The
Non-Dispersive Infrared (“NDIR”) technique has long been considered as one of
the best methods for gas measurement. In addition to being highly specific,
NDIR gas analyzers are also very sensitive, stable, reliable and easy to
maintain and service. Ever since the NDIR technique of gas measurement was
first introduced and practiced in the mid 1950's, a large number of improved
measurement techniques based upon the NDIR principle for gas detection have
been proposed and successfully demonstrated. The most notable advances over the
years in this field are summarized as follows.
Burch
et al. (U.S.
Pat. No. 3,793,525) and Blau et al. (U.S. Pat. No. 3,811,776) in 1974
were the first to advance a so-called “Double Beam” technique for NDIR gas
measurement by taking advantage of the principle of nonlinear absorption for
some strongly absorbing gases such as CO2 to create a reference
channel. Shortly thereafter, this “Double Beam” NDIR gas sensor technique was
greatly simplified with the use of two interposed spectral filters (one
absorbing and one neutral) to create a sample and a reference detector channel.
Subsequent NDIR gas sensors, designed using this technique, have enjoyed good
performance alluded to briefly above.
In U.S. Pat. No.
4,578,762 (1986) Wong advanced the first self-calibrating NDIR CO2 analyzer
using a novel two-wheel chopper and mirror arrangement. Another improved type
of such gas analyzer is shown and described in U.S. Pat. No. 4,694,173 (1987)
by Wong. This gas sensor has no moving parts for effecting the interposition of
spectral filters to create both a sample and reference detector channel as in
the NDIR gas analyzers described earlier.
In U.S. Pat. No.
5,163,332 (1992), Wong advanced the so-called “wave-guide” sample chamber
concept for simplifying NDIR gas sensors into ones that are compact, rugged and
low-cost while still maintaining their superior performance characteristics.
This concept has subsequently been widely adopted in the design of today's NDIR
gas sensors, particularly in low-cost and high volume versions.
All of
the NDIR gas analyzers described above for the measurement of the
concentrations of one or more gases in a mixture perform well functionally and
have contributed successfully to the overall technical advancement in the field
of gas analysis during the past two decades. They have been widely accepted in
both the medical and industrial communities. Despite their undisputed success
over the years, there still remain a number of important sensor performance
characteristics that need to be greatly improved in order to further extend the
useful applications of these devices in a number of areas.
By far
the most deficient performance characteristic of gas sensors of today,
inclusive of NDIR gas sensors, is the sensor output stability over time. Unlike
the temperature controller or thermostat device which just about everybody is
familiar with at home or in their workplaces for sensing temperature that never
requires output adjustment or recalibration over time, such is not the case for
gas sensors irrespective of their operational principle, functional design,
material construct or even costs. Dependent upon the type of gas sensors, just
about every one of them requires recalibration once every six months to a year
without exception in order that they remain accurate over time. While this
performance deficiency has been well tolerated over the years, it remains as a
significant drawback for gas sensors and even precludes their use in a number
of vital applications and therefore there has been a long-felt need for
elimination of this problem.
The
second most prominent performance deficiency for gas sensors of today
irrespective of their operational principle is their output dependence as a
function of the temperature of the environment wherein the sensors are located.
This performance deficiency for just about all gas sensors is universally,
albeit reluctantly, dealt with by specifying the output correction per degree
of temperature change with respect to the output stipulated at a standard
temperature. In some gas sensors these output temperature corrections are quite
large and in many cases severely limit the use of these sensors outdoors. It
would be a significant step forward in the development of future gas sensors,
particularly for the NDIR type, because of its prevalent use in most
industries, that this performance deficiency be also overcome and, again, there
has been a long-felt need for overcoming this problem.
The
afore-mentioned serious NDIR gas sensor performance deficiencies, namely sensor
output drift over time and output dependency as a function of exposed sensor
temperature, have earlier been addressed by the present inventor in a
provisional patent application 61/274,874 to the US Patent Office filed on Aug.
21, 2009 and entitled “Absorption Biased NDIR Gas Sensing Methodology.” In this
recent patent disclosure, the present inventor takes advantage of the fact that
if the spectral content of radiation from the source and/or convoluted with
those from the surroundings be always kept the same for both the reference and
the signal channels of an NDIR gas sensor, assuming that this sensor uses the
most widely deployed dual-channel methodology, the output of the sensor taken
as the ratio of the signal output over the reference output can always be kept
constant or unchanged over time except when the gas of interest is present in
the sample chamber.
In
order that this recently disclosed Absorption Biased methodology be
implemented, both the signal and the reference channel must be provided with
exactly the same spectral narrow band pass filter designed for detecting the
gas of interest in front of the respective infrared detectors. In order to
differentiate between the signal and the reference channel outputs from the
respective detectors in the presence of the gas of interest, an absorption bias
is designed between the two channels via the use of different sample chamber
path lengths for the two channels. Thus, if the sample chamber path length for
the signal channel is longer than that for the reference channel, the signal
channel detector output will change greater (or be reduced more) than that for
the reference channel when the same concentration level of the gas of interest
is present in the sample chamber. In other words, the sensor output will change
as the concentration level of the gas of interest changes in the sample chamber
as reflected by the calibration curve which can be prepared for the sensor.
The
fact that both detection channels have the same narrow band pass spectral
filter and they receive radiation from one and the same single infrared source
as taught by the widely deployed dual-channel NDIR gas detection methodology,
they are all affected in the same way to first order when there are spectral
changes caused by temperature variations in the sample chamber and/or by the
short or long-term operational changes (e.g. aging) of the infrared source.
Thus the outputs of the dual-channel NDIR gas sensor for the detection of any
gas of interest implemented using the inventor's recently disclosed Absorption
Biased methodology will stay virtually drift-free over time without the need
for any periodic re-calibration or software correction.
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