Rudolf

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Experimental methods E181101
EXM3
Temperature(part II. Optical) or how to measure
temperature in microwave ovens or without direct
contact with measured surface
Some pictures and texts were copied from
www.wikipedia.com
Rudolf Žitný, Ústav procesní a zpracovatelské
techniky CVUT FS 2010
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T-optical methods (using optical fibres and
temperature sensitive crystals)
EXM3
Optical thermometers (excited luminiscence of
phosphor or GaAs crystal) Optical fibre
(Luxtron, Nortec, Fibronic) Laser excited GaAs
crystal in the thermometer Reflex (manufacturer
Nortec)
Optical fibre
Crystal
Emitted lightl
evaluated spectrum of reflected radiation Time
decay of intensity of reflected radiation is a
function of temperature.
Reflex Neoptix T1 accuracy 0.8C, ?0.1 s (time
constant)
These thermometers are suitable for temperature
measurement in microwave owens, transformers or
ohmic heaters environment with strong elmag.
fields
Rudolf Žitný, Ústav procesní a zpracovatelské
techniky CVUT FS 2010
3
T-infrared thermometers
EXM3
Infrared thermometers measure temperature by
detecting infrared radiation emitted from
objects. Radiation is focused by lens to
photosensitive elements (CCD-charge coupling
device) that transforms thermal radiation to
voltage.
Wavelength Typical range 1 15 ?m
Example Infrared thermometer Raytek Inc. Raynger
PM3, spectral range 8 to 14 ?m, response time
0.25 s, temperature range -18 to 540 0C, accuracy
1 fullscale Video Raytec
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T-infrared thermometers
EXM3
Theoretical fundamentals (radiation emitted by
surface at temperature T) Stefan Boltzmann
radiation flux W/m2 emitted
from surface having emissivity ? Wiens law
temperature x wavelength at max.power
constant Plancks law spectral
emissive power E as a function of
wavelength ? and temperature T Total
emitted energy at 1000K Is shown as shaded area
(integral of Plancks equation). This area is
proportional to the 4th power of temperature as
described by Stefan Boltzmann equation. This
would be detected value as soon as no band
filter is used (broad band detector)
This graph corresponds to blackbody radiation
?1
?max for T1000K
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Emissivity of surface
EXM3
Previus diagram holds only for blackbody surface
that reflects no radiation. All incoming
radiation is absorbed and at the same time
maximum energy is emitted (Kirchhoff law
absorptivity emissivity). Real surfaces of
plastics, wood, etc are close to the behaviour of
blackbody (non reflecting) surfaces having ?0.9,
while metals (polished) are characterised by much
less ?0.1 and signal of a detector must be
amplified according to the estimated ?. Surfaces
reducing emissivity uniformly (? is independent
of wavelength) are grey-bodies.
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Emissivity
EXM3

• There are several ways how the emissivity of
  measured sample can be identified
• From table or table
• Calibrating by a thermometer (e.g. thermocouple)
  attached to the surface
• Masking tape (attach a paper stick of known ? and
  compare results)
• Boring a deep hole into a sample (this is
  optically blackbody part of surface)

material ?
paper 0.93
brick 0.5 0.9
wood 0.8 0.9
water 0.67
Paint white 0.9 0.95
Aluminium 0.05
Stainless steel (polished) 0.22
Steel (polished,oxidised) 0.08 0.8
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Emissivity - tutorial
EXM3
Identify emissivity of surface of aluminium
cylinder
Pt100 T C
RAYTEC
Masking tape
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Wavelength - filter
EXM3
IR thermometers detect radiation only in a
wavelength band. Selection of optimal wavelength
depends upon emissivities of material. Metals
short wavelengths are recommended Plastic
material (for thin sheets it is recommended to
select wavelength with the highest emissivity and
the smallest transmissivity)
glass absorbs at 4.6 ?m, therefore sensors
looking through a glass should operate at
1-4?m. Polyethylene absorbs at 3.43 ?m.
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Absorption - tutorial
EXM3
Explain why no temperature increase was recorded
by Raytec
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Dual beam IR thermometers
EXM3
Effect of unknown emissivity can be eliminated
(for gray body surfaces) using dual wavelength IR
thermometers. Incoming radiation is split into
two beams and each beam is filtered. Spectral
emissive powers corresponding to wavelengths ?1,
?2 depend upon temperature T and emissivity ?1 ?2
according to Plancks equation. Processor
calculates ratio of spectral powers R, of the two
beams
Value R depends upon temperature and is
independent of emissivity for gray body
radiation, when ?1?2 .
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Dual wavelength IR
EXM3
Applications Read paper Hijazi et al A
calibrated dual-wavelength infrared thermometry
approach with non-greybody compensation for
machining temperature measurements, Meas. Sci.
Technol. 22 (2011) 025106 (13pp)
Short review Barron