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A gas sensor of the type that detects the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test comprises an optical source for emmiting radiation therefrom and a detector sensitive to radiation emitted from t

A gas sensor of the type that detects the presence of a specific gas by monitoring the absorption of optical radiation transmitted through a chamber containing a sample of gas under test comprises an optical source for emmiting radiation therefrom and a detector sensitive to radiation emitted from the source at opposing ends of a circumferential chamber, having optically reflective surfaces, extending around the periphery of a sensor housing The optical pathway between the source and detector may include a radial portion as well as a circumferential portion to allow one or other of the source and detector to be located in a central chamber of the sensor housing.

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1. A gas sensor comprising:an optical source for emitting radiation therefrom;a detector sensitive to radiation emitted from the source;an optical pathway extending between the source and the detector;a chamber having optically reflective surfaces defining a substantially circular portion of the opt

1. A gas sensor comprising:an optical source for emitting radiation therefrom;a detector sensitive to radiation emitted from the source;an optical pathway extending between the source and the detector;a chamber having optically reflective surfaces defining a substantially circular portion of the optical pathway and a substantially radial portion of the optical pathway; andat least one reflector oriented generally at an oblique angle to the substantially circular portion of the optical pathway to separate the substantially circular portion of the optical pathway and the substantially radial portion of the optical pathway. 2. The gas sensor of claim 1 in which the chamber is defined by outer and inner circumferential walls of a substantially cylindrical housing. 3. The gas sensor of claim 2 further including a first end wall, extending radially between the outer and inner circumferential walls to define a first end of the chamber. 4. The gas sensor of claim 3 further including a second end wall, extending generally radially between the outer and inner circumferential walls and at an oblique angle to a tangent of the outer or inner circumferential walls, to form the at least one reflector to reflect light through a gap in the inner circumferential wall into a central chamber. 5. The gas censor of claim 4 in which the central chamber is defined by an internal surface of the inner circumferential wall. 6. The gas sensor of claim 4 or claim 5 in which the detector is located within the central chamber. 7. The gas sensor of claim 4 or claim 5 in which the source is located within the central chamber. 8. The gas sensor of claim 4 in which the detector is located within the central chamber and comprises two detector elements spaced apart along an axis substantially parallel to the central axis of the circumferential walls. 9. The gas sensor of claim 3 in which the source is located adjacent said first end wall. 10. The gas sensor of claim 3 in which the detector is located adjacent said first end wall. 11. The gas sensor of claim 2 further comprising a chamber cover, forming a closure for the cylindrical housing, the chamber cover including a reflective inner surface in combination with a gas permeable member. 12. The gas sensor of claim 11 in which the gas permeable member comprises a flame arresting material. 13. The gas sensor of claim 11 or claim 12 in which the gas permeable member covers an annular portion of the circumferential chamber. 14. The gas sensor of claim 13 in which the gas permeable member comprises a disc having a radius greater than a radius of said inner circumferential wall and less than a radius of said outer circumferential wall. 15. The gas sensor of claim 1 wherein the optical source emits infra red radiation. 16. The gas sensor of claim 1 wherein the detector senses infra red radiation. 17. The gas sensor of claim 1 wherein the detector is a pyroelectric detector. 18. A method of forming a gas sensor comprising the steps of:providing an optical source for emitting radiation therefrom,providing a detector sensitive to radiation emitted from the source,providing an optical pathway extending between the source and the detector,providing a chamber having optically reflective surfaces defining a substantially circular portion of the optical pathway and a substantially radial portion of the optical pathway; andproviding at least one reflector oriented generally at an oblique angle to the substantially circular portion of the optical pathway to separate the substantially circular portion of the optical pathway and the substantially radial portion of the optical pathway. 19. A gas sensor comprisingan optical source for emitting radiation therefrom;a detector sensitive to radiation emitted from the source; anda circumferential chamber having optically reflective surfaces and extending between the source and the detector, the chamber being defined by outer and inner circumferential walls of a substantially cylindrical housing, the chamber including a first end wall, extending radially between the outer and inner circumferential walls to define a first end of the chamber, and a second end wall, generally radially between the outer and inner circumferential walls and at an oblique angle to a tangent of the outer or inner circumferential walls, to reflect light through a gap in the inner circumferential wall into a central chamber, thereby forming an optical pathway between the source and detector thereby comprising a substantially circumferential portion and a radial portion. 20. The gas censor of claim 19 in which the central chamber is defined by an internal surface of the inner circumferential wall. 21. The gas sensor of claim 19 or claim 20 in which the detector is located within the central chamber. 22. The gas sensor of claim 19 or claim 20 in which the source is located within the central chamber. 23. The gas sensor of claim 19 in which the detector is located within the central chamber and comprises two detector elements spaced apart along an axis substantially parallel to the central axis of the circumferential walls. 24. A gas sensor comprising:an optical source for emitting radiation therefrom;a detector sensitive to radiation emitted from the source;a circumferential chamber having optically reflective surfaces and extending between the source and the detector, the chamber being defined by outer and inner circumferential walls of a substantially cylindrical housing; anda chamber cover forming a closure for the cylindrical housing, the chamber cover including a reflective inner surface in combination with a gas permeable member, the gas permeable member covering an annular portion of the circumferential chamber and comprising a disc having a radius greater than a radius of said inner circumferential wall and less than a radius of said outer circumferential wall. 25. The gas sensor of claim 24 in which the gas permeable member comprises a flame arresting material.