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Manually Operated Pyrometers Disappearing Filament Pyrometers Principle of operation The first disappearing filament pyrometer was built in 1901 by L. Holborn and F . Kurlbaum (Holborn and Kurlbaum, 1903) . Disappearing filament pyrometers are spectral pyrometers where the brightness of a lamp filament is changed by adjusting the lamp current until the filament disappears against the background of the target, whose temperature is to be measured. In pyrometers of this kind, the eye of the observer is itself the detector . In another seldom applied type of pyrometer the brightness match is achieved by the attenuation ofthe target brightness, using a neutral grey. | Temperature Measurement Second Edition L. Michalski K. Eckersdorf J. Kucharski J. McGhee Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-86779-9 Hardback 0-470-84613-5 Electronic 9 Manually Operated Pyrometers 9.1 Disappearing Filament Pyrometers 9.1.1 Principle of operation The first disappearing filament pyrometer was built in 1901 by L. Holbom and F. Kurlbaum Holbom and Kurlbaum 1903 . Disappearing filament pyrometers are spectral pyrometers where the brightness of a lamp filament is changed by adjusting the lamp current until the filament disappears against the background of the target whose temperature is to be measured. In pyrometers of this kind the eye of the observer is itself the detector. In another seldom applied type of pyrometer the brightness match is achieved by the attenuation of the target brightness using a neutral grey filter. In the first type shown in Figure 9.1 the observer sees the filament of the lamp against the target background through the eyepiece and red filter. The lamp current is adjusted by the resistor until the filament picture disappears when the brightness or radiance L of equation 8.23 of filament and target are identical. The measured value of temperature is read from the ammeter A calibrated in temperature units. A comparison of the radiance of the filament and the target occurs at one wavelength. A grey filter can be used to increase the measurement range. Disappearing filament pyrometers which are calibrated for black-body targets have a lower limit of temperature range of about 700 C determined by the long wave visibility limit of the human eye. Figure 9.1 Principle of the disappearing filament pyrometer 164 MANUALLY OPERATED PYROMETERS 9.1.2 Red filter A red filter is used for the following reasons and advantages. The spectral transmissivity of a red filter and the spectral relative sensitivity of a standard human eye are displayed in Figure 9.2 as a function of wavelength Âe. It may be clearly seen that there is a well .