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An infrared absorption gas detector uses the amount of IR radiation of a particular wavelength to indicate the presence of a particular component or gas. A monitoring head for such a detector includes a source (8) of IR radiation, dual IR detector elements (17, 18), an optical path which extends bet

An infrared absorption gas detector uses the amount of IR radiation of a particular wavelength to indicate the presence of a particular component or gas. A monitoring head for such a detector includes a source (8) of IR radiation, dual IR detector elements (17, 18), an optical path which extends between the source and the elements and which includes a gas sample volume, and selectively transmitting spectral filters (20, 21) immediately in front of the detector elements (17, 18) to filter the radiation impinging on them. The optical path also includes a concave mirror (9) to provide a converging beam of radiation concentrated onto both elements of the detector. The location of the gas sample volume, the separation of the two detector elements (17, 18) of the detector, and the mirror are arranged so that IR radiation emitted by the source and impinging upon each of the detector elements follows a substantially common path through the gas sample volume. The monitoring head operates, in effect, as a single beam in both space and time while having no moving parts.

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1. In a monitoring head for an infrared absorption gas detector including a source (8) of infrared radiation, two detectors (17, 18) for detecting infrared radiation, a gas sample volume, an optical path extending between said source and said detectors and passing through said gas sample volume, and

1. In a monitoring head for an infrared absorption gas detector including a source (8) of infrared radiation, two detectors (17, 18) for detecting infrared radiation, a gas sample volume, an optical path extending between said source and said detectors and passing through said gas sample volume, and a selectively transmitting spectral filter (20, 21) located in said optical path leading to at least one of said detectors, the improvement wherein said two detectors are two elements of a dual element detector and both of said elements are identically formed, closely proximate each other, on a common substrate, wherein said filter is located immediately in front of one of said dual elements of said detector, and wherein said optical path includes converging means (9) to provide overlapping and converging beams of radiation individually concentrated on said both elements of said dual element detector; the location of said gas sample volume, the separation of said two elements of said dual element detector, and said converging means being arranged such that infrared radiation emitted by said source and impinging on each of said elements of said dual element detector follows a substantially common path through said gas sample volume. 2. The monitoring head of claim 1, wherein one selectively transmitting spectral filter is located in front of each of said elements of said radiation detector, the pass band of said selectively transmitting spectral filters being different. 3. The monitoring head of claim 2, wherein said dual element radiation detector is selected from a group consisting of a dual element pyroelectric detector, and a dual element thermopile detector. 4. The monitoring head of claim 2, wherein said dual elements of said detector are spaced less than 2 mm apart. 5. The monitoring head of claim 2, wherein said beams of radiation impinging on said two elements of said radiation detector have at least 50% of their area in common when they pass through an entrance window (5) defining a downstream end of said gas sample volume. 6. The monitoring head of claim 4, wherein said beams of radiation impinging on said two elements of said radiation detector have at least 50% of their area in common when they pass through an entrance window (5) defining a downstream end of said gas sample volume. 7. The monitoring head of claim 2, wherein said converging means includes a spherical concave mirror. 8. The monitoring head of claim 7, wherein said infrared source and said radiation detector are located side by side in a common plane and are shielded from one another to prevent said radiation emitted by said source impinging directly on said detector. 9. The monitoring head of claim 6, wherein said converging means includes a spherical concave mirror. 10. The monitoring head of claim 9, wherein said infrared source and said radiation detector are located side-by-side in a common plane and are shielded from one another to prevent radiation emitted by said source impinging directly on said detector. 11. The monitoring head of claim 2, which also includes means to prevent cross talk between said two elements of said radiation detector. 12. The monitoring head according to claim 2, which also includes pump means (27-29) to draw an atmosphere to be monitored through said gas sample volume. 13. The monitoring head of claim 6, which also includes pump means (27-29) to draw an atmosphere to be monitored through said gas sample volume. 14. A monitoring head for an infrared absorption gas detector comprising a source (8) of infrared radiation; a dual element radiation detector for detecting infrared radiation, both elements of said detector being identically formed, closely proximate each other, on a common substrate; a gas sample volume; two selectively transmitting spectral filters (20, 21) individually located in front of said elements (17, 18) of said radiation detector, the pass band of said two selectively transmitting spectral filters being different, said infrared source and said radiation detector being located side-by-side in a common plane and shielded from one another to prevent radiation emitted by said source impinging directly on said detector; converging means including a mirror (9) to provide overlapping and converging beams of radiation individually concentrated on said both elements of said dual element detector; an optical path extending between said source and said dual element detector and passing through said gas sample volume; the separation of said two elements of said dual element detector, said converging means and said gas sample volume being arranged such that beams of radiation impinging on said two elements of said radiation detector have at least 50% of their area in common when they pass through an entrance window (5) defining a downstream end of said gas sample volume, whereby said beams follow a substantially common path through said gas sample volume. 15. The monitoring head of claim 14 wherein said dual element radiation detector is selected from a group consisting of a dual element pyroelectric detector and a dual element thermopile detector. 16. The monitoring head of claim 14, which also includes pump means (27-19) to draw an atmosphere to be monitored through said gas sample volume. 17. A gas detector including a monitoring head in accordance with claim 8, wherein electrical and electronic elements of said gas detector are housed with said source and said dual element radiation detector in a common, completely sealed housing, and said housing including an infrared transparent window to enable infrared radiation emitted by said source to leave said housing and pass through said gas sample volume and return to said detetor located in said housing. 18. A gas detector including a monitoring head in accordance with claim 14, wherein electrical and electronic elements of said gas detector are housed with said source and said dual element radiation detector in a common, completely sealed housing, and said housing including an infrared transparent window to enable infrared radiation emitted by said source to leave said housing and to pass through said gas sample volume and return to said detector located in said housing.