Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

Temperature(part II. Optical) or how to

measure temperature in microwave ovens or without direct contact with measured surface

Experimental methods E181101 EXM3

Some pictures and texts were copied from www.wikipedia.com

Rudolf Žitný, Ústav procesní a zpracovatelské techniky ČVUT FS 2010

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)

Reflex Neoptix T1 accuracy 0.8C, =0.1 s (time constant)

evaluated spectrum of reflected radiation

Time decay of intensity of reflected radiation is a function of temperature.

These thermometers are suitable for temperature measurement in microwave owens, transformers or ohmic heaters – environment with strong elmag. fields

Optical fibre Crystal Emitted lightl

T-infrared thermometersEXM3

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% fullscaleVideo Raytec

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

T-infrared thermometersEXM3

Theoretical fundamentals (radiation emitted by surface at temperature T)

Stefan Boltzmann radiation flux [W/m2] emitted from surface having emissivity

Wien’s law temperature x wavelength at max.power = constant

Planck’s 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 Planck’s equation). This area is proportionalto 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)

4Tq

mKT 00289.0max

)1(

)(5

1

T

C

e

CTE

max for T=1000K

This graph corresponds to blackbody radiation

=1

Emissivity of surfaceEXM3

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.

4Tq

EmissivityEXM3

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 surfaceMasking tape (attach a paper stick of known and compare results)Boring a deep hole into a sample (this is optically blackbody part of surface)Coating with special black matte paint

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

Emissivity - tutorialEXM3

Identify emissivity of surface of aluminium cylinder

Pt100

T [C]

RAYTEC

Masking tape

Wavelength - filterEXM3

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 1-4m.

Polyethylene absorbs at 3.43 m.

Absorption - tutorialEXM3

Explain why no temperature increase was recorded by Raytec

RAYTEC

V

1

1

2

2

1 )(

v

v

T

T

Dual beam IR thermometersEXM3

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 Planck’s equation. Processor calculates ratio of spectral powers R, of the two beams

D1

D2 ))11

(exp()(21

5

1

2

2

1

2

1

T

C

L

LR

Value R depends upon temperature and is independent of emissivity for gray body radiation, when 1=2 .

)1(

)(5

1

T

C

e

CTE

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