Patent GB-1350231-A (GB)
Title: Fluorescent source non-dispersive infrared gas analyser
IPC: G01N21/35
Filed: 1971-06-08
Abstract: 1350231 Infrared gas analyzer ARKON SCIENTIFIC LABS 8 June 1971 19446/71 Headings G1A and G1N A non-dispersive infrared gas analyzer for detection of the presence in a sample of a gaseous compound, e.g. Co or CO2, utilizes a source comprising a container containing the same gaseous compound to be detected in the sample and means for stimulating molecular fluorescent infrared emission from the said compound in the container. The sample is contained in or passed through a test chamber and absorption of the fluorescence radiation from the source measured by a number of methods. System variations. In Fig. 1, a black body infrared source and reflector 11 illuminates the container 14. Characteristic emitted infrared radiation is absorbed by any gaseous compound to be detected in the test sample flowing through combined test cell and detector 18. This is of the pneumatic type. In the arrangement of Fig. 5 (not shown) the sample flows through a separate sample cell, and a solid state detector is used to monitor infrared radiation passing through the cell. In Fig. 6 (not shown) a two-path system is provided by the use of a source radiating a sample test/detector cell and a standard/detector cell (containing a known gas mixture). In Fig. 7 (not shown) separate test and standard flow cells are used, radiation passing through which is detected in a two part differential pneumatic cell. Selectivity is increased in the arrangement of Fig. 8 by providing after the test chamber, a fluorescence filter 53 filled with the gaseous compound to be detected. The filter absorbs, and re-emits, radiation to a solid state device, a parabolic mirror being used for focusing. In the embodiment of Fig. 9 a comparison standard is provided by the use of a further source container containing a different isotope of the compound to be detected e.g. in the case of C<12>O<16>, the isotope C<12>O<17> may be used. A stimulating source 62 is provided with a chopper for directing radiation to one and then to the other container. The apparatus also includes a fluorescence filter as in Fig. 8, but containing both isotopes. As an alternative the source Fig. 10 (not shown) may compact a single container for both isotopes, and two fluorescence filters containing the respective isotopes, is used. The chopper is then between these and the detector. A three isotope system Fig. 11 (not shown) is also envisaged. Supplementary chopping of the radiation so as to be able to compensate for source drift etc. , may be provided by a cylindrical chopper drum 104, Fig. 13, which alternately interrupts radiation entering and leaving the container 100. In Fig. 14 (not shown) the stimulating source is supplied with a square wave current. In Fig. 15, two rotating source containers 115, 116 alternately are irradiated by source 121 and themselves emit fluorescence radiation. In Fig. 16 (not shown) a closed loop flow of the gaseous compound is irradiated at one point and emits fluorescence radiation at another. In a modification of this system, Fig. 17, the emitting section of the closed loop is at the centre of a pipe along which the test gas sample is flowed. Temperature sensors before and after the irradiation position are used to determine the absorption. Source construction. Various arrangements of gaseous compound containers and stimulating sources, are described. In one the container is a flat box and the infrared heater an elongated heater element and parabolic reflector. Heaters may be provided on one or more (including all) edges of the box, Figs.2A, 2E (not shown). The containers may be cylindrical and has a coaxial heater, Figs.2C, 2G (not shown) or end heaters, Figs. 2B, 2F (not shown). The containers may be spherical with a central filament, Figs.2D, 2I (not shown) or a flat cylinder with an annular filament, Fig. 2H (not shown). To provide multiple passes a container may be internally reflective (e.g. coated with gold) an exit aperture or slit for fluorescence radiation, being provided. The source may also be energized by pressure excitation or electrical discharge.