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Sensing Systems and Signal Processing
Dr Richard
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Thermo-electric sensors
Seebeck effect – Electro Motive Force is generated across a material with a temperature gradient
Size of effect depends on the conductor and their Seebeck parameter.
Seebeck series is : Bi,Ni,Co,Pt,Cu,Mn,Hg,Pb,Sn,Au,Ag,Zn,Cd,Fe,As,Sb,Te
Can’t measure directly as you add connections which changes things.
Thermo-electric sensors
The EMF can be measured between the junctions
EMF for this arrangement is ~4mV per 100 difference.
This EMF is determined by the difference in the Seebeck coefficients for the materials
Constantan
Cold Junction
Hot Junction
Thermocouple combines two materials
Thermo-electric sensors
Semi-conductors have very high values – strongly dependant on the doping, but they don’t make good wires, so most thermocouples made from metallic wires.
S (μV/K)* S (μV/K)
Selenium 900 Lead 4.0
Tellurium 500 Aluminium 3.5
Silicon 440 Carbon 3
Germanium 330 Mercury 0.6
Antimony 47 Platinum 0 (by definition)
Nichrome 25 Sodium -2.0
Molybdenum 10 Potassium -9.0
Cadmium, tungsten 7.5 Nickel -15
Gold, silver, copper 6.5 Constantan -35
Rhodium 6 Bismuth -72
Tantalum 4.5
*Seebeck coefficient relative to platinum (μV/K)
Thermo-electric sensors
Standard Thermocouples have letter designations and are made from specific combinations of materials.
The combined Seebeck coefficient is the difference between the coefficients for the materials used.
What is S for a T type device ?
For a 100K temperature difference the measured voltage will be?
Type Materials S (μV/K)
T Copper-Constantan ?
S (μV/K)* S (μV/K)
Selenium 900 Lead 4.0
Tellurium 500 Aluminium 3.5
Silicon 440 Carbon 3
Germanium 330 Mercury 0.6
Antimony 47 Platinum 0 (by definition)
Nichrome 25 Sodium -2.0
Molybdenum 10 Potassium -9.0
Cadmium, tungsten 7.5 Nickel -15
Gold, silver, copper 6.5 Constantan -35
Rhodium 6 Bismuth -72
Tantalum 4.5
Thermo-electric sensors
Standard Thermocouples have letter designations and are made from specific combinations of materials.
The combined Seebeck coefficient is the difference between the coefficients for the materials used.
For example: T type device
S = 6.5-(-35) = 41.5,
For a 100K temperature difference the measured voltage will be
41.5 μV/K x 100K = 4.15mV
This is only true for small changes in temperature.
Type Materials S (μV/K)
E Chromel-Constantan 60
J Iron-Constantan 51
T Copper-Constantan 41
K Chromel-Akumel 40
N Nicrosil-Nisil 38
S Pt(10%Rh)-Pt 11
B Pt(30%Rh)-Pt (6%Rh) 8
R Pt(13%Rh)-Pt 12
Thermo-electric sensors
S is not usually constant as it is a function of temperature so the produced EMF is not linear with temperature, it can be expressed as a power expansion in terms of temperature
Neutral temperature is when
, which is when T=Tn = -a/2b
Inversion Temperature is when T = Ti= 0 = T0-a/b
Here the EMF continues to change but now has opposite sign.
Tn is property of the thermocouple, due to materials used and is independent of cold junction temperature.
Tc is the cold junction temperature, (zero in example)
Ti is dependant on Tc as Tn is (Ti+Tc)/2
S is the local gradient of this curve.
Thermo-electric sensors
temperature
Reference Junction
Copper wire
Voltage out goes to measuring circuit
Reference junction is a known temperature – often an ice bath is used for scientific measurements
Thermo-electric sensors
Thermocouple Applications depend on the type of thermocouple as the different material compositions have different operating ranges.
Type K (-270 to +1370°C)
The K Type is a very general thermocouple and is found in many different application areas, in particular for process plants e.g. chemical production and petroleum refineries. General used for monitoring appliance safety.
Type J (-210 to +760°C )
The Type J is a Thermocouple typically used to monitor inert materials and for vacuum applications.
Typically used at slightly lower temperatures than K type if accuracy needs to be maintained. Not very good at low temperature or in damp conditions due to risk of oxidation. Often used in plastics and resin manufacture.
Type T (-270 to +400°C)
The Type T is a high accuracy thermocouple often found in the food industry applications as it doesn’t oxidise in humid environments. Often used in monitoring in food processing and production also used in low temperature and cryogenic applications.
Type N (-270 to +1300°C)
The N type has good high temperature oxidation performance so tends to get used for high temperature applications, such as furnace/kiln/oven or hot gas measurements from engine or turbine exhausts.
Type Materials S (μV/K)
J Iron-Constantan 51
T Copper-Constantan 40
K Chromel-Akumel 40
N Nicrosil-Nisil 38
measure the surface temperature of objects by measuring the thermal radiation emitted by the object.
Typical pyrometers use an optical system to focus the emitted thermal radiation onto a detector.
The thermal radiation ( in W/m2) is related to the temperature by the emissivity () of the object and a constant of proportionality ()
Using the emitted radiation means the temperature can be determined without making contact with the object.
σ = 5.670367×10−8 W⋅m−2⋅K−4
emissivity – efficiency of the surface of a material to emit energy as thermal radiation.
Aluminium foil 0.03
Asphalt 0.88
Concrete, rough 0.91
Copper, polished 0.04
Copper, oxidized 0.87
Glass, smooth (uncoated) 0.95
Limestone 0.92
Marble (polished) 0.89 to 0.92
Paint (including white) 0.9
Plaster, rough 0.89
Silver, polished 0.02
Water, pure 0.96
measure the surface temperature of objects by measuring the thermal radiation emitted by the object.
Typical pyrometers use an optical system to focus the emitted thermal radiation onto a detector.
The thermal radiation ( in W/m2) is measured to be 130 kW/m2 from a copper block.
What is the temperature of the object?
σ = 5.670367×10−8 W⋅m−2⋅K−4
emissivity – efficiency of the surface of a material to emit energy as thermal radiation.
Aluminium foil 0.03
Asphalt 0.88
Concrete, rough 0.91
Copper, polished 0.04
Copper, oxidized 0.87
Glass, smooth (uncoated) 0.95
Limestone 0.92
Marble (polished) 0.89 to 0.92
Paint (including white) 0.9
Plaster, rough 0.89
Silver, polished 0.02
Water, pure 0.96
measure the surface temperature of objects by measuring the thermal radiation emitted by the object.
Typical pyrometers use an optical system to focus the emitted thermal radiation onto a detector.
The thermal radiation ( in W/m2) is measured to be 130 kW/m2 from a copper block.
What is the temperature of the object?
130k = 0.87x 5.670367×10−8 xT^4
1274K or 1001 o C
Assumed oxidized as at elevated temperature.
σ = 5.670367×10−8 W⋅m−2⋅K−4
emissivity – efficiency of the surface of a material to emit energy as thermal radiation.
Aluminium foil 0.03
Asphalt 0.88
Concrete, rough 0.91
Copper, polished 0.04
Copper, oxidized 0.87
Glass, smooth (uncoated) 0.95
Limestone 0.92
Marble (polished) 0.89 to 0.92
Paint (including white) 0.9
Plaster, rough 0.89
Silver, polished 0.02
Water, pure 0.96
non contact measurements – can use on hazardous things
Fast response time
Good stability
Can be expensive
Accuracy effected by environment (dust / smoke and background thermal radiation)
A sailor checking the temperature of a ventilation system.
https://en.wikipedia.org/wiki/Pyrometer#/media/File:Pyrometer_040824.jpg
By Tls60 at en.wikipedia, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=18156600
A bolometer uses an absorptive layer to capture incoming energy, this is connected to a body at a constant temperature (thermal reservoir) via thermal link.
When energy is absorbed this causes the device to change temperature above that of the reservoir, the greater the energy input the greater the temperature.
The temperature change can be measured directly with an attached resistive thermometer, or the resistance of the absorptive element itself can be used as a thermometer.
They are not usually used with cooling – the thermal reservoir gradually absorbs the added energy and the device returns to its equilibrium state until energy is absorbed again. This can make them slow devices.
Bolometers are directly sensitive to the energy left inside the absorber. For this reason they can be used not only for ionizing particles and photons, but also for non-ionizing particles, any sort of radiation,
A microbolometer is a specific type of bolometer used as a detector in a thermal camera. Each pixel of a thermal camera is a microbolometer.
They are designed to absorb wavelengths in the 7.5-14 micron range, when photons in this range are absorbed they cause the absorbing material to heat up which changes it’s electrical resistance.
This resistance change is measured and processed into temperatures which can be used to create an image.
Relative changes of resistance of ~2%/K are common. Unlike other types of infrared detecting equipment, microbolometers do not require cooling. So they can be made small and compact and used in groups to make cameras.
By FeuRenard – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=46553556
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