Temperature measurement optical 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... 

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). 

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. 

We use commercial Ti02 crystals (Pi-Kent) cut and polished to within 0.3° of the (110) face and we prepare them further with cycles of Ar + bombardment and U H V annealing to approximately 950-1100 K, typically 5-10 min for each cycle. The samples are mounted onto tantalum back-plates via strips of tantalum spot-welded to the back-plate. Annealing is performed by high-energy electron bombardment of the back-plate from a hot filament. Temperatures are measured from optical pyrometers (Minolta) focused on the back-plate. The temperatures are not measured directly from the samples because they are translucent and get darker with each sputter/anneal cycle. 

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. 

Various approaches to measuring flame temperature are well described in Gaydon s book on flames (see Appendix C). The best methods are spectroscopic rather than those which use thermocouples. The sodium line reversal method is perhaps the easiest. Sodium is added to the flame and the sodium D lines viewed against a bright continuum source (e g. a hot carbon tube). When the flame is cooler than the source the lines appear dark because of absorption. When the flame is hotter than the tube, the bright lines stand out in emission. The current to the tube, which will have been precalibrated for temperature readings by viewing the tube with an optical pyrometer, is adjusted until the lines cannot be seen. At this reversal point, the flame and tube temperature should be equal. 

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 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. 

In deflagration, the limiting temperature is the adiabatic flame temperature. Figure 11 (41) presents some temperature measurements made with an optical pyrometer on the exit wall of the same ceramic combustion chamber for which pressure loss data were presented in Figure 5. Temperatures w ithin 500° F. of the adiabatic flame temperature were obtained. 

Pyrometer, optical -role m temperature measurement [TEMPERATURE MEASUREMENT] (Vol 23)... 

The limiting load for cylindrical pellets 4 mm in size, which worked for a short time under atmospheric pressure and at 10% NH3 in the mixture with air, was near to 10,000 1 mixture (NTP)/cm2 hr. The measurements of the temperature of pellets with an optical pyrometer showed that it is nearly independent of load up to the limiting load. 

It is also possible to obtain excited neutral species by heating the molecules in a furnance. This method was employed to obtain a vibrationally excited N2 beam that was reacted with 0+ ions.127 Since the molecules undergo a large number of collisions with the walls of the furnace before escaping into the beam, a Boltzmann distribution of internal-energy states is established. With such an apparatus, the source temperature is measured by an optical pyrometer and is typically in the range 1000-3000° K. Several reactions of ions with excited neutrals are listed in Table III. 

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. 

All boiler temperatures were measured just prior to and after the collection of the coal samples and their respective fly ashes, since it was physically impractical to collect the samples and measure the temperatures at the same times. In all cases, the temperatures remained essentially constant. An optical pyrometer was used to measure flame temperatures, and water-cooled jacketed thermocouples were used to monitor the boiler temperatures. The wall effects on the temperature measurements were minimized by insertion of the thermocouple into the boiler until temperatures remained constant with distance upon further insertion of the thermocouple into the boiler. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

Optical Pyrometers. The optical pyrometer can be used for the determination of temperatures above 900 K, where blackbody radiation in the visible part of the spectrum is of sufficient intensity to be measured accurately. The blackbody emitted radiation intensity at a given wavelength A in equilibrium with matter at temperature Tis given by the Planck radiation law,... 

Fixed points for the calibration of the optical pyrometers are the silver, gold, and copper points and higher temperature secondary fixed points such as those given in Table 2. Calibration at other temperatures can be accomplished by use of a rotating sector or a filter of accurately known transmission factor between the fixed-point source and the pyrometer, in order to simulate a source of lower temperature in accordance with the Planck equation. Such sectors or filters are used also to permit the optical pyrometer to be used for the measurement of temperatures much above 2000 K. 

Optical-pyrometer measurements are most reliable when the object being examined is the interior of a furnace or cavity of uniform temperature viewed through a small opening. Readings for an exposed surface are dependent upon the emittance e of the substance concerned, which for an ideal blackbody is unity and for actual materials is less than unity. The emittance in the visible range is near unity for carbon (e — 0.85) and oxidized metals, but it is considerably less for platinum (e = 0.3) and other unoxidized metals, especially when they are polished. The difference between the brightness temperature Tb obtained from the optical pyrometer and the actual temperature can be approximated by... 

In the regions below 4000°K., the disappearing-filament optical pyrometer, the two-color pyrometer, or the photoelectric pyrometer can be used to measure International Scale Temperatures. Since optical pyro-metry has been well covered in the literature (F5, Rl), further discussion here would serve no purpose. 

Substrate temperature The substrate temperature is most frequently measured by a single-wavelength optical pyrometer without correction of emissivity. The emissivity from specimen is usually unknown so that it is assumed to be unity. Thus, is only a read of the pyrometer display. Even so, the numbers are useful to reproduce the substrate temperature in the process conditions. 

Although the optical pyrometer is essential for the measurement of temperatures above 1,500°C. its usefulness is by no means confined to the high temperature range. The thermocouple cannot be adapted to many processes at low temperatures for example, the measurement of the temperature of steel rails as they pass through the rolls, ingots and forgings in the open and small sources such as a heated wire or lamp filament. The temperatures used in the above processes may be accurately measured by the optical pyrometer. The temperature of a microscopic sample of any material can be measured by a modified form of the Leeds Northrup pyrometer. Also in many processes a thermocouple is not so convenient to use as an optical pyrometer, especially when the temperature is not required often enough to warrant a permanent installation of thermocouples. 

Actually it is at about the same temperature as the less bright surrounding wall. On account of reflection a corresponding bright patch appears on the opposite wall although this wall may be free from coke. It is evident that the measurement of temperature of a portion of a non-uniformly heated furnace by means of an optical pyrometer is difficult unless the precautions suggested above are taken. As soon as the furnace attains temperature uniformity and equilibrium the optical pyrometer gives the true temperature very easily and readily. 

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