperature expands the gasDiaphragm pushes on a “sensor” (strain
gauge, potentiometer) or even a switchThe sensor’s output is graduated in
temperature
Rise in temperature expands the gasDiaphragm pushes on a “sensor” (strain
gauge, potentiometer) or even a switchThe sensor’s output is graduated in
temperature
Thermo-pneumatic sensorThermo-pneumatic sensor
Called a Golay cellGas expands in a flexible cellMotion moves a mirror and deflects lightExtremely sensitive device
Called a Golay cellGas expands in a flexible cellMotion moves a mirror and deflects lightExtremely sensitive device
Thermal expansion of metalsThermal expansion of metals
All metals expand with temperature
Volume stays constant - length changes
Each metal has a coefficient of linear expansion .
is usually given at T1, temperatures in C.
All metals expand with temperature
Volume stays constant - length changes
Each metal has a coefficient of linear expansion .
is usually given at T1, temperatures in C.
l2 = l11 + α T2 − T1 m
Coefficients of linear expansion
Coefficients of linear expansion
Table 3.10. Coefficients of linear expansion for some. Coefficients are given per ° .C Material Coefficien t of expansio na, x0 Aluminum 2.0 Chromium 30.0 Copper 6.6 Gold 4.2 Iron 2.0 Nickel .8 Platinum 9.0 Phosphor-bronze 9.3 Silver 9.0 Titanium 6. Tungsten 4. Zink 3
Table 3.10. Coefficients of linear expansion for some. Coefficients are given per ° .C Material Coefficien t of expansio na, x0 Aluminum 2.0 Chromium 30.0 Copper 6.6 Gold 4.2 Iron 2.0 Nickel .8 Platinum 9.0 Phosphor-bronze 9.3 Silver 9.0 Titanium 6. Tungsten 4. Zink 3
Thermal expansion of metalsThermal expansion of metals
Coefficients of linear expansion are small
They are however measurableCan be used to directly operate a lever
to indicate temperatureCan be used to rotate a shaftIn most cases the bimetal configuration
is usedServe as sensors and as actuators
Coefficients of linear expansion are small
They are however measurableCan be used to directly operate a lever
to indicate temperatureCan be used to rotate a shaftIn most cases the bimetal configuration
is usedServe as sensors and as actuators
Example: direct dial indicationExample: 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
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
Bimetal sensorsBimetal sensors
Two metal strips welded togetherEach metal strip has different coefficient of
expansionAs 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
Two metal strips welded togetherEach metal strip has different coefficient of
expansionAs 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
Bimetal sensors (cont.)Bimetal sensors (cont.)
To extend motion, the bimetal strip is bent into a coil. The dial rotates as the coil expands/contracts
To extend motion, the bimetal strip is bent into a coil. The dial rotates as the coil expands/contracts
Bimetal sensors (cont.)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
Displacement for the bar bimetal: r - radius of
curvature T2 - sensed
temperature T1 - reference
temperature (horizontal position)
t - thickness of bimetal bar
r =
2 t
3 u
− l
T2
− T
d = r 1 − cos80 L
π r
md = r 1 − cos80 L
π r
m
Bimetal switch (example)Bimetal switch (example)
Typical uses: flashers in cars, thermostats)Operation
Left side is fixed Right side moves down when heated Cooling reverses the operation
Typical uses: flashers in cars, thermostats)Operation
Left side is fixed Right side moves down when heated Cooling reverses the operation
Bimetal coil thermometerBimetal coil thermometer
Bimetal switch (car flasher)Bimetal switch (car flasher)