GB873576A - Improvements in or relating to methods and apparatus for gas mixture analysis - Google Patents

Abstract

873,576. Photo-electric gas analysis. UNION CARBIDE CORPORATION. Feb. 6, 1959 [Feb. 10, 1958], No. 4184/59. Class 40(3) In an arrangement for determining the proportion of a gas in a gas mixture (e.g. the concentration of nitrogen in an argon-nitrogen mixture) the mixture is excited to produce its characteristic emission spectrum in an electrical discharge tube 10 (Fig. 1) the radiation beam from which after chopping by a rotating disc 28 is optically divided by a beam splitter 30 and directed on to a first photo-electric cell 32, via a filter 37 passing the spectral range characteristic of nitrogen, and on to a second photo-electric cell 34 (an argon spectral range filter being utilized if the concentration of nitrogen is appreciable) the signal produced by which is employed as a reference signal with which the signal output of photoelectric cell 32 is compared in a ratio-recorder 40. The latter automatically adjusts a potentiometer 42 to equalize the two signals and the amount of such adjustment is recorded by the pen of the recorder which records the nitrogen content of the mixture in parts per million. In a second embodiment (Fig. 5), which avoids errors due to unequal performance of the two photo-electric cell channels, the radiation beam from the discharge tube 10 is directed on to a single photoelectric cell 44 over two optical paths the first of which includes a filter 37 passing the spectral range of the nitrogen component and the second of which includes an adjustable optical attenuator 52. Additionally, both paths include a rotating (20 r.p.m.) semicircular shutter 46 having the side opposite the cell 4 silvered. In this manner cell 44 receives radiation, alternately, from the two paths any difference in intensity in which is manifest as a 20 c.p.s. modulation of the signal output from the cell the amplitude of which is proportional to the difference in concentration of the two gases and the phase of which relative to a reference 20 c.p.s. from a generator 60 (driven by the motor 48 which drives the shutter 46) is determined by which of the two beam intensities is the greater. This phase difference is detected in 62 and utilized to adjust the attenuator 52 in the appropriate sense to equalize such difference the amount of adjustment also operating the recorder 64. A simplified embodiment useful for spot checks where absolute accuracy is not necessary consists in filtering out the spectral component of the gas whose concentration is required to be measured and applying the output of a photo-electric cell impinged by such radiation component to a suitable indicator (Fig. 9, not shown). The discharge tube 10 is provided with external electrodes to prevent contamination of the gas mixture thereby and the latter, which may be at atmospheric or higher pressure, may be fed continuously through the tube. A suitable construction of the latter is described with reference to Fig. 2 (not shown).