Crossed biased filtering NDIR gas sensing methodology

A process for detecting a concentration of a first gas in the presence of water vapor when the first gas and water vapor have overlapping absorption bands and the first gas and water vapor are in a gas sample of a gas chamber of an Non-Dispersive Infrared (“NDIR”) gas sensor having a first detection channel, a second detection channel and a third detection channel, all of the first, the second and the third detection channels having a narrow band pass filter with a central wavelength and a full width half-maximum bandwidth that are identical and the central wavelength falls within an absorption band of the first gas and water vapor, said process comprising:

• measuring a first output of the first detection chamber that is modulated by the presence of both the first gas and water vapor in the sample chamber;
• measuring a second output of the second detection chamber that is independent of any concentration of the first gas and only slightly responsive to an amount of water vapor in the sample chamber;
• measuring a third output of the third detection chamber that is independent of an concentration of the first gas in the sample chamber;
• and
• using the first, the second and the third outputs to calculate the concentration of the first gas through use of a set of calibration data for the NDIR sensor.

A process, comprising:

• generating a calibration curve A by plotting a ratio of a first output of a Non-Dispersive Infrared (“NDIR”) gas sensor divided by a second output of the NDIR gas sensor versus concentration of a first gas alone when the first and the second outputs are detected using the first gas alone in a sample chamber, wherein the NDIR gas sensor is comprised of:
• an infrared source compartment containing an infrared source;
• the sample chamber;
• a detector compartment, said sample chamber being located optically between the infrared source compartment and the detector compartment, said detector compartment comprising:
• a first detection channel comprising a first narrow band pass filter hermetically sealed with a non-absorbing gas that does not have an absorption band overlapping the first gas or water vapor, said first detection channel having the first output modulated by presence of the first gas and water vapor in the sample chamber;
• a second detection channel comprising a second narrow band pass filter hermetically sealed with the first gas and a water cell located between the infrared source and the second narrow band pass filter, said second detection channel having the second output that is independent of any amount of the first gas in the sample chamber and partially responsive to the amount of water vapor in the sample chamber;
• and
• a third detection channel comprising a third narrow band pass filter hermetically sealed with the first gas, said third detection channel having a third output that is independent of the amount of the first gas in the sample chamber;
• and
• electronics for processing signals from the first detection channel, the second detection channel and the third detection channel;
• wherein the first narrow band pass filter, the second narrow band pass filter and the third narrow band pass filter have a central wavelength and a full width half-maximum bandwidth that are identical and the central wavelength falls within an absorption band of the first gas and water vapor;
• generating a calibration curve B by plotting a ratio of the first output divided by the second output versus concentration of water vapor alone when the first and the second outputs are detected using water vapor alone in the sample chamber;
• generating a calibration curve C by plotting a ratio of the third output divided by the second output versus concentration of water vapor alone when the second and the third outputs are detected using water vapor alone in the sample chamber;
• generating a normalized calibration curve NA by normalizing the calibration curve A with only the non-absorbing gas in the sample chamber;
• generating a normalized calibration curve NB by normalizing the calibration curve B with only the non-absorbing gas in the sample chamber;
• generating a calibration curve ND by multiplying together calibration curves NA and NB;
• obtaining an output “a” for a gas sample in the sample chamber from the first detection channel;
• obtaining an output “b” for the gas sample from the second detection channel;
• obtaining an output “c” for the gas sample from the third detection channel;
• and
• calculating a detected concentration of the first gas in the gas sample by use of the outputs “a,” “b” and “c” and the calibration curves C, NB and ND.