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4.6 Radiative Temperature Measurements. Radiation refers to the emission of electromagnetic wave from the surface of an object. s = 5.67•10 -8 W/m 2 K 4.
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4.6 Radiative Temperature Measurements Radiationrefers to the emission of electromagnetic wave from the surface of an object. s = 5.67•10-8 W/m2K4 This radiation has characteristics of both waves and particles , which leads to a description of the radiation as being composed ofphotons. ＊The thermal radiation emitted from an object is related to its temperature , and has wavelengths ranging from approximately . cary_wang@sohu.com
s = 5.67•10-8 W/m2K4 cary_wang@sohu.com
s = 5.67•10-8 W/m2K4 cary_wang@sohu.com
Principles ofradiation Max Planck developed the basis for the theory of quantum mechanics in 1900 as a result of examining the wavelength distribution of radiation. He proposed the following equation to describe the wavelength distribution of thermal radiation for an ideal or blackbody radiator: cary_wang@sohu.com
Planck radiation formula: As the body increases in temperature, its emissive power increases, and the peak of the spectrum shifts to higher frequencies (lower wavelengths) cary_wang@sohu.com
式中，W为波长；c1为普朗克第一辐射常数, c2为普朗克第二辐射常数，h为普朗克常数；c为光速；k为玻耳兹曼常数 cary_wang@sohu.com
This equation creates the following Laws: ①. Stefan-Boltzmann’s Law : s = 5.67•10-8 W/m2K4 cary_wang@sohu.com
②. Wien’s displacement law: • The wavelength of maximum irradiance for a black body cary_wang@sohu.com
Radiative Temperature Measurements cary_wang@sohu.com
Wien’s displacement law (cont.) • Sun; wavelength of maximum emission at 0.5 x 10-6 meters (micrometers), assuming an equivalent black body temperature of 5780 K • Earth; wavelength of maximum emission of 11.4 x 10-6 meters assuming an equivalent black body temperature of 255 K cary_wang@sohu.com
Blackbody • An ideal emitter of electromagnetic radiation • opaque • non-reflective • for practical blackbodies ε = 0.9 • Cavity effect • em-radiation measured from a cavity of an object cary_wang@sohu.com
Cavity effect • Emissivity of the cavity increases and approaches unity • According to Stefan-Boltzmann’s law, the ideal emitter’s photon flux from area a is • In practice: cary_wang@sohu.com
Cavity effect • For a single reflection, effective emissivity is • Every reflection increases the emyssivity by a factor (1-ε) cary_wang@sohu.com
Cavity effect cary_wang@sohu.com
Practical blackbodies • Copper most common material • The shape of the cavity defines the number of reflections • Emissivity can be increased cary_wang@sohu.com
对非黑体: cary_wang@sohu.com
Radiative heat is transferred via photons which travel at the speed of light. When this energy strikes a surface, it can either be absorbed, reflected, or transmitted. For a non-ideal radiator, The radiative heat transfer between two ideal bodies A and B cary_wang@sohu.com
If A is not ideal, In our case, the detecting element will be B, and from this we will determine the heat flux (and thus the temperature) of A. Calibration is required to account for unknown quantities like the view factor and the body emissivity. cary_wang@sohu.com
Radiation Detectors—Two Broad Categories Some radiative temperature measurements are made by detecting photons emitted by the hot source. We’ll call these Photon Detectors. There is essentially no difference between this and a CCD camera. A Thermal Detector produces a rise in temperature at some detector cary_wang@sohu.com
Photon(quantum) Detectors Photon’s energy cary_wang@sohu.com
Optical Pyrometry One or two wavelengths of light are selected using a series of optical filters. For a photon detector, we can determine the temperature from If two colors (wavelengths) are examined, the influence of the unknown emissivity of the object, which may be independent of wavelength, can be eliminated. cary_wang@sohu.com
Thermal detectors • Response to heat resulting from absorption of the sensing surface • The radiation to opposite direction (from cold detector to measured object) must be taken into account cary_wang@sohu.com
Thermal radiation from detector cary_wang@sohu.com
Pyrometres(高温计) • Disappearing filament pyrometer • Radiation from and object in known temperature is balanced against an unknown target • The image of the known object (=filament) is superimposed on the image of target cary_wang@sohu.com
Pyrometres • The measurer adjusts the current of the filament to make it glow and then disappear • Disappearing means the filament and object having the same temperature cary_wang@sohu.com
Disapperaring filament pyrometer cary_wang@sohu.com
Pyrometres • Two-color pyrometer(比色高温计) • Since emissivities are not usually known, the measurement with disappearing filament pyrometer becomes impractical • In two-color pyrometers, radiation is detected at two separate wavelengths, for which the emissivity is approximately equal cary_wang@sohu.com
Two-colour pyromerer cary_wang@sohu.com
Pyrometers • The corresponding optical transmission coefficients are γx and γy Displayed temperature cary_wang@sohu.com
Measurements • Stefan-Boltzmann’s law with manipulation: • Magnitude of thermal radiation flux, sensor surface’s temperature and emissivity must be known before calculation • Other variables can be considered as constants in calibration cary_wang@sohu.com
Error sources • Errors in detection of the radiant flux or reference temperature • Spurious heat sources • Heat directly of by reflaction into the optical system • Reflectance of the object (e.g. 0.1) But does not require contact to surface measured! cary_wang@sohu.com
Pyrometer Pros and Cons • PROS • Can measure high temperatures without melting or oxidizing • Can also be used at lower temperatures • CONS • Most bodies are not black bodies • Are not as accurate as other methods of measurement cary_wang@sohu.com
Total Radiation Pyrometry cary_wang@sohu.com
Infrared Thermometry • Infrared thermometers measure the amount of radiation emitted by an object. • Peak magnitude is often in the infrared region. • Surface emissivity must be known. This can add a lot of error. • Reflection from other objects can introduce error as well. • Surface whose temp you’re measuring must fill the field of view of your camera. cary_wang@sohu.com
透射式光学系统的部件是用红外光学材料制成的，根据红外波长选择光学材料。透射式光学系统的部件是用红外光学材料制成的，根据红外波长选择光学材料。 测量温度 高温 中温 低温 >700℃ 100℃～700℃ < 100℃ 有用波段0.76～3 μm 3～5 μm 5～14 μm 近红外区 中红外区 中远红外 材料 一般光学玻璃 氟化镁、氧化镁等 锗、硅、热压 或石英等材料 热压光学材料 硫化锌等材料 并通常还在镜片表面蒸镀红外增透层，一方面滤掉不需要的波段，另一方面增大有用波段的透射率。 cary_wang@sohu.com
Benefits of Infrared Thermometry • Can be used for • Moving objects • Non-contact applications where sensors would affect results or be difficult to insert or conditions are hazardous • Large distances • Very high temperatures cary_wang@sohu.com
Field of View • On some infrared thermometers, FOV is adjustable. cary_wang@sohu.com
Emissivity • To back out temperature, surface emissivity must be known. • You can look up emissivities, but it’s not easy to get an accurate number, esp. if surface condition is uncertain (for example, degree of oxidation). • Highly reflective surfaces introduce a lot of error. • Narrow-band spectral filtering results in a more accurate emissivity value. cary_wang@sohu.com
Ways to Determine Emissivity • Measure the temperature with a thermocouple and an infrared thermometer. Back out the emissivity. This method works well if emissivity doesn’t change much with temperature or you’re not dealing with a large temperature range. • For temperatures below 500°F, place an object covered with masking tape (which has e=0.95) in the same atmosphere. Both objects will be at the same temperature. Back out the unknown emissivity of the surface. • Drill a long hole in the object. The hole acts like a blackbody with e=1.0. Measure the temperature of the hole, and find the surface emissivity that gives the same temperature. • Coat all or part of the surface with dull black paint which has e=1.0. • For a standard material with known surface condition, look up e. cary_wang@sohu.com
Spectral Effects • Use a filter to eliminate longer-wavelength atmospheric radiation (since your surface will often have a much higher temperature than the atmosphere). • If you know the range of temperatures that you’ll be measuring, you can filter out both smaller and larger wavelength radiation. Filtering out small wavelengths eliminates the effects of flames or other hot spots. • If you’re measuring through glass-type surfaces, make sure that the glass is transparent for the wavelengths you care about. Otherwise the temperature you read will be a sort of average of your desired surface and glass temperatures. cary_wang@sohu.com
Price and Accuracy • Prices range from $500 (for a cheap handheld) to $6000 (for a highly accurate computer-controlled model). • Accuracy is often in the 0.5-1% of full range. Uncertainties of 10°F are common, but at temperatures of several hundred degrees, this is small. cary_wang@sohu.com
Choice Between RTDs, Thermocouples, Thermisters • Cost – thermocouples are cheapest by far, followed by RTDs • Accuracy – RTDs or thermisters • Sensitivity – thermisters • Speed - thermisters • Stability at high temperatures – not thermisters • Size – thermocouples and thermisters can be made quite small • Temperature range – thermocouples have the highest range, followed by RTDs • Ruggedness – thermocouples are best if your system will be taking a lot of abuse cary_wang@sohu.com
Pyroelectric thermometres • Generate electric charge in response to heat flux • Crystal materials • Comparable to piezoelectric effect: the polarity of crystals is re-oriented cary_wang@sohu.com
Infrared Thermography (红外热象仪) Thermal imaging cameras detect radiation in the infrared range of the electromagnetic spectrum (roughly 9000–14,000 nanometers or 9–14 µm) and produce images of that radiation, called thermograms. cary_wang@sohu.com
Infrared Thermography (红外热象仪) • An Infrared Camera Senses Infrared Radiation • A Processor In The Camera Assigns Color To The Infrared Radiation-Different Color Equals Different Temperature • Enabling Us To Visualize The Thermal World cary_wang@sohu.com
Principle of measurement by infrared thermography equipment http://www.avio.co.jp/english/products/tvs/what_thermo/index.htm
