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PYROMETERS. Submitted To: Dr. Ing Naveed Ramzan Submitted By: Usama Malik 2007-chem-11. What is a Pyrometer??. Pyrometer, an instrument for measuring temperature. As the name itself signifies, they are high temperature measuring devices
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PYROMETERS Submitted To: Dr. IngNaveedRamzan Submitted By: UsamaMalik 2007-chem-11
What is a Pyrometer?? • Pyrometer, an instrument for measuring temperature. • As the name itself signifies, they are high temperature measuring devices • They measure the tempereatue of the surface of objects. • Does not involve any physical contact
The amount of thermal energy or heat leaving a body by radiation and the wavelength of that radiation are functions of the temperature of the body. • This dependence on temperature of the characteristics of radiation is used as the basis of temperature measurement in these instruments. • Here the temperature is measured by sensing the heat radiated from a hot body through a fixed lens that focuses the heat energy on to a thermopile
Black-Body • An ideal blackbody is one that at all temperatures will absorb all radiation falling on it without reflecting any whatever in the direction of incidence. • A perfect Black Body is the one that has an absorptive power of one • Bodies that donot absorb any incident radiations at all or only some portion of it have an absorptive power of less than unity.
Stefan-Boltzmann Law • It states that “the total power of radiant flux of all wavelengths R emitted into the frontal hemisphere by a unit area of a perfectly black body is proportional to the fourth power of the temperature Kelvin” • Mathematically given by, R = ∂T4 • This law has great deal of importance since most, if not all pyrometers are based primarily on this principle.
Types of Pyrometers • Four basic types based on technique of measurement • Total radiation, • Pyroelectric. • Photo-electric • Optical Pyrometers • The first three are often simply classified as radiation pyrometer due to the similarity in construction and working principle
Usually made up of a cylindrical metal body made of aluminum alloy, brass, or plastic. • One end of the body carries a lens, which, depending on the wavelength range required, consists of germanium, zinc sulfide, quartz glass, and sapphire. • The opposite end carries the electrical terminations for connecting the sensing head to its signal conditioning module (as shown in the figure in the previous slide)
Total Radiation Pyrometers • Here the radiation emitted by the radiant body or fluid whose temperature is to be measured is focused on a thermal receiving surface. • The receiving element may be a resistance element, usually it is a blackened platinum, thermopile or a thermopile • A thermopile is a set of thermocouples connected in series or side by side to form a wheel • Thermopile thermal detecting elements are most commonly used in total radiation detectors.
Pyro-electric Technique Thermometers • They are relatively new forms of pyrometers. • Usually made up of ceramic material • The molecular dipoles are fixed in these materials at ambient temperature and the molecules are present in a “mish-mash” manner. • When the temperature is increased, a point is reached that the molecules start to freely rotate • This temperature is called CURIE TEMPERATURE.
Working • If it is placed between a pair of electrodes, at ambient temperature the molecules will be fixed.(shown in first figure) • By applying voltage across electrodes and increasing temperature beyond curie temperature, molecules start to align in a certain orientation and electric field is generated as a result.(shown in fig 2) • As the ceramic temperature is further increased, the free rotation also increases.
Photo-electric Pyrometer • They are used in places where the radiations of the measured object are of shorter wavelength i.e. at very high temperatures. • They are also very similar in construction to total radiation pyrometers . • The one major difference in construction though is the use of photodiode as the detector rather than thermopile.
What is a photo-diode?? • A photodiode is usually a semiconductor diode, it could be made of germanium or silicon. • The diode is constructed in such a manner that the incident radiations can reach the junction region of the semiconductor. • If germanium is used, the diode will be a simple P-N junction, but if silicon is used it could be a P-N or P-I-N junction
Working… • As the radiant energy impacted upon the surface of the photoelectric diode increase, more electrons cross the barrier and hence more voltage reading will be obtained. • This will obviously happen at higher source temperature, thus the temperature is measured indirectly by measuring the voltage reading
Optical Pyrometers • Optical radiation thermometers are a simple in construction and they are accurate for temperature measurement between 600 oC to 3000 oC. • They require the eye and the decision making of the viewer (operator), thus they are not a suitable device for recording or control purposes. • They are very effective for point measurements and for calibration of total radiation thermometers
Working • In terms of construction, they are similar to a telescope. Here a tungsten filament lamp is placed at the focus of the objective lens. • To use the instrument the point where the temperature is required to be known is viewed through the pyrometer. • The current passing through the filament of the lamp is adjusted in such a way that the filament disappears in the image.
Represent the manner in which the filament appears in the eyepiece against the background of the radiant object whose temperature is being measured. • In (a) the current through the filament(i.e the temperature) is too high and it looks bright against the light coming from the radiant object. • In (c) the current through the filament is too low. • In (b) the filament is at the same temperature as the radiant object indicated by the fact that the filament has disappeared from the image
References… • 1.Boyes, W. (2002). Instrumentation Reference Book Third Edition (Vol. 1). Burlington: Elsevier Science Publications. • 2.Dunn, W. C. (2005). Fundamentals of Industrial instrumentation and process control (Vol. 1). Chicago: The McGraw Hill Publications. • 3.Michalski, L., Eckersdorf, K., Kucharski, J., & McGhee, J. (2001). Temperature Measurement Second Edition. West Sussex: John Wiley & Son Ltd.
4.Sirohi, R. S., & Krishna, H. C. (1991). Mechanical Measurements Third Edition. New Delhi: New Age International Limited Publisher. • 5.Sutko, D. A., & Faulk, D. J. (1996). Industrial Instrumentation.MidWest State University: Delmar Thompson Learning. • 6. www.en.wikipedia.org/wiki/Pyrometer (Retrieved on November 09, 2010) • 7.http://science.howstuffworks.com/dictionary/physics-terms/pyrometer-info.htm (Retrieved on November 09, 2010)
