A gas sensor comprises a body on which is mounted an optical source and a detector means sensitive to light from the source. The body further comprises a channel arranged to admit gas, the channel comprising an elongated groove having reflective surfaces defining a folded optical path for light from the source. The groove is arranged as a serpentine shape or spiral shape so that light from the source to detector travels around the groove being internally reflected within the shape.
1. A gas sensor comprising a body, the body comprising:
a channel having a first end and a second end and arranged to admit a gas, the channel comprising an elongated groove having reflective surfaces defining a folded optical path for light from the source;
an optical source located at the first end of the channel;
a first detector, located at the second end of the channel, to detect light from the source; and
a second detector located to detect light from the source that has been tapped off partway along the channel so that light detected by the second detector travels a shorter optical path than has light detected by the first detector.
2. A sensor as claimed in claim 1 , wherein the second detector is connected to the channel by an optical pathway arranged to redirect a portion of light from a predetermined region of the channel to the second detector.
3. A sensor as claimed in claim 1 , in which a portion of the elongated groove forms a spiral optical path.
4. A sensor as claimed in claim 1 , in which a portion of the groove forms a serpentine optical path.
5. A sensor as claimed in claim 1 , in which a portion of the groove forms a helical optical path.
6. A sensor as claimed in claim 5 , in which the body is cylindrical and the helical optical path extends around the exterior of the body.
7. A sensor as claimed in claim 6 , in which the cylinder includes a hollow region and a portion of the groove comprises a helical optical path around the interior of the hollow region.
8. A sensor as claimed in claim 5 , in which the body includes a hollow cylindrical region and a portion of the groove comprises a helical optical path around the interior of the hollow region.
9. A sensor as claimed in claim 1 , in which the body comprises a base arranged to accommodate the source and detector(s) and at least one wall extending transversely from the plane of the base.
10. A sensor as claimed in claim 9 , in which the walls are arranged substantially to bisect each other transversely.
11. A sensor as claimed in claim 9 , in which a portion of the elongated groove is located on the at least one wall and a portion of the groove is located on the base.
12. A sensor as claimed in claim 1 , further comprising a cover for the channel including a gas admittance member.
13. A sensor as claimed in claim 12 , in which the cover has an interior surface facing the channel, which surface is arranged to reflect radiation.
14. A sensor as claimed in claim 12 , in which the gas admittance means includes sintered material.
15. A sensor as claimed in claim 12 , in which the gas admittance means includes a particulate filter.
16. A sensor as claimed in claim 1 wherein the optical source is an infrared source.