2015-8-29cary_wang@sohu.com1 4.6 radiative temperature measurements = 5.6710 -8 w/m 2 k 4 radiation...
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23/4/21 cary_wang@sohu.com 1
4.6 Radiative Temperature Measurements
= 5.67•10-8 W/m2K4
Radiation refers to the emission of electromagnetic wave from the surface of an object.
This radiation has characteristics of both waves and particles , which leads to a description of the radiation as being composed of photons.
* The thermal radiation emitted from an object is related to its temperature , and has wavelengths ranging from approximately .
7 310 10to m
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Eb T 4
= 5.67•10-8 W/m2K4
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Eb T 4
= 5.67•10-8 W/m2K4
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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:
Principles of radiation
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As the body increases in temperature, its emissive power increases, and the peak of the spectrum shifts to higher frequencies (lower wavelengths)
Planck radiation formula:
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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
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This equation creates the following Laws:①. Stefan-Boltzmann’s Law :
= 5.67•10-8 W/m2K4
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The wavelength of maximum irradiance for a black body
②. Wien’s displacement law:
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Radiative Temperature Measurements
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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
Wien’s displacement law (cont.)
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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
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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:
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Cavity effect For a single reflection, effective emissivity
is
Every reflection increases the emyssivity by a factor (1-ε)
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Cavity effect
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Practical blackbodies
Copper most common material The shape of the cavity defines the number of
reflections Emissivity can be increased
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对非黑体 :
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1
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
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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.
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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
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Photon(quantum) Detectors
Photon’s energy
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Optical PyrometryOne 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.
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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
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Thermal radiation from detector
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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
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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
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Disapperaring filament pyrometer
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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
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Two-colour pyromerer
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Pyrometers The corresponding optical transmission
coefficients are γx and γy
Displayed temperature
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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
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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!
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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
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Total Radiation Pyrometry
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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.
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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。
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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
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Field of View On some infrared thermometers, FOV is
adjustable.
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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.
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Ways to Determine Emissivity1. 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.
2. For temperatures below 500°F, place an object covered with masking tape (which has =0.95) in the same atmosphere. Both objects will be at the same temperature. Back out the unknown emissivity of the surface.
3. 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.
4. Coat all or part of the surface with dull black paint which has =1.0.
5. For a standard material with known surface condition, look up .
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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.
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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.
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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
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Pyroelectric thermometres
Generate electric charge in response to heat flux Crystal materials Comparable to piezoelectric effect: the polarity
of crystals is re-oriented
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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.
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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
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Principle of measurement by infrared thermography equipment
http://www.avio.co.jp/english/products/tvs/what_thermo/index.htm
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Temperature distribution image data
Temperature distribution -matrix of pixels (number of
detector: for example, 320 horizontal X 240 vertical pixels)
- thermal image data can be transferred to PC.
- the data can be calculated and utilized freely.
- thermal image data is colored up pixel by pixel based on temperature
http://www.meto.gov.uk/satpics/disk_eur.html
This image is updated every 6 hours and comes from a satellite positioned 36,000 kilometer above the Equator.
The infrared image shows cold areas in white and warm areas in black, and the cool clouds can be seen clearly against the darker oceans and land.
Example of IR imagery
Military application
Diagram of IR-guided missile
• Primary and secondary mirrors focus energy through a reticule onto an IR sensing cell.
• A spinning reticule pattern rotates around the optical axis of the seeker
http://www.deastore.com/pdf/artech/1580536867.pdf
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IR seeker
A spinning reticule pattern rotates around the optical axis of the seeker.
Seek can sense the direction to the target from the timing of the square wave portion of waveform.
http://www.deastore.com/pdf/artech/1580536867.pdf
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Infrared countermeasures (1)
Principal countermeasure against an IR-guided missile has been a high temperature flare ejected from an aircraft to break the lock of current types of missile.
Missile will home on the flare. Flare radiates significantly more IR energy
http://www.deastore.com/pdf/artech/1580536867.pdf
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Infrared countermeasures (2)
IR jammer generate IR signal which attack the guidance signals passed to the sensor in IR-guided weapons.
IR laser generates a modulated high level jamming signals.
http://www.deastore.com/pdf/artech/1580536867.pdf
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Specialized thermal imaging cameras use focal plane arrays (FPAs) that respond to longer wavelengths (mid- and long-wavelength infrared). The most common types are InSb, InGaAs, HgCdTe and QWIP FPA. The newest technologies use low-cost, uncooled microbolometers as FPA sensors. Their resolution is considerably lower than that of optical cameras, mostly 160x120 or 320x240pixels, up to 640x512 for the most expensive models. Thermal imaging cameras are much more expensive than their visible-spectrum counterparts, and higher-end models are often export-restricted due to the military uses for this technology. Older bolometers or more sensitive models such as InSb require cryogenic cooling, usually by a miniature Stirling cycle refrigerator or liquid nitrogen.
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Infrared Image Color Palette Range Is Chosen For Clarity
Color Palette 2White = T>95FBlack = T<72F
Color Palette 1White = T>85FBlack = T<72F 1 2
Palette 2 Is Better Suited For This Image
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Thermal_Image.jpg (760 × 480 pixels, file size: 67 KB, MIME type: image/jpeg)
Thermal image of human body
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Some Uses Of Infrared Thermography Electrical Inspections Hot
Connections Failing Components
Overloaded Circuits Mechanical Inspections
Motors, Bearings, Drives, Steam Systems, Boilers
Fire Hot Spots People Rescue
Building Thermal EnvelopeRoof Leaks
Military
Medical Severe Acute Respiratory
Syndrome (SARS) Manufacturing Processes
Plastics Carpet Heat/Chilling Processes
Mill OperationsSlag Detection
Police Work Fugitive Search Drug Enforcement Missing Persons Search
Animal Equine Hot Joints Ligaments
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Applications:
Portable, variable temperature blackbody radiation source for precision calibration of radiation thermometers.
Portable thermal imager.
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Advantages of thermographyIt shows a visual picture so temperatures over a large area can be comparedIt is capable of catching moving targets in real timeIt is able to find deteriorating, i.e., higher temperature components prior to their failureIt can be used to measure or observe in areas inaccessible or hazardous for other methodsIt is a non-destructive test methodIt can be used to find defects in shafts, pipes, and other metal or plastic partsIt can be used to detect objects in dark areas
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Limitations and disadvantages of thermography
Quality cameras often have a high price range (often US$6,000 or more)Images can be difficult to interpret accurately when based upon certain objects, specifically objects with erratic temperatures, although this problem is reduced in active thermal imagingAccurate temperature measurements are hindered by differing emissivities and reflections from other surfacesMost cameras have ±2% accuracy or worse in measurement of temperature and are not as accurate as contact methodsOnly able to directly detect surface temperatures
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新型温度传感器及其测温技术石英晶体温度传感器( quartz thermometer )
It measures temperature by measuring the frequency of a quartz crystal oscillator.
高分辨力 :0.0001℃ 高线性度 :0.002% 高稳定性 测温范围 : 中低温段Resolutions of .0001 °C, and accuracy of .02 °C from 0-100 °C are achievable. The high linearity makes it possible to achieve high accuracy over an important temperature range that contains only one convenient temperature reference point for calibration, the triple point of water.
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光纤测温技术光纤传感器可分为功能型和非功能型两种型
式。 功能型传感器:利用光纤的各种特性,
由光纤本身感受被测量的变化,光纤既是传输介质,又是敏感元件;
非功能型传感器又称传光型:由其他敏感元件感受被测量的变化,光纤仅作为光信号的传输介质。
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① 对采用普通测温仪表可能造成较大测量误差,甚至无法正常工作的强电磁场范围内的目标物体进行温度测量。
如金属的高频熔炼与橡胶的硫化、木材与织物、食品、药品等的微波加热烘烤过程的炉内温度测量。光纤测温技术在这些领域中有着绝对优势,因为它既无导电部分引起的附加升温,又不受电磁场的干扰,因而能保证测量温度的准确性。
光纤温度传感器适用场合
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② 高压电器的温度测量。 最典型的应用是高压变压器绕组热点的
温度测量。英国电能研究中心从 70 年代中期就开始潜心研究这一课题,起初是为了故障诊断与预报,现在由于计算机电能管理的应用,便转入了安全过载运行,使系统处于最佳功率分配状态。另一类可能应用的场合是各种高压装置,如发电机、高压开关、过载保护装置等。
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③ 易燃易爆物的生产过程与设备的温度测量。
光纤传感器在本质上是防火防爆器件,它不需要采用隔爆措施,十分安全可靠。
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④高温介质的温度测量。 在冶金工业中,当温度高于 l 300℃ 或 l
700℃时,或者温度虽不高但使用条件恶劣时,尚存在许多测温难题。充分发挥光纤测温技术的优势,其中有些难题可望得到解决。例如,钢水和铁液在连轧和连铸过程中的连续测温问题。
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1 、膨胀式温度计 就地指示
2 、热电偶温度计 基于热电效应工作。 基本定律 冷端温度补偿
3 、热电阻温度计 三线制连接
温度检测仪表
5 、非接触式温度计 特别在高温时,响应快
4 、热敏电阻和集成温度传感器 价廉,在非工业生产过程领域中广泛使用
集中显示、记录和控制接触式(常用)
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各种温度检测方法及其测温范围表
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