1. 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

2. 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)luminiscence 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)Neoptix T1 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

3. T-infrared thermometers EXM3 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% fullscaleRaytek Inc. Video Raytec Infrared thermometers 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

4. T-infrared thermometers EXM3 Theoretical fundamentals (radiation emitted by surface at temperature T) Stefan Boltzmann radiation flux [W/m 2 ] 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 proportional to the 4 th 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) max for T=1000K This graph corresponds to blackbody radiation  =1

5. 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.

6. Emissivity EXM3 There are several ways how the emissivity of measured sample can be identified  From table or tabletable  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)  Coating with special black matte paint material  paper0.93 brick0.5 – 0.9 wood0.8 – 0.9 water0.67 Paint white0.9 – 0.95 Aluminium0.05 Stainless steel (polished)0.22 Steel (polished,oxidised)0.08 – 0.8

7. Emissivity - tutorial EXM3 Identify emissivity of surface of aluminium cylinder Pt100 T [C] RAYTEC Masking tape

8. 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.

9. Absorption - tutorial EXM3 Explain why no temperature increase was recorded by Raytec RAYTEC V

10. 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 Planck’s equation. Processor calculates ratio of spectral powers R, of the two beams D1 D2 Value R depends upon temperature and is independent of emissivity for gray body radiation, when  1 =  2.

11. 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)paper Short review BarronBarron