22/6/18 [email protected]1 4.6 Radiative Temperature Measurements = 5.67•10 -8 W/m 2 K 4 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 3 10 10 to m
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
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
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.
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 .
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.
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
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.
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
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.
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
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
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.
