EPA-650/2-74-094
DECEMBER 1974
Environmental Protection Technology Seri
es
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EPA-650/2-74-094
INFRARED GAS FILTER CORRELATION
INSTRUMENT FOR IN-SITU MEASUREMENT
OF GASEOUS POLLUTANTS
by
D. E. Burch and D. A. Gryvnak
Philco-Ford Corporation
Newport Beach . CA 92663
Contract No. 68-02-0575
Program Element No. 1AA010
ROAPNo. 26AAP
Project Officer: William F. Herget
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park , North Carolina 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
December 1974
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This report has been reviewed by the Environmental Protection Agency
and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Agency,
nor does mention of trade names or commercial products constitute
endorsement or recommendation for use.
11
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ABSTRACT
An infrared analyzer employing gas cell correlation techniques has been
designed and constructed to measure the concentrations of carbon monoxide,
nitric oxide, sulfur dioxide, hydrogen chloride, and hydrogen fluoride
in the effluent of stationary sources. An infrared beam is directed across
the stack to a retroreflector and back so that the instantaneous average
concentration is measured continuously without disturbing the constituents
of the effluent. A small, removable, fixed-position grating monochromator acts
as a unique optical filter that passes narrow spectral intervals that are
centered at wavelengths where the gas to be detected will absorb. One grating
monochromator is used for CO and NO, another for S02 and HC1, and a
third for HF. The useful ranges of concentration times path length, in atm
cm, over which each gas can be measured are: 0.005 to 0.4 to NO;
0.0013 to 0.15 for CO; 0.001 to 4.0 for S02; 0.0003 to 0.2 for HCl and
0.0001 to 0.02 for HF. The discrimination against other gases in the
effluent is excellent.
This report was submitted in fulfillment of contract number 68-02-0575 by
Philco-Ford Corporation, Aeronutronic Division, under the sponsorship of
the Environmental Protection Agency.
iii
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CONTENTS
PAGE
ABSTRACT ill
ILLUSTRATIONS V
LIST OF TABLES vii
ACKNOWLEDGEMENT . viii
SECTION
1 INTRODUCTION AND SUMMARY 1
Background 1
Brief Description of Instrument 2
Definitions, Symbols and Units 3
2 SPECTROSCOPIC PRINCIPLES 5
Correlation in Simple Spectral Models 5
Energy and Voltage Relationships 11
Design Factors 18
Two-Cell System 20
Multiple Correlation Cell Systems 24
3 INSTRUMENT DESIGN AND PERFORMANCE 25
Optical Layout of Basic Instrument 25
Instrument Employing Grating Assembly 29
Spectral Properties of the Gases 35
Laboratory Tests and Calibration Data 40
Field Tests 47
4 REFERENCES 53
LV
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ILLUSTRATIONS
FIGURE PAGE
1 Simplified optical diagram of a basic gas-cell
correlation spectrometer. 6
2 Plots of transmittance for a model gas illustrating
the spectroscopic principles of detection. 8
3 Plots of transmittance illustrating interference by a
gas species other than the one being measured. 9
4 Plots of transmittance illustrating the effect of an
absorbing gas of species y whose spectrum is not
correlated with that of species x. 10
5 Simplified diagram of a gas-cell correlation spectro-
meter that employs high-frequency chopping and low-
frequency modulation. 12
6 Calculated spectral plots of transmittance for a
model spectrum similar to a portion of a CO band. 14
7 Plots of calculated TT • T and T tt « T versus
i , . i „ , L sam att sam , ..
sample absorber thickness. 16
8 Plot of calculated A/2 versus sample absorber thickness. 17
9 Calculated spectral plots of transmittance for a model
spectrum illustrating the principle of detection of
the two-cell system. 21
10 Calculated spectral plots of transmittance demon-
strating the principles of discrimination against
continuum absorption by the two-cell and the one-cell
attenuator systems. 22
11 Optical diagram of a gas-cell correlation instrument
for across-the-stack measurements. 26
v
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ILLUSTRATIONS (continued)
FIGURE PAGE
12 Optical diagram of a retroreflector assembly used
to redirect the radiant energy beam back across
the stack to the gas-cell correlation instrument. 28
13 Optical diagram of a gas-cell correlation instru-
ment incorporating a grating assembly. 30
14 Optical diagram of a retroreflector and grid
assembly. oo
15 The grating assembly for S02 and HCl. 33
16 The power supply unit above the electronics unit. 34
17 Photographs of the optical assembly. 35
18 Spectral curves of transmittance of H,0 and of H00 + NO
between 1862 and 1928 cm'1. 37
19 Spectral curves of transmittance between 2465 and
2525 cm'1. 39
20 Curves of V1 versus normalized absorber thickness
for CO and NO. 4!
21 Curves of V1 vs normalized absorber thicknesss for
S02 and HCl. 42
22 Curve of V1 vs normalized absorber thickness for HF. 43
23 The optics assembly and the retroreflector mounted
on the smokestack of the Duke Power Co. at
Charlotte, N. C. 48
24 Plots of NO concentration vs time for 3 different
days at the Duke Power Co., Charlotte, N. C. 49
25 Plot of CO concentration vs time of day for
25 June 1974 at Duke Power Co., Charlotte, N. C. 50
26 Plots of S02 concentration vs time of day for
3 different days at Duke Power Co. In Charlotte, N. C. 52
vi
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LIST OF TABLES
TABLE PAGE
1 Comparison of the sensitivity and discrimination
of two-cell and one cell-attenuator system. 24
2 Instrument parameters and performance. 46
vii
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ACKNOWLEDGEMENT
The authors would like to express their gratitude to two of their co-workers,
Mr. Francis J. Gates and Mr. John D. Pembrook, for their cooperation and
assistance. Mr. Gates' ingenuity and skills in setting up test apparatus and
in assembling small components were invaluable. Mr. Pembrook is responsible
for designing and testing the electronic components.
viii
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SECTION 1
INTRODUCTION AND SUMMARY
BACKGROUND
Most commercially available instruments for measuring the concentrations
of gaseous pollutants in the effluent of stationary sources require that a
sample be drawn from a stack or duct through a probe into a separate container
for analysis. The analysis may be made continuously on-line or at a later
time, possibly in a laboratory. Such sampling techniques have two major
drawbacks: (1) concentration changes may occur along the sampling train; and
(2) concentration gradients may exist in the flow field so that several
samples may need to be taken from different positions in order to determine
the average concentration. By employing in-situ absorption spectroscopy, the
first difficulty can be eliminated and the second one greatly reduced. In
absorption spectroscopy radiant energy from a thermal source is transmitted
through the effluent from one side of the duct to the other side. By compar-
ing the amount of transmitted energy at characteristic wavelengths where a
particular gas species absorbs strongly to that at wavelengths where it does
not absorb, or absorbs weakly, it is possible to determine the instantaneous
average concentration across the flow field. The effluent flow is undisturbed,
and the measurement along the path across the duct adequately represents the
average unless there are unusually large gradients in the concentration.
The required wavelength selection for absorption spectroscopy may be achieved
in a variety of ways. Narrow band interference filters provide a simple,
relatively inexpensive method, but in most cases they do not provide the high
spectral resolution required for good specificity, or discrimination against
other gas species that may absorb in the same spectral interval. Dispersing
instruments employing a prism or grating and a scanning mechanism provide
better spectral resolution, but are more complex and, in turn, more expensive
and more subject to misalignment.
A class of instruments employing gas-cell correlation spectroscopy to
provide good sensitivity and specificity at relatively low cost have been
developed for a variety of applications. Gas-cell correlation instruments
of various types have been discussed in References 1-5. In one form, the
energy beam transversing the sample gas is alternately directed through
either a correlation cell or an attenuator. The correlation cell, also
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frequently called a gas filter, contains the gas species to be measured, and
the attenuator has nearly constant transmittance at all wavelengths of interest.
An interference filter restricts the spectral bandpass to a region of absorp-
tion by the gas species being measured. The gas in the correlation cell is
essentially opaque at the wavelengths of maximum absorption for the gas species
being measured. Therefore, as the correlation cell and the attenuator are
alternated in and out of the beam, the energy of wavelengths of strong absorp-
tion by the gas are modulated. In the following section, some very simple
models of absorption spectra are used to describe the spectroscopic principles
of detection and discrimination of gas-cell correlation spectrometers. The
illustrations used are helpful in understanding many of the factors involved
in designing a gas-cell correlation instrument for a particular application.
BRIEF DESCRIPTION OF INSTRUMENT
The primary purpose of the work reported here has been to perform the
required research and to design and construct a gas-cell correlation instru-
ment for across-the-stack monitoring of stationary source emissions. Any of
the five gases, NO, CO, S02, HF, or HC1, can be measured over the concentra-
tion ranges ordinarily of interest for stationary sources. SO was originally
included, but subsequent research indicated that this gas coula probably be
measured better by methods other than gas-cell correlation spectroscopy.
Conversion of the instrument from one gas species to another is relatively
simple and involves changing a correlation cell and a bandpass filter. Chang-
ing the radiant energy source and/or the detector may also be required, de-
pending on the gas species involved. The instrument has been designed for
versatility, it is necessarily more complex than an instrument designed to
use the same principle of operation for a single gas species on a particular
type of stack. In some cases, the stability, the signal-to-noise ratio and
the ease of operation have been compromised in order to retain the versatility.
The instrument is intended primarily as a tool for the Environmental Protection
Agency to evaluate gas-cell correlation methods for a variety of across-the-
stack measurements. By testing it under various conditions with different
instrument parameters, valuable information can be obtained for the design
of simpler, single-purpose gas-cell correlation instruments and to determine
the factors that limit their performance.
By combining gas-cell correlation techniques along with dispersion
techniques to carefully control the spectral bandpass, it has been possible
to greatly improve the discrimination and the stability over that attainable
with either technique used separately. A small, permanently aligned assembly
employing a grating as the dispersing element is included for each of the 5
gases with the spectral bandpass selected to give optimum discrimination,
stability, and signal-to-noise ratio. Each assembly uses a multiple-slit
grid in the focal plane of the dispersed radiant energy to pass narrow spectral
intervals that contain strong absorption by the gas. Discrimination and
stability are improved by rejecting much of the energy at wavelengths where
the gas does not absorb strongly. The improved discrimination against 1^0
is particularly important when measuring NO. The instrument can be used
with a grating assembly or without it, as in the more conventional manner.
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Stack diameters between approximately 1.5 m and 10 m can be accounted
for with only minor adjustments of a few optical components. In normal
operation, the stack is "double-passed" by using a retroreflector on the
side opposite the radiant energy source to reflect the energy back through
the stack to the remainder of the instrument. However, by moving the source
and some associated optics to the opposite side, it is possible to "single-
pass" the stack.
The instrument has been tested in the laboratory with synthetic samples
of effluent to provide calibration data at various temperatures and to measure
the interference by other gases typically in stack effluent. Field tests
have also been performed on the stack of a coal-burning power plant while the
gas concentrations of interest were simultaneously being measured by other
instruments. The data obtained on the stack and in the laboratory indicate
that gas-cell correlation instruments can be built with adequate sensitivity
and discrimination to monitor any of the 5 gases listed above. The ultimate
performance of any instrument of this type for across-the-stack measurements
will probably be limited by the uncertainty in determining and maintaining
the "zero-balance" that corresponds to no absorbing gas in the stack. This
is a much more serious problem for across-the-stack instruments than for
instruments in which the sample area can be evacuated or flushed with a non-
absorbing gas.
DEFINITIONS, SYMBOLS AND UNITS
T transmittance at a particular wavenumber (or wavelength) that would
be measured with a spectrometer having infinite resolving power.
Subscripts_indicate the transmittance of different components or gas
samples. T indicates the average transmittance over a specified
interval.
T transmittance of the sample gas being analyzed.
S cUu
T transmittance of the gas species x to be measured in the sample gas.
x This is equivalent to T if only species x is present in the sample
-- sam
cell.
T transmittance of an interfering gas of species y in the sample gas
y that may produce a false indication of species x.
T transmittance of the spectral bandpass filter.
T transmittance of the gas of species x at low pressure in a correla-
tion cell.
T transmittance of an attenuator, assumed to be constant over the
a spectral bandpass of interest.
T transmittance of the gas of species x at high pressure in a correla-
H tion cell.
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v wavenumber of radiant energy (cm ).
v wavenumber of the center of an absorption line. Other subscripts or
superscripts indicate particular values. (cm ) .
^ wavelength of radiant energy expressed in micrometers (|j,m) .
f carrier frequency (high) (Hz) .
f alternator frequency (low) (Hz).
a
p partial pressure of a particular gas species, usually species x (atm) .
P total pressure of a gas mixture (atm).
^ geometrical path length of the radiant energy beam through a gas
sample (cm) .
j concentration of a gas species, usually expressed in parts per
million (ppm) or percent.
u, w pC- (atm cm), absorber thickness of a particular gas species, u
refers to gas in the sample cell and w to gas species x in a correla-
tion cell. One atm cm is equivalent to 10 ppm meters.
k absorption coefficient at wavenumber v of an absorption line (atm cm) ,
S Pkdv, intensity of an absorption line (atm cm" cm ) .
V voltage component of amplified detector signal at carrier frequency f
c c
V voltage component of amplified detector signal at alternating
frequency f .
cL
V' V /V , normalized voltage. Normalization is made so that V' = 1
when there is 100% modulation of the beam at frequency f . See
Equation (5) and related text. a
D.R. discrimination ratio, ratio of the concentration of an interfering
gas species to the concentration of the species being measured that
produces the same reading. This ratio may be positive or negative.
F lim V1 Pembrook Factor (atm cm)
u-»0 u
E chopped radiant energy. Equations (1) and (2) and related text
define E and E and explain the subscripts.
LI att
M constant relating radiant energy to transmittances. See Equations (1)
and (2).
A and Z are defined and explained by Equations (3) and (4) and related text.
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SECTION 2
SPECTROSCOPIC PRINCIPLES
CORRELATION IN SIMPLE SPECTRAL MODELS
Gas-cell correlation spectrometers of the type described here depend
for their sensitivity on the correlation between the structure in the spectrum
of the gas species x to be measured in the sample cell and that of the gas in
a correlation cell. The desired correlation is achieved by filling the
correlation cell with the same gas species x. Generally, the spectral band-
pass of the instrument includes several absorption lines so that the gas in
the correlation cell has large fluctuations in the transmittance at different
wavenumbers. Figures 1-4 are based on a simple instrument and a greatly
simplified model spectrum to illustrate the spectroscopic principles of
detection and discrimination in such a way that the reader can easily follow
the mathematics involved.
An optical diagram of a simple gas-cell correlation spectrometer is
illustrated in Figure 1. Radiant energy from a source passes through either
a correlation cell or an attenuator, then through the sample cell and a band-
pass filter to the detector. The sample cell may be a stack or a closed cell
with windows, or it may simply be an open space such as an atmospheric path.
The correlation cell is filled with gas species x to be measured. The attenu-
ator and the correlation cell are alternately moved into the beam at the
alternator frequency f . An ac amplifier measures the component of the
detector signal at frequency f that results from a difference between the
amounts of radiant energy on tne detector during the two halves of the alterna-
tor cycle.
For simplicity, we assume that the transmittance of the bandpass
filter is unity from wavenumber v to v +1 cm" , and is zero elsewhere. A
transmission spectrum of the gas in the correlation cell is illustrated in
the upper-left corner of Figure 2. The upper-right portion shows the trans-
mittance spectrum of the attenuator over the same spectral region. Note_that
~T , the average transmittance of the correlation cell, is made equal to T ,
the average transmittance of the attenuator. When there is no absorbing gas
in the sample cell, the amplifier output is zero because the amount of radiant
energy incident on the detector is the same during both halves of alternator
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vSOURCE CORRELATION
CELL
BAND-PASS
FILTER
X1
ATTENUATOR
SAMPLE CELL
DETECTOR
AMPLIFIER
&
METER
FIG. 1. Simplified optical diagram of a basic gas-cell
correlation spectrometer.
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cycle. A sample of gas of species x in the sample cell is represented by the
middle panel of Figure 2. Because the same gas species exists in both the
correlation cell and the sample cell, the absorption occurs over the same
spectral interval in both cells. The difference in the transmit tance results
from different amounts of gas in the two cells.
The transmittance of the correlation cell and the sample in series is
given by TL . Tsam. In this case, the sample does not affect the transmittance
because the correlation cell absorbs all of the energy at wavenumbers where
the sample absorbs. During the correlation-cell half of the alternator cycle,
the energy on the detector is proportional to J TL . Tgam dv, which is
numerically equivalent to TL . Tsam in the 1 cm"1 interval. The lower right-
hand portion of Figure 2 illustrates the spectrum of the energy during the
attenuator -ha If of the cycle. Note that the two halves are no longer balanced
with the sample cell. TL . Tsam = 0.50 >Tatt • T = 0.375. The differ-
ence is measured by the a.c. amplifier and meter and is related to the concen-
tration of the gas in the sample. Because Tatt is a constant equal to
Tsam' ^en there is a P°sitive correlation
between the spectral structures of the gases in correlation cell L and the
sample cell, TL . Tsam > T£ . T^^, as in the example above. If TL . Tsam <
TL * ^sam' tne correlation is negative. The two terms are equal when there
is no correlation.
Figures 3 and 4 are based on the same correlation cell and attenuator
as the previous example and illustrate the interference by two different
absorbing gas species other than species x in the sample. The gas species
illustrated in the upper panel of Figure 3 absorbs, but at different wave-
length numbers than does species x. The correlation between the spectra of
the sample and species x is obviously negative with the resulting misbalance
opposite to that when only species x is in the sample. TL . Tsam = 0.40 <
Tatt • 'I'sam = 0«45. If the interfering gas is alone in the sample cell,
as indicated by Figure 3, the instrument will indicate a negative amount of
species x. When both species x and the interfering gas are present, the
indicated concentration of species x is too low.
The interfering gas of species y illustrated in Figure 4 has two
absorption lines, one inside the absorption line of species x and one outside.
No correlation exists between the spectra of this gas and species x, and
f, . TJT = 0.4 = Tatt • Ty» The interfering gas decreases, by the same
fraction, the energy incident on it during both halves of the cycle, and the
system remains balanced. The lower panels of Figure 4 correspond to a sample
consisting of species y, whose transmittance is plotted in the upper panel,
and species x, represented by the middle panel of Figure 2. We note that
T — . Tsam - Tatt . Tsam = 0.4 - 0.3 = 0.1. This difference is less than
tne 0.125 produced by the same amount of species x, but with no species y
present (Figure 2) . Although the interfering gas y by itself does not
produce a misbalance, it reduces the misbalance produced by species x to 0.8
its original value. This factor is, of course, equal to TT.
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J.
J
H
0
i ,
g
to
H"
0
1,
1
i
co
H
~
—
—
—
—
••-
D
i 1
I 1
I I
^
I
>~ V
i
4J
0
1_
B
(0
u
I
(cm"1)
1 | 1 1
n r
1 1 1 1
0 1
FIG. 2. Plots of transmittance for a model gas illustrating
the spectroscopic principles of detection. The average trans-
mittances are as follows:
Upper panels, T = 0.50 and f = 0.50. Middle panel, T = 0.75.
•k ate sam
Lower pane Is, T
sam
0.50 and f
att
sam
0.375.
-------
e
cd
CO
H
•
o
to
H
S
n
1
—
—
0
1 i
i i
V
L
|
L
j
'Va
i
01
4-
n
H
0
(cm )
1
cd
01
H
•
4J
4->
cd
H
o
1 I I 1 .
FIG. 3. Plots of transmittance illustrating interference by a
gas species other than the one being measured.
Upper panels, T =0.9. Lower panels, T
sam L
sam
0.40 and
"att
sam
0.45.
-------
o
I I I
J I
4J
«B
I I I
I
(0
V-V (cm" ) .
FIG. 4. Plots of transmittance illustrating the effect of an absorbing
gas of species y whose spectrum is not correlated with that of species x.
The upper panel represents gas species y alcme in the sample cell. The
combined transmittances of species y with the correlation cell and with
the attenuator are shown in the two middle panels. The corresponding two
plots in the two lower panels are for a sample consisting of both species
x and species y.
Upper panels, T = 0.80. Middle panels, T_
y
Lower panels, T
T =0.40 and T
sam att
Tv = 0.40 and T
= 0.30.
att
T = 0.40.
10
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A similar reduction in the misbalance due to a sample of species x would
be produced by continuum absorption or by dirt on the windows of the sample
cell. The reduction in the misbalance and in the resulting detector signal
can be accounted for by processing the detector signal in such a way that
the output is proportional to (TL . Tgam - Tatt . T8am)/(TL . Tsam + l'att . T ).
For example, from Figure 2, we see that this ratio for species x, (0.50 - 0.375)/
(0.50 + 0.375) = 1/7, is equal to the corresponding ratio for the same quantity
of species x plus the uncorrelated species y represented by Figure 4: (0.40 -
0.30)7(0.40 + 0.30) = 1/7. The instrument described in the following section
measures a quantity proportional to this ratio. In most real situations, the
spectral band includes several absorption lines of the gas being measured and
may include several lines of one or more interfering gases. If the positions
and intensities of the lines of the interfering gas are completely uncorrelated
with those of species x, the interfering gas will behave as the one illustrated
in Figure 4 and not produce a false indication. In practice, the spectrum of
a gas absorbing within the spectral bandpass will usually have some correlation
either positive or negative, with that of species x and consequently will in-
terfere with the measurement.
ENERGY AND VOLTAGE RELATIONSHIPS
Figure 5 illustrates a gas-cell correlation spectrometer that is more
complex than the basic one shown in Figure 1. It is similar in operating
principle to the one built by us for across-the-stack measurements and can be
used with two correlation cells or with one correlation cell and an attenuator.
The alternator consists of correlation cell L and either an attenuator or
correlation cell H along with the optical and mechanical components that alter-
nately direct the beam through them. The one-cell attenuator system employs
correlation cell L and an attenuator. In the two-cell system, the attenuator
is replaced by correlation cell H, which contains the gas species to be
measured at high pressure. In the alternator, the beam may remain fixed
while the correlation cells (or attenuator) move as indicated, or the correla-
tion cells may remain fixed while the beam is alternately directed through
them by moving mirrors. In either case, the beam passes through the sample
cell during both halves of the attenuator cycle.
Before passing through the alternator, the energy beam is chopped at a
relatively high frequency f , called the carrier frequency. The difference
in the energy during the two halves of the alternator cycle modulates the
amplitude of the carrier signal. Employing the high frequency chopping in
addition to the low frequency modulation makes it possible to measure a
quantity proportional to V /V , where these voltages are proportional to the
components of the detector signal at frequencies f and f , respectively.
The ratio V /V can be related experimentally to tne gas concentration and
is relatively independent of the source brightness, detector sensitivity,
dirt on windows or mirrors, etc.
11
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CORRELATION CELL L
SOURCE
\
CHOPPER
(Frequency f )
SAMPLE CELL
ATTENUATOR
OR
T —' CORRELATION CELL H
ALTERNATOR I
(frequency f )
cl
Ref
I Ref
o
OUTPUT
(Proportional
to
Va/V
SYNCHRONOUS I
DEMODULATOR
CONTROL
BANDPASS FILTER
J [
1
1
1
D
DETECTOR
PREAMPLIFIER
IUNED FILTER (f )
AGC AMPLIFIER
&
SYNCHRONOUS
DEMODULATOR
FIG. 5. Simplified diagram of a gas-cell correlation spectrometer that employs high-
frequency chopping and low-frequency modulation.
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Figure 6 contains several plots of calculated transmittance for a model
spectrum used to illustrate the spectroscopic principles of the one cell-
attenuator system illustrated in Figure 5. The curves correspond closely to
those of Figure 2, except that the rectangular-shaped spectral lines of
Figure 2 are replaced by two absorption lines with spacings, intensities, and
half-widths typical of a portion of the fundamental CO band. The curves of
Figure 6 are quite realistic, but the effects of correlation are less obvious
than in Figure 2. For simplicity, we assume that the bandpass filter passes
only the spectral region between the centers of the absorption lines (T = 1
for 0 s v - v 54 cm'1, and T = 0 for all other v).
3 I
The radiant energy chopped at frequency f incident on the detector
during the half-cycle that the correlation eel? L is in the beam is given by
E = M pT, T_ T dv. (1)
L .) f L satn
The corresponding energy during the attenuator-half of the cycle is
E = M PT. T ^ T dv. (2)
att .1 f att sam
Although T- is constant in the present example, it is left under the integral
sign so that the equations will be valid for all situations. The constant M
depends on source intensity and size, aperture sizes, window transmittance,
mirror reflectivities, etc. For simplicity, we assume that the spectral
emissivity of the source and sensitivity of the detector are both constant
over the spectral interval of interest. The remainder of the discussions
on spectroscopic principles is based on the condition that E = E with
no sample in the sample cell. (T =1.)
S cLUl
The curve in the upper panel of Figure 6 labeled (T - TL) represents
the spectral response of the system at frequency f . Sample absorption where
(T - T ) >0 produces a positive V . On the other hand, a gas in the sample
ceftSnth a strong absorption line wftere (T - TL) 0 would have a negative correlation with the gas in
the correlation cell and would produce a negative V . If the spectrum of the
sample gas is completely uncorrelated with the spectrum of the gas in the
correlation cell, the absorption producing negative V balances that producing
positive V .
The spectrum shown in the center panel of Figure 6 corresponds to 0.01 atm
cm of CO at 1 atm pressure in the sample cell. During the half of the cycle
when correlation cell L is in the beam, the transmittance of the beam that has
passed through the sample is given by TT . T . This quantity is indicated
in the lower panel along with the corresponding curve that applies during the
attenuator-half of the alternator cycle. The sample absorbs only a very small
portion of the radiant energy that has passed through correlation cell L
because little energy remains at the wavenumbers where the sample absorbs.
During the attenuator half of the cycle, the sample absorbs a larger portion
of the energy with the result that E >E . A detector voltage component V
LI att a
proportional to (E - E ) appears as a modulation of the carrier voltage V .
13
-------
o
1
i
CO
•ZL
<
cm
1.0
4 0.5
• 0
sam
1.0
0.5
0
\
—
i
1
\
\
<>•-•'<
s'
n
i
f- TL.
1
!
1
— • ] — — — • - . i_ .__
' v
X Tatf rsam \^
i
T \
Tsam
i
\
,
(
i
i
i
\J
0
1 2 3
- v WAVENUMBER (cm-1)
a
FIG. 6. Calculated spectral plots of transmittance for a model spectrum
similar to a portion of a CO band. The curves illustrate the principle of
detection of the one cell-attenuator system. The plot of TL is based on a
correlation cell containing 0.2 atm cm of CO at 1 atm pressure.
14
-------
The curves in Figure 7 illustrate the dependence of Eatt an<^ EL on sample
absorber thickness for the spectral model illustrated in Figure 6. Each pair
of curves correspond to a different absorber thickness w of species x in the
correlation cell; the pressure is maintained at 1 atm. Because If = 1 over
the region of interest, 0 ^ v - va ^ 4 cm'1, TL . T8am = EL/4M, and Tatt. Tsam
Ea*-«-/4M. As the sample absorber thickness u approaches zero, 1'sam apPr°aches
unity, and TL . Tsam and Tatt • 'I'sam approach T^
We define two useful quantities.
TL - Tatt> Tsam dV = (EL ' Eatt)/M'
E -J* VTL + Tatt > Tsam dV = ( EL + Eatt)/M'
For the spectral model of Figure 6, A is proportional to V and to the
difference between values plotted in Figure 7 for given values of u and w.
Similarly, Z is proportional to the sum of the same values and to the average
value of V . Neither A nor £ is easy to measure directly; however, the ratio
can be measured with an instrument of the kind illustrated in Figure 5. In
order to calibrate the instrument, the radiant energy beam through the attenu-
ator is temporarily blocked so that T and, in turn, E , equal zero. Under
this condition A/Z = 1, and the amplifier gain settings are adjusted so that
the output voltages V and V are equal. We define
£L C
V = V /V (5)
a c
with the gain settings adjusted as just described. If the gain settings
remain fixed, it follows that V1 = V /V = A/I! when the attenuator beam is
unblocked and the voltages are measured.
An automatic gain control circuit that maintains V constant can further
simplify the measurement. When this is used, V" is proportional to V , which
is measured directly. Thus, only one voltage, V , need be read in order to
determine A/2 .
Curves based on calculated values of V1 = A/Z are shown in Figure 8 for
5 values of w, including the 2 represented in Figure 7. Recall that the
calculated curves are based on a portion of a model spectrum similar to a CO
spectrum and that the spectral interval corresponds to the spacing between
two adjacent lines. Because the curves of Figure 8 are based on ratios of
integrated transmittance, it follows that the same curves would be obtained
if the spectral interval of the model spectrum were widened to include
several identical lines with the same spacings. Such a model is similar to
the 8 or 10 strongest lines in either the P branch or the R branch of the
fundamental CO band. We have employed a gas-cell correlation spectrometer
with a spectral bandpass that included approximately 10 of the strongest lines
in the R branch of the CO band. Each experimental curve we obtained was very
similar to the one for the corresponding value of w in Figure 8. The differ-
ences between the experimental and the calculated curves could be explained
15
-------
1.0
LU
o
0.5
CO
^
<
oe:
0
__ w = 0.1 atm cm
w = 2 atm
TL' Tsam
1
0.001
0.01 0.1 1
SAMPLE ABSORBER THICKNESS u (atm cm)
10
FIG. 7. Plots of calculated T
T and T
sam att
T versus sample absorber thickness.
Sain
The
curves are based on the spectral model similar to CO that is illustrated in Fig. 6.
-------
0.001
0.001
0.01 0.1 1
SAMPLE ABSORBER THICKNESS u (atm cm)
FIG. 8. Plot of calculated A/£ versus sample absorber thickness. Each curve is identified by w, the
absorber thickness of gas species x in correlation cell L. T for each value of w is: 2 atm cm, 0.461;
1 atm cm, 0.600; 0.5 atm cm, 0.711; 0.2 atm cm, 0.817; and O.I atm cm, 0.875.
-------
by the difference between the model spectrum and the real CO spectrum and by
the shape of the transmission curve of the bandpass filter used in the in-
strument. A "rectangular" bandpass in which T = 0 or 1 has been assumed for
the calculations.
An experimental curve of V vs u can be obtained without adjusting the
amplifier gains by the procedure described above. The shape of the curve on
a logarithmic scale is the same regardless of changes in the gain setting,
which shift the curve vertically. However, adjusting the gains as outlined
makes it possible to relate V directly toA/S. For small values of u, the
curves of Figure 8 have unityaslope, indicating that A/E is proportional to u.
Furthermore, for a given value of u, A/Z varies by a factor of only about 1.5
for variations of a factor of 20 for w.
If w were further reduced to values much less than those indicated, T
would no longer approximate zero near the line centers, and A/Z would reduce
more rapidly. In many situations, the major factor limiting the accuracy of
a gas-cell correlation instrument is "zero-drift", the slow variation in the
signal output when u = 0. An important instrumental parameter is given by
= lira — = lim -&- (atm cm)"1. (6)
u-»0 U ir»0
For lack of an unambiguous and descriptive name, we have named F the
Pembrook Factor after one of our colleagues, John Pembrook, who has done con-
siderable work in developing methods to evaluate instrument performance. For
example, from Figure 8 we see that A /T s- 0.001 when u = 0.001 atm cm and
w = 1 atm cm. Thus, F = 1.00 (atm cm)" , and a zero-drift corresponding to
a misbalance of A/Z » 0.001 produces an error equivalent to 0.001 atm cm of
the gas being measured. From Equations (3) and (4), we see that this corres-
ponds to a zero-drift such that 0.998 as much chopped radiant energy is trans-
mitted through the alternator during one-half of its cycle as during the
other half. [U- 0.998)7(1 + 0.998) ~ 0.001D. It follows that the spectral
bandpass and correlation cell parameters should be chosen to produce a large
value of F if the sample cannot be removed so that zero drift can be checked
and accounted for regularly.
The slopes of the curves of Figure 8 decrease with increasing u. For
values of u more than 1 or 2 times w, the dependence of V" on u is so slight
that it establishes a practical upper limit of u for an instrument.
DESIGN FACTORS
From Figures 6, 7, and 8, we can establish some general "rules of thumb"
and factors to be considered in determining the optimum spectral bandpass,
sample cell length, and correlation cell parameters for a particular applica-
tion. The spectral bandpass should be chosen to include strong absorption
lines of gas species x to be measured and a minimum of absorption by other
gas species that may be present in the sample gas. Interference by other
gas species can generally be decreased by employing a selected narrow bandpass.
However, a lower practical limit on the width of the spectral bandpass may be
determined by filters that are available or by detector noise. If the band-
pass is very narrow, the signal output, which is proportional to A, (Equation (3))
for small u may be less than the detector noise.
18
-------
The absorber thickness w in the correlation cell should be great enough
that the gas is essentially opaque at the wavenumber of strong absorption
near the line centers. The curves of Figure 8 show that by employing a
relatively large w, a larger Pembrook Factor F can be achieved along with an
extension of the dynamic range to larger values of u. Two factors establish
a practical upper limit on w. If the correlation cell is opaque over too wide
of a spectral interval near each line center, the "sharp" structure is lost
and discrimination_against other gases may suffer. Furthermore, as w increases
to the point that T is small, the signal output resulting from a small u
decreases, and detector noise may become a problem. The decrease in signal
output with increasing w can be observed in Figure 7 in which A is proportional
to the separation between a pair of curves. For a given u less than approxi-
mately 0.1 atm cm, A is larger for w = 0.1 atm cm than for w = 2 atm cm. If
w were increased further, A would continue to decrease, but the ratioA/£
would continue to increase slightly as indicated in Figure 8 because 2 would
also decrease.
The optimum combination of sensitivity and discrimination are generally
achieved when the pressure of the gas in the low pressure correlation cell is
equal to or somewhat less than the pressure in the sample cell. This ensures
that the widths of the pressure-broadened lines are approximately the same
in both cells. We have shown experimentally and analytically that the per-
formance is seldom affected significantly when the correlation cell pressure
varies from the sample pressure down to less than half of its value. If the
spectral bandpass contains only spectral lines of approximately equal intensity,
the dependence on pressure is less than if the lines cover a wide range of
intensities. It is understood that as the pressure is varied, the absorber
thickness is adjusted to maintain near opacity near the centers of the strong
lines. The desired combination of total pressure and absorber thickness is
achieved by varying the length of the correlation cell or by adding a non-
absorbing gas such as N_ to collision broaden the lines.
For a fixed sample cell length, the range of concentrations that can be
measured is limited on the low end by noise or by uncertainty in the correct
zero reading and on the upper end by the "saturation" indicated by the curves
of Figure 8. If it is practical, the sample cell length is selected to be
consistent with the range of concentrations to be measured. Two or more
sample cell lengths may be required to cover the entire range of concentrations.
In some applications, such as across-the-stack measurements of stationary
source pollutants, the optical length through the sample gas may be difficult
to change. Double-passing the energy beam through a stack to increase the
length may be practical but limits on window sizes and on the mechanical
stability of the optical platforms make it difficult to pass the beam across
the stack more than twice. If one pass of the stack produces too much
absorption, it may be necessary to select a different spectral bandpass in
which the absorption is weaker. Another solution is to use an optical arrange-
ment that passes the radiant energy beam across only part of the stack. This
method does not provide the averaging of a path that extends across the entire
stack.
19
-------
Narrowing the absorption lines by decreasing the sample pressure to less
than 1 atmosphere can greatly improve the discrimination and sensitivity for
some gases. This is particularly true for SO-, whose absorption lines are so
closely spaced that they overlap and smooth out much of the spectral structure
when the sample is near 1 atm. At pressures less than about 0.1 atm, the
spectral structure of SCL is greatly increased so that the performance of a
gas-cell correlation spectrometer is improved. The optimum sample cell length
and correlation cell parameters for any application obviously depend on the
sample pressure. The half-width of a collision-broadened absorption line is
proportional to the total gas pressure and inversely proportional to the
square root of the temperature. Therefore, the spectral lines of a hot gas
in a smokestack are narrowed by the higher temperature although the pressure
is approximately atmospheric.
TWO-CELL SYSTEM
The two-cell system is illustrated schematically by Figure 5 when a high-
pressure correlation cell H is used in place of the neutral density attenuator.
Figure 9 illustrates the spectroscopic principles of this system with the same
low-pressure sample in correlation cell L as in the case illustrated in
Figure 6. The model is based on correlation cell H filled to 5 atm with CO,
the gas to be detected. The absorber thickness of the CO in this cell, 0.108
atm cm, was adjusted so that fTfTRdv = jTfTLdv. Because of the pressure
broadening of the absorption lines, the gas in cell H absorbs less near the
centers of the lines, but more in the wings of the lines, than does the low-
pressure gas in cell L. The curve labeled (T - T ) represents the spectral
response of the system at frequency f . By comparing Figure 9 with Figure 6,
we see that the response |TR - TL | , of the two-cell system is less than
' Tatt " TLr°f the one'ceH attenuator system in the region midway between the
lines. Thus, the two-cell system would discriminate better against an absorp-
tion line of an interfering gas in this region. The two-cell system is only
about 3/5 as sensitive as the one cell-attenuator system because of the
smaller value of (TR - TL) near the centers of the lines.
In matching fT^dv with fTfTRdv , the pressure of the gas in cell H could
have been made higher with a corresponding shorter path to maintain the
balance between the two beams. This change would further broaden the lines,
increasing TR near the line centers and decreasing it in the wings. As a
result, the spectral response would be greater near the centers of the lines.
However, this would be accompanied by an increased negative response in the
wings of the lines, causing the system to respond more to absorption by other
gases with absorption lines between the CO lines. The optimum pressure in
the cells depends on the spectra of the gas species to be measured and any
interfering gases in the sample. In many cases, the decrease in response of
the two-cell system is more than compensated for by an increased discrimination
over the one cell-attenuator system.
Figure 10 illustrates model spectra for which the discrimination by the
two-cell system is far superior to that by the one cell-attenuator system.
The curves labeled T and T in the upper panel correspond to the transmit-
tances of low and high pressure correlation cells, respectively. Both have
20
-------
1.0
o
I
00
0
-0.2
v-.v WAVENUMBER(cnrl)
3
FIG. 9. Calculated spectral plots of transmittance for a model spectrum
illustrating the principle of detection of the two-cell system.
21
-------
1.0
0.5
0
0
v - v
2345
WAVENUMBER (cm"1)
FIG. 10. Calculated spectral plots of transmittance demonstrating the
principles of discrimination against continuum absorption by the two—
cell and the one-cell attenuator systems.
22
-------
same average transmittance, which is represented by the curve labeled T tt«
Thus the two curves T^ and T^ correspond to the two-cell system, and the
curves TL and Tatt correspond to the one-cell attenuator system. Both of the
absorption lines have a half-width 01 of 0.05 cm for the low pressure sample,
and 0.5 cm~l for the high pressure sample. The intensities of the lines at
(v-va) = 1, and 3 cm"^- are 4 atm'^-cm'-'-cm"-'- and 1 atm~lcm"i'-cm~-'-, respectively.
The absorber thicknesses of gas x in the low and high pressure cells are 0.5
and 0.24074 atm cm, respectively. It is assumed that Tf = 1 for 0 < (v-va)
< 6 cm~l and Tf = 0 for all other v.
Curves A, B and C in the lower panel represent three simple types of
continuum absorption by interfering gases in the sample. Table 1 summarizes
the relative sensitivities of the two systems to the three types of continuum
and to a small amount of gas species x at the same pressure as the gas in
correlation cell L. The equivalent error corresponds to the amount of gas
species x that produces the same A/£ as that produced by the interfering con-
tinuum. Continuum A has constant transmittance and produces no error, whereas
Continua B and C produce signals in the one-cell attenuator system that cor-
respond to a sample containing approximately 0.108 atm cm of the gas x. The
equivalent error for the two-cell system is only about 2% as much. Sample B
produces an error that corresponds to a positive amount of gas x because the
general slope of the transmittance curve is similar to that of gas x; i.e.,
the transmittance increases with increasing wavenumber for both gas x and
continuum B. As expected, the opposite is true for continuum C, whose trans-
mittance curve slopes in the opposite direction.
A small amount of gas species x produces a smaller signal in the two-
cell system than in the one-cell attenuator system; however, the increase in
discrimination may be more important than the loss in sensitivity. Similar
results would be obtained if the continua were replaced by spectra with the
same general slopes, but consisting of many lines. Thus, curves B or C may
be representative of the spectra near the edge of an absorption band of an
interfering gas. The model spectrum of gas x in the correlation cells is
also typical of the edge of many absorption bands in which the lines get pro-
gressively weaker with increasing distance from the band center. As an
example, interference by the wing of a C02 band when measuring CO is similar
to the model illustrated in Figure 10. In such a case, the discrimination
by the two-cell system can be much better than that by one-cell attenuator
system.
23
-------
TABLE 1
COMPARISON OF THE SENSITIVITY AND DISCRIMINATION
OF TWO-CELL AND ONE-CELL ATTENUATOR SYSTEM
0.001 atm cm
of gas
Continuum A
Continuum B
Continuum C
Two-Cell
0.0002066
0 0
0.0004778 +0.00231
-0.0004778 -0.00231
One-Cell Attenuator
Sample
A
Z
Equivalent
Error
(atm cm)
A
z
Equivalent
Error
(atm cm)
0.0003203
0
0.03474
-0.03474
0
+0.1084
-0.1084
MULTIPLE CORRELATION CELL SYSTEMS
The discussion above has been restricted to relatively simple instruments
for which the processing of the detector signal is not excessively complicated
By using more than two correlation cells with different amounts of gas either '
at different pressures or at the same pressure, it may be possible to obtain
more information about the sample absorption and to better account for inter-
fering gases, particulate matter, and variations in sample temperature or
pressure. In general a measurement with a correlation cell containing a
small amount of gas at low pressure provides information about the absorption
near the centers of the lines. Similarly, a correlation cell with more gas
absorbs over a wide interval near the line centers and provides information
about absorption in the wings of the lines. An alternate method is to use a
single correlation cell and vary the gas pressure in it. Processing the
detector signal for an instrument with more than two correlation cells, or with
one of varying pressure, is necessarily more complicated than for two cells or
for one cell and an attenuator. The added complication is usually not re-
quired for absorption measurements, but it may be justified for passive
measurements in which the hot gas being measured serves as the radiant energy
source. &y
24
-------
SECTION 3
INSTRUMENT DESIGN AND PERFORMANCE
OPTICAL LAYOUT OF BASIC INSTRUMENT
Figure 11 shows the optical diagram of the instrument that was built
and tested for EPA. It employs the principles described above and
can be used to compare different methods of gas-cell correlation spectroscopy
under a variety of situations. Different gas species can be measured by in-
terchanging the gas correlation cells, a bandpass filter, and possibly the
source and detector. The instrument is equipped with correlation cells and
filters to measure NO, CO, SO,, HC1, or HF. All of the optical components
shown in Figure 11 are mounted on a baseplate approximately 47 cm x 53 cm.
Except for a preamplifier, all of the electronic components are mounted
separate from the optics.
In Figure 11, mirrors indicated by an N are in the entrance section that
directs the beam of radiant energy into the stack. A tungsten filament bulb
in a quartz envelope serves as the radiant energy source for wavelengths less
than approximately 3.6 ^m. Either a Nernst glower or an electrically heated
nichrome wire covered with a ceramic material serves as a source for longer
wavelengths. Two sources can be kept in place. Mirror Nl is removed or put
in a place as shown, depending on the source being used. In normal operation,
bypass mirrors Bl, B2, and B3 are not in place. Mirror N2 forms an enlarged
image of the source that overfills aperture Apl; a further enlarged image of
the source is formed on mirror N5, which directs the beam through a CaF2
window into the stack. A field lens near aperture Apl images mirror N_ on
mirror N . A retroreflector described in the following paragraph, placed
on the opposite side of the stack directs the beam back through the stack
and forms another image of the source on mirror XI. Mirror XI images mirror
C3 of the retroreflector onto aperture Ap2 just ahead of the alternator. A
rotating mirrored chopper Al, with two blades and two open sectors, alter-
nately directs the beam through one of two paths. One alternate path contains
correlation cell CC, and the other contains an attenuator. As discussed
previously, the attenuator may be replaced by a high-pressure correlation cell.
Mirror A5 directs the beam upward through a bandpass filter to the detector.
Either of two liquid-nitrogen cooled detectors is used: PbS for \^ 4 u.m, and
pbSe for ^ between 4 and 6 u-m. The bandpass filter passes a spectral band that
contains absorption by the gas being measured.
25
-------
•Window
X2
From Stack
To Stack
10 Hz
Mirrored
Chopper
435 Hz .
Source u Chopper
r
^Alternate Source
FIG. 11. Optical diagram of a gas-cell correlation instrument for across-the-stack
measurements.
26
-------
Figure 12 shows an optical diagram of the retroreflector assembly, which
is a modified version of a "cat's eye" reflector. Mirror Cl forms an image of
mirror N5 on the small spherical mirror C2, which is adjusted to correspond
to the distance between Cl and N5. Mirror C2 images Cl near C3. Mirrors Cl
and C3 are cut from a single spherical mirror and mounted so that their
centers of curvature are at the same height but separated slightly in an
azimuthal direction in order to image N5 on XI. Provided the relative posi-
tions of the three mirrors Cl, C2, and C3 are rigidly fixed, the position of
the image formed near mirror XI relative to the effective source near N5 is
insensitive to small angular or linear displacements of the retroreflector
assembly.
The radii of curvature of the mirrors are chosen to be optimum for a stack
approximately 3 m in diameter. For this size of stack, mirrors N2, N3, Cl,
and C3, aperture Ap2, and the detector are conjugate to each other. The
source is conjugate to Apl, N5, C2, and XI.
The instrument is very sensitive to differences in the energy passing
through the two paths of the alternator section. Therefore, after the alter-
nator has been balanced with no absorbing gas in the beam, it is very important
that any changes in aperture or angular field of view affect both paths of the
alternator in the same way. This is best accomplished by ensuring that the
limits of the beam are well defined outside of the alternator and that the
alternator transmit to the detector all of the beam entering it. The image
of C3 at Ap2 is larger than the aperture, and the image of Ap2 at the detector
is smaller than the sensitive element. Therefore, Ap2 forms the effective
aperture as long as any misalignment of the optical components preceding it is
sufficiently small that Ap2 remains completely filled. The windows and mirrors
in the alternator section are oversized so that a mask in front of mirror XI
limits the angular field-of-view of the detector. The optical components
preceding mirror XI are designed so that the opening in the mask on mirror XI
is completely filled.
In order to balance the alternator, the bypass mirrors Bl, B2, and B3
are easily positioned as indicated in Figure 11 to bypass the stack and provide
an optical path with no absorbing gas. Bypass mirror B3 replaces XI, which
has a different radius of curvature, and sits in the same position; the same
mask is used for XI and B3. When the bypass optics are in place, Ap2 is con-
jugate to mirrors N2 and N3 as it is when the beam is traversing the stack.
Similarly, B3 is conjugate to the source and Apl as is XI when it is in place.
Thus, the instrument has the same field-of-view while it is being balanced as
it has during a measurement. This method of bypassing the stack in order to
balance the alternator matches the aperture and field-of-view about as well as
is possible without passing the beam through the smokestack. Errors due to
non-uniformities in the transmittance of stack windows or in the reflectivity
of mirrors in the entrance section are minimized, but they cannot be eliminated
completely. The best balance could probably be achieved by placing a pipe
across the stack with the radiant energy beam passing through the pipe. The
absorption could be eliminated by flushing the pipe with a non-absorbing gas.
However, this method of balancing is frequently not practical because of space
limitations or because of possible interference with the operation of the plant
27
-------
NJ
c»
C3
FIG. 12. Optical diagram of a retroreflector assembly used
to redirect the radiant energy beam back across the stack to
the gas-cell correlation instrument.
-------
being monitored. It seems probable that the performance of gas-cell correla-
tion instruments for stack measurements will ultimately be limited by the un-
certainty in the balance corresponding to no absorption in the stack.
The radiant energy from the source incident upon the detector is chopped
at 435 Hz with a 10 Hz modulation that is related to the concentration of the
gas being measured. The detector signal is amplified and processed in such a
way that the 435 Hz signal serves as a carrier for the 10 Hz signal as indi-
cated by Figure 5. In this way, the system is insensitive to 10 Hz variations
in the radiant energy on the detector unless the energy is first chopped at
435 Hz. Thus, emission by hot gases in the stack, or by any optical compon-
ents that follow the 435 Hz chopper, does not influence the instrument output.
INSTRUMENT EMPLOYING GRATING ASSEMBLY
The stability and discrimination of an instrument of the type shown in
Figure 11 could frequently be improved if the spectral bandpass could be
controlled more accurately that can be done with an interference filter.
Figure 13 shows another version of the instrument described above with a
grating assembly to provide the desired bandpass. Either CO or NO can be
studied with the grating assembly shown by blocking the beam at the appropriate
retro-reflector Rl or R2. The bandpass filter near the detector eliminates
overlapping orders of shorter wavelengths that are passed by the grating
assembly. It is also necessary to change the correlation cell, and possibly
the attenuator, when changing from one gas to another. A second grating
assembly is built for SO- or HC1, and a third one serves for HF. Each
grating assembly is constructed in its own box, and they can be interchanged
quickly with little or no realignment of optical components required. The
box for each assembly is mounted to the main base plate on a 3-point system
that will not transfer stresses from the base plate to the box. All of the
parts in the grating assembly are bonded in place with epoxy cement to avoid
misalignment.
Mirrors Tl, T2, and T3 are bonded to a block that can be removed or put
back in place easily. This transfer optics assembly directs the beam into an
entrance slit SI that is below the optic axis of the alternator. The beams
of the selected bandpass exits from the grating assembly through slit S2 at
the height of the optics axis of the alternator. Mirror T3 directs the beam
into the alternator. When the transfer optics are installed, it is necessary
to tilt mirror XI to direct the beam to the lower level of mirrors Tl, T2, and
slit SI. Otherwise, no adjustments are required when the transfer optics
assembly is installed or removed.
Mirror T2 images mirror XI, and thus the source, on the entrance slit SI.
For a stack diameter of 3 m, mirror C3 of the retroref lector is imaged on
mirror T2. As the diameter changes with the resulting change in distance
between mirrors C2 and XI, the conjugate to mirror C3 moves slightly from
mirror T2. A mask on the grating limits the field-of-view of the detector.
29
-------
Detector
Window
X2
From Stacl^
10 Hz.
Mirrored
CHopper
435 Hz
Source " Chopper
^Alternate Source
>'IG. 13. Optical diagram of a gas-cell correlation instrument incorporating a
grating assembly.
30
-------
The grating assembly shown in Figure 13 disperses the radiant energy and
directs that of wavelengths near 5.25 p-m toward the retroreflector Rl.
Energy from a portion of the spectral band selected for the measurement of NO
is redirected back to the grating, which undisperses it and directs it to
mirror Gl and the exit slit S2. Similarly, wavelengths near 4.6 y,m fall on
retroreflector R2, which returns a portion selected to measure CO. Only
one gas is measured at a time; the retroref lector not being used is covered
with a mask.
A previous paper by one of us(°/ describes the method by which the radia-
tion is dispersed and a selected portion is undispersed and imaged. The
original paper described the method for a prism as a dispersing element;
however, the obvious application to a grating and to systems with a "tailored"
bandpass were pointed out. Decker(7) has also described an application of the
method to Hadamard spectroscopy.
A view of the retroref lector and grid assembly such as the ones indicated
by Rl or R2 in Figure 13 is shown in Figure 14. A converging beam of dis-
persed radiant energy from mirror G2 (Figure 13) is incident upon mirror G4
whose reflecting plane is 45° from the vertical. The horizontal grid is in
the focal plane where monochromatic energy forms an image of the entrance
slit Si. Openings in the grid, such as the two shown in Figure 14, pass the
selected spectral intervals to mirror G5. Mirrors G4 and G5 are normal to
each other so that each ray of the beam returns to mirror G2 parallel to, but
displaced vertically downward from, the ray before it struck mirror G4. The
returning beam is completely undispersed by the grating, and the image formed
at the exit slit S2 is undispersed and contains only those wavelengths passed
by the grid in the retroref lector and grid assembly. Because an image of
slit Si is formed at slit S2, only one of these slits is required to provide
the desired spectral resolution. Slit S2 is intentionally made somewhat
wider than slit SI; the effective part of S2 is only that portion conjugate
to the narrower SI. Slit S2 reduces scattered energy reaching the detector;
having it oversize does not degrade the resolution and makes the positioning
of the slits much less critical.
A photograph of the grating assembly for SOo and HCl is shown in
Figure 15. A single mirror, G2, reflects the dispersed energy to both retro-
ref lector and grid assemblies. Two separate mirrors cannot be used as G2
and G3 are for the NO and CO instrument shown in Figure 13. At mirror G2
of the grating assembly for S02 and HCl, the beams of dispersed energy for
the two wavelengths of interest, 4.0 and 3.6 u-m, overlap each other. The
two beams near 5.2 and 4.6 (j-m used to measure NO and CO, respectively, are
separated enough that two separate mirrors, G2 and G3, are used to focus
the beams on the retroreflector and grid assemblies. By rotating G3 rela-
tive to G2, the two retroref lector and grid assemblies can be placed close
to each other as shown in Figure 13. The grating assembly for HF contains
a single retroreflector and grid assembly.
The power supply unit and the electronics unit shown in Figure 16 are
designed for rack mounting and are connected to the optics assembly by
approximately 15 meters of electrical cables. Power supplies and appropriate
31
-------
OJ
ro
G?
GRID
FIG. 14. Optical diagram of a retroreflector and grid assembly.
-------
Slits
FIG. 15. The grating assembly for S02 and HCl. The
two retroreflector and grid assemblies are
labeled according to the gas they are used
to measure.
J3
-------
| .
«^PP
u.
t ••*• t *
..% *
Q
FIG. 16 The power supply unit above the electronics unit.
-------
switches are provided for three different radiation sources: a Nernst glower,
a nichrome-heated ceramic glower, and a tungsten-iodide bulb. Also included
in the power supply unit are amperite ballast bulbs for the Nernst glower
and a switch so that either 1, 2, or 3 of the ballasts can be connected in
parallel to maintain a current of approximately 0.27, 0.54, or 0.80 amps.
Motors for the two choppers and a heater to start the Nernst glower are also
controlled from the power supply unit.
The amplified detector signal from the preamplifier on the optics
assembly is fed into the electronics unit shown in Figure 16. All of the
signal processing indicated by the electronics diagram of Figure 5 takes
place in the electronics unit. Included on the front panel are gain con-
trols and adjustments for the phases of the reference signals used in the
synchronous demodulators. An electronic balance control, not shown in
Figure 5, on the front panel of the electronics unit makes it possible to
properly account for a possible small misbalance between the two beams in
the alternator section when the stack is being by-passed to determine a
"zero" reading.
By setting a switch to the appropriate position, the panel meter can be
made to read either of five different voltages: signal input, carrier level,
signal output, carrier reference, or modulation reference. The signal input
represents the ac input at all frequencies and makes it possible to measure
unwanted "pickup" when the chopped beam is blocked from the detector. The
carrier level represents the signal at the output of the carrier-frequency
demodulator. After the instrument has been properly calibrated, the absorber
thickness of the gas being measured can be determined from the signal output,
which can be read from the panel meter or from a recorder connected to an
output jack. Nine different ranges exist for the signal output. The panel
meter is also used when adjusting the levels of the carrier reference and
the modulation reference signals. Time constants of 0.3, 1, and 3 seconds
are available.
Two photographs of the optics assembly, with and without the cover,
appear in Figure 17. The chopped radiant energy leaves the optics assembly
before traversing the stack and returns from the stack through the open port
in the front of the cover. The bracket directly beneath the opening in the
cover fastens to a flange on the port of the stack. The detectors are filled
with liquid nitrogen through an opening in the top of the cover; a cylin-
drical cap covers the opening and protects the detector. A small fan mounted
above the source housing and visible in the lower photograph of Figure 17
circulates air and carries away much of the heat from the source. A pre-
amplifier and a box for the bias battery are mounted on the baseplate near
the front of the instrument and adjacent to the grating assembly.
SPECTRAL PROPERTIES OF THE GASES
Figure 18 shows two experimental curves of transmittance in the region
of the Q-branch and the R-branch of the fundamental NO band. The absorption
represented by the upper curve is due to approximately 1.5% H~0 in 8 m of
35
-------
'JflHhb.
,<^^^l^^^
FIG. 17. Photographs of the optical assembly. The top picture
shows the assembly with the cover installed.
36
-------
1920
1910
1900
1890
I860
1870
WAVENUMBER (cm-1)
FIC. 18. Spectral curves of transmittance of H20 and of HO + NO between 1862 and 1928 cm
width = 0.35 cm~l. The transmittance scales of
'1
"the two curves differ by 0.20.
Spectral slit-
-------
room air. The lower curve, which is displaced downward for clarity, repre-
sents the same amount of H20 and a sample of 0.03 atm of NO in a 10 cm
sample cell. The need to reduce interference by H20 is apparent. Absorption
by H20 is even stronger in the lower-wavenumber P-branch of the NO band. The
cross-hatch areas correspond to the 6 narrow spectral intervals passed by
the retroreflector and grid assembly for NO. Interference by H?0 is reduced
by choosing the intervals with a minimum of H^O absorption. Each interval
contains a pair of NO lines.
Blocking the spectral intervals where there is little or no NO absorp-
tion decreases £ with essentially no decrease in A. (See Equations 3 and 4.)
Thus, the Pembrook Factor F is increased with a resulting increase in the
stability of the zero balance. The blocked energy, if allowed to pass,
would provide no information about NO concentration, but it would add to
any misbalance of the alternator. Information on the positions, intensities,
and haIf-widths of the NO lines can be obtained from two articles by Abels
and Shaw.(B'y;
Two transmittance curves of the S02 band employed are shown in Figure 19.
The structure in Curve A is due to "clusters" of many closely-spaced lines
that are not resolved. When the total pressure of the sample is greater than
about 1 atm, the lines are broadened enough that they overlap considerably
and much of the structure in a spectrum of 1 atm sample that could be observed
with a very narrow spectral slitwidth is not greatly different from Curve A.
Further smoothing of the spectrum with increased pressure is illustrated by
Curve B of Figure 19. At the high pressure, the widths of the S02 lines are
comparable to the spacings between the clusters of lines.
The relatively "shallow" structure in the spectrum of S02 near 1 atm
contributes little to the sensitivity of a gas cell correlation instrument
for thisgas when the sample is near 1 atm, as in a smokestack. The greatest
sensitivity can be achieved by making use of the coarse, deep structure. In
order to do this, the grid for S02 has been made with only two openings that
pass two spectral intervals, each about 4.7 cm"1 wide. One is centered in
the region of strong absorption near 2513 cm'1, and the other is centered
outside the region of significant absorption near 2538 cm"1.
Enough S02 is contained in the correlation cell that it is essentially
opaque in the 2513 cm"1 interval while it is nearly transparent in the other
interval. It follows from the previous discussions on the spectroscopic
principles that the transmittance of the attenuator is approximately 50%
over both spectral intervals. The amount of radiant energy passing through
the correlation cell is only very slightly dependent on the amount of SOo
in the sampling region. When the attenuator is in the beam, the SO- in the
sampling section absorbs in one of the two spectral intervals passed to the
detector. Approximately twice the contrast in the two beams of the alter-
nator could be achieved if the alternator transmitted only in one of the two
intervals during one-half of each cycle and only in the other interval during
the other half. Because of the relatively wide spectral intervals involved,
4.7 cm"1, it would not be difficult to accomplish this by modulating the
energy with a chopper in the plane of the dispersed energy in the grating
38
-------
1.0
o
CO
0
6
1
1
2500 WAVENUMBER(cm'l) 2470
—,
T
4.0 WAVELENGTH (Mm) 4.5
FIG. 19. Spectral curves of transmittance between 2465 and 2525 cm"1. The
total pressures of the two dilute, room-temperature, mixtures of SO, in N2
are 1.00 atm for Sample A and 15.0 atm for Sample B. u = 0.78 atm cm for
Sample A; u for Sample B was reduced a few percent to produce nearly the same
average transmittance as Sample A. The spectral slitwidth ^ 0.35 cm"l.
39
-------
assembly. In this case a correlation cell would not be required. Alternating
between two similar intervals could also be done with narrow -bandpass filters.
It follows from the above discussion that gas -cell correlation methods do
not provide the advantages in terms of effectively high spectral resolution
for S02 that they do for other gases such as HCl, HF, NO, CO, or C0? whose
spectra consist of distinct, well-separated lines.
Two bands of S02 centered near 1150 cm and 1360 cm~l are stronger
and would provide higher sensitivity than the one used. However, H20 inter-
ference is much more troublesome and detectors are less sensitive in the
regions of the two lower -wavenumber bands. Data on the absorption by these
bands are given in Reference 10.
Data on the absorption properties of CO, HCl, and HF are available in
a number of reports and papers. Line parameters used in the design of the
present instrument are from Benedict, et al(H) for CO; Benedict, et al(!2)
for HCl; Meredith and Kent(-'U-) and Herget , et al(14) for HF.
LABORATORY TESTS AND CALIBRATION DATA
Figures 20, 21, and 22 show plots of V1 = V /V for each of the 5 gases
with the appropriate grating assembly in place as indicated by Figure 13.
These curves can be used for calibration to convert the instrument output to
the partial pressure p of the gas species being measured. The ordinate for
each of the figures corresponds to the ratio A/E plotted in Figure 8, and
the abscissa is the absorber thickness of the gas normalized to room tempera-
ture, 296 K. JThe curves for CO and NO in Figure 20 are based on samples at
296 K, and u = pi, where p is the partial pressure of the absorbing gas in
atm and I is the geometrical path length in cm. All of the samples used to
obtain the data contained the absorbing gas mixed with N2 at 1 atm total
pressure.
Samples of CO + N2 and NO + N2 were also investigated at approximately
420 K (279°F) which is typical of smokestack exhaust. We found that a single
curve can be used if the value of u is normalized to 296 K by
„/„,.„ \ - (cm) 296 U6
u(atm cm296) - - , or p
where 6 is the stack temperature. By normalizing in this manner, the system-
atic error introduced by using a single curve is not more than approximately
5% over the temperature range investigated. The validity of this simple ex-
pression for normalizing indicates that the absorption is nearly independent
of temperature changes provided the number of molecules in unit cross -sectional
area of the beam remains constant.
The S02 data obtained at the two different temperatures cannot be repre-
sented by a single curve. Thus, accounting for the stack temperature is
somewhat more complicated for S02 than for CO or NO. Concentrations of SO,
could be determined graphically from the two curves of Figure 21 by interpolation
40
-------
0.000!
0.00!
0.01 O.I
u (atm cm
10
FIG. 20. Curves of V1 versus normalized absorber thickness for CO and NO. The
correlation cell parameters are: p = 0.461 atm, L = 2 cm for
CO; and p = 0.414 atm, L = 2 .cm for NO.
-------
to
0.00!
0.0! 0.!
u (atm cm296)
FIG. 21. Curves of V1 vs normalized absorber thickness for S02 and HCl. The results
for the elevated temperature 422 K for SO, are shown as a broken curve. The
correlation cell parameters are: p = 0.539 atm, L = 1 cm for HCl; and p = 2 atm,
L = 5 cm for S02»
-------
U)
O.I
0.01
0.001
0.0001
0.00001 0.0001 0.001 0.01
u ( atm crr,296)
O.I
1-0
FIG. 22. Curve of V1 vs normalized absorber thickness for HF. The
solid portion is based on laboratory data, the dashed portion on
calculations. The correlation cell'parameters are p = 0.164
'atm, L = 1 cm.
-------
for temperatures between 296 K and 422 K. The temperature can also be
accounted for empirically by use of the follwoing expression:
u(atm cm . ) 6 g
p =
2%
_|-
The uncorrected value of u1 is determined from V1 and the solid S00 curve of
Figure 21. '
No calibration data were obtained for HCl or HF at other than room
temperature. For elevated temperatures less than approximately 420 K, it
is probably safe to account for temperature in the simple manner used for CO
and NO.
Table 2 summarizes the important parameters and the performance of the
instrument for the 5 gases of interest when the appropriate grating assembly
is being employed. The second column lists the spectral intervals passed by
the grid in the retroreflector and grid assembly. As indicated in the dis-
cussion of Eq. (6), the Pembrook Factor given in the third column relates
the absorber thickness of the gas in the sample to the resulting percentage
misbalance in the two alternator beams.
A Nernst glower source heated by 0.8 amp of alternating current was
used in measuring the absorber thickness required to produce a signal equiva-
lent to the peak-to-peak noise arising from the detector. A 3-second time
constant was employed. The laboratory optical path simulated a double-passed
stack 3 m in diameter with 5 cm diameter windows. Both the PbSe and PbS de-
tectors have 2 mm x 2 mm sensitive elements cooled with liquid nitrogen.
Water vapor produces the most serious interference when measuring NO
The discrimination ratio (10,000:1) is based on a 3 m diameter stack contain-
ing 5% H20. Because of a non-linear relationship between the H00 absorber
thickness and the instrument response, the discrimination ratio depends on
the amount of H20. Unrealistically high discrimination ratios can be observed
with high concentrations of the interfering gas. A meaningful discrimination
ratio should be measured with approximately the same absorber thickness of the
interfering gas as will be encountered during a measurement.
Several measurements of H-O interference were made while measuring NO
with a different instrument employing various bandpasses that included several
of the H20 lines seen in Figure 18. Typically, the H?0 interference was
10-100 times as great as that observed with the grating assembly with a band-
pass of 6 narrow spectral intervals. By employing the grating assemblies to
carefully select the bandpasses, it has been possible to reduce the inter-
ference by other gases to a negligible amount while measuring CO, SO , HCl
or HF. 2 '
44
-------
Hydrogen fluoride is highly reactive, and it adsorbs and desorbs on and
off of the walls of the gas-handling system and correlation cell. Therefore,
concentrations of the dilute HF mixture were difficult to determine accurately.
Samples of HF concentration between 0.1% and 1% were prepared in a 1 cm sample
cell in the following manner to provide calibration sample absorber thick-
nesses between approximately 0.001 and 0.01 atm 0*1295. A. stainless steel
cylinder was used to make a 1% HF mixture in N2. The cylinder was precondi-
tioned for adsorption by introducing pure HF into the cylinder at a slightly
higher pressure than would be used in the mixture and allowing the gas to
react with the walls for approximately one hour. The HF was pumped out and
the desired 1% HF in N2 mixture made.
The cylinder was attached to the sample cell with a short piece of
tubing to reduce adsorption in the lines. In order to obtain samples with
concentrations below 1%, the sample cell was filled with the 1% mixture to
a predetermined pressure, always less than 1 atm. The total pressure in the
sample cell was then increased to 1 atm by adding N2« This method gave con-
sistent results for concentrations as low as approximately 0.1%. At lower
concentrations, apparent variations in the amount of adsorption and desorp-
tion made the results unreliable. A shorter sample cell was not available
to enable us to make samples of smaller absorber thickness while maintaining
an accurately measurable concentration.
In order to extend the calibration curve to smaller values of absorber
thickness, we employed published values for the HF absorption line para-
meters to calculate the expected instrument response. The calculations also
involved the spectral bandpass, the amount of HF in the correlation cell,
and use of the Ladenberg-Reiche functionC15) for absorption by a spectral line.
For very small absorber thicknesses, V1 is proportional to u, as is indicated
by the broken line of unity slope in Fig. 22. The calculated values agreed
with the experimental values for u between approximately 0.001 and 0.005 atm
cm296.
We have used this calculation method previously for other gases whose
line parameters are well-known. Generally, the shape of the curve can be
calculated quite accurately for values of u up to where the slope of a cali-
bration curve such as the one in Fig. 22 has decreased to about 0.5. The
calculated curve of V1 vs u may require a slight shift up or down to match
an experimental curve, but the shapes of the two curves on log-log scales
are nearly identical. Because of our previous success in calculating cali-
bration curves, and because of the agreement between the calculated and
measured values for several values of u, we believe that both the solid and
dashed portions of the calibration curve in Fig. 22 are accurate to less than
* 10%.
When measuring HF, the discrimination against H20 is 50,000 : 1. Most of
the interference by HoO is due to the longest wavelength interval which is
passed by the grid silt nearest the grating. If more discrimination against
HoO is needed, the grid slit that passes the longest wavelength could be
blocked and only the other two grid slits used. New HF calibration data
would have to be obtained for the assembly used in this fashion.
45
-------
TABLE 2
INSTRUMENT PARAMETERS AND PERFORMANCE
Gas to be
Measured Spectral Band Pass
NO 6 intervals, each centered
Pembrook
Factor j
(atm cm)
0.81
Detector and "~
Noise-Equivalent
Absorber Thickness
(atm cm)_ Remarks on Interfering Gases
PbSe HO D.R. = 10,000:1 with H-0 at 125°C.
0.003 „
on a pair of NO lines near
1900 cm"1
5 ppm NO. The 6 pairs of NO lines in
the spectral bandpass are relatively
free of HjO absorption. Inclusion of
other lines would increase the inter-
ference by HjO.
CO 11 intervals, each centered
on a CO line
and 2200 cm"
1.85
on a CO line between 2150
-1
PbSe
0.0008
C00
Interference too small to measure.
D.R. > 200,000:1 with O>2 at 125°C.
107. CO2 produces error corresponding
to less than 0,5 ppm CO
D.R. S- . 50,000:1 with H.O at 125°C.
57. H20 produces error corresponding
to -1 ppm CO.
SO- 2 intervals, near 0.088
2538 and 2513 cm"1.
HC1 4 intervals, each includes 0.7
corresponding lines of 35_
L>
PbS
0.011
PbS
0.003
No significant interference by other gases
in stacks of coal-burning plants in this
spectral region.
Intervals in the spectral bandpass are
essentially free of interfering absorption.
and 38 isotopes. Inter-
C
vals are between 2750 and
2850 cm"1.
absorbs near the adjacent lines.
SO
might interfere with one of the HC1 lines.
HF
3 intervals, each passes a
line between 3995 and
4080 cm
-1
4.2
PbS
0.0001
D.R. = 50,000:1 with H20 at 150"C.
Improved discrimination can be obtained by
blocking the long wavelength grid slit.
46
-------
No data have been obtained with the instrument shown schematically in
Figure 11 without the grating assembly. However, from other data obtained in
our laboratory with instruments employing narrow bandpass filters in the
place of a grating assembly, we can conclude that the simpler filter instru-
ment provides a higher ratio of signal to detector noise at the expense of
poorer discrimination and a lower Pembrook Factor.
FIELD TESTS
Field tests of the instrument measuring NO, CO, and S02 were performed
on a smokestack of a coal-burning power plant in Charlotte, N.C. Photographs
of the optics assembly and retroreflector mounted on the stack are shown in
Figure 23; the cover of the optics assembly has been removed. The side of
the optics assembly next to the stack is connected by a hinge to a flange
welded on the stack. Two adjustable legs support the back side of the optics
assembly; the legs can be adapted to a variety of stacks.
Two ports with 10 cm openings are welded permanently on opposite sides of
the stack at approximately 1.5 meters above the level of the flat roof. The
effluent temperature was approximately 422°K (300°F). The two-way optical
path length through the effluent was approximately 580 cm. A small building
approximately 6 meters from the stack housed the power supply unit and the
electronics unit.
A CaFo window is mounted on a shutter to prevent stack effluent from
escaping into the instrument. Compressed air blown over the inside surface
of the window prevents the accumulation of dirt on the window. The air then
passes into the stack through a cylindrical tubing that extends approximately
30 cm into the stack. Because of the air flow through the tubing, very little
effluent reaches the window. Although the required air flow was greater than
expected, the windows remained clean enough for operation for several hours.
The window is mounted on a sliding shutter that can be closed easily to pro-
tect the window when the instrument is not in use. A similar window arrange-
ment with plumbing for compressed air is mounted on the retroreflector assem-
bly on the side of the stack opposite the optics assembly.
Smoothed plots of the NO concentration measured at four different times
are shown in Figure 24. During three of the four periods, measurements were
compared with valves obtained with a DuPont Model 460-1 analyzer. We note
that the data obtained with the two instruments agree quite well. The DuPont
instrument extracts a portion of the effluent from the stack and employs the
uv to measure the N0£ concentration. The NO is then oxidized to N02, and
another NO? measurement is made. From the difference between the N0£ concen-
trations, the original NO concentration in the effluent can be determined.
The N02 concentration in the effluent before it leaves the stack is generally
much lower than that of the NO.
Similar smoothed plots of the CO concentration are shown in Figure 25.
The relatively low concentration varying between 5 ppm and 20 ppm indicates
that the combustion was nearly complete. Simultaneous measurements made with
an extractive NDIR CO analyser resulted in average values near 9.5 ppm during
47
-------
FIG. 23. The optics assembly and the retro-
reflector mounted on the smokestack
of the Duke Power Co. at Charlotte,
N. C.
48
-------
200
§
£ 100
i
945
23 July 1974
T
23 July 1974
950
955
1610
Tla*
1615
200
100
26 July 1974
1535
27 July 1974
1540
T1JM
1545
940
945
FIG. 24. Plots of NO concentration vs time for 3 different days at the Duke Power
Co., Charlotte, N.C. The peak-to-peak noise was 14 ppro. The symbol Q)
represents the data obtained with a DuPont Model 460-1 analyzer.
49
-------
20
c
o
o
B
O
U
10
1000
1010
Time
1020
20
8
a
o
jjt
'10
c
o
u
1440
1450
1500
1510
1520
Time
FIG. 25 Plot of CO concentration vs time of day for 25 June 1974 at Duke Power Co.
The peak-to-peak noise was 2.8 ppui.
Charlotte, N. C.
-------
the 3 day period of 25, 26 and 27 June 1974. Several months earlier we had
employed a small grating spectrometer to scan transmission spectra of the
same stack. From these spectra we estimated the average CO concentration to
be 15 ppm.
Data on the S0~ concentration are shown in Figure 26 for periods during
three different days. Agreement with the data by the DuPont instrument is
good. Effluent was extracted from the stack by the DuPont instrument and uv
spectroscopic methods were employed to determine the S0~ concentration.
During the beginning of the period of measurement on 26 June 1974, both
instruments indicated essentially the same concentration. After about 9:10,
the DuPont instrument indicated a nearly constant concentration while the
Philco-Ford instrument indicated that the SO,, concentration was decreasing
slowly. No satisfactory explanation for the difference is given.
The high level of noise observed while measuring S0« was due to the
PbSe detector being used. Ordinarily a liquid-nitrogen cooled PbS detector
is used for 802? however, the dewar for this detector had lost its vacuum
and could not be used. The detectivity of the PbS detector is only about
one-tenth that of the PbSe detector at 4 |im.
The noise-equivalent concentration of NO and CO were both approximately
twice as high as would be expected on the basis of the laboratory data repre-
sented in Table 2. For example, the noise-equivalent absorber thickness of
0.003 (atm 0^295) of NO corresponds to approximately 7 ppm for the 5.80 meter
two-way path across the stack at 422 K. Any one of several factors may have
attributed to the lower signal-to-noise ratios observed in the field: dirt
on the windows, turbulence and vibration, or improper alignment. Exposure
of the detector to sunlight during the alignment also probably increased the
detector noise for several hours after the exposure. This latter factor seems
quite probable since the detector did appear to behave abnormally for a while
after it had been exposed to the sun.
No field tests have been made for HCl or HF to date, although such tests
are planned by EPA personnel in the near future.
In general, the instrument performed quite satisfactorily during the
field tests. The sensitivity, detectivity, and discrimination against inter-
ference by other gases are more than adequate for most applications. As
indicated previously, the largest contribution to the measurement errors
probably comes from uncertainty in the zero setting. A few minor problems
were encountered, but these can be remedied easily. Among these were the
accumulation of dirt on the windows, binding of the window shutters, and
difficulties in changing the detector, or refilling it with liquid nitrogen,
without exposing it to the sunlight.
It is anticipated that additional tests performed with this versatile
instrument will provide information that is helpful in the design of simpler
gas-cell instruments.
51
-------
1000
to
c
o
4J
c
0)
o
C!
O
u
CM
O
25 June 1974 ;
-1530-
Time
T701
T7TO 10TKJ
27 June 1974
TOTD-
—roio-
Time
1530
1000
T
e
o
26 June 1974
0! L_
900
910
920
930
Time
940
950
1000
FIG. 26. Plots of S02 concentration vs time of day for 3 different days at Duke Power Co. in Charlotte, N.C.
The time during zero adjustment and checking is represented by the dashed lined in the lower figure.
The peak-to-peak noise was 120 ppm. The symbol ffi represents data obtained with the DuPont
Model 460-1 analyzer.
-------
SECTION 4
REFERENCES
1. Burch, D. E., and J. D. Pembrook. "Instrument to Monitor CH , CO, and
CO in Auto Exhaust." Prepared by Philco-Ford Corp. for EPA under
Contract No. 68-02-0587. EPA Report No. 650/2-73-030, Oct. 1973.
2. Ludwig, C. B., Bartle, and M. Griggs. "Study of Air Pollutant Detection
by Remote Sensors." Prepared for National Aeronautics and Space
Administration under Contract NAS12-630 by Convair Division of General
Dynamics. Report No. GDC-DBE68-011, Dec. 1968.
3. Bartle, E. R., S. Kaye, and E. A. Meckstrath. "An In-Situ Monitor for
HC1 and HF," AIAA Paper No. 71-1049, presented at the Joint Conference
on Sensing of Environmental Pollutants, Palo Alto, Calif., Nov. 1971.
4. Goody, R. "Cross-Correlating Spectrometer," J. Opt. Soc. Am. 58, 900
(1968).
5. Advances in Environmental Science and Technology. John Wiley and Sons,
Publishers, 1971. Edited by J. N. Pitts and R. Metcalf. Discussion on
gas cells appears in a section "Spectroscopic Methods for Air Pollution
Measurements," by P. L. Hanst.
6. Burch, D. E. "Adjustable Bandpass Filter Employing a Prism," Appl.
Opt. 8, 649 (1969).
7. Decker, J. A., Jr. "Experimental Realization of the Multiplex Advantage
with a Hadamard-Transform Spectrometer," Appl. Opt. 10. 510 (1971).
8. Abels, L. L., and J. H. Shaw. "Widths and Strengths of Vibration-
Rotation Lines in the Fundamental Band of Nitric Oxide," J. Mol.
Spectrosc. .20, 11 (1966).
9. Shaw, J. H. "Nitric Oxide Fundamental," J. Chem. Phys. 24, 399 (1956).
10. Burch, D. E., J. D. Pembrook, and D. A. Gryvnak. "Absorption and
Emission by SO. between 1050 and 1400 cm"1 (9.5 - 7.1 M-m ," prepared by
Philco-Ford Corp. for EPA under Contract No. 68-02-0013. Philco-Ford
Report No. U-4947, (ASTIA PB 203523), July 1971.
11. Benedict, W.S., R. Herman, G. E. Moore, and S. Silverman. "The Strengths,
Widths, and Shapes of Lines in the Vibration-Rotation Bands of CO."
Astrophys. J., 135. 227 (1962).
53
-------
REFERENCES (Cont'd.)
12. Benedict, W. S., R. Herman, G. E. Moore, and S. Silverman. "The
Strengths, Widths, and Shapes of Infrared Lines II. The HC1
Fundamental." Can. J. Phys. 34, 850 (;956).
13. Meredith, R. E., and N. F. Kent. "Line Strength Calculations for the
°~*"1' 0—*2, 0->-3, and l-*-2 Vibration-Rotation Bands of Hydrogen
Fluoride." Prepared for the Advanced Res. Proj. Agency under Contract
No. SD-91 by The University of Michigan. Report No. 4613-125-T
April 1966. '
14. Herget, W. F., W. E. Deeds, N. M. Gailer, R. J. Lowell, and A. H.
Nielsen. "Infrared Spectrum of Hydrogen Fluoride: Line Positions and
Line Shapes. Part II. Treatment of Data and Results." J. Opt. Soc.
15. Goody, R. M., "Atmospheric Radiation", 1964, Oxford University Press.
54
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BIBLIOGRAPHIC DATA
SHEET
1. Report No.
EPA-650/2-74-094
3. Recipient's Accession No.
5. Report Date
December 1974
Title and Subtitle
Infrared Gas Filter Correlation Instrument for in situ
Measurements of Gaseous Pollutants
6.
. Author(s)
D. E. Burch and D. A. Gryvnak
I. Performing Organization Kept.
No. U6121
Performing Organization Name and Address
Philco-Ford Corporation, Aeronutronic Division
Ford Road
Newport Beach, California 92663
10. Project/Task/Work Unit No.
1AA010/26AAP/14
11. Contract /Grant No.
EPA 68-02-0575
"2. Sponsoring Organization Name and Address
Chemistry and Physics Laboratory
National Environmental Research Center
Environmental Protection Agency
Research Triangle Park, N. C. 27711
^^Supplementary Notes
13. Type of Report Si Period
Covered Final Report
7/72 to 6/74
14.
infrared analyzer employing gas cell correlation techniques has been designed!
n constructed to measure the concentrations of carbon monoxide, nitric oxide, sulfur I
ioxide, hydrogen chloride, and hydrogen fluoride in the effluent of stationary sources.
n infrared beam is directed across the stack to a retroreflector and back so that the
"nstantaneous average concentration is measured continuously without disturbing the con-
tituents of the effluent. A small, removable, fixed-position grating monochromator acts
as a unique optical filter that passes narrow spectral intervals that are centered at
,avelengths where the gas to be detected will absorb. One grating monochromator is used
-.or CO and NO, another for S02 and HC1, and a third for HF. The useful ranges of concen-
ion times path length, in atm cm, over which each gas can be measured are: 0.005 to
. to NO; 0.0013 to 0.15 for CO; 0.001 to 4.0 for S02; 0.0003 to 0.2 for HC1 and 0.0001
-o 0.02 for HF. The discrimination against other gases in the effluent is excellent.
report was submitted in fulfillment of contract number 68-02-0575 by Philco-Ford
(-.orp.» Aeronutronic Div., under the sponsorship of the Environmental Protection Agency.
Key Words and Document Analysis. 17o. Descriptors
Exhaust Gases
NO, CO, S02, HC1 and HF
Infrared Analysis
Correlation Techniques
•|7b. Identifiers/Open-Ended Terms
Gaseous Pollutants
Gas Filter Correlation
Effluent Concentration
Stationary Source
C- COSATI Field/Group
, Availability Statement
Release Unlimited
19. Security Class (This
Report)
:LASSIFII
20. Security Class (This
Page
UNCLASSIFIED
21. No. of Pages
22. Price
NTIS-38 110-70)
USCOMM-DC 40328-P71
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