Patent GB-762285-A (GB)
Title: Improvements in non-dispersion analyzers
IPC: G01N21/37
Filed: 1954-01-11
Abstract: 762,285. Infra-red gas analysis. PERKIN. ELMER CORPORATION. Jan. 11, 1954 [Jan. 30, 1953], No. 743/54. Class 40 (3). In a radiation absorption gas analyser -in which radiation travels to a detector by three separate paths constituting an analysis, comparison and nulling beam, the beams are interrupted periodically, the analysis and nulling beams being interrupted simultaneously and out of phase with the comparison beam. As shown, infra-red radiation produced by a spiral filament mounted in a reflecting cavity in a housing 28 passes through three divergent paths 23, 24, 25, respectively, the analysis, comparison and nulling beams and through windows 33. One beam passes into a sensitizing cell 37 another into a compensating cell 43 and the third into a reference path containing cell 49. Radiation leaving sensitizing cell 33 enters one half of a two-part sample cell 73 while that leaving compensating cell 43 enters the other half. The two sample cells are interconnected by a tube 80 so that the gas under test may be circulated through both and may be pivoted together out of the radiation path for cleaning. Each of the three beams then enters one of three convergent paths 86, 87, 89 contained in a housing 88 finally leaving through a single window 98 and entering a detector chamber 95. All three beams are continuously interrupted by a rotating " chopper " plate 54 (Fig. 5) which interrupts both the analysis beam and the nulling beam simultaneously but interrupts the compensating beam out of phase with the other two. Motor 59 drives the chopping plate driven through a toothed belt by a shaft 55 and is used as the driving shaft for a generator 60 producing a reference voltage. The generator may be moved transversely of the radiation paths by means of a screw 65 thus altering the position in the cycle at which the compensating beam is interrupted. Each of the radiation paths is provided with a trimmer 93 consisting of a mask which may be screwed down into the radiation beam. The quantity of radiation in the nulling path may be adjusted by an optical wedge comprising a variable width circular strip 117 rotated about its centre 119 by a motor 123 positioned beneath the base of the instrument. The detector (Fig. 12) consists of three chambers, a central one 97 which the radiation enters and two identical side ones 99, 100. All three chambers are filled with the gas to be detected and the centre chamber is separated from each of the side chambers by a diaphragm 103. Adjacent the diaphragms are situated rigid members 107 the distance from the diaphragm of which is adjustable. The fixed plates and diaphragms constitute capacitors which respond to the intermittent expansion of the gas in the central chamber. The diaphragms are both earthed and the fixed plates interconnected. In use the chambers in housing 95 are filled with non-interfering and preferably non-infra-red absorbing gas and the nulling path is blanked off. The passages in housing 85 the sample cells 72 and the cells 37 and 43 are thus evacutaed or filled with a non-absorbing gas. The manual trimmer 92 in the reference path is then adjusted to reduce the detector output to zero. The sensitizing cell 37 in the analysis path is then filled with the gas of interest and the interfering component present in the mixture under test enters the compensating cell. Trimmer 117 in the nulling path is then adjusted so as to place a given point in the nulling path. A known sample is thus introduced into the sample cells and the manual trimmer adjusted to bring the detector signal to zero. The unknown mixture now replaces the known sample in the sample cells any variation in the amount of the gas of interest producing an output signal which may be reduced to zero by movement of trimmer 117 this movement being a measure of the concentration of the gas of interest. In a modification double sample tube 72 is replaced by a funnel-shaped tube having two windows at its entrance end through one of which the analysis beam enters and the other the reference beam. Both beams then emerge through a single window entering a housing corresponding to housing 85 but having only two tubes. Specification 756,729 is referred to.