Diffusion-type gas sample chamber

An improved diffusion-type gas sample chamber for transmitting radiation through gases present in the chamber by ambient pressure diffusion, the gases including an analyte gas having an absorption band and including an interfering gas having an absorption band that partially overlaps the absorption band of the analyte gas, said improved diffusion-type gas sample chamber comprising in combination:

• a) an elongated hollow tube having an inner wall and having a closed end and an open end, composed of a gastight material and having a specularly-reflective surface on the inner wall and on the inwardly-facing side of the closed end;
• b) said tube including a plurality of filtering apertures arrayed along said tube for improving the diffusion into and out of the space within said tube;
• c) a sheet of a semipermeable membrane covering each of said plurality of filtering apertures, said semipermeable membrane permitting gases to diffuse through it under ambient pressure into and out of the space within said tube and preventing airborne particles larger than a predetermined size from entering said space;
• d) a source of radiation;
• e) a detector of radiation;
• f) means for mounting both said source of radiation and said detector of radiation proximate said open end and facing said closed end, whereby some of the radiation emitted in various directions from said source of radiation is conducted by at least one reflection from the specularly-reflective surface on the inner wall to the specularly-reflective surface on the inwardly-facing side of the closed end and from the latter by at least one reflection from the specularly-reflective surface on the inner wall to said detector of radiation;
• g) a gas filter cell located between said closed end of said tube and said detector so that radiation traveling from said source to said detector must pass twice through said gas filter cell;
• and,
• h) said gas filter cell enclosing a quantity of the interfering gas which substantially absorbs that part of the radiation generated by said source which lies in the absorption band of the interfering gas, whereby said detector receives substantially no radiation at wavelengths where the absorption band of the interfering gas overlaps the absorption band of the analyte gas, thereby rendering said improved diffusion-type gas sample chamber substantially immune to the presence of the interfering gas.