Filament pyrometers, disappearing

One advantage of a spectral radiation pyrometer is that the emissivity or emittance at only a specific wavelength (e.g. 0.653 pm) is of importance. A non-blackbody source will be less luminescent than a blackbody source at the same temperature. Thus, a falsely low temperature will be determined by sighting a calibrated disappearing filament pyrometer on the non-blackbody. This temperature has been referred to as the brightness temperature . 

By measuring the brightness temperature using a disappearing filament pyrometer at two wavelengths, the blackbody (actual) temperature can be calculated. 

While disappearing filament pyrometers are convenient and accurate, they require human interaction and hence are not well suited for use in feedback control systems. In a total radiation pyrometer, a lens system focuses incoming radiation onto... 

Disappearing filament pyrometers were among the first pyrometers used. The original optical pyrometers used red filters to limit the wavelength around 0.65 p,m. A tungsten filament is located inside the pyrometer and will initially... 

A disappearing filament (Figure 8.7) pyrometer is a form of a spectral radiancy pyrometer, which is a device that evaluates... 

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. 

Although many methods have been used to establish the temperature of a flame, the most widely accepted technique is the line reversal method. The flame is heavily doped with an element, usually sodium, which has a conveniently placed and easily excited resonance line. This resonance line is then viewed by spectroscope against the continuous background of a lamp, whose operating temperature is adjusted until the resonance lines disappear. If the flame is hotter than the lamp, the lines appear in emission (bright) while if the lamp is the hotter, the lines appear in absorption (dark). When the lines are not visible, the lamp and flame are believed to be at the same temperature. The temperature of the lamp filament is then measured independently with a pyrometer. The technique is discussed in detail, and the necessary precautions are outlined by Wolfhard and Gaydon or Lawton and Weinberg. ... 

Optical pyrometers allow direct measurement of the temperature of an object. The disappearing filament optical pyrometer works by matching the intensity of radiant energy coming from an incandescent source to the intensity of a calibrated filament when both the source and the filament are viewed through a red filter. When the filament and the source intensities are the same, the image of the filament disappears as it is superimposed on the image of the source. An obvious requirement of this technique is that it can be used only to measure temperatures that produce visible incandescence, above about 750°C. 

The disappearing filament optical pyrometer is slow and manual. However, other pyrometers such as the photoelectric optical pyrometer can be automated. 

Morse, Samuel patented the disappearing filament optical pyrometer in 1899. 

Optical pyrometers are suitable for measuring temperatures up to 3000 °C and are usually of the disappearing filament type. The heated object is viewed through a telescope which is fitted with an electric lamp. The brightness of the lamp filament is adjusted until its temperature equals that of the object, when the lamp filament disappears . At this point the temperature of the filament, and hence the object, can be obtained from calibrated scales. 

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