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1
Temperature sensors

• Introduction, temp. scales
• Thermoresistive sensors
• 2.1 Resistance temperature detectors (RTD)
• 2.2 Thermistors
• 3. Thermoelectric sensors
• 4. Semiconductor p/n junction sensors
• Optical temperature detectors
• 5.1. Pyrometers
• 5.2. Fiber optic detectors

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Introduction

• From historical point of view the most widely
  used phenomenon for temperature sensing was
  expansion (mercury thermometers).
• At present one uses detectors with electrical
  signal at the output.
• Temperature detectors can be classified basing of
  several criteria.
• From the point of view of generated power we
  have
• generation-type sensors (eg. thermoelectric)
• parametric-type (eg. resistance R(T), magnetic
  µ(T), dielectric e(T))
• From the point of view of other criterion we
  have
• contact-type sensors (eg. thermoresistors)
• noncontact sensors (eg. pyrometers)
• In practice we expect that temp. sensors will be
• accurate (for a given temp. range)
• reliable (important in process control)
• inexpensive (in consumer applications)

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• Growing perspectives of temperature sensors
• expansion of layer technologies and
  micromachining
• increased application of fiber optic sensors
• increased role of microprocessors and sensors
  with digital output.
• Temperature scales
• Celsius scale (1742)
• Based on two equilibrium points of water
  freezing (00C) and boiling (1000C)
• Thermodynamic scale
• Based on the Carnot engine
• T Ttr Q/Qtr Q heat absorbed from the
  source of temp.T
• Qtr heat discharged to the source of temp.
• Ttr 273.16 K
• International Temperature Scale 1990 (ITS90)
• Based on thermodynamic scale which is
  conneted with Celsius scale as follows
• t(oC) T(K) 273,15 then 1oC 1K.
• There are introduced 17 fixed points (phase
  equilibrium points) with defined temperatures,
  interpolation formulae between the fixed points
  and 4 standard thermometers for temperature
  measurements. For example between the triple
  point of equilibrium hydrogen (13.8033K) and the
  freezing point of silver (961.78 0C) T90 is
  defined by means of platinum resistance
  thermometer.

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Thermoresistive sensors
Thermoresistive sensors can be divided into
metallic - type, called RTD (resistance temperatur
e detectors) and semiconductor - type known as
thermistors. Metallic temperature detectors
(RTDs) The resistance of metallic snsors in a
narrow temp. range can be given as a linear
dependence R(t) Ro1 a(t - to)
a temp. coefficient of resistance
TCR Ro resistance at to (mostly 0oC or
25oC) In a wider temp. range higher order
polynomials should be used. For example for
platinum the good approximation in a range fom
00C to 8500C is a second order polynomial (PN-EN
60751 in accordance with ITS90)
R(O) Ro(1 39,08310-4 T 5,775 10-7 T2)
Ro resistance at 00 C T temp. in
Kelvin scale
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RTDs, cont.

• The requirements for metallic thermoresistors are
  as follows
• high sensitivity, i.e. high a
• linearity (constant a )
• miniaturization (high resistivity ?)
• chemical inertness and long-term stability

Resistivities and TCR for selected metals
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Platinum resistance thermometers
Platinum is the most popular material for
resistance thermometers. Pure Pt is obtained in
a form of wires with a diameter lower than
0.05mm, what is necessary to obtain the required
magnitude of a thermometer resistance. A lot of
possibilities give thin and thick film
technologies, which reduces the sensor
fabrication costs. Typical Pt sensor is known as
Pt100 (100 O at 0oC). Relative resistance of a
platinum wire given as R100/Ro 1 a ?t is
a measure of TCR and depends on wire purity.
For a very pure Pt wire one
obtains R100/Ro 1,3927 in a precise
thermometry one uses R100/Ro 1,3910
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Platinum resistance thermometers
In USA SAMA standard for Pt a value
specifies R100/Ro 1,3925 In
Eurpe the standard for Pt thermometers in
technical applications (DIN 43760, IEC 751)
is R100/Ro 1,3850 IEC standard defines
additionally two classes of precision for Pt
thermometers A for the range -200 do 650oC
(more rigorous) possible error oC (0,15
0,002t) B for the range -200 do 850oC
possible error oC (0,3 0,005t).
Typical dimensions of Pt wire thrmometers 3,2 x
10 mm dla 100 O, 500 O, 1000 O 2 x 10 mm dla
100 O, 500 O, 1000 O 2 x 2,5 mm dla 100 O 1 x
5 mm dla 100 O
Outside view of Pt wire thermometers
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Thin film Pt sensors
Thin film of Pt is deposited on the ceramic
substrate and the resistance corrected for the
required magnitude.
Laser cut thin film of platinum with bonded lead
wires (view of a sensor without covering
protective layers).
Ready for use thin film Pt thermometer
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Semiconductor resistance temperature
sensors (thermistors)
Thermistors are mostly obtained as sintered
oxides, sulfides and selenides of elements such
as Co, Mn, Ti, Fe, Ni, Cu, Al, fabricated in a
form of bars, droplets, discs and also thick
films.
Thermistors can be divided into two groups NTC
(negative temperature coefficient) PTC (positive
temperature coefficient).
Characteristics of NTC and PTC thermistors as
compared to metallic RTD thermoresistors.
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NTC thermistors
Conventional oxide resistors have a negative TCR
and their resistance as a function of temperature
can be written with a good approximation as RT
A exp ß/ T Constant A depends on sample
dimensions, ß is a material constant which
determines the sensitivity (ß 3000
4500K). Introducing the reference resistance Rref
at a reference temp. Tref 25oC, one
obtains RT Rref exp ß(1/T 1/Tref) The
values of Rref vary in a range 500O 10MO. In a
wide temp. range the sensitivity is better
characterised by TCR a 1/RT dRT/dT -
ß/T2 The values of a are ca. 6 10 times higher
than those for metals but decrease quickly with
temperature.
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Measurements of thermoresistors resistance
Compensation of leads
resistance (in this case 1 and 3) 1,2,3
identical thermoresistor leads
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Thermoelectric sensors
For this kind of sensors the generated thermo EMF
is based on the Seebeck effect. Seebeck effect
(1821) In a circuit consisting of two
conductors A and B, which junctions have
temperatures T ?T and T, the thermoelectric
voltage is generated and a thermoelectric current
is flowing.
A() metal A positive in respect to
B
For a given conductor the absolute Seebeck
coefficient is defined as and connects the
generated electric field Ea with temperature
gradient
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Thermocouples
As the thermocurrent flows in a circuit
consisting of at least two different conductors,
the differential Seebeck coefficient is
introduced and accordingly For the small
temperature change we can then write If
temperature T0 of the reference junction is
known then from the measurement of thermovoltage
V the temperature T of a measurement junction
can be determined (thermocouple). In practice we
do not use a which is temperature dependent but
in calculation of a thermovoltage for a given
thrmocouple we exploit the tabulated
values. Example Calculate the thermovoltage for
Au-Pd thermocouple in the case t0 00C, t
2000C given thermovoltage for a junction Au-Pt
1,84 mV given thermovoltage for a junction
Pd-Pt - 1,23 mV Calculated thermovoltage for
Au-Pd thermocouple 1,84 (-1,23) 3,07 mV
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Thermocouples

• Thermocouples widely used are standardised. They
  are manufactured from alloy materials with
  compositions which are often restricted by the
  producer.
• Designation (ANSI) Materials
• E Chromel/Constantan
• J Fe/Constantan
• K Chromel/Alumel, known also as NiCr/NiAl
• T Cu/Constantan
• R Pt/Pt-13Rh
• S Pt/Pt-10Rh
• B Pt-6Rh/Pt-30Rh
• Properties of K-type thermocouple
• 1-st thermoelectrode NiCr (), composition 85
  Ni, 12 Cr and other elements in
• small quantities
• 2-nd thermoelectrode NiAl (-), composition
  95 Ni, 2 Al, 2 Mn, 1 Si,
• nearly linear thermometric characteristics,
• resistant to oxidizing atmosphere, at elevated
  temperatures sensitive to reducing
• atmosphere,
• working temp. range from 2700C to 11500C,
  average sensitivity 41 µV/K.

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Thermocouples
Typical thermocouples are manufactured in
insulation. With the help of micromachining
technology the thermocouples are manufactured on
membranes. The small heat capacity and good
thermal isolation enable the registration of
infrared radiation spectra. In the solution
shown in the figure the cold junction is placed
in the area of a good heat conductivity. The hot
junction is placed in a central part of a
membrane with low thermal conductivity. Additional
y the hot junction is placed under the IR
absober. Serial connection of thermocouples,
called the thermopile, enhances the sensitivity.
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Measurements with the help of thermoelements
The basic measurement circuit of a thermocouple.
Reference junction temperature variation introduce
s the measurement error. In a reference temp. 00C
one measures et. In a reference temp. tr one
measures ea et - er
Compensation of reference temperature variation

• The reference junction is put at a distance from
  the heat source with the help of
• compensation leads
• PX, NX - the leads with thermoelectrical
  properties identical with those of thermoelements
  
• (for PtRh-Pt one uses alloys of copper and
  nikel).

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Compensation of reference temperature variation,
cont.
2. One uses a thermostat stabilising the
reference temp., eg. 500C . Traditionally a
reference ice bath was used to maintain 00C,
which presents some limitations for the
practical uses.
3. Automatic correction of the influence of
reference temperature variation
The reference
temp. increases from t0 to t1 . Response of a
thermometer Rt R0 1 a (t1 t0)
Response of a thermocouple ?e k (t1
t0) Compensation condition ?e - UN The
compensation condition is fulfilled
if Uz 4k/a
4. Actual temperature is calculated by a
microprocessor as t td C tr
td , tr measured temp., C thermocouple
constant
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Pyrometers

• This kind of thermometers is used for distant
  (noncontact) measurements of temperature.
• It is based on the analysis of thermal radiation
  emitted by the objects.
• Monochromatic pyrometers are used as standard
  thermometers from the freezing temp. of silver
  (961,780C).
• Classification of pyrometers
• total radiation pyrometers (wide bandwidth)
• monochromatic pyrometers
• two-color pyrometers
• multicolor pyrometers
• Basic laws of thermal radiation
• Plancks law
• Radiation flux density, i.e. power of radiation
  per unit of area and unit of wavelength
• (Wm-2µm-1) is equal
• ?- wavelength, c1,c2 radiation constants
• e? monochromatic emissivity of a
  source (for a blackbody equal 1)

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Pyrometers, cont.

• Stefan-Boltzmann law
• Radiation is absorbed by the detector in a
  limited range of wavelength. Integration of the
  Plancks formula vs. wavelength gives a power per
  unit of area which is emitted by the object with
  temperature T
• s 5.67x10-8 W/m2K4
• e - emissivity, depending on
• the surface
  condition and temperature
• Above formulae, called S-B law, is a base of the
  wide
• band pyrometry.
• In the analysis of radiation exchange between
• an object and a sensor the radiation reflected
• and emitted by the sensor must be taken into
• account. This leads to the dependence
• eS, TS emissivity and temp.
• of a sensor

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Two color pyrometer
The analysis of radiation flux density F? as a
function of source emissivity e indicates,
that for neighbour wavelength one can write
Therefore measurement of a signal in two
neighbour narrow spectral ranges elliminates the
necessity of determination the source emissivity
e. This is a base of the so called two color
pyrometry.
Emission spectrum for the source at temperature
of 600oC and for three different emissivities e
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Pyrometers construction
Two color pyrometer
Wide band pyrometer
Thermal detectors are used (bolometers or
thermopiles). Wide band entrance window is
necessary.
For this purpose photonic detectors (photovoltaic
or photoconductive) are developed, ?1, ?2
determine narrow bands placed in a
neighbourhood.
Four colour pyrometers were developed for uses
where the emissivity is very low and not stable
during processing. Four color pyrometers measure
the radiation intensity simultaneously in four
different spectral areas and they are able to
adapt and make a correction of the emissivity
setting.
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p/n junction as temperature sensor
Semiconductor p/n junction (of eg. diode
connected transistor) is polarized in a forward
direction. I IS exp(qUBE/kT)
1 for qUBE gtgt kT
UBE (kT/q) ln (I/IS) f(T)
for I const one obtains quite good linearity
in a range from - 500C to
1500C For silicon bipolar transistors the
sensitivity is equal ? UBE/ ? T - 2,25 mV/K
for T300 K and I10 µA However the saturation
current IS depends weakly on temperature which
gives the nonlineatity error. This error is
quite small and in many cases no linearity
correction is required. Better linearity one
obtains in a differential configuration.
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p/n junction as temperature sensor, cont.
Voltage drops at the junctions powered from
constant current sources are equal The
differential voltage is equal For a given
technology of transistors one can assume that
emitter current densities are equal JS1
JS2. Labelling the emitters cross-sections ratio
as r AS2/AS1 one obtains For IF1 IF2 and
r 4 one gets

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Junction as temperature sensor, integrated circuit
The circuit in the figure illustrates the
practical solution of the described
differential method employing semiconductor p/n
junctions. This system is often manufactured as
integrated ciruit in a silicon substrate in
monolithic sensors requiring temperature
compensation (eg. in micromechined membrane of
pressure sensors). Transistors Q3 i Q4 form the
so called current mirror which secures the
equality of currents IC1 IC2 I Drop of
voltage VT on resistor R is equal and then is
proportional to the absolute temperature. The
sensors of this type are called PTAT
(proportionalto-absolute-temperature) sensors.
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Optical temperature sensors
Fiber optic detector
With the increse of temperature, semiconductor
absorption edge shifts to the longer wavelengths
and the transmitted light intensity
decreases. Emission spectrum of the source
diode is also shown.
Light intensity decreases after passing the
seniconductor.