Two detectors of the same kind, each having an identical
neutral band-pass filter to the target gas, are installed next to Signal
channel and Reference channel detectors as pairs in an AB designed NDIR gas
sensor layout, which are called Standard Signal channel detector and Standard
Reference channel detector. “Standard” GAMMA is the ratio of Standard signal
channel detector output over that of Standard Reference channel detector.
“Standard” GAMMA is independent of the measurement Physics of NDIR gas sensors,
is dependent only upon the performance characteristics of the sensor component
and is also independent of the presence of any amount of target gas in the
sample chamber. Consequently, “Standard” GAMMA can be used to proportionally
correct and update GAMMA of the sensor as its components age over time thereby
rendering such an AB designed NDIR gas sensor self-commissioning or staying
accurate over time after initial calibration.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part application of
U.S.
patent application Ser. No. 13/149,738, the disclosure of which is specifically
incorporated herein by reference.
FIELD OF THE INVENTION
The present invention is in the field of measuring
instruments, and specifically relates to a configuration design and method for
an NDIR gas sensor.
BACKGROUND OF THE INVENTION
Output instability or drift over time leading to
measurement inaccuracies has long been a major deficiency for gas sensors
irrespective of what technology or methodology is used for their conception or
realization. Output software correction may alleviate the problem somewhat but
it is in many instances inaccurate and not even always applicable. It has long
been the objective of many researchers in this field to overcome this problem
fundamentally and for good.
Recently the present author in U.S. Pat. No. 8,143,581,
the disclosure of which is specifically incorporated by reference herein,
advanced the teaching of an Absorption Biased NDIR Gas Sensing Methodology
which is capable of eliminating substantially all the NDIR gas sensor output
drifts over time without the need for re-calibration. As it turns out, the
solution to solving this output drift problems for gas sensors actually lies
deeper than the availability of superior NDIR gas sensor types even though they
can indeed be designed to be capable of maintaining measurement accuracy over
time. The fact of the matter is that people have experienced gas sensor output
instability for such a long time in the past that when output stable sensors
really come along nobody believes it. Until such time that stable gas sensors
become widely available and users begin to consider their performance as
trustworthy and truly believable, the real need today must be viewed from a
completely different perspective, which is to be able to come up with a fast,
inexpensive and simple methodology that can easily check the accuracy of gas
sensors and inexpensively re-calibrate them when they are found to be
inaccurate.
In U.S.
application Ser. No. 13/149,738, filed May 31, 2011, of which this application
is a continuation-in-part application, the present author advanced the teaching
of a novel Re-calibration Methodology for simply and easily re-calibrating
Absorption Biased (AB) designed NDIR gas sensors without the need of standard
gases. With the recent advent of the Absorption Biased (AB) gas sensing
methodology for realizing NDIR gas sensors whose outputs are significantly
drift-free over time and also the advent of a complementing methodology that
can check and re-calibrate AB designed NDIR gas sensors simply and easily
without the need of standard gases, one would think that the gas sensor
industry at large, particularly the HVAC industry, would be relatively
satisfied and happily go forward in growing its business. But, unfortunately,
this is not the case at all. While the HVAC industry is still trying to deal
with their old and on-going problem of sensor inaccuracies over time, already
the industry is pushing forward in finding new and better solutions for
optimizing energy expenditure and achieving superior comfort level for
occupants in buildings. One rather obvious approach widely being investigated
and considered everywhere today is the grouping of all sensors in a building
together into a computer network. These sensors can actually interact and work
with one another in an efficient manner with self-commissioning, self-tuning,
self-diagnostic and correction, and even self-configuring features. By so doing
the energy requirement for buildings can be reduced to an absolute minimum
while the comfort level and safety for occupants in the buildings can also be
greatly increased.
No doubt from the standpoint of computer networking
hardware and smart software availability today, this approach is clearly
workable. However, when all the sensors are to be left alone by themselves to
interact with one another over time in buildings, the obvious question to ask
is whether these sensors are indeed ready to take on this self-policing task of
always staying accurate. In other words, who is there to check whether the
outputs of some of these sensors are actually staying accurate over time and if
not, what are the consequences for the maintenance status of the buildings and
the comfort level and safety of their occupants? Thus, while computer hardware
and system networking software may be ready for this futuristic approach to
building controls, it is very clear that not all the sensors needed to perform
perfectly in this approach are here today to meet the challenge. In particular,
gas sensors such as CO2 and dew point might be relatively accurate
over time but for how long before they become inaccurate? But would there be
anybody or any mechanism scheduled in the networking controls system to perform
the checking or re-calibrating tasks for them? To put it bluntly, until such
time that all the required sensors in the networking controls system can be
self-commissioning or in other words can render themselves capable of
automatically staying accurate all the time, the futuristic building controls
approach with the use of computer networking and relevant software to connect
all the sensors in the system together working interactively simply will not
work.
It is the object of the present invention to advance a
configuration design and methodology for AB designed NDIR gas sensors such that
they can become self-commissioning or in other words capable of automatically
maintaining their measurement accuracy indefinitely over time after initial
calibration. This invention is achieved via extending the previously disclosed
Absorption Biased methodology of U.S. Pat. No. 8,143,581 and Re-calibration
methodology without the need of standard gases (U.S. Ser. No. 13/149,738, Wong)
for NDIR gas sensors.
SUMMARY OF THE INVENTION
The present invention is generally directed to a
self-calibrating NDIR gas sensor and its use in which an infrared source
illuminates a signal channel that is longer than a reference channel while
electronics are used to calculate a chosen gas concentration in a sample
chamber containing the two channels. The difference in length between the two
channels creates an absorption bias between outputs of a signal detector and a
reference detector, each of the two detectors having an identical narrow band
pass filter with the same Center Wavelength (“CWL”), Full Width Half Maximum
(FWHM) and transmittance efficiency at the CWL. A second pair of detectors,
called standard detectors, are placed in the two channels, and both of these
standard detectors have an identical standard narrow band pass filter with the
same Center Wavelength (“CWL”), Full Width Half Maximum (FWHM) and
transmittance efficiency at the CWL and the CWL of the standard narrow band
pass filter is a neutral wavelength. The electronics of the sensor is
calibrated by use of a calibration curve generated by using a normalized ratio
of the signal channel output to the reference channel output that starts at
unity when there is zero concentration of the chosen gas. The calibration curve
is self-calibrated by using a stored standard gamma ratio obtained at a first
period of time and a measured standard gamma ratio obtained at a second period
of time after the first period of time, the standard gamma ratio being the
ratio of a standard signal output from a standard signal detector to a standard
reference output from a standard reference detector.
Such an NDIR gas sensor can be made to detect a second
gas by including a second signal detector and a second reference detector that
function similarly to the signal and reference detector, except that they are
designed to detect a different gas. This additional pair of detectors will each
have an identical second chosen gas narrow band pass filter with the same Center
Wavelength (“CWL”), Full Width Half Maximum (FWHM) and transmittance efficiency
at the CWL and will have its own calibration curve generated by using a second
chosen gas normalized ratio of the second chosen gas signal output to the
second chosen gas reference output that starts at unity when there is zero
concentration of the second chosen gas. As was the case with a single gas
detection sensor, the second gas calibration curve is self-calibrated by using
the stored standard gamma ratio and the measured standard gamma ratio.
The NDIR gas sensor can also be recalibrated by comparing
the sample concentration of a gas it is detecting to a second gas measurement
of such gas determined by a secondary gas standard and then adjusting the
normalized ratio of the signal output to the reference output for the gas based
upon a reversed calibration curve algorithm that is a non-linear equation if a
difference between the sample concentration of the gas and the second gas
measurement exceeds a preselected threshold.
Accordingly, it is a primary object of the present
invention to provide an NDIR gas sensor that self-calibrates itself.
This and further objects and advantages of the present
invention will be apparent to those skilled in the art in connection with the
drawings and the detailed description of the invention set forth below.
