Patent GB-1531844-A (GB)
Title: Method and device for the non-dispersive optical determination of the concentration of gas and smoke components
IPC: G01N21/33
Filed: 1976-05-14
Abstract: 1531844 Spectrametric gas analysis ERWIN SICK GmbH 14 May 1976 [16 May 1975] 20004/76 Heading G1A Apparatus for investigating mixed gas and smoke components comprises: means directing radiation over a path passing through a measurement region; means reflecting light to pass again through the region towards a photo-detector; means which can be intermittently arranged to intercept the directed light and to reflect it to the photo-detector without passing through the measurement region; and means for calculating the concentrations of the components from photodetector signals. Light from a low-pressure Hg vapour lamp 16, Fig. 1, condensed at 19, is transmitted 50 across a stack 72 via apertures 74, 75 to illuminate a field just larger than the face of a triple retroreflector 13, which redirects the light along an autocollimation path 78 including a lens 20. A semireflecting mirror 77 directs the returning light to a wheel 18 provided with arcuate tinted glass filters 23, 24, 25 and driven to provide cyclic illumination of a photo-multiplier 17 at wavelengths of 313, 435 and 546 nm. A dark zone of the wheel also enables the quiescent current of the photo-multiplier to be sampled. Between the sides of the stack 72 and the lens 20 and reflector 13 respectively are passages 76, 14 purged with clean air 71. At intervals of e.g. 10 minutes a second retroreflector 22 is moved into position adjacent the collimation lens 20 to illuminate the photomultiplier 17 with reference light which has not passed through the stack 72. The outputs of adjustable amplifiers 41-47 driven in parallel with light-responsive signals, are sorted by gating means 27, 28 controlled by light barriers 37, 35 viewing apertures in the filter wheel 18 and by a contact 30 sensing the displacement of the reflector 22. Signals U 1 -U 3  and U<SP>1</SP> 1 -U<SP>1</SP> 3  represent respectively transmitted and reference light at the three wavelengths; and a dark reference signal at 57 is fed back to the common input of the amplifiers 41-47 to zero the quiescent signal. The logarithmic ratio of each transmission signal U 1 -U 3  to its corresponding stored reference signal U<SP>1</SP> 1 -U<SP>1</SP> 3  is determined in a circuit 33 to provide an extinction coefficient signal; the extinction signals are further combined with spectral absorption coefficients of the suspected gases and smoke to provide signals representing their respective concentrations to a recorder 69. For good reflectance at UV wavelengths the retroreflectors 13, 22 each comprise the back face of a plastics pyramid reflector Fig. 6 (not shown) coated with aluminium. Either or both reflectors may have a pre-settable diaphragm to augment the adjustment range provided by the amplifiers 41 - 47. The apparatus may be calibrated using a reference filter 73 representing the absorption due to predetermined component concentrations. In a second embodiment Fig. 8 (not shown) a chopper wheel (99) mounted on the filter-wheel shaft provides a dark reference. Alternatively the dark reference may be obtained by pulsing the light source.