Patent GB-758255-A (GB)
Title: Improvements in non-dispersion analyzers
IPC: G01N21/37
Filed: 1953-09-07
Abstract: 758,255. Radiation absorption gas analysers. PERKIN - ELMER CORPORATION. Sept. 7,1953 [Nov. 20, 1952], No. 24692/53. Class 40(3). In a radiation absorption gas analyser for discovering the amount of a component of interest present in a mixture with an interfering component three radiation beams are produced one an analysis beam, one a reference beam, and the other a pulling beam the entirety of which is adjusted by scan of the detector output signal to equalize the difference in entirety of the other two beams and to operate an indicator in accordance with this equalization. As shown, radiation from a heater 10, which may be of " Nichrome " (Registered Trade Mark) and is situated in a chamber 12, is divided nto three beams 15, 16, 17. Beam 15 traverses he analysis path comprising chamber 18, analysis cell 23 which is connected with cell 35 for the circulation of the gas under test through both, and filter cell 28, finally impinging on detector 52. The path of reference beam 16 comprises cell 30, sample cell 35 and filter 28, while the nulling beam 17 passes to filter cell 28 through tube 43. A rotating chopping plate 66 is provided which intercepts beams 15 and 17 together but alternately with reference beam 16. Each beam is provided with a screwadjusted shutter 65. A servo-operated member 71 (Fig. 4), is provided to adjust the intensity of radiation in the nulling path 17 and consists of a rotatable circular strip 71 of varying width. The detector 52 receives all three radiation beams and consists of three chambers separated by pressure-sensitive diaphragms 55, 56 interconnected by capillary tubes to prevent static pressure differences between the chambers. Radiation absorbed in the central chamber deforms the diaphragms which form with fixed plates 61, 62 pressuresensitive condensers. The changes of capacity of these condensers are converted into voltage pulses in a pre-amplifier 75. These pulses are amplified in amplifier 76 and rectified in a phasesensitive rectifier 77 supplied. with a reference voltage is synchronized with the rotation of the chopping plate. The D.C. output varies in sense according to whether the pulses due to the reference beam are larger or smaller than those due to the joint effect of the analysis and nulling beams. The D.C. signal is amplified in a servo-amplifier 80 and drives a servomotor 81 to adjust an optical wedge 71 to rebalance the beams. At the same time a slide wire 84 is adjusted the changes in length being recorded. When setting up the instrument, chamber 12 is filled with the gas of interest. Cell 43 is filled with a non-interfering and preferably an absorbing gas and the nulling path blanked off by trimmers 65. Filter cell 29 is evacuated and filled with a non-absorbing gas. With the source 10 and chopper plate 66 functioning, the trimmer 65 in the reference path is then adjusted until the output of the A.C. amplifier is zero. Cell 18 in the analysis path is now filled with the gas of interest to sensitize the apparatus and the interfering component is introduced into cell 30 in the reference path at such a pressure as to render the instrument unresponsive to the component. The nulling trimmer 71 is then adjusted until a particular part lies in the nulling path and the servomotor shut off. A standard sample is then introduced to sample cells 23, 35 and manual trimmer 65 adjusted in the nulling path until the output of the A.C. amplifier is again, zero. The standard sample is then removed, the servomotor switched on and the unknown mixture introduced into cell 23, 35, when the recorder indicates directly the gas of interest in the sample. Compensation for the effect of the interfering component may be assisted by introducing this component to filter cell 18. In a modification (Fig. 5, not shown), a single diaphragm condenser only is used. Radiation received from the reference path acts on a chamber directly connected to one side of the diaphragm while radiation from the analysis and nulling paths acts on the other. All three beams are intercepted simultaneously by the chopper plate the pressure difference producing deflections of the condenser diaphragm. In a further arrangement (Fig. 7, not shown), two separate chambers each with its own condenser is used, these condensers feeding a subtracting network. Specification 756,729 is referred to.