Surface pyrometer

Temperature measurements ranging from 760 to 1760°C are made usiag iron—constantan or chromel—alumel thermocouples and optical or surface pyrometers. Temperature measuriag devices are placed ia multiple locations and protected to allow replacement without iaciaerator shutdown (see... 

A surface pyrometer, often with an extension probe to penetrate through the insulation, can be used. Success with this technique was demonstrated in a couple of cases (34, 121). Temperature differences greater than 15°F are usually indicative of a maldistri-buted pattern temperature differences smaller then 5°F suggest no evidence for a maldistributed pattern. 

Using a surface pyrometer (find a probe with a fiat, flexible head), carefully and accurately measure the external temperature of each downcomer. An infrared thermometer will also do nicely (see Chapter 29). 

Pyrometers can be used to monitor temperature in a cavity to eliminate the variable of emissivity of a surface. Pyrometers may also be programmed to only record maximum temperatures so that, if a series of surfaces and voids are passed in front of the pyrometer, only the temperatures of the hot surfaces wiU be indicated. This can be useful when observing a rotating fixture such as is used in coating drill bits (Figure 3.13). 

The deterrnination of surface temperature and temperature patterns can be made noninvasively using infrared pyrometers (91) or infrared cameras (92) (see Infrared technology and raman spectroscopy). Such cameras have been bulky and expensive. A practical portable camera has become available for monitoring surface temperatures (93). An appropriately designed window, transparent to infrared radiation but reflecting microwaves, as well as appropriate optics, is needed for this measurement to be carried out during heating (see Temperature measurement). 

No object can radiate more energy than can a blackbody at the same temperature, because a blackbody ia equiUbrium with a radiation field at temperature T radiates exacdy as much energy as it absorbs. Any object exhibiting surface reflection must have emissivity of less than 1. Pyrometers are usually caUbrated with respect to blackbodies. This can cause a serious problem ia use. The emissivities of some common materials are fisted ia Table 4. 

Red Brass Alloys. In forming red brass alloys, which iaclude leaded red and leaded semired brasses, caution should be exercised to prevent gas absorption by flame impingement or the melting of oily scrap, or metal loss through excessive oxidation of the melt surface. To prevent excessive 2iac volatilization, the melt must be poured as soon as it reaches the proper temperature. The melt should be finally deoxidized and cast at ca 1065—1230°C as measured with a pyrometer. Fluxing is usually not needed if clean material has been melted. 

Pyrometers Planck s distribution law gives the radiated energy flux qb(X, T)dX in the wavelength range X to X -1- dX from a black surface ... 

Accuracy of Pyrometers Most of the temperature estimation methods for pyrometers assume that the objec t is either a grey body or has known emissivity values. The emissivity of the nonblack body depends on the internal state or the surface geometry of the objects. Also, the medium through which the therm radiation passes is not always transparent. These inherent uncertainties of the emissivity values make the accurate estimation of the temperature of the target objects difficult. Proper selection of the pyrometer and accurate emissivity values can provide a high level of accuracy. 

HREELS experiments [66] were performed in a UHV chamber. The chamber was pre-evacuated by polyphenylether-oil diffusion pump the base pressure reached 2 x 10 Torr. The HREELS spectrometer consisted of a double-pass electrostatic cylindrical-deflector-type monochromator and the same type of analyzer. The energy resolution of the spectrometer is 4-6 meV (32-48 cm ). A sample was transferred from the ICP growth chamber to the HREELS chamber in the atmosphere. It was clipped by a small tantalum plate, which was suspended by tantalum wires. The sample was radia-tively heated in vacuum by a tungsten filament placed at the rear. The sample temperature was measured by an infrared (A = 2.0 yum) optical pyrometer. All HREELS measurements were taken at room temperature. The electron incident and detection angles were each 72° to the surface normal. The primary electron energy was 15 eV. 

These results provide clear evidence for the existence of selective heating effects in MAOS involving heterogeneous mixtures. It should be stressed that the standard methods for determining the temperature in microwave-heated reactions, namely with an IR pyrometer from the outside of the reaction vessel, or with a fiber-optic probe on the inside, would only allow measurement of the average bulk temperature of the solvent, not the true reaction temperature on the surface of the solid reagent. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

The temperature measurement devices which do not contact the hot surfaces, for example, optical -, radiation pyrometers, and infrared techniques, are not typical for high-pressure application. 

The Czochralski Technique. Pulling from the melt is known as the Czochralski technique. Purified material is held just above the melting point in a cmcible, usually of Pt or Ir, most often powered by radio-frequency induction heating coupled into the wall of the crucible. The temperature is controlled by a thermocouple or a radiation pyrometer. A rotating seed crystal is touched to the melt surface and is slowly withdrawn as the molten material solidifies onto the seed. Temperature control is used to widen the crystal to the desired diameter. A typical rotation rate is 30 rpm and a typical withdrawal rate, 1—3 cm/h. Very large, eg, kilogram-sized crystals can be grown. 

The methods of temperature measurement of graphite filaments are also subject to criticism. Temperature must be measured by an optical pyrometer. Duval (19) admits a possible error of 50° C. due to uncertainty in the calculated emissive power of a dull graphite surface (60). Furthermore, the temperature range of investigation cannot be extended far below 1000° C. without making arbitrary extrapolations of temperature vs. voltage curves. 

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

Another thermocouple and pyrometer indicate the flame temperature at the coating surface. The burner setup is designed to apply the desired temperature for the given period of time. For example, a temperature of 1750 F. can be reached in 1 minute and then held constant tor 0.5 hour. Figure 3 is a photograph of a typical fire-retardant test setup. 

A temperature of 1750 F. (or 2000 F.) is reached in 1 minute on the coating surface and maintained for 0.5 hour. The pyrometers indicate the temperature differential (insulative ability of the fire-retardant coating) between the name temperature and the bare metal temperature. Results should be recorded on a graph indicating time vs. temperature and evaluated on a relative basis. 

Fig. 12.9 Melt surface temperature rise at the capillary exit, calculated for ABS Cycolac T and measured ( ) with an infrared pyrometer Tq = 505 K, Dq = 0.319 cm, L/Dq — 30. The relationships Nu = C(GZ) 3 are used to estimate h. [Reprinted by permission from H. W. Cox and C. W. Macosko, Viscous Dissipation in Die Flow, AIChE J., 20, 785 (1974).]...

Ignition properties can be determined by direct measurements. Tig can be measured with fine thermocouples attached to the exposed surface of ignition test specimens, or by using an optical pyrometer. ASTM D 1929 is a bench-scale furnace test method to determine piloted ignition and... 

Another area of research that could be profitably explored is the use of remote sensing instruments to measure surface temperatures of textile assemblies. Infrared thermovision cameras have been used to visualize temperature distributions over clothed and nude persons in order to study the transport of microorganisms by convective heat flow (112). A variety of less expensive radiometers and radiation pyrometers that are used to measure and automatically control the temperature of textiles during drying and texturing (113, llU, 115) could also assess the thermal behavior of apparel and clothing assemblies and thus elucidate their contribution to thermal comfort indoors. 

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