Patent GB-1380841-A (GB)
Title: Analysis of gas by light absorption
IPC: G01N21/31
Filed: 1972-12-13
Abstract: 1380841 Photo-electric analysis of gases SOURIAU & CO 13 Dec 1972 [4 Feb 1972] 57514/72 Heading G1A In a method of gas analysis using the selective absorption of radiation, a beam of radiation is passed into a tank containing the gas, a narrow waveband of the radiation is transmitted by an interference filter to a detector the remainder of the beam being reflected by the filter within the tank. The reflected beam may be incident on a second filter having a different pass band, the radiation transmitted by the two filters being applied to the same detector. As shown, Fig.1, I.R. radiation from an incandescent filament lamp 12 passes through a CaF entry window 11 into a U-shaped tank 10, 16, 19 through which the gas specimen is passed. Radiation of a wavelength selectively absorbed by a first constituent of the gas is transmitted by an interference filter 14 to give a measure of the concentration of this constituent. The remainder of the beam is reflected by filter 14 towards a second filter 17 which transmits radiation of a second wavelength corresponding to another constituent of the gas. The non-transmitted radiation is reflected by filter 17 towards a third filter 20 the passband of which corresponds to a further constituent of the gas. The radiation transmitted by the three filters 14, 17, 20 is passed to a detector 23 via a system of mirrors 24 to 29 two of which (27 and 28) are movable to select the radiation beam transmitted by one of the filters to be applied via a chopper 30 to the detector 23. The detector may be a photodiode, photo-transistor, photo-resistor or pyroelectric device and the output is synchronously demodulated. The order in which the filters are arranged is chosen according to the relative absorptions of the corresponding wavelengths by the different path lengths of the gas. The third filter may be arranged to reflect the remaining radiation towards a non-selective exit window of the tank. The arms of the tank may be arranged in a zig-zag configuration or in the form of a polygon and the sequence of arms need not lie in the same plane. The interference filters may be positioned outside the tank which is then provided with windows through which the radiation passes at normal incidence. In a two path system, Fig.5 (not shown), using a U-shaped tank the radiation transmitted by one or other of two interference filters is passed to a detector by a selectively positioned mirror system. This beam and a reference beam provided by the light exiting from the tank through a third filter, the pass band of which does not correspond to an absorption line of any of the constituents of the gas sample, are alternately chopped and applied to a detector the output of which is synchronously detected to produce a signal which controls the position of an optical wedge attenuator (40) in the reference radiation beam. Since the intensity of the radiation transmitted by the two constituent filters differs a second wedge attenuator is provided which is brought into use when the mirror (36) is positioned for selection of the second filter. The position of the relevant attenuator is a measure of the concentration of the corresponding constituent of the gas sample. Alternatively the attenuators may be omitted, the outputs of the synchronous detector being used as a measure of the constituent concentrations.