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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