2014-2015 Sensors & Actuators - H.Sarmento 1
Outline
• Non-electric devices.
• Contact:
– RTD - Resistance temperature detectors.
– Silicon resistive sensors
– Thermistors.
– Thermocouples.
– Semiconductor pn junction.
– Piezoelectric temperature sensors.
• Non contact
– Infrared thermometer.
2014-2015 Sensors & Actuators - H.Sarmento 2
Non-electric devices
• Read visually, do not produce electrical signals.
– Molecular change-of-state devices.
– Fluid expansion devices.
– Bimetallic devices.
• Disadvantages: less accurate than electrical sensors, and
temperature values not easily recorded.
• Advantages: portability and independence from a power
supply.
2014-2015 Sensors & Actuators - H.Sarmento 3
Molecular change-of-state devices
• Devices whose appearance changes once a certain temperature
is reached: labels, crayons, lacquers and liquid crystals.
[Source: Omega]
2014-2015 Sensors & Actuators - H.Sarmento 4
Fluid expansion devices
• the volume of a liquid or a gas changes as a function of
temperature.
[Source: Labon][Source: Omega]
2014-2015 Sensors & Actuators - H.Sarmento 5
Bimetallic devices (1)
• Two metals with dissimilar thermal expansion coefficients.
• Manufactured in different forms for cooking, refrigerators and
freezes, industrial applications.
[Source: globalspec]
2014-2015 Sensors & Actuators - H.Sarmento 6
Bimetallic devices (2)
• Two metals with similar moduli of elasticity and thicknesses.
• The radius of curvature r changer with temperature.
• Complex sensor: use of a displacement sensor to measure r.
thermal expansion coefficients aA,aB
BA aa
1T
t
2T
12 TT
21 TT
BAr
tTT
aa
3
212
2014-2015 Sensors & Actuators - H.Sarmento 7
Thermo resistive sensors (1)
• RTD (Resistance Temperature Detector), silicon resistive sensors
(KTY) and NTC (Negative Temperature Coefficients) thermistors.
RTD: Metal (Pt)KTY: doped siliconNTC: doped ceramic
2014-2015 Sensors & Actuators - H.Sarmento 8
Thermo resistive sensors (2)
• Power supply requirements: active (modulating).
• Stimulus perception: contact.
• Stimulus detection: absolute.
• Complexity: direct sensor.
• Type of stimulus: thermal.
• Transduction principle: resistive.
• Energy conversion: thermal electrical.
2014-2015 Sensors & Actuators - H.Sarmento 9
• Metallic: almost platinum (linear, predictable response, long-
term stability, and durability) used, but also nickel, copper,
tungsten (rare) and alloys.
• Resistivity increases with temperature.
Resistance temperature detectors
RTD
2014-2015 Sensors & Actuators - H.Sarmento 10
• Metallic: resistivity of metals increases with temperature.
• Approximate transfer function (Callendar–van Dusen):
– From -200ºC to 0ºC
– From 0ºC to 630ºC
RTD transfer function
n
nt TTTTTTRR 0
2
02010 ...1 aaa
1001 320 TCTBTATRRt
20 1 BTATRRt
2014-2015 Sensors & Actuators - H.Sarmento 11
• Most popular: base resistance of 100 (Pt100) , 500 (Pt500) or
1000 (Pt1000) ohms.
• The most common is Pt100 (resistance of 100 ohms specified
at 0°C).
[Source: rdt-products]
Platinum RTD
Pt100
2014-2015 Sensors & Actuators - H.Sarmento 12
RTD advantages
• Accurate (±0.1 ºC)
• Highly linear over limited temperature range (platinum)
• Wide temperature range
• Long term stability
• Repeatable
• Standardized.
• Resistant to contamination /corrosion
Material Temp Range
PLATINUM -260 º +650 º
NICKEL -100 º +300 º
COPPER -75 º +150 º
2014-2015 Sensors & Actuators - H.Sarmento 13
Standardized RTD
• Platinum RTDs are standardized:
IEC (International Electrotechnical Commission)
DIN (Deutsches Institut für Normung)IEC-751.
2014-2015 Sensors & Actuators - H.Sarmento 14
RTD disadvantages
Disadvantages:
• Expensive wire wound, but low cost thin film
• Low sensitivity (Pt100 0.4 ohm/ºC)
• Exist in limited values (Pt 100, Pt500, Pt1000)
• Self-heating
• Slow response time
• Sensitive to shock and vibration
2014-2015 Sensors & Actuators - H.Sarmento 15
RTD field of applications
Industrial applications:
• Oil & Gas industry: thermowells.
• Food & beverage, pharmaceutical and bio-technology plants:
Temperature dryers in food processes, Pasteurization, Heat
exchangers, Material storage tanks, Cheese vats, Brewhouse /
cellar, Cookers / freezers, Dehydrator, Fermentor / bio-
reactor control.
• Etc.
2014-2015 Sensors & Actuators - H.Sarmento 16
Commercial RTD
wound wire (probes) thin film components
[Source: pyromation]
[Source: Omega][Source: smartsensors]
2014-2015 Sensors & Actuators - H.Sarmento 17
Silicon Resistive Sensors (1)
• Pure silicon (without impurities):
ni – concentration of electrons
q – electron charge
m – mobility
Eg – Band gap
K – Boltzman constant
• Resistivity decreases with temperature.
1
pniqn mm
2
3
Tm 22
3
kT
E
i
g
eTn
T 2kT
Eg
e
2014-2015 Sensors & Actuators - H.Sarmento 18
Doped silicon
» PTC-
Silicon Resistive Sensors (2)
[Source: Jacob Fraden, 2010]
PTC
2014-2015 Sensors & Actuators - H.Sarmento 19
Silicon Resistive Sensors (3)
• Silicon doped with an n-type impurities.
• Resistivity increases with temperature in a limited to arelatively small temperature range.
• Discrete silicon sensors: KTY (originally from Philips)
2014-2015 Sensors & Actuators - H.Sarmento 20
KTY transfer function
• Resistivity increases with temperature.
• Values usually specified at 25º C
2
000 1 TTTTRRt a
2014-2015 Sensors & Actuators - H.Sarmento 21
Commercial Silicon Resistive Sensors
• KTY
– Discrete
– MEMS
[Source: NXP]
2014-2015 Sensors & Actuators - H.Sarmento 22
Silicon Resistive Sensors advantages
• Linear
• Moderate cost.
• High sensitivity
• Low weight
• Very long operation life
• Medium long term stability
2014-2015 Sensors & Actuators - H.Sarmento 23
Silicon Resistive Sensors disadvantages
• Highly non-linear at temperatures below 0°C and greater than
70°C. Can ne compensated.
• Limited range of temperatures (silicon):
– Maximum: –55C to +150C.
– Typical: - 45C to +85C or 0C to +80C
• Slow response time
2014-2015 Sensors & Actuators - H.Sarmento 24
Silicon Resistive Sensor field of applications
• Industrial applications:
Overheating protection, Protection for power supplies,
Process temperature control, Exhaust control, Toaster
control, Temperature compensation for microprocessors
• Automotive applications:
Oil temperature, Oil level, Water temperature, Diesel
injection, Transmission, Engine coolant, Engine air, Air
conditioning
2014-2015 Sensors & Actuators - H.Sarmento 25
KTY linearization
• When further linearization becomes necessary a resistor (RL)
to shunt the sensor (RT) can be used.
• Linearization in a temperature range.
TLeq RRR //
2014-2015 Sensors & Actuators - H.Sarmento 26
RTD/KTY (1)
• Similar characteristics to RTD, but more sensitive and less
linear.
2014-2015 Sensors & Actuators - H.Sarmento 27
RTD/KTY (2)
• KTY with small SPAN than RTD
Material Temp Range
PLATINUM -260 º +650 º
NICKEL -100 º +300 º
COPPER -75 º +150 º
Sensor Span
KTY-81-1 -55 ºC 150 ºC
KTY-81-2 -55 ºC 150 ºC
KTY-83-1 -55 ºC 150 ºC
KTY-84-1 0 ºC 300 ºC
2014-2015 Sensors & Actuators - H.Sarmento 28
NTC thermistors
• Ceramic materials, normally highly resistive, made semi-conductive
by the addition of dopants.
• Small doping: Negative Temperature Coefficients (NTC).
NTC
2014-2015 Sensors & Actuators - H.Sarmento 29
NTC transfer function
• Transfer function highly nonlinear.
• Simpler model (lower the accuracy):
• Traditionally, thermistors are specified at temperature of 25ºC
(T0 =298.15 K).
• For a relatively narrow temperature range, β can be considered
temperature independent.
3
3
2
210ln
T
A
T
A
T
AARt
TARt
0ln
Tt eAR
2014-2015 Sensors & Actuators - H.Sarmento 31
NTC advantages
• Inexpensive
• High sensitivity
• Fast response time.
• Self heating can be useful for certain applications (PTC).
2014-2015 Sensors & Actuators - H.Sarmento 32
NTC disadvantages
• Non linear (can be compensated).
• Fragile.
• Self heating.
• Limited range of temperatures.
2014-2015 Sensors & Actuators - H.Sarmento 33
NTC field of applications
• Consumer and household appliances: Burglar alarm and fire
detectors, for your oven, air conditioning, refrigerator
temperature control, or fever thermometer.
• Fibre and photographic processing, solar, meteorological,
geological, and oceanographic equipment.
• Motor winding compensation, transistor temperature
compensation, infrared sensing compensation, gain
stabilization and piezoelectric temperature compensation.
2014-2015 Sensors & Actuators - H.Sarmento 34
NTC linearization
• LTN –Linear Thermistor Network
vv
opBTA
V
V
2014-2015 Sensors & Actuators - H.Sarmento 35
Resistive temperature sensors: comparison
NTC RDT (Pt) Silicon Resistive
Temp range -55ºC +125ºC -200ºC +850ºC -50ºC +125ºC
Linearity Exponential Linear (range) Linear
Sensitivity High Low Moderate
Response time Fast Slow Slow
Long-term stability Low High Medium
Cost Low High (wire wound) Low-Medium
2014-2015 Sensors & Actuators - H.Sarmento 36
NTC self-heating (1)
• Voltage or current in the sensor generates self heating (Joule effect).
• At equilibrium:
• P dissipated:
• P generated:
– Current excitation
– Voltage excitation
00 TTPP
TT dissipated
dissipated
generateddissipated PP
22 IAeIRP Ttgenerated
22
Tt
generated
Ae
V
R
VP
2014-2015 Sensors & Actuators - H.Sarmento 37
NTC self-heating (2)
• With current excitation:
• Final temperature (self limiting).0
2 TIeA
T Tfinal
RT
P generated
0T finalT
RT PR TP
2 IAeP Tgenerated
0TTPdissipated
2014-2015 Sensors & Actuators - H.Sarmento 38
P generated
0T finalT
NTC self-heating (3)
• With voltage excitation:
• Thermistor can be destroyed.
RT PR
dissipatedPTT 0
TP
2
T
generated
Ae
VP
2014-2015 Sensors & Actuators - H.Sarmento 39
• Over Curie temperature, resistivity increases with temperature.
• Most PTC thermistors with Curie temperature between 60°C
and 120°C. Can be manufactured for Curie temperature as low
as 0°C or as high as 200°C.
PTC Thermistors
PTC
2014-2015 Sensors & Actuators - H.Sarmento 40
Voltage current characteristic
[Source: Jacob Fraden]
V RI Ohm’s law
no self-heating
resistance negative
VRTPI
limiting self
2014-2015 Sensors & Actuators - H.Sarmento 41
PTC commercial devices
• PTC (Positive Temperature Coefficient)
[Source: apitechnologies]
2014-2015 Sensors & Actuators - H.Sarmento 42
PTC Thermistors applications
• Not to measure temperature.
• Applications:
– Self-Regulating Heaters
– Over-Current Protection
– Liquid level sensing
– Constant current
– Time delay
– Arc suppression
2014-2015 Sensors & Actuators - H.Sarmento 43
Self-Regulating Heaters
• Initially at NTC region:
• If the voltage is high enough, the unit will self-heat until it
passes into the PTC region of resistance.
• In the PTC region, if the temperature decreases
2
IRT
RR
VP
LT
TPIR
TPIRT
2014-2015 Sensors & Actuators - H.Sarmento 44
Over-Current Protection
• Under normal conditions RT is low and IL depends on V.
• A short circuit or over-current condition (IL) RT causes heating.
• At Curie Point, the PTC transforms into a high resistance element,thereby limiting current to the load.
• Removing the fault condition decreases the current flow and allowsPTC to cool to its normal resistance mode.
V
TL
LRR
I
2014-2015 Sensors & Actuators - H.Sarmento 45
Liquid Level Sensing
• When a PTC thermistor heated in air is immersed into a liquid (orair flow condition), a larger amount of heat is dissipated than in air.
• I > Imin relay is actuated.
[Source: Epcos] IRT PTCth
thAirthLiquid
2014-2015 Sensors & Actuators - H.Sarmento 46
Constant current
• It is possible to obtain a nearly constant current (IS) by
connecting a PTC thermistor in parallel with a resistor.
PTCPTCPTCPTCPTC IRTPI
constantI and I sPTCRp I
PTCo IV
I RpoV
2014-2015 Sensors & Actuators - H.Sarmento 47
Time delay
• The relay stays energized until the PTC switches from low to
high resistance.
• The relay will only be energized after the time necessary for
the PTC to switch from low to high resistance.
[source: Spectrum Sensors]
2014-2015 Sensors & Actuators - H.Sarmento 48
Arc Suppression
• When the switch is opened, the PTC changes from low
resistance to high resistance, suppressing the arc.
[source: Spectrum Sensors]
2014-2015 Sensors & Actuators - H.Sarmento 49
Thermocouples classification
• Power supply requirements: passive (modulating).
• Stimulus perception: contact.
• Stimulus detection: relative.
• Complexity: direct sensor.
• Type of stimulus: thermal.
• Transduction principle: thermoelectric.
• Energy conversion: thermal electrical.
2014-2015 Sensors & Actuators - H.Sarmento 50
Thermocouples transfer function
[Source: Jacob Fraden, 2010]]
ABE
CT º00
20
2201
0
TTCTTCETTAB
Emf in tables, usually for
2014-2015 Sensors & Actuators - H.Sarmento 51
Commercial thermocouples
[Source: allproducts] [Source: coleparmer]
2014-2015 Sensors & Actuators - H.Sarmento 52
Thermocouples advantages
• Inexpensive.
• Wide temperature range (200 oC to 2600 oC).
• Most types non-linear.
• Fast response time.
• Standardized.
• Moderate cost.
2014-2015 Sensors & Actuators - H.Sarmento 53
Standardization
• ANSI standard (wires with different colors).
• T, J, and K are most commonly used.
[Source: University of Cambridge ]
Type Composition
J Iron/Constantan (Nickel Copper)
K Nickel chromium/Nickel Aluminum
N Nickel chromium Silicon/Nickel Silicon
T Copper/Constantan
E Nickel chromium/Constantan
R Platinum Rhodium/Platinum
S Platinum Rhodium/Platinum
B Platinum Rhodium/Platinum Rhodium
2014-2015 Sensors & Actuators - H.Sarmento 54
Thermocouples disadvantages
• Small sensitivity.
• Small repeatability.
• Requires two temperatures be measured (cold junction).
• Output wire in the same thermocouple material.
• Long term stability: prone to aging.
• Susceptibility to electrical noise if not shielded.
2014-2015 Sensors & Actuators - H.Sarmento 55
Thermocouples field of applications
• Industrial applications: gas turbine exhaust, diesel engines,
furnaces, etc.
• Rocket engines and amunitions.
• Homes, offices and businesses: thermostats, flame sensors
in safety devices for gas-powered appliances.
2014-2015 Sensors & Actuators - H.Sarmento 56
Operation principle of thermocouples
• The Seebeck emf generated in a thermoelectric circuit results
from Thomson and Peltier effects.
• Peltier effect: existence of an EMF due to the contact of two
dissimilar metals dependent on junction temperature.
• Thomson effect: Existence of an EMF due to temperature
gradients along conductors in a circuit.
• Thomson effect normally much smaller than the Peltier effect.
2014-2015 Sensors & Actuators - H.Sarmento 57
Seebeck effect (1)
• A current flows in a circuit with two dissimilar homogeneous
metals A and B, having the two junctions at different
temperatures.
[Source: Jacob Fraden, 2010]
2014-2015 Sensors & Actuators - H.Sarmento 58
Seebeck effect (2)
• A thermally induced potential exists across the broken
conductor, which only depends on the materials and the
temperature difference.
[Source: Jacob Fraden, 2010]
21 TETEE ABABAB
2014-2015 Sensors & Actuators - H.Sarmento 59
Law of intermediate metals (1)
• The algebraic sum of emf in a circuit composed of any number
of dissimilar materials is zero if all of the circuit is at a
uniform temperature.
• Therefore, inserting any type of wire into a thermocouple
circuit has no effect on the output as long as both ends of that
wire are at the same temperature, or isothermal.
21 TETE ABAB
21
3321
TETE
TETETETE
ABAB
CAACBAAB
2014-2015 Sensors & Actuators - H.Sarmento 60
Law of intermediate metals (2)
• Another consequence: the emf of the combination of two
metals is the sum of their emf against the reference material.
ABE
BCE
BCABAC EEE
2014-2015 Sensors & Actuators - H.Sarmento 61
Law of intermediate temperatures
• E1-2 between A and B with junctions at T1 and T2
• E2-3 between A and B with junctions at T2 and T3
• Between A and B with junctions at T1 and T3
32 TTE
21 TTE
322131 TTTTTT EEE
2014-2015 Sensors & Actuators - H.Sarmento 62
Reference temperature (1)
• To use a thermocouple to measure temperature, one junction
must remain at a fixed reference temperature.
• Ice Baths: reference temperature immersed into a melting ice
bath.
– Accurate and inexpensive.
– Serious limitations for many practical uses.
CABTABifino EEvvvº0
CT º00
TCTCTCETAB 1
2
21
1C
ET AB
2014-2015 Sensors & Actuators - H.Sarmento 63
Reference temperature (2)
• Electronically Controlled References
– Require periodic calibration and are generally not as stable
as ice baths, but are more convenient.
ambT
TABcompifino Evvvv ambTT 0
TCTCTCETAB 1
2
21
1C
ET AB
ambTABifcomp Evv
2014-2015 Sensors & Actuators - H.Sarmento 64
Cold junction compensation
• LT1025 - Micropower Thermocouple Cold Junction Compensator
[Source: Linear Technology]
2014-2015 Sensors & Actuators - H.Sarmento 65
Semiconductor PN junction classification
• Power supply requirements: active (modulating).
• Stimulus perception: contact.
• Stimulus detection: absolute.
• Complexity: direct sensor.
• Type of stimulus: thermal.
• Transduction principle:
• Energy conversion: thermal electrical.
2014-2015 Sensors & Actuators - H.Sarmento 66
• The voltage across a forward biased junction biased by a
constant current generator can provide a measure of the
junction temperature.
Semiconductor PN junction
q
kTVT
kT
E
S
g
eII 20
kT
qv
kT
EIi Dg
D 2
lnln 0
kT
qv
V
v
S
DD
T
D
eeI
i
D
g
D iIq
kT
q
Ev lnln
20
2014-2015 Sensors & Actuators - H.Sarmento 67
• Sensitivity
Semiconductor PN junction transfer function
D
g
D iIq
kT
q
Ev lnln
20
μA) (10 º32 1 /º2 CmV/. mA)(CmV
2014-2015 Sensors & Actuators - H.Sarmento 68
Commercial semiconductor PN junction
• AD590, LM335, LM35, LM3911, TMP100/101, LM75
2014-2015 Sensors & Actuators - H.Sarmento 69
Semiconductor PN junction advantages
• Linear
• Low cost.
• Easily integrated in ICs at low cost (temperature sensing of
microprocessors thermal-shutdown in power-supply chips).
• Fast response time.
2014-2015 Sensors & Actuators - H.Sarmento 70
Semiconductor PN junction disadvantages
• Junctions cannot support high temperatures (LM35 -55 ºC
- 150 ºC; LM3911- 25ºC – 85 ºC).
• Sensitivity depends on bias current.
2014-2015 Sensors & Actuators - H.Sarmento 71
PN junction field of applications
• Remote sensing (connected by PVC cables).
• Domestic appliances: refrigerators, freezers, water heater,
dishwasher, bread maker, radiator, drying machine, etc.
• Cellular phones.
• Hard disk drivers, personal computers.
• Process control: vehicle-mounted refrigerators, storage tanks
for cosmetics, disinfecting machine, etc.
2014-2015 Sensors & Actuators - H.Sarmento 72
More PN junctions
• Increase of linearity by using more PN junctions:
• Identical characteristics:
12
21
2
2
1
1
21 lnlnSD
SDT
S
D
S
D
TDDDIi
IiV
I
i
I
i
Vvvv
12
2121 ln
Ai
Ai
q
kTvv
D
DDD
mA
A
1
2
21 DD ii
Tmq
kvv DD ln21
2014-2015 Sensors & Actuators - H.Sarmento 73
AD590 (1)
AD590 (output current):
• Output current proportional to absolute
temperature.
• Sensitivity: 1 μA/K
• SPAN: −55°C to +150°C
• Non linearity: ±0.3°C over full range
(AD590M)
(A) T01 -6TI
2014-2015 Sensors & Actuators - H.Sarmento 74
AD590 (2)
• Use of transistors:
1
221 ln -
A
A
q
kTVVV BEBER
mRq
kT
R
VVI BEBE ln
2
-2 21
4343 CC IIAA
2314 CCCC IIII
12 mAA KAI /1m
21243 2 CCCCC IIIIII
2R
VI R
C
2014-2015 Sensors & Actuators - H.Sarmento 75
LM35
• Output current proportional to temperature in ºF.
• Accuracy (at +25˚C): 0.5˚C.
• Sensitivity: + 10.0 mV/˚C
• SPAN: −55˚ to +150˚C
• Nonlinearity: ± 1⁄4˚C typical.
FmVV /º10
PTAT: Proportional to Absolute Temperature
2014-2015 Sensors & Actuators - H.Sarmento 76
Piezoelectric temperature sensors
classification
• Power supply requirements: passive (self generating).
• Stimulus perception: contact.
• Stimulus detection: absolute.
• Complexity: direct sensor.
• Type of stimulus: thermal.
• Transduction principle: piezoelectric
• Energy conversion: thermal electrical.
2014-2015 Sensors & Actuators - H.Sarmento 77
Piezoelectric temperature sensors
• The oscillating frequency is highly dependent on thecrystallographic orientation of the plate (angle of cut).
• The angle of cut depends on the temperature.
• Change in resonant frequency between 25 °C and 600 °C dependingon the cut-angle.
• GaPO4 (gallium orthophosphate): belongs to the same point groupas quartz.
2014-2015 Sensors & Actuators - H.Sarmento 78
Piezoelectric sensors transfer function
• Temperature dependence:
fT - crystal frequency at temperature T (in °C)
f0 - crystal frequency at reference temperature T0
202010 TTaTTaffT
2014-2015 Sensors & Actuators - H.Sarmento 79
Commercial piezoelectric temperature sensors
• RKTV06, RKOV206 (AXTAL)
• PTK01(AXTAL)
2014-2015 Sensors & Actuators - H.Sarmento 80
Piezoelectric temperature sensors advantages
• Low power consumption.
• Miniature size
• Operating temperature range ‐50°C to +180°C (standard) and
optionally up to +320°C
• High resolution down to µK range.
• Short time constant due to low thermal mass
• High shock and vibration resistance.
2014-2015 Sensors & Actuators - H.Sarmento 81
Infrared thermometer classification
• Power supply requirements: passive (self generating).
• Stimulus perception: non-contact.
• Stimulus detection: absolute.
• Complexity: complex sensor.
• Type of stimulus: thermal.
• Transduction principle: depending on the
detector/sensor
• Energy conversion: thermal electrical.
2014-2015 Sensors & Actuators - H.Sarmento 82
Infrared thermometer
• An optical system and a detector:
• Infrared radiation emitted by the object is picked up by the
optical system that focuses it on the sensor.
• The detector (sensor) converts the infrared radiation received
into electrical signals.
Optical system sensor
2014-2015 Sensors & Actuators - H.Sarmento 83
Infrared radiation
• Atom and molecule of an object above absolute zero temperaturevibrate emitting electromagnetic radiation, called thermal radiation.
• Blackbody spectral radiant emission
increases with temperature.
• An infrared sensor intercepts a portion of the infrared energyradiated by an object.
[Source: Hamamatsu]
2014-2015 Sensors & Actuators - H.Sarmento 84
Electromagnetic Radiation Spectrum
• Emitted thermal radiation is in the infrared region.
• The spectrum of this radiation ranges from 0.78 to 1000 µm
wavelength.
[Source: Raytek]
2014-2015 Sensors & Actuators - H.Sarmento 85
IR optical system
• For accurate temperature measurement, the target should be
larger than the instrument’s field of view or spot size.
2014-2015 Sensors & Actuators - H.Sarmento 86
Infrared detectors
• There are two main groups of infrared detectors:
– Thermal detectors: the temperature of the sensitive element
varies because of the absorption of electromagnetic
radiation.
– Quantum detectors: the striking photons of the infrared
radiation lead to an increase of the electrons inside the
semiconductor material.
2014-2015 Sensors & Actuators - H.Sarmento 87
IR detectors: advantages and disadvantages
• Thermal detectors:
– Low detection capability.
– Independent of wavelength.
– Slow response time (ms).
– Do not require cooling.
• Quantum detectors:
– High detection capability
– Dependent of wavelength.
– Fast response time (ns and ms).
– Must be cooled, except for near IR region.
2014-2015 Sensors & Actuators - H.Sarmento 88
Thermal detectors
• Impacting photons are absorbed by a thermally isolated
detector resulting in an increase in the temperature of the
element.
• Temperature can be sensed by:
– Thermocouple elements.
– Element with a change in charge due to the pyroelectric
effect: infrared pyrometer.
– Element with a change in resistance (metal or
semiconductor): infrared bolometers.
2014-2015 Sensors & Actuators - H.Sarmento 89
Quantum detectors
• Impacting photons are absorbed and generate free carriers
which are sensed by an electronic circuit:
– Photovoltaic.
– Photoresistive.
2014-2015 Sensors & Actuators - H.Sarmento 90
Infrared field of applications
• Manufacturing processes for metals, glass, cement, ceramics,semiconductors, plastics, paper, textiles, coatings.
• Fire-fighting, rescues and detection of criminal activities(intrusion).
• Measurement of human body temperatures (1 second timeresponse).
• Building heating.
• Electrical power generation and distribution (hot spotdetection).
2014-2015 Sensors & Actuators - H.Sarmento 91
IR temperature sensors advantages
• No contact.
• Fast response times.
• High repeatability.
• Good stability over time.
2014-2015 Sensors & Actuators - H.Sarmento 92
IR temperature sensors disadvantages
• Cost.
• Complexity.
• Emissivity variations affect temperature measurementaccuracy.
• Accuracy affected by dust, smoke, background radiation, etc.
2014-2015 Sensors & Actuators - H.Sarmento 93
Commercial IR temperature sensors (5)
[Source: Omega]
[Source: Raytek]
[Source: Texas instrument, Dec.2012]
[Source: Ge-sensors and measurement]
2014-2015 Sensors & Actuators - H.Sarmento 94
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http://support.fluke.com/raytek-sales/Download/Asset/IR_THEORY_55514_ENG_REVB_LR.PDF.
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http://www.optris.com/applications?file=tl_files/pdf/Downloads/Zubehoer/IR-Basics.pdf.
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