GB747602A - Improvements in photo-electric methods for the estimation of gases or vapours - Google Patents

Abstract

747,602. Photo-electric gas analyser. SOC. ANON. DES MANUFACTURES DES GLACES ET PRODUITS DE ST.-GOBAIN, CHAUNY ET CIREY. Jan. 17, 1952 [Jan. 18, 1951; March 19, 1951], No. 1407/52. Class 40 (3). In a method of analysing a gas or vapour wherein radiations of the characteristic wavelength which the gas absorbs are passed through it and received on a photo-cell sensitive to these wavelengths the radiations are intermittently interrupted to facilitate amplification of the photo-cell output signal. The interruption of the light may be effected by a mechanical chopper or the light source may be a gasdischarge tube fed from the output of a lowfrequency oscillator. As shown (Fig. 3), light from a source 20 passes through a tube 21 containing the gas under test to a photoelectric cell 22 while a second beam from the same source passes to a second cell 23. Both beams are chopped simultaneously by a rotating perforated disc the output of each cell being amplified and fed in opposition to a meter 30 recording instrument 31 and a relay 32. A filter F2 may be used to limit radiation to the band 3,600 to 4,000Š where a coloured gas is being analysed. The Specification describes a method of detecting moisture (Fig. 5, not shown) in a gas by allowing it to rise through a column packed with Raschig rings into which is fed an oleum from a container. The fog formed by the humid gas then passes through a tube containing further Raschig rings in which excessively large particles are deposited into an absorption tube in which the:unit is detected photo-electrically as already described.. Circuits.-A differential circuit for eliminating source variations from the output is described with reference to Fig. 4. In this embodiment the light source is a high pressure mercury vapour lamp supplied with alternating current. The analysing and comparison cells C 1 and C2 are connected in apposition as shown. A voltage is developed across a potentiometer P1 the output of which is amplified at L1 and coupled to successive amplifying stages L2, L3 by a transformer T1. The voltage developed across a cathode resistor of tube L3 is fed to one grid of a pair of cathode coupled triodes L4, the other grid of which is earthed and a meter, recorder, and relay are connected between the anodes. To eliminate all harmonics of the modulating frequency degenerative feed-back is fed from the cathode of valve L3 through a twin T circuit which passes all frequencies except the fundamental to the grid of tube L2. In the embodiment of Fig. 2 (not shown), a single cell feeds the first amplifying valve and the double triode stage following tube L3 is omitted.