Method and apparatus for negating measurement effects of interferent gases in non-dispersive infrared analyzers

A method and apparatus is disclosed for selectively determining by non-dispersive infrared techniques the concentration of a gaseous constituent in a sample gas mixture while determining and discounting deleterious effects of spectrally interfering gases in the sample gas mixture.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for increasing 
accuracy in non-dispersive infrared analyzer systems for determining the 
concentration of a preselected gas in a homogeneous sample gas of 
spectrally overlapping constituents, and more particularly to a method and 
apparatus for determining the concentration of an interferent gas and 
removing its effect on the measurement of a preselected constituent gas in 
a gas sample. 
Prior art has disclosed many methods and apparatus for measuring 
concentration content of a preselected component in a sample gas, the most 
spectrally specific of which is a non-dispersive infrared analyzer 
utilizing a pneumatic detector. 
While pneumatic detectors charged with the component of interest will give 
generally accurate indications of the concentration of the selected 
component, a problem is many times encountered where other gas components 
present in the sample stream have absorption bands which either overlap or 
lie within the major absorption band of the selected components. For 
example, carbon dioxide has an infrared absorption band which overlaps the 
major infrared absorption band of carbon monoxide while water vapor has a 
minor absorption band which lies within the major absorption band of 
carbon monoxide. Thus, these gases, when present, interfere with the 
measurement of carbon monoxide by non-dispersive infrared techniques. In 
order to compensate for the infrared energy absorbed by water vapor and 
carbon dioxide, prior art has used additional pneumatic detectors 
specifically sensitized to determine the concentration of carbon dioxide 
and/or water vapor and in turn subtract the effect of these concentrations 
from the concentration measured by the carbon monoxide detector. This 
solution to the problem results in a very complex, bulky, and expensive 
carbon monoxide detectors for testing the sample gas. In order to 
accurately measure the concentration of a gas such as carbon monoxide, an 
additional pneumatic detector is required for each interferent to be 
measured, resulting in a multiple in the cost of a detector to measure one 
gas. 
It is therefore an object of the present invention to provide a simple dual 
beam, non-dispersive infrared analyzer utilizing a single pneumatic 
detector which measures the component concentration in a sample gas with 
increased accuracy. 
It is another object of the present invention to provide an inexpensive 
accurate measurement of a component gas in a sample gas stream. 
It is a further object of the present invention to provide a gas 
concentration analyzer which has a high degree of accuracy. 
SUMMARY OF THE INVENTION 
The foregoing objects are satisfied and the foregoing deficiencies are 
overcome by the present invention which comprises solid state detectors 
disposed within the sample cell of a dual-beam, non-dispersive infrared 
analyzer utilizing a pneumatic detector. These solid state detectors are 
situated behind narrow band filters which pass infrared radiation of a 
wavelength corresponding to the wavelength of interfering species which is 
different from the wavelength of the infrared waves absorbed by the 
constituent gas to be measured. The energy incident upon the solid state 
detectors is converted into an electrical signal. This signal is processed 
further to represent concentration of interferent gases. The processed 
signal is algebraically combined with the signal produced by the pneumatic 
detector. The resulting signal is an indication of the concentration of a 
preselected constituent of a sample gas without the inaccuracies 
introduced by interferent gases.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a prior art dual beam non-dispersive infrared analyzer 9 
comprising infrared sources 10A and 10B, a reference cell 12A, a sample 
cell 12B, a pneumatic detector having chambers 14A and 14B, indicator 16 
and narrow band filters 18A and 18B. 
Narrow band filters 18A and 18B are not essential but are recommended to 
provide an improved measurement of a constituent gas by limiting the 
number of spectrally interferring species to gases which have absorption 
bands within the bandwidth of the filters. Therefore, narrow band filters 
18A and 18B are included as part of the non-dispersive infrared analyzer 
with which the present invention is used. However, it is understood by one 
skilled in the art that narrow band filters 18A and 18B are not essential. 
In practice, reference cell 12A is filled with an inert gas or may be 
evacuated, the essential characteristic being that the contents of 
reference cell 12A contains a gas which does not absorb infrared energy or 
a gas for which the infrared energy absorption remains constant for the 
specific application of interest. The sample gas stream containing the 
component gas to be measured flows through the sample cell 12B. Two 
reasonably matched narrow band filters 18A and 18B are located in front of 
pneumatic detector chambers 14A and 14B to pass a narrow band of infrared 
radiation which corresponds with a strong absorption band of the 
constituent gas to be measured. Narrow band filters 18A and 18B may be of 
any type commercially available. However, interference filters on sapphire 
substrates are preferred. The narrow band filters cover detector windows 
which are in line with reference cell 12A and sample cell 12B. Detector 
chambers 14A and 14B are generally filled with the same kind of gas as 
that to be measured, in this case carbon monoxide, and are separated by a 
diaphragm 20. Diaphragm 20 is a very thin sheet of some conductive 
material such as aluminum foil which serves as the variable plate of a 
capacitor. Disposed next to diaphragm 20 is an electrode 22 which serves 
as the fixed plate of a capacitor. 
In operation, infrared sources 10A and 10B emit a wide spectrum of infrared 
radiation. A rotating chopper blade 24 is operated to interrupt the 
infrared sources 10A and 10B simultaneously, preferably at about ten times 
per second. The action of chopper blade 24 produces pulsating infrared 
radiation which travels synchronously through reference cell 12A and 
sample cell 12B. A broad band of infrared energy, including that which is 
characteristic of the component gas to be measured, in this case carbon 
monoxide, travels down reference cell 12A and is incident upon narrow band 
filter 18A. Narrow band interference filter 18A transmits only those 
wavelengths which coincide with the absorption band of the constituent gas 
to be measured, such as the 4.7 micron absorption band of carbon monoxide. 
Inside pneumatic detector reference chamber 14A, a proportional amount of 
the infrared radiation is absorbed by the carbon monoxide contained 
therein and produces a heating effect. The infrared energy from source 10B 
similarly travel down sample cell 12B. However, the infrared radiation 
transmitted to chamber 14B will be diminished by absorption by any carbon 
monoxide which may be present in sample cell 12B as illustrated in FIGS. 1 
and 3. Infrared energy within the absorption bandwidth, characteristic of 
carbon monoxide, which has not been absorbed by the component gas in 
sample cell 12B will be absorbed proportionately by the carbon monoxide a 
pneumatic detector chamber 14B. The carbon monoxide in pneumatic detector 
chamber 14A will heat to a greater extent than that in pneumatic detector 
chamber 14B since an amount of the infrared energy has been absorbed by 
carbon monoxide contained in sample cell 12B. The unequal heating in 
pneumatic detector chambers 14A and 14B will produce a pressure 
difference, causing deflection of diaphragm 20 which in turn will change 
the capacitance established between electrode 22 and disphragm 20. 
Indicator 16, which in essence measures this change in capacitance between 
electrode 22 and disphragm 20, will indicate the concentration of the 
component gas, carbon monoxide. 
Referring now to FIG. 2, a graph representing the infrared spectrum, 
portions of which are absorbed by carbon monoxide, carbon dioxide and 
water vapor, is illustrated. Carbon dioxide absorption bands are indicated 
by the dotted lines marked "CO.sub.2 ". Water vapor absorption band are 
indicated by the spiked solid lines marked "H.sub.2 O". Carbon monoxide 
absorption bands are indicated by the solid line marked "CO". Bandwidths 
illustrated as having center points at 2.6 microns, 2.694 microns (2.7 
microns), 2.76 microns and 4.7 microns are representative of optical 
filters which may be used with the present invention. Absorption by carbon 
monoxide, carbon dioxide and water vapor can be seen on the graph between 
points a and b. Carbon monoxide will absorb infrared radiation within a 
narrow bandwidth having its center at 4.7 microns. However, also at this 
bandwidth, carbon dioxide and water vapor will weakly absorb some of the 
infrared energy. Between points "c" and "d", having its center at 2.694 
microns, carbon dioxide and water vapor are strong absorbers whereas 
carbon monoxide is essentially a non-absorber of infrared energy. Between 
points "e" and "f", having its center at 2.6 microns, water vapor is a 
strong absorber while carbon monoxide and carbon dioxide are negligible 
absorbers of infrared energy. Hence, water vapor may be independently 
measured at wavelengths centered at 2.6 microns and an independent 
measurement of carbon dioxide may be obtained by subtracting the effect of 
water vapor measured at 2.6 microns from the measurement of carbon dioxide 
and water vapor measured at 2.7 microns. Although measurements of water 
vapor at 2.6 microns and water vapor and carbon dioxide at 2.7 microns are 
preferred, carbon dioxide and water vapor are also absorbers of infrared 
energy in a bandwidth having its center at 2.76 microns in addition to the 
one centered at 2.7 microns where carbon monoxide is again essentially a 
non-absorber. Thus, the measurement of the concentration of carbon 
monoxide will also include trace amounts of carbon dioxide and water vapor 
(carbon monoxide measurement error) whereas the concentration of water 
vapor and carbon dioxide can be measured without absorption of infrared 
radiation by carbon monoxide. 
Referring now to FIG. 3, a non-dispersive infrared analyzer is illustrated 
constructed in accordance with the teaching of the present invention. 
Portions of the analyzer of FIG. 3 which are identical to the analyzer of 
FIG. 1 have identical numbers and will not be described further. Solid 
state detectors 30 and 31 are illustrated as being disposed within sample 
cell 12B and having optical filters 32 and 33 in front thereof and 
conductors 34 and 35 leading to electronic circuit 36, respectively. Solid 
state detectors 30 and 31 may be either thermistors, lead sulfide sensors, 
lead selenide sensors, pyroelectric devices or any solid state black body 
detector. Pyroelectric devices are preferred due to their sensitivity and 
low cost. Optical filters 32 and 33 may be of any type commerically 
available. However, narrow band, interference type infrared filters on a 
quartz substrate are preferred. Filters 32 and 33 may have a bandwidth 
center point of any value which coincides with the absorption band of the 
interferent gases. In the case where carbon monoxide is to be determined 
in the presence of water vapor and carbon dioxide, optical filter 32 
preferably has a bandwidth of 0.05 microns with a center point at 2.6 
microns. Optical filter 33 preferably has a bandwidth of 0.03 microns with 
its center point of 2.694 microns although a bandwidth of 0.04 microns 
with its center point at 2.76 microns is equally suitable. Electronic 
circuit 36 also receives an input from diaphragm 20 through conductor 38 
and issues an output to indicator 16 through conductor 40. 
Infrared radiation in the 2.6 and 2.694 micron regions, the preferred 
center point for the bandwidths passed by the optical filters 32 and 33, 
respectively, travels down sample gas channel 12B and is partially 
absorbed by any carbon dioxide and/or water vapor contained in the sample 
gas. The portion of the infrared energy not absorbed by water vapor or 
carbon dioxide will be passed by optical filters 32 and 33 to solid state 
detectors 30 and 31, respectively. The energy from the passed infrared 
radiation will cause a signal to be produced by solid state detectors 30 
and 31. The signals produced by solid state detectors 30 and 31 are, 
respectively, related to the concentration of water vapor alone and water 
vapor and carbon dioxide in the sample gas being tested. The infrared 
radiation within the principal absorption bandwidth of the component gas 
being analyzed, in this case carbon monoxide, will not pass through 
optical filters 32 and 33. Infrared radiation within the abosrption 
bandwidth of carbon monoxide will pass narrow band filter 18B and be 
absorbed by the carbon monoxide contained within pneumatic detector 
chamber 14B. The infrared radiation within the absorption bandwidth of 
carbon monoxide centered at 4.7 microns does not affect the output of the 
solid state detectors 30 and 31 and the infrared radiation within the 
absorption bandwidth of carbon dioxide and water vapor centered at 2.6 and 
2.7 microns does not affect the output of pneumatic detector 14. The 
operation of the non-dispersive infrared analyzer 9 remains essentially 
unchanged from its operation in prior art. The signals, however, produced 
by pneumatic detector 14 and solid state detectors 30 and 31 are further 
processed through electronic circuit 36 prior to display at indicator 16. 
FIG. 4 illustrates electronic circuit 36 in block form as comprising a 
charge detector and amplifier 50 connected to electrode 22 through 
conductor 38 on its input side and its output connected to chop frequency 
bandpass filter 52 through conductor 54. The output of chop frequency 
bandpass filter 52 is connected to the input of demodulator 56 through 
conductor 58. Demodulator 56 is connected to filter 60 through conductor 
62. Filter 60 is connected to summing circuit 64 through conductor 66 
which is connected to scaling amplifier 68, through conductor 70. Scaling 
amplifier 68 has a zero control 69 and is connected to low pass filter 72 
through conductor 74 which is connected to carbon monoxide display unit 16 
through conductor 40. Electronic circuit 36 also comprises amplifiers 80 
and 82 having their inputs connected to solid state detectors 30 and 31 
through conductors 34 and 35, respectively. Amplifier 80 is connected to 
chop frequency bandpass filter 88 through conductor 90 which is connected 
to demodulator 92 through conductor 94. Demodulator 92 is connected 
through conductor 98 to low pass filter 96 which is connected to summing 
amplifier 100 and H.sub.2 O/CO.sub.2 control 102 through conductors 104 
and 106, respectively. Summing amplifier 100 receives an additional input 
from zero control 110 and produces an output to H.sub.2 O control 112 
through conductor 114. H.sub.2 O control 112 produces an output which is 
the second input to summing circuit 64 through conductor 116. 
The amplifier 82 is connected through conductor 120 to chop frequency 
bandpass filter 118 which is connected to demodulator 122 through 
conductor 124. Demodulator 122 is connected through conductor 128 to low 
pass filter 126 which furnishes one input to summing amplifier 130 through 
conductor 132. Summing amplifier 130 receives an input from H.sub.2 
O/CO.sub.2 control 102 and zero control 133 through conductors 134 and 
136, respectively. Summing amplifier 130 produces the input to 
linearization circuit 138 through conductor 140. Linearization circuit 
138, having linear control 139, supplies the input to CO.sub.2 control 142 
through conductor 144. CO.sub.2 control 142 has a CO.sub.2 +H.sub.2 O 
control 143 and supplies the third input to summing circuit 64 through 
conductor 146. Although electronic circuit 36 is illustrated as comprising 
discrete components, it is understood by one skilled in the art that many 
variations are possible and the functions of electronic circuit 36 may be 
accomplished by a microprocessor or the like. 
In operation, charge detector 50 receives an indication of capacitance 
through conductor 38 from electrode 22. This signal will be an a.c. signal 
which is fed to chop frequency bandpass filter 52 which removes spurious 
a.c. signals and feeds a clean a.c. signal having the frequency determined 
by chopper 24, preferably 10 hertz, to demodulator 56. Demodulator 56 
functions as a full wave rectifier and will feed a d.c. signal with an 
a.c. ripple to filter 60. Filter 60 smooths out the d.c. signal with a 10 
Hz ripple so that summing circuit 64 receives a smooth d.c. signal through 
conductor 66. Thus, the signal on conductor 66 into summing circuit 64 is 
a pure d.c. signal whose amplitude is proportional to the concentration of 
the constituent gas of interest (in the example, carbon monoxide) plus the 
concentration of any interferent gases having absorption bands in the same 
wavelength region being utilized to detect the constituent gas (in the 
example, carbon dioxide and water vapor). 
Amplifier 80 receives an a.c. signal from solid state detector 30 through 
conductor 34. This a.c. signal is amplified and fed to chopper frequency 
bandpass filter 88. Chopper frequency bandpass filter 88 removes all 
spurious a.c. signals to produce a 10 Hz a.c. signal, the frequency of 
chopper blade 24, to demodulator 92. Demodulator 92 serves the same 
furnction as demodulator 56, that is to produce a full wave rectified a.c. 
signal to filter 96 which smooths the d.c. with the a.c. ripple into a 
pure d.c. signal. The d.c. signal from filter 96 is fed to summing 
amplifier 100 where it is combined with a d.c. signal from zero control 
110. The d.c. signal from filter 96 is also fed through the H.sub.2 
O/CO.sub.2 control 102 to summing amplifier 130 which will be described in 
detail later. Summing amplifier 100 produces a signal to H.sub.2 O control 
112 which further processes the signal and inverts the signal to assure a 
signal with opposite polarity to that fed to summing circuit 64 from 
filter 60. The signal on conductor 116 to summing amplifier 64 is a pure 
d.c. signal having an amplitude proportional to the concentration of water 
vapor in the sample gas. Amplifier 82 receives the signal from solid state 
detector 31 and amplifies this input to produce a signal to chopper 
frequency bandpass filter 118 which serves the same function as filters 88 
and 52, that is to remove spurious a.c. signals from the desired 10 hertz 
signal. The clean a.c. signal is then fed to demodulator 122, which acts 
as a fully wave rectifier, to produce a d.c. with an a.c. ripple to filter 
126 which removes the ripple and produces a smooth d.c. signal to summing 
amplifier 130. Summing amplifier 130 receives a portion of the signal from 
filter 96 through H.sub.2 O/CO.sub.2 control 102 and the d.c. input from 
zero control 133. CO.sub.2 /H.sub.2 O control 102 provides a signal to 
correct for any water vapor absorption which may affect the signal 
produced by solid state detector 31. Summing amplifier 130 combines these 
three signals and produces an output to linearization circuit 138. 
Linearization circuit 138 linearizes the signal from summing amplifier 130 
to assure a linear relationship between the signal from carbon dioxide 
sensor 31 and the error in the carbon monoxide measurement signal from 
conductor 66 due to the amount of carbon dioxide present in the gas sample 
(see FIG. 5). Amplifier 142 has a gain adjustment which supplies the 
required amount of signal from linearization circuit 138 necessary to 
match the carbon monoxide signal from conductor 66 due to carbon dioxide 
in the test sample. Amplifier 142 also inverts this signal to again assure 
a signal of opposite polarity to that produced by filter 60 and feeds this 
inverted signal to summing circuit 64 where it is combined with the d.c. 
signals indicating the amount of carbon monoxide and the amount of water 
vapor in the sample. The signal from summing circuit 64 is fed through 
scaling amplifier 68 which sets up the proportional relationship between 
the amount of carbon monoxide in the sample and the voltage of the 
combined signals from conductors 66, 116, and 146. The signal from scaling 
amplifier 68 is passed through low pass filter 72 to remove any remanent 
spurious a.c. signals and feeds this signal to carbon monoxide display 16 
wherein the amount of carbon monoxide in the sample is indicated. 
FIG. 5 is a graphical representation of carbon dioxide concentration versus 
voltage in a test sample. Solid curve A represents the detected 
relationship between carbon dioxide concentration and voltage from the 
solid state detector 31, as seen at the output of summing amplifier 130. 
Dashed line B represents the linear relationship between carbon dioxide 
concentration and voltage from linearization circuit 138 which may be any 
suitable linearization circuit known in the art. Both lines A and B are 
for zero water vapor content. Broken line C represents the relationship of 
the output of linearization circuit 138 and the carbon dioxide content 
with a constant amount of water vapor content without the signal from the 
H.sub.2 O/CO.sub.2 control 102 through conductor 134. Dotted line D 
represents the relationship of the output of linearization circuit 138 and 
the carbon dioxide content with a constant amount of water vapor content 
with the signal from the H.sub.2 O/CO.sub.2 control 102 through conductor 
134 and with control 102 adjusted such that the response to water vapor is 
the same with or without carbon dioxide present. The voltage produced by 
sensor 31 does not have a linear relationship with the carbon monoxide 
measurement signal at conductor 66 due to the presence of carbon dioxide 
in the test sample and thus the linearization circuit 138 is used to 
improve the capability to remove the error due to carbon dioxide. Once the 
voltage from sensor 31 is linearized at conductor 144, the response is 
essentially linear with either carbon dioxide or water vapor, when only 
one of the two are present. However, without the H.sub.2 O/CO.sub.2 
control and the water vapor signal on conductor 134, a linear relationship 
between the carbon monoxide measurement error and the response at 
conductor 144 due to carbon dioxide and water vapor when both are present 
in varying amounts is essentially limited in range. The control 102 is 
adjusted to remove some of the water vapor signal such that the response 
at conductor 144 is linear for an appreciable range of variation for 
carbon dioxide and water vapor when both are present to the carbon 
monoxide measurement error signal at conductor 66 due to carbon dioxide 
and water vapor. 
The voltage produced by sensor 30 is essentially linear with the water 
vapor content and thus does not need a linearization circuit for the most 
general application. 
As was indicated previously, pneumatic detector 14 will produce a signal 
indicative of the carbon monoxide content in the sample gas; however, this 
signal will also contain inaccuracies due to carbon dioxide and water 
vapor which will absorb infrared energy within the same bandwidth as 
carbon monoxide (see FIG. 2). Solid state detector 30, on the other hand, 
will produce a signal indicative of the amount of water vapor in the 
sample since optical filter 32 has its bandwidth centered at 2.595 
microns (2.6 microns) with a preferred bandwidth of 0.05 microns. The 
signal from solid state sensor 31 will produce a signal indicative of the 
carbon dioxide in the sample and also an amount of water vapor in the 
sample since optical filter 33 has its bandwidth center at 2.692 microns 
with a preferred bandwidth of 0.03 microns, although an alternate center 
point of 2.764 microns with a preferred bandwidth of 0.04 microns may be 
used. Summing circuit 64 will receive an indication of the carbon monoxide 
plus carbon dioxide and water vapor present in the sample from pneumatic 
detector 14 and will receive a signal representative of the water vapor in 
the sample from solid state detector 30 and a signal representative of the 
carbon dioxide and water vapor in the sample from solid state detector 31. 
The signal from pneumatic detector 14 is positive, whereas the signal from 
solid state detectors 30 and 31, after processing by electronic circuit 
36, are negative so that the amount of carbon monoxide plus carbon dioxide 
plus water vapor minus the signal representative of water vapor and minus 
the signal representative of carbon dioxide will yield a signal 
representative of carbon monoxide alone to scaling amplifier 68. Signals 
from scaling amplifier 68 is processed further and fed to carbon monoxide 
display 16 as a more accurate determination of the carbon monoxide present 
in the gas sample being tested. 
The foregoing embodiment discloses an apparatus and a method for removing 
measurement errors due to water vapor and carbon dioxide. However, 
simplification of the disclosed apparatus is easily achieved by removal of 
the sensor and associated circuitry for the measurement of water vapor. By 
deleting optical filter 32, solid state sensor 30, amplifier 80, filter 
88, demodulator 92, filter 96, summing amplifier 100, H.sub.2 O control 
112, H.sub.2 O/CO.sub.2 control 102, and linearization circuit 138, a 
circuit is disclosed for removing the effects of small amounts of water 
vapor or carbon dioxide. If the amount of water vapor increase above a 
level of approximately 3% or carbon dioxide increases above a level of 
approximately 7%, linearization circuit 138 is necessary and must be 
inserted in its position illustrated in FIG. 4. A correction circuit 
comprising optical filter 33, solid state sensor 31, amplifier 82, chop 
frequency band pass filter 118, demodulator 122, filter 126, summing 
amplifier 130, linearlization circuit 138 and CO.sub.2 control 142 in 
conjunction with the circuitry associated with pneumatic detector 14 will 
yield a system that will correct for either carbon dioxide error or water 
vapor error provided that either carbon dioxide or water vapor and not 
both is present in the sample gas stream to be analyzed. 
In the alternative, optical filter 32 with sensor 30, amplifier 80, chop 
frequency band pass filter 88, demodulator 92, filter 96, summing 
amplifier 100, and H.sub.2 O control 112 may be used to remove the error 
of water vapor in the sample gas stream when used in conjunction with 
pneumatic detector 14 and its associated circuitry with summing circuit 
64. 
While a preferred embodiment has been shown by way of illustration, it is 
not to be construed as a limitation on the present invention, the present 
invention only being limited by the claims contained herein.