NDIR gas analysis using spectral ratioing technique

In a ratioing type of NDIR gas analyzer for use in determining the concentration of a particular gas that might be present in the air based on the propensity of the particular gas to absorb radiation in a characteristic wavelength band in which air not containing the particular gas does not absorb radiation, having a source of blackbody radiation, having a first filter that transmits only radiation in a spectral band that includes the characteristic wavelength band of the particular gas, having a first detector that receives only radiation that has passed through said first filter, having a second filter that transmits only radiation in a spectral band that does not include the characteristic wavelength band of the particular gas, and having a second detector that receives only radiation that has passed through said second filter, the improvement comprising:

• a tube having an open first end in which said source is located, having an open second end, having an inside surface that is specularly reflective for conducting the radiation along said tube by multiple reflections, having a wall, and having at least one aperture extending through the wall;
• a membrane spanning said at least one aperture and permitting the particular gas to diffuse into said tube from the air around the instrument while preventing unwanted particles from entering said tube;
• and,
• a housing attached to the second end of said tube, supporting said first filter, said first detector, said second filter, and said second detector, said housing further including a baffle interposed between said first filter and said second filter, and between said first detector and said second detector, preventing radiation that has passed through said first filter from reaching said second detector and preventing radiation that has passed through said second filter from reaching said first detector.

In a ratioing type of NDIR gas analyzer for use in determining the concentration of a particular gas that might be present in the air based on the propensity of the particular gas to absorb radiation in a characteristic wavelength band in which air not containing the particular gas does not absorb radiation, having a source of blackbody radiation, having a first filter that transmits only radiation in a spectral band that includes the characteristic wavelength band of the particular gas, having a first detector that receives only radiation that has passed through said first filter, having a second filter that transmits only radiation in a spectral band that does not include the characteristic wavelength band of the particular gas, and having a second detector that receives only radiation that has passed through said second filter, the improvement comprising:

• a tube having an open first end in which said source is located, having an open second end, having an inside surface that is specularly reflective for conducting the radiation along said tube by multiple reflections, having a wall, and having at least one aperture extending through the wall;
• a membrane spanning said at least one aperture and permitting the particular gas to diffuse into said tube from the air around the instrument while preventing unwanted particles from entering said tube;
• a first canister connected to the second end of said tube, having an aperture to admit radiation, containing said first detector, and supporting said first filter so that it spans the aperture of said first canister;
• and,
• a second canister connected to the second end of said tube, having an aperture to admit radiation, containing said second detector, and supporting said second filter so that it spans the aperture of said second canister.