An Absorption Biased (AB) methodology for NDIR gas sensors is used with a single infrared source and a detector to detect a single gas of interest by using a motion device to change the path length between that of the signal and reference channels.
As in the case of the AB designed NDIR gas sensor, the ratio of the output of the Signal channel, measured during location arrangement X, over that of the Reference channel, measured during location arrangement Y, will be used to process the gas measurement.
Multiple gases of interest can be detected by using one detector to detect multiple gases and/or by locating a second detector to detect multiple gases more distant from the source than the first detector, thereby creating longer path lengths for the second detector.
BACKGROUND OF THE INVENTION
All molecules vibrate and rotate at characteristic frequencies in the electromagnetic spectrum. These vibration/rotational frequencies cause asymmetric molecules such as CO2 and H2O, but not symmetric molecules like N2 or O2, to absorb light at very specific wavelengths, particularly in the infrared. The NDIR gas measurement technique targets these characteristic absorption bands of asymmetric molecules of gases in the infrared for their detection. The term “non-dispersive” which actually implies “non-spatially-dispersive” as used herein refers to the apparatus used, typically a narrow-band infrared transmission filter instead of a spatially-dispersive element such as a prism or diffraction grating, for isolating for the purpose of measurement the radiation in a particular wavelength band that coincides with a strong absorption band of a gas to be measured.
The NDIR technique has long been considered as one of the best methods for gas measurement. In addition to being highly specific, NDIR gas sensors are also very sensitive, relatively stable and easy to operate and maintain. In contrast to NDIR gas sensors, the majority of other types of gas sensors today are in principle interactive. Interactive gas sensors are less reliable, short-lived and generally non-specific, and in some cases can be poisoned or saturated into a nonfunctional or irrecoverable state.
Despite the fact that interactive gas sensors are mostly unreliable and that the NDIR gas measurement technique is one of the best there is, NDIR gas sensors still have not enjoyed widespread high volume usage to date. The main reasons for this can generally be attributed to their high unit production cost, relatively large size and output drifts over time.
Just about all gas sensors ever designed and manufactured to date, irrespective of what technology is being employed, invariably have significant output drifts over time. While NDIR gas sensors can be recalibrated as part of a periodic maintenance program or service, the cost of such recalibration has prevented NDIR gas sensors from being widely adopted for many applications.
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