EPA-650/2-75-041
May 1975
Environmental Protection Technology Series
$$^
^
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EPA-650/2-75-041
INFRARED SENSOR
FOR THE REMOTE MONITORING
OF S02
by
E. R. Bartle
SAI, Inc.
P. O. Box 1393
La Jolla, California 92037
Contract No. 68-02-1208
ROAP No. 26AAP
Program Element No. 1AA010
EPA Project Officer: Dr. H. M. Barnes
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
U. S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
WASHINGTON, D. C. 20460
May 1975
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EPA REVIEW NOTICE
This report has been reviewed by the National Environmental Research
Center - Research Triangle Park. Office of Research and Development,
EPA, and approved for publication. Approval does not signify that the
contents necessarily reflect the views and policies of the Environmental
Protection Agency, nor does mention of trade names or commercial
products constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has been assigned to the ENVIRONMENTAL PROTECTION
TECHNOLOGY series. This series describes research performed to
develop and demonstrate instrumentation, equipment and methodology
to repair or prevent environmental degradation from point and non-
point sources of pollution. This work provides the new or improved
technology required for the control and treatment of pollution sources
to meet environmental quality standards.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-650/2-75-041
11
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ABSTRACT
A prototype passive infrared sensor for the measurement of sulfur dioxide
emissions from stationary sources is described. The infrared radiation
emitted by gases in a plume originating from smokestacks may be detected,
and from this the SC>2 concentration in the plume may be determined. In
general, the radiation received by the sensor is a function of the intervening
and background atmosphere. Thus, the problem of quantitative measurements
is generally complex. A technique is described, based upon the principle of
Gas Filter Correlation, which minimizes these effects.
This report presents a detailed description of the sensor, it's specifications,
and performance characteristics. The basic unit is battery operated and
weighs only 10 kgms; thus, it is readily portable. It's sensitivity is presently
limited to about 70 ppm-m for source plume temperatures of 270 C and about
290 ppm-m for temperatures of 170 C, but this can be improved.
The results of field testing at both oil and coal-burning power plants are com-
pared with extractive sample data. In general, the remote measurements
agree with the extractive data within ± 25 percent over SC>2 concentrations
ranging from 150 ppm to 1300 ppm from slant ranges of 130 to 400 m.
This report is submitted in partial fulfillment of contract number 68-02-1208
by JRB Associates, a division of Science Applications, Inc. under the spon-
sorship of the Environmental Protection Agency.
iii
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CONTENTS
PAGE
ABSTRACT Hi
TABLE OF CONTENTS iv
ILLUSTRATIONS v
TABLES vi
ACKNOWLEDGEMENT vii
SECTION
1 INTRODUCTION AND SUMMARY 1
Statement of the Problem 1
Brief Description of the Sensor 1
Definitions, Symbols, and Units 2
2 DESCRIPTION OF THE RGFC TECHNIQUE
FOR REMOTELY DETECTING SO0 4
£i
Phenomenology 4
Signal Calculations 6
Theory of the GFC Technique 9
3 SENSOR DESIGN AND PERFORMANCE 14
Mechanical-Optical Design 14
Electronics Design 16
Theoretical Performance 18
Laboratory Test Results and Calibration 21
Field Testing 29
Discussion of Field Test Data Reduction 32
Conclusions 34
4 REFERENCES 36
APPENDDC - Optimization of Gas Cell Parameters 37
Theoretical 37
Experimental 42
iv
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ILLUSTRATIONS
PAGE
FIGURE
1 Schematic of viewing geometry 5
2 Theoretically computed radiances of hot N« -diluted SOg 8
3 Schematic diagram of remote gas filter correlation sensor 10
4 Optical system schematic 15
5 Signal processing block diagram 17
6 Radiance difference detected by sensor (assumes chopper's
radiance is a 300 K blackbody) 20
7 Radiometric mode calibration for V2R channel 22
8 Radiometric data compared with theoretical values 24
9 Data obtained with sensor operating in GFC mode
(T = 270 C) 25
10 Data obtained with sensor operating in GFC mode
(T = 170 C) 26
11 Data showing sensitivity of ratio to SO2 temperature
and concentration 27
12 RGFC ratio mode calibration 28
13 Limiting sensitivity of the sensor for t = 90 seconds 30
14 Photograph of SO« remote sensor during initial field testing 31
15 Comparison of remote sensing data with extractive data
obtained from the DuPont analyzer and from EPA Method 6 33
A-l Effect of specifying and reference cell optical thickness
on sensor sensitivity 41
A-2 Theoretical calculation and measurement of Ng-diluted
SO« pressurized to one atm for SOg sensor 44
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TABLES
PAGE
TABLE
1 Summary of instrument parameters 19
2 Summary of remote SCL field measurements made at two
oil-burning power plants 29
3 Summary of. remote SO2 field measurements made at a
coal-burning power plant 32
vi
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ACKNOWLEDGEMENT
This report documents research and development performed by JRB
Associates, a wholly-owned subsidiary of Science Applications,
Incorporated, under Contract 68-02-1208 between 25 June 1973 and
28 February 1975. The work was sponsored by the Environmental
Protection Agency, Research Triangle Park, North Carolina. The
technical monitor was Dr. H. M. Barnes, Jr.
During this program valuable contributions were made by G. Houghton
(mechanical-optical design), L. Acton (optical analysis), E. Meckstroth
(electronics design, instrument assembly, testing, calibrations, and
field measurements), G. Hall (electronics consultation), Dr. W. Malkmus
(theoretical programming), and, especially, Dr. C. B. Ludwig (theoretical
calculations, field testing, data analysis, and reporting).
vii
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SECTION 1
INTRODUCTION AND SUMMARY
STATEMENT OF THE PROBLEM
The goal of the program was to design, fabricate, calibrate, field
test, and reduce the data for an infrared sensor that remotely measures
sulfur dioxide emissions from stationary sources. The design goals were
for the instrument to achieve the following performances:
Remote Range: 100-1000 meters (slant range)
Weight: less than 35 pounds
Volume: less than one cubic foot
SO2 Concentration Range: 100-1000 ppm
Accuracy: ± 5% for stack diameters of 1-10 meters.
The phenomenology for remote detection of pollutants emitted from
stacks is dependent upon specific spectral measurements of infrared radiation.
The infrared radiation emitted by gases in a plume originating from smoke-
stacks may be detected, and from this the SO2 concentration in the plume may
be determined. In general, the radiation received by the sensor is a function
of the intervening and background atmosphere. Thus, the problem of quanti-
tative measurements is generally complex. A technique has been developed,
based upon the principle of Gas Filter Correlation, which minimizes these
effects.
BRIEF DESCRIPTION OF THE SENSOR
The infrared sensor for the remote monitoring of SOo is based upon
the technique of Gas Filter Correlation described in Section 2 and elsewhere^ '
in detail. Briefly, it is a modification of the Non-Dispersive Infra-Red
(NDIR) technique that has been known for some time. It is based on the
concept that a sample of gas provides a selective filter for radiation absorbed
by a polluted mixture of atmospheric species. The radiation at the sensor
is chopped so that it alternately passes through two optical paths, one through
a cell containing the specific gas and one transparent. Thus, the radiation
is modulated only at the wavelengths at which the pollutant absorbs and high
specificity results.
It has been shown ^ that the signal generated by chopping between
two cells is a non-linear function depending upon the SO2 in the plume and
fixed instrument parameters and the difference between the radiance emitted
by the plume and background atmosphere. By ratioing two GFC signals
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obtained using different amounts of SO£ in the specifying cells, the effects
of plume and atmospheric radiance are greatly minimized. As is shown,
it is desirable to operate the sensor as a pure radiometer also. This is
accomplished by blanking off the radiance from passing through the specifying
gas cells.
Extensive laboratory testing was conducted and the gas cell para-
meters optimized. The sensitivity limits of the sensor were determined.
Field tests were conducted at two oil -burning power plants and one coal-
burning power plant under a variety of weather conditions. Extractive
samples were taken and analyzed using EPA Method 6; these results were
compared with the remote data and the two sets of data agreed within 25
percent.
DEFINITIONS, SYMBOLS, AND UNITS
a ratio of spectral line half-width to line spacing
aQ "a" divided by the equivalent pressure of the gas (atm )
f aperture adjustment parameter, see Equations (15) and (16)
Af noise bandwidth of the sensor (Hz)
f(u), F(u) defined by Equations (A-8) and (A-12)
k monochromatic absorption coefficient (atm -cm )
k mean absorption coefficient over a prescribed spectral interval
AX
* integration limits, see Equation (1) [length]
p pressure (atm)
t integration time (seconds)
u optical thickness (atm -cm or ppm-m)
x integration length
A, B defined by Equation (A-6)
o
Ad detector area (cm )
AQ sensor entrance aperture (cm^)
C gas volumetric concentration
C(to) normalized spectral definition function of the sensor
D* detector detectivity (cm-Hz^/W)
-------
o
E spectral energy arriving at sensor (W/cm -(/m-sr)
L optical pathlength (cm or m)
N blackbody radiance (W/cm^-jjm-sr)
R instrument responsivity (V/W cm'^-sr'1)
T temperature (°C or °K)
V voltage generated at the detector by the energy passing through
either the reference or specifying gas cell (V)
AV ac voltage generated by alternately passing the energy through
the two cells (V)
W X
"'£' defined by Equation (A- 17)
i, f->t
c emissivity of a particular radiator
77 sensor overall efficiency
X wavelength (jim)
AX spectral interval
r monochromatic transmittance of a particular instrument
component or gaseous species
o> wavenumber
v vibration transition parameter
&Q solid angle entrance to the sensor (sr)
The sub and superscripts used are self-explanatory in the text.
A bar over a parameter indicates that it is the mean value over a
spectral interval AX.
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SECTION 2
DESCRIPTION OF THE REMOTE
GAS FILTER CORRELATION TECHNIQUE
FOR REMOTELY DETECTING SO0
ti
PHENOMENOLOGY
To illustrate the problem of remotely detecting SO2 in stack plumes,
consider a sensor receiving energy from a plume with the sky as the back-
ground (see Figure 1). From the basic theory of radiative transfer^4), an
expression is developed which describes the monochromatic radiation re-
ceived by the sensor for the background, the effluent plume, and the inter-
vening atmosphere:
E<*> = TPra/
(Tb«)
Ox
dx
(1)
where the first term is the emission of the atmosphere from infinity to the
far edge of the plume, the second term is the emission of the plume, and
the third term is the emission of the atmosphere between the plume and the
sensor.
N (T(x)) represents the blackbody function at temperature T, which
is, in general, a function of x along the line of sight and, of course, is also
a function of wavelength X. The atmospheric transmission is indicated by
the terms ra and T^; it consists of the transmissivities of all of the normal
atmospheric species, i. e.,
x T
(2)
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Plume /Background
Intervening
Atmosphere
i i i i i 'i >• i j
X«0
FIG. 1. Schematic of viewing geometry.
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The transmissivity of the plume is similarly formulated
T = r (S02) x r (CO2) x r (H2O) x r (NgO) x f (CH4)x ... (3)
This may be written as Tp= TTJ, where T is the transmissivity due to S02
and TJ is the transmissivity of all interfering species. The SO2 transmissivity
T is given by
H
T = exp-/ k(X)C«pt (x)dx (4)
where k(X) is the spectral absorption coefficient of SO2, C is the unknown SO2
concentration in the plume and pt is the total pressure of the plume.
Consideration of Equations (1-4) shows that the task of quantifying the
SO2 concentration in the plume is complex. The radiation received by the
sensor is a function of the atmospheric and plume temperatures as well as
the emissivity of SO2 and interfering species in the atmosphere and plume.
In principle, it appears that a number of simple radiometric measurements
over carefully chosen wavelength intervals and a suitable computer program
may be used to obtain quantitative results. However, this is a very complex
procedure and generally not sufficiently accurate. Therefore, we have
devised a technique which is independent of the plume and atmospheric tem-
peratures, and which does not require a computer to reduce the data.
SIGNAL CALCULATIONS
Sulfur dioxide possesses many infrared-active bands, two of which
are the most promising ones. These are the v± + v% combination band
centered at about 4 urn and the v-± fundamental band centered at about 8. 6 ym.
Although the band at 8. 6 urn is about 4 times stronger, it is heavily inter-
fered by water vapor and— to a lesser extent— by CH4, N2O and 03. On
the other hand, only very weak interference occurs at 4 ym due to the presence
of N2O, CO2 and CH4 and the H2O continuum. In addition, more sensitive
IR detectors are available for operation at 4 um than at 8. 6 ym. Thus,
measurements at 4 nm are preferred.
In order to gain insight into the spectral emission levels of hot
smokestack plumes containing SO2, we have calculated the radiance and
emissivity of plumes, having various optical depths and temperatures.
These calculations were performed with our line-by-line computer program,
whose input parameters were taken from the AFCRL atlas of atmospheric
lines and from data we have generated under different contracts^1).
6
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From the listing of the SC^ line parameters, it is determined that
this band consists of many thousand lines. Besides SOj lines, there are
those of N2O and a few CO2 and CH4 lines. Water is present through the
continuum, which is composed of the tails of strong lines originating in
the 2. 7 um and 6. 3 urn band systems.
In order to gain a better overview about the distribution and strength
of the SO2 band, we have generated the band model parameters, k and an
averaged over 5 cm-1 intervals. They are based on the statistical band
model with exponential line strength distribution, viz. ,
r(S02) = exp[-ku/(l+ku/4a)'1/2J (5)
where u is the optical thickness (= CptL) and a = aope where pe is the equi-
valent pressure for N2 broadened SO2.
It is found that the SO2 band at 4 jum is quite weak. It's band strength
is only 22 cm^atm'1 at 300 K. In comparison, the f i - SO2 band at 8 7 Wm
is about 4 times stronger, the CO fundamental band at 4. 6 urn is about 10 times
stronger and the CO2 band at 4. 3 ym is over 100 times stronger. Thus, the
emission of the hot smokestack gas is relatively weak at low SO2 concentrations.
By multiplying the emissivities with the blackbody functions, the ap-
propriate radiances are obtained. These calculations have been made for the
We have also used our computer program to calculate the typical
transmission between a smokestack and an observer on the ground. For the
conditions
Height of smokestack: 54 m
Horizontal Distance: 122 m
Relative Humidity: 85%
Temperature: 16 C
Concentration of N2O: 0. 3 ppm
Concentration of CO2: 320 ppm
Concentration of CH4: 1. 4 ppm
the transmission becomes 0. 9979, which indicates an insignificant loss due
to atmospheric absorption. Even if the atmosphere were heavily polluted by
S02, the transmission is decreased only slightly. As an example, 100 pnb
of SO2 reduces the transmission to only 0. 9977.
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10
10"
Optical Thickness, ppm-m
FIG. 2. Theoretically computed radiances of hot N0-diluted SO0.
t> £i
8
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THEORY OF THE GAS FILTER CORRELATION TECHNIQUE
Since 1969, we have been developing the Gas Filter Correlation
(GFC) technique^1-3) which is a modification of the Non-Dispersive Infra-
Red (NDIR) technique that has been known for some time(5X In contrast
to pure radiometry or dispersive spectroscopy, a GFC (non-dispersive)
device uses the gas itself to obtain the ultimate high-spectral resolution
filter (provided by the natural line-width of the gas). High spectral re-
solution is the most important parameter in obtaining specificity and
accuracy in pollutant analysis.
Even quite narrow spectral bandpass radiometers are low-resolution
instruments and specificity is difficult to obtain.
High-resolution instruments depend upon finding a single line of
the pollutant to obtain specificity, but this necessitates the use of narrow
apertures. Thus, sensitivity is difficult to obtain.
GFC combines the high energy throughput feature of radiometers
and the high-resolution features of dispersive instruments. It makes use
of the contributions of all spectral lines of a band system of a particular
species to obtain sensitivity. Specificity is obtained by making use of
random correlation between spectra arising from the particular species
and interfering species; the principle of random correlation has been
establishedU-o; for most pollutant species and interfering species occurring
naturally and in polluted atmosphere. In addition, a ratioing technique may
be employed that eliminates effects of changes in source intensity, back-
ground radiation, and continuum absorption due to aerosols, water vapor,
or other molecular species.
A remote GFC sensor consists basically of a single detector, a
light chopper, a lens, a gas cell containing SOo and a reference cell (see
Figure 3).
The chopper alternately passes the entering radiation through the
gas cell and the reference cell. When the chopper is in the position indicated
in Figure 3, the signal generated at the detector is
Vl = /{Er2ToT3 + NcT2TrT3 + €lN?T2roT3 + ins} RdX
AX
where T, N and e refer to transmissivity, blackbody function and emis-
sivity respectively; the numerical subscripts are indicated in Figure 3
and ins refers to the instrument which is maintained at a constant
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Chopper
Detector
To
Tr
*
f
*
e
Tt^t n«ii
\
•
EM \
vA;
Front Window
2 C
FIG. 3. Schematic diagram of remote gas filter correlation sensor.
10
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temperature; R is the overall responsivity of the detector, optical effi-
ciency and electronics. All symbols are, of course, a function of X ,
but this is not noted for the sake of brevity.
Similarly, when the chopper blocks off the gas cell, the signal
generated at the detector is
2 = /{EVoT3 + NcT2TrT3 + ClN?T2ToT3 + ins} RdX
The signal generated at the frequency of the forward chopper is the
difference between V^ and Vj:
AV = /T2T3 tTl E + el Nr N
c
AX
The instrument is balanced by replacing the incoming energy E by
two calibration blackbody sources of temperatures TBB and TBB and r
is adjusted such that AVT =AV*T • This implies fr « f0 where
iBB1 1BB2 °
the bars denote mean values over the interval AX . But, since the values
anc* ^ TQO are not electronically zero, in general, the value AV
662
given in Equation (8) is always referenced against level AV T-.-. or AV
Since R, T% , and 73 are only slowly varying functions of X , an overall
effective responsitivity Ro may be defined by
Ro = T2 T3 R
where the bar denotes the mean value over the interval AX . Thus,
AV
l E(X) + C]L Nj(X) - N^(X)] [T - T 00] dX
AX
where E(X) is given by Equation (1). e.,N°(X) and N°(X) are effectively
eliminated through the balancing procedure; thus, c
g = /E(X)[rr-ro(X)]dX
0 AX
11
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When E(X) given by Equation (1) is introduced into Equation (10) and the
integration over AX carried out, one obtains, using the mean value theorem
AV - 7 N^ (f -
-- i b v a
a a
) (f T -
' v r
r -7ft)
r o
(11)
Since by balancing the instrument, irosrrt Equation (11) may be
simplified:
AV _ - r-
R- :
(12)
Thus, an expression results that shows the ac signal is effectively
a product of a modulation function that is only related to the SO2 transmissivity
and fixed instrument transmissivities and of the difference between the ra-
diance emitted by the plume and background atmosphere.
If we now consider a second cell pair with TQ" t r"p and chopped at a
different frequency, but using the same detector and optical components, a
similar expression is derived:
f = VaNp-
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temperature if the gas cell and plume temperatures are the same. Any
mis-match in these temperatures will cause only relatively small errors
in determining SO2 concentration due to the second order effects of tem-
perature on the absorption coefficient of SO2 and changes in spectral slope
of the blackbody function with temperature.
The signal as a function of SO2 concentration in the plume can be
adjusted depending upon the amounts of SO2 in the specifying cells. In
addition, we have observed that the addition of -a small amount of SC>2 to
the reference cell will greatly enhance the sensitivity. In this case the
signal is given by
TTn9 - FT f0
c _ v* _ r 2
where fj and f2 are the dimensionless aperture attenuators used to balance
or zero the sensor; viz.
= frf2 and
The optimum values for rJJ, r~02, and r were studied in detail.
These results are presented in the Appendix.
13
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SECTION 3
SENSOR DESIGN AND PERFORMANCE
MECHANICAL-OPTICAL DESIGN
The two-cell GFC pairs are contained in a single optical system as
illustrated in Figure 4. Separate tuning fork choppers are used for each cell
pair. The cells are 10 cm in length. The image of the stack plume is
focused by an f/3 lens on a 1. 0 x 1. 0 mm detector which defines the field-
of-view to be about 8 milliradians (8 m at 1 km). An ambient temperature
operation (ATO) PbSe detector is used; but, a single-stage thermoelectric
cooler is used to provide temperature control. A passband optical filter is
located in front of the detector. This filter is centered at 4. 00 microns
with a half-transmission width of about 0.1 microns.
Sapphire optics, antireflection-coated, are used. A transmission
of greater than 0. 9 is obtained. The objective lens has a diameter of 5 cm
and a focal length of 15 cm.
Section A-A of Figure 4 shows the configuration of the dual split cell.
The two gas cells contain different partial pressures of SO2 and are pres-
surized to 1 atm with pure N£ to pressure-broaden the SO2 lines. The re-
ference cell contain a lesser amount of 803 pressurized to one atm with pure
N2.
The tuning fork choppers, made by American Time Products (Bulova)
are stable, reliable and low power. Because the frequency is dependent only
on the mechanical resonance of the fork, no stable-frequency AC power source
is needed and battery operation is possible, as is true of the entire electronics
system.
Section B-B of Figure 4 shows the dual tuning fork configuration.
Fork-1 is shown closed, allowing radiation to pass through the AV-1 reference
cells; Fork-2 is shown open, allowing radiation to pass through the AV-2 gas
cell. Fork-1 operates a frequency of 40 Hz and Fork-2 at a frequency of
100 Hz. Because a single lens serves both AV-1 and AV-2 systems and super-
imposes the image of each on the same detector, both systems have exactly
the same field-of-view at the stack plume, as is essential for proper can-
cellation of the stack effluent temperature factor. The superimposed image
signals of the two AV systems are electronically separated by signal processing
described later.
A third tuning fork chopper, operating at 800 Hz, is located immediately
ahead of the detector aperture. This chopper obstructs the entire beam when
14
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Entrance Window
B » A
r
Dual Split Cell -
U\ L
Windows
Objective Lens
Plane of AV Tuning
Fork Choppers
AV-1
Section B-B
AV-2
T. E. Cooler
PbSe Detector
4. 0 nm Filter
800 Hz Tuning Fork
Rel
Gas-1
Ref
•f
Rel
Gas-2
Ref
Section A-A
FIG. 4. Optical system schematic.
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closed. The purpose of this chopper is to eliminate the low frequency 1/f
detector noise from the AV signals.
Because of the small (8 mr) field-of-view, accurate aiming is
essential. A 4-power Bushnell riflescope, accurately aligned with the
GFC optics, is an integral part of the instrument. An illuminated retical
in the riflescope permits accurate aiming at dusk or under other adverse
lighting conditions.
ELECTRONICS DESIGN
An electronic signal processing technique that involves phasing,
gating (sample-hold) and ratioing of different frequency signals has been
developed. A block diagram of the electronics is shown in Figure 5.
In this instrument, two AV signals are used resulting from the use
of two gas/reference cells systems containing different partial pressures
of SOo. Each cell pair is chopped by a tuning fork chopper of different
frequency. Because of the common objective lens the image at the detector
consists of both AV signals at their respective chopping frequencies. These
are later separated by electronic filtering. The frequencies are 40 Hz and
100 Hz. These are not harmonically related and are far enough apart to
allow good separation by filtering, yet are low compared with the 800 Hz
"carrier" system created by the third chopper, described below. This
tuning fork chopper, operating at 800 Hz, is located immediately in front
of the aperture plate and obstructs the entire light beam when closed. The
detector signal is therefore the 800 Hz "carrier" signal amplitude modulated
by the two AV chopper frequencies.
The detector is followed by a preamp and a 800 Hz bandpass filter.
The filter passes the 800 Hz signal and its modulation side-bands at 700,
760, 840 and 900 Hz while rejecting the DC component of the detector output
which is due to the bias current. Thus, even though low frequency AV
chopping is needed in order to obtain large-amplitude fork oscillations,
only the detector noise at the 800 Hz passband is processed, and the large
1/f detector noise at the low AV chopping frequencies is rejected and high
effective detector D*'s are obtained.
The filter output feeds a lock-in amplifier. Receiving its reference
signal from the 800 Hz fork driving circuit, the lock-in synchronously detects
the 800 Hz wavetrain and provides a DC output on which are superimposed
the two AV signals.
16
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Bias
Preamp
0
PbSe
Detector
Thermal
Control
800 Hz
40 Hz
100 Hz
800 Hz
Tuning
Fork
Drivers
Filter
Lock-In
Amp
Phase &
Gating
Control
40 Hz
Filter
100 Hz
Filter
Lock-In
Amp
Lock-In
Amp
Low-Pass
Filter
Divider
Low-Pass
Filter
JL
Visua
Read-Out
AV2AV2AV1
Recorder
Output
FIG. 5. Signal processing block diagram.
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THEORETICAL PERFORMANCE
The signal-to-noise ratio for a GFC radiometer is given by
nr A O D*AE AX
SNR = ° °° (17)
where
77 is the instrument efficiency
•F is the transmissivity through the SO2 gas cell
A is the entrance aperture of the gas cell (cm )
O is the instrument acceptance solid angle (sr)
D* is the detector detectivity (cm-Hz1'2/W)
2
A, is the detector area (cm )
Af is the electronic bandpass (Hz)
AE is the difference in radiance between the instrument
2
gas and vacuum cells (W/cm -y-sr)
AX is the bandpass of the optical filter
A figure of merit for any radiometric instrument is the noise-
equivalent-spectral-radiance which is given by Equation (17), setting
SNR = 1; thus,
NEN = — - — - - , W/cm -sr (18)
A summary of the instrument parameters is given in Table 1.
Calculations of the radiance integrated over the actual filter band-
pass were made. The results are presented in Figure 2. Operating as a
pure radiometer (channel 2), the difference between these radiances and
the radiance of the forward chopper is indicative of the signal to be detected.
For example, if the chopper is emitting as a 300 K blackbody, the radiance
difference is plotted in Figure 6. Since the theoretical NEN for Channels 1
and 2 are 1. 22 x 10-7 and 8. 14 x 10-8W/cm2sr, respectively, the theo-
retical radiometric sensitivities are on the order of 150 ppm-m and 100 ppm-m
for a temperature of 350 K.
18
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TABLE 1
SUMMARY OF INSTRUMENT PARAMETERS
Parameter
Tl jfc
T **
O
f, Hz
A 2
AQ, cm
n , sr
D*, cm Hz /W
2
A,, cm
Af, Hz
2 ***
NEN,W/cm -sr
Channel 1
0.37
0.44
100
1.54
6. 4 x 10"5
q
2. 7 x 10y
io-2
2.8x 10"3 to 0.25
1.221x 10"7
Channel 2
0.37
0.66
40
1.54
6.4x 10"5
q
2. 7 x 10y
io-2
2. 8 x 10"3 to 0. 25
8. 143 x 10"8
77 Twindow x Tgas cell window x Tlens x Tfilter x ^electronics
= . 98 x (. 90)2 x . 95 x . 5 x . 98 = 0. 37
** See Appendix.
-3
*** Assumes integration time = 90 seconds; Af = 1/4 t = 2. 8 x 10 Hz.
19
-------
330-81
CO
o
|*r,(S02', T S) - Nu(300 K)
-ILL
10
SO, Optical Thickness, ppm-m
FIG. 6. Radiance difference detected by sensor (assumes chopper's radiance is a 300 K blackbody).
-------
LABORATORY TEST RESULTS AND CALIBRATION
Testing of the sensor was performed to determine the optimum
specifying and reference cell SO2 concentration. The optimum concen-
trations and corresponding transmissivities are:
uQ1 = 5. 0 atm-cm TQ- = 0. 44
uQ2 = 2. 0 atm-cm f™ - 0. 66
u = 0. 5 atm-cm 7 = 0. 88
r r
Measurements were made of the signal and noise of the sensor,
while operating in the radiometric mode and in the GFC mode.
A Barnes Model 1140T Field Source was used to calibrate channel 2
(V2R) operating as a pure radiometer. The results, covering a temperature
range from 50 to 225 C, are presented in Figure 7. Note, these results are
for the sensor normalized to—- Gain = 10 and Attenuation = 10.
In order to compare the experimental data with theoretical pre-
dictions, we have to determine the instrument responsivity. The radio-
inetric responsivity RQ is defined through
V2R = R02 C(w)[N(w, T) - N(W, TQ)] dco (19)
Aw
where V is the radiometer signal in volts, Ro is the responsivity in V/
Wcm-^sr'1, C(w) is the normalized filter function, and N°(w, T) and
N°(w, TC) are the blackbody functions of the calibration source and chopper
blade, respectively.
From the measurements of V2R, the radiometric responsivity was
determined —
RQ2 = 7. Ox 103V/Wcm"2sr"1 (20)
The measured peak-to-peak noise level with an integration time of
90 seconds is 5 x 10~6 V. This is equivalent to 5 x 10~3/5 (= 10-3 Vrms),
assuming a factor of 5 to convert peak-to-peak random noise to rms noise;
i. e. , 99 percent of the noise energy exists within a voltage range of 5 x
Vrms- T1\f theoretically predicted noise, given by the NEN in Table 1, is
8. 14 x 10-° x 7. 0 x 10J = 5. 7 x 10'4 Vrms. Thus, the sensor's noise limit
is about two times higher than predicted. The actual rms noise measure-
ments converted into radiance is 7. 1 x 10~8W/cm2-sr. This defines the
minimum detectable SO2 optical thickness.
21
-------
FR/+FH
O.Ot-
50
FIG. 7. Radiometric mode calibration for Vor, channel.
ZJtx
22
-------
Measurements were also made of N2-diluted SC>2 mixtures heated
to 170 and 270 C in a 50 cm calibration cell with 7. 5 cm dia. sapphire
windows (transmissivity = 0. 88). The experimental data corrected for
window emission are plotted in Figure 8; also shown are the theoretically
predicted values obtained by multiplying Ro(= 7 x 10^ V/Wcm'^-sr"*)
with the radiance differences given in Figure 6. Excellent agreement is
obtained.
Thus, by using the curves presented in Figure 2, the radiance of
the choppers, and the empirical value of Ro (Equation (20)), the SC>2 optical
thickness may be determined from field radiometric measurements, if the
plume temperature is known, or vice-versa.
Operating in the GFC mode, the sensor is first balanced by adjusting
the reference cell apertures such that the same AV signal levels are obtained
when viewing the field source at temperatures of 350 and 500 K. Note, this
is not necessarily electronic zero. After balancing, measurements were
made of N2-diluted SC>2 mixtures heated to 170 and 270 C in the 50 cm cali-
bration cell.
Preliminary measurements were made with a small (0. 25 x 0. 25 mm)
detector which provides a sensor fov of 2 mrads. The initial measurements
were made with pure N2 in the reference cells and with UQI » 4. 3 atm-cm
and UQ2 w 1. 3 atm-cm. Tests were conducted by using the laboratory atmo-
sphere as the background and by placing a dry ice block behind the cell
simulating a cold sky background; no discernible differences in the data
were observed in the two test procedures.
Experimental data of AV^ and AV2 were obtained at temperatures of
270 C and 170 C and are reproduced in Figures 9 and 10. The abscissa is
the optical thickness of SO2 in ppm-m. In dividing AV2 by AVj, the ratio
which is independent of the gas temperature, is obtained. The results are
shown in Figure 12. As expected, the results are independent of the tem-
perature. These experiments demonstrate the viability of the ratio technique
for the measurement of hot SO2 emission without knowing the gas temperature.
The resulting calibration data (Figure 11) were used to interpret field mea-
surements at two oil-burning plants, as described in the following section.
Latter field measurements and laboratory experiments showed that
the AV2/AVj ratio could be made more sensitive by optimizing the gas-
reference cells' SO2. optical thicknesses. Using near optimum values, a
calibration curve was obtained (see Figure 12) that was applicable to field
measurements at a coal-burning power plant.
23
-------
N .
si
ii
I
^ rheoretical Data '••
FIG. 8. Radiometric data compared with theoretical values.
24
-------
10 20 SO
SO Optical Thickness, ppm-m x 10"
50
FIG. 9. Data obtained with sensor operating in GFC mode (T = 270 C).
25
-------
0.20
0.16
CO
? 0.12
•a
c
> 0.08
0.04
0 *
Gain = 10
Att. = 10
10
20
30
40
SO2 Optical Thickness, ppm-rm x 10
-3
AV
AV.
50
FIG. 10. Data obtained with sensor operating in GFC mode (T = 170 C).
26
-------
O T = 270 C
A T = 170 C
FIG. 11. Data showing sensitivity of ratio to SCL temperature and concentration.
27
-------
K-S
•CMI-COOAHtTHMte 40 4973
CO.
r r
. S
CO
CO
; ; : : ;-p rr
-; ;ii!ii i>i> il;
10*
U (SO2), ppm-m
FIG. 12. RGFC ratio mode calibration.
-------
From the measurements of signal and rms noise of the sensor, its
limiting sensitivities as a function of plume temperature have been estab-
lished. The results are given in Figure 13. These results are based on
an integration time equal to 90 seconds and, for the radiometric mode,
assume the chopper radiates as a blackbody at a temperature of 300 K; they
are based upon the condition when the signal-to-rms noise ratio equals one.
FIELD TESTING
Preliminary field measurements were made using the 0. 25 x 0. 25 mm
detector and non-optimized gas cell parameters at two oil-burning power
plants near San Diego, CA. A photograph of the first site is shown in
Figure 14. A summary of these measurements is presented in Table 2.
Extractive measurements analyzed using EPA Method 6 were made during
all tests except the first one.
TABLE 2
SUMMARY OF REMOTE SO2 FIELD MEASUREMENTS
MADE AT TWO OIL-BURNING POWER PLANTS
Date
3/25/74
5/20/74
5/21/74
5/21/74
5/23/74
Range
130 m
185 m
185 m
130 m
130 m
Stack
Dia.
4.25 m
4.0m
4.0m
4.0m
4.0m
Fuel
oil
oil
oil
oil
oil
Temp.
650 K
410 K
420 K
420 K
440 K
SO2 Cone.
Range
~ 200 ppm
350-450 ppm
260-340 ppm
170-230 ppm
260-340 ppm
Weather Conditions, Time
20 C, calm, partial clouds, p. m.
21 C, light wind, clear, p.m.
26 C, light wind, partial clouds, p. m.
26 C, light wind, partial clouds, p. m.
22 C, calm, clear, dusk
The data taken on 3/23/74 are consistent with a nearly steady SO2 concen-
tration in the stack of about 200 ppm, which was a calculated value based
upon the known sulfur content of the fuel oil'''. (No extractive data were
available during these first tests.) However, the accuracy at these low SO2
concentrations is low (see calibration curve in Figure 11). Radiometric
mode data were also taken and reduced, using laboratory calibration curves
and the measured in-stack temperature; the results indicate a concentration
range from 150 to 210 ppm SO«.
The data taken on 5/20, 5/21 and 5/23 were obtained at the second
site. In these cases the plume temperatures were lower, as seen by the
data in Table 2, and the signal levels were correspondingly low, giving
the larger indicated uncertainties in the data. Extractive data were taken
29
-------
K-C SEW-IOOABITHMIC « CYCLES X 70 DIVISIONS
C. (turret, t CMC* eo. ..«..»>.
46 6013
650
600
300
10
102"
SO2 Optical Thickness, ppm-m
F'G. 13, Limiting sensitivity of the sensor for t = 90 seconds.
-------
FIG. 14. Photograph of SCX, remote sensor during initial field testing.
31
-------
and analyzed following EPA Method 6; these data indicated SO2 concen-
trations ranging from 30 to 50 ppm. However, the data were not believed
reliable because of a leak in the sampling apparatus. Furthermore, the
power plant company assumed 200 to 300 ppm were being emitted^,
which is consistent with the observed results.
Field measurements were also made using the 1. 0 x 1. 0 mm detector
and nearly-optimized gas cell parameters at a coal-burning power plant near
Charlotte, N. C. A summary of the measurements is presented in Table 3.
TABLE 3
SUMMARY OF REMOTE SO2 FIELD MEASUREMENTS
MADE AT A COAL-BURNING POWER PLANT
Date
••' ' •
6/25/74
6/26/74
6/27/74
6/27/74
6/27/74
—
Range
^MMM.U_
210
170
170
4CO
210
m
m
m
m
m
Stack
Dia.
2.
2.
2.
2.
2.
0
0
0
0
0
m
m
m
m
m
Fuel
coal
coal
coal
coal
coal
Temp.
425 K
425 K
425 K
425 K
425 K
SO2 Cone.
Range
700-1300 ppm
700-1300 ppm
500-700 ppm
500-700 ppm
500-700 ppm
Weather Conditions, Time'
32 C,
30 C,
24 C,
24 C,
24 C,
light wind, scattered
light wind, cloudy, a.
calm, foggy, a. m.
calm, foggy, a. m.
calm, rain, p.m.
— — — _
clouds, p.
ro. and p.
Both radiometric and GFC mode of operation data were taken. In
addition, extractive data were taken and analyzed following EPA Method 6
and continuously monitored by a DuPont Model 460-1 Analyzer (8). All of
the data are presented graphically in Figure 15. As seen, all of the data
are in agreement within + 100 ppm.
DISCUSSION OF FIELD TEST DATA REDUCTION
The data presented in Figure 15 were reduced by measuring the
individual AVj and AV2 signals because the signal levels were too low for
the electronic divider module to function properly. In addition, the sensor
was not perfectly balanced during the measurements. This means the
terms (c. f. Equation (11))
N° f (r -T )
p a v r o'
and
are not zero. Of these two terms the first term is much larger than the
second term, and, the data were corrected by subtracting the contribution
32
-------
1200
1100
1000
900
s
800
700
600
500
6/25/74
o
•
0°
•
o
13 14 15 16 17
HOUR
6/26/74
9 10 11 12 13 14 15
HOUR
6/27/74
O DuPONT
A METHOD 6
O REMOTE CfC
o
9 10 11 12 13
HOUR
.SO
.55
.60
.W2
.65
.70
.75
FIG. 15. Comparison of remote sensing data with extractive data obtained
from the DuPont Analyzer and from EPA Method 6.
33
-------
due to the first term. This contribution was measured in the laboratory
using a blackbody field source both before and after the field measurements.
The majority of the measurements were averaged over about a 15
minute time period. In cases where the two AV signals were changing rapidly
with time, the analyzed data was erratic and, thus, not used.
In the GFC mode, two principle sources of error arise. The first is
due to the basic sensor sensitivity (signal-to-noise ratio) and the second due
to the temperature dependent correction applied because of sensor imbalance,
as noted above.
In the first case, the SNR averaged about 13 and 25 for channels AV2 and
AVi, respectively. Thus, for u = 1600 ppm-m, AV^/AVj = 0. 55, and in terms
of error, this is equivalent to
AV2 _ 0. 55(1 + 1/13)
1.0(1 ±1/25)
A
which gives
AV2
-g^- = 0.55 ±0.095 (22)
Or, u = 1600 ± 350 ppm-m and, at Location 2, this gives an uncertainty of
± 220 ppm in SOg concentration.
In the second case, it is believed that the plume temperature was
known within + 25 C. Note, in the cases where radiometric data were taken,
it is believed to be known within ± 10 C. For temperature uncertainties of
+ 10 and +25 C, the equivalent uncertainties in 563 concentration are about
+ 70 and + 175 ppm, respectively (on the average).
In summary, since both of these sources of uncertainties may be
considered to be random, the maximum RSS uncertainty is about + 280 ppm.
CONCLUSIONS
The GFC dual channel technique has been proven that it can remotely
determine SO2 concentrations in hot plumes with only minimal effects due
to plume temperature and backgrounds. However, the prototype sensor
was found to have certain limitations. These are:
34
-------
1. Limited sensitivity due to detector noise.
2. A susceptibility to becoming unbalanced, which
necessitates rather large temperature corrections.
3. Lack of provision for in-field rebalance adjustment
capability.
4. Subject to erratic behavior due to overheating when
operating in direct sunlight on very warm days.
5. The electronics divider module used does not permit
electronic ratioing of the AV signals because of low
signal levels.
6. A fov (8 mrads) that is larger than desired; 2 mrads
would be better.
The deficiencies above were corrected by a modified design; this
was done under EPA Contract 68-02-1696.
35
-------
SECTION 4
REFERENCES
1. NASA Contracts 12-2109, 1-10466, 1-11111, and 1-12048.
2. Ludwig, C. B. et al., "Remote Measurement of Air Pollution
by Nondispersive Optical Correlation", AIAA Paper No. 71-1107,
November 1971; AIAA Jr. 11, 899, 1973.
3. Bartle, E. R. et al., "An In-Situ Monitor for HC1 and HF", AIAA
Paper No. 71-1049, November 1971; J. Spacecraft and Rockets,
11, 836, 1972.
4. Chandrasekhar, S., Radiative Transfer, Dover Publications, Inc.
New York (1950).
5. Considine, D. M., Editor., Process Instruments and Controls
Handbook, McGraw-Hill, New York (1957).
6. "Determination of Sulfur Dioxide Emissions from Stationary Sources",
Federal Register 36, 24890, December 1971.
7. Hardway, J., SDG&E (private communication), March 1974.
8. The data obtained by the DuPont Model 460-1 Analyzer were provided
by Dr. W. Herget of EPA.
36
-------
APPENDIX
OPTIMIZATION OF GAS CELL PARAMETERS
THEORETICAL
A comparison of the two GFC calibration curves (Figures 11 and 12)
shows a large increase in sensitivity to changes in SO2 optical thickness
when SO2 is added to the reference cell. The signal-to-noise is actually
decreased, but the sensitivity obtained when chopping between two cell
pairs containing different optical thicknesses of SO2 and ratioing is much
greater than when chopping between two cell pairs, one of each containing
different optical thicknesses of SO2 and reference cells containing N«.
For the latter case, the ratio is given by Equation (15); viz.
rrn9 - r £„
_ . _02 _r2
1 TTorTTr£l
Equation (A-l) reduces to Equation (14) when
V2 = f02 and Vl = ?01 •
The mean value for transmissivity is given by
" dX = ^ f± .UfL [k(X)f u-
- e dX = . [k(X) uOX (A-2)
AX AX n=0
where u(cm-atm) is the optical thickness and k(X)(cm"1-atm"1) is the
spectrally dependent absorption coefficient.
Since u is independent of wavelength,
=
n!
n=0
Using Equations (A-l) and (A-3) for the case where rr = 1 (ur = 0)
37
-------
= [l-k(u
AV = l-k(u+u ) +
O ft \j u O I
(A-4)
IF
- ku + . u2 - . u3
c\ f\
Simplifying, and considering only second order terms in u, gives
AV = u[uo(7-k2)- n02
-------
Considering now the case where there is gas in the reference cell
of optical thickness, Uj., and transmissivity, rr, similar expressions can
be derived. For this case, the modulation function, AV', is given by
AV' = TT -TT(?o/f) (A-lO)
Performing a similar series expansion to the second order in u gives
[•^w ""I f*
-KuI / s1 c> o o 1 ~o 5" }
r I/ \ /, £• T \ i £> £\tl\f:-,4 . O» I
e J = u|(uo-ur)(k -k )-(uQ -ur )(2-)(kk -k ) + .. .j
(A-ll)
Again, considering the second cell pair, and forming the ratio between
the two signals results in
_ 'Uo2 - V
Differentiating Equation (A-12) with respect to u gives
Flu) = (U°2 " "r ) (Uol " U02}
W (u -u \ ' ~ - 7
%1 Ur} [A/B - (uol + ur + u)]2
Dividing Equation (A-13) by Equation (A -9) shows the relative effect
in terms of sensitivity for the two cases; i. e. ,
. (U02 - V . .
-
Equation (A- 14) can be examined to show the increase in sensitivity
to u in terms of the various cell optical thicknesses. Also, the fundamental
equations, (A-7) (A-8), (A-ll) and (A-12) can be examined to show the effect
on signal levels.
The constant A/B was evaluated using the data presented in Figure 12.
The results are as follows.
39
-------
A/B (cm-atm)
8.6
14.6
9.1
Evaluated at
F(0) = 0. 95
F(u = 2000 ppm-m) = 0.47
F'(0) = -2. 5 cm"1-atm"1
This evaluation indicates A/B is not a constant and that the sim-
plified analysis inadequately describes the true performance of the sensor.
Nevertheless, calculations were made, assuming A/B = 12 cm-atm. The
results are presented in Figure A-l. These curves indicate the dramatic
improvement in sensitivity as the reference cell's optical thickness is in-
creased. This can be seen by examination of Equation (A-14) since when
U01 + u + ur = A/B
(A-15)
the ratio of the sensitivity with u * 0 and sensitivity with u_ = 0 (= F'(u)/f'(u))
becomes infinite.
However, as was pointed out, this simplified analysis does not
adequately describe the sensor's performance. This is principally due
to not considering the higher order terms in the series expansion.
A similar analysis has been performed to include third order terms.
The resulting equation is
s Uo2-ur 1+(u02+Vu)w+uo2urX + ^of^oZ"^^* (uo2+ur)uZ
Uol"Ur MuQl-Hir+u)W+ uQlurX + (uQ1 +uolur-Hi
2 2;
(uol+ur)uZ
The constants W, X, Y and Z have been evaluated by solving a 4 x 4
matrix using determinants developed from fitting the results presented in
Figure 12. Their definitions and numerical values are given below.
W =
y _
Y =
Z =
—JT —K —K- 2
(kV -kd)/(k^-k ) = -0.3118
° ~5 J
)/(k^ - k ) = -0. 0260
k ) = 0.0343
k ) = 0.0435
(A-17)
(A-l
40
-------
too
F'(l. 8) = oo
I __-L
000 ppm-m
., cm-atm
FIG. A-l. Effect of specifying and reference cell optical thickness
on sensor sensitivity.
41
-------
Using these values, the calibration data presented in Figure 12 is des-
cribed within i 2 percent over the range of 100 to 5000 ppm-m of SOp.
Calculations of AV for u = 1, 2, 3,... 10 atm-cm and u = 0, 0. 5,
1. 0, 1. 5, and 2. 0 atm-cm have been made. These results show how the
addition of SC>2 to the reference cell affects the slope, but also reduces the
signal intensity. It is calculated that at large specifying cell optical thick-
nesses, the signal levels are only reduced by about a factor of 2, even for
large (2 atm-cm) amounts of SO£ in the reference cell. It should also be
noted that the results for intermediate specifying cell optical thicknesses
(4 and 6 atm-cm) and reference cell optical thicknesses greater than 1 atm-cm,
do not appear to be physically reasonable. This indicates that the use of the
analytical expression is not quantitative enough. Apparently even higher order
terms in the series approximation should be included. Nevertheless approxi-
mate optimum cell conditions have been calculated.
The expression for F(u) = AV2/AVJ has been differentiated and
numerically evaluated for u = 1000 ppm-m. These results show the rapid
increase in sensitivity as the reference cell's optical thickness is increased
and the optimum value for the second specifying cell's optical thickness.
Furthermore, they indicate that the sensitivity, F'(U), continues to increase
as un-i increases.
Since the length of both specifying cells in the sensor is 10 cm and
it is undesirable to pressurize the SOo beyond one atmosphere due to pressure
broadening the 803 lines, the practical maximum optical thickness for UQI
is 10 atm-cm. For this condition, the calculations indicate the optimum value
for the second specifying cell's optical thickness is about 4. 5 atm-cm.
We have also calculated the values for the reference cell's optical
thickness that forces F'(u) - » for u = 1000 ppm-m; the result is u =
1. 8 atm-cm.
In summary, calculations made using the third order series expansion
analysis indicate the sensor would have optimum performance with
UQ- = 10 atm-cm
U02 = ^'
u = 1. 8 atm-cm .
r
EXPERIMENTAL
Measurements have been made using the sensor to determine SO2
transmissivity as a function of optical thickness. The sensor was operated
in the radiometric mode by blocking off the reference cell apertures. The
42
-------
lower specifying cell was evacuated, and the signal (I ) recorded; then,
pre-mixed SOo diluted with N2 was admitted to the ceft at one atmosphere
pressure and the signal (I) recorded. The ratio I/IO is, of course, the
transmissivity. The results of this experiment was presented in Figure
A-2. Also shown are the results generated by a line-by-line computer
program using the actual filter function of the sensor.
Laboratory calibrations of the sensor operating in the GFC mode
are difficult. We used a 50 cm long cell with sapphire windows on both
ends providing a clear aperture of 6. 2 cm. The windows are uncoated
and have a transmissivity of 0. 88. The maximum entrance aperture
dimension is (0. 775 + 0. 867) (2. 54) = 4.17 cm. Locating the sensor
100 cm away from the far end window of the cell gives a projected maxi-
mum dimension of 4.17 + (. 008) (100) = 4. 97 cm and, thus, no direct con-
tribution is given by the cell walls. However, three major difficulties
arise in the calibration.
One. Multiple reflections internal to the cell and near field-of-view
thermal non-uniformities generally give unreliable AV signals.
Two. The cell is heated by external strip heaters, cold pre-mixed
SO2-N2 test gas mixtures are admitted to the cell and the temperature
monitored by a thermocouple in the cell's interior. It is difficult to main-
tain the same temperature as different SO2-N2 concentrations are sequentially
admitted to the cell and maintained at one atm pressure. Since the tempera-
ture strongly affects the magnitude of the AV signals and relatively long time
constants (30 seconds) are required to get adequate signal-to-noise, the data
tend to be erratic.
Three. Field data give AV signals that are larger than those simu-
lated in the laboratory and indicate a greater sensitivity to changes in SC>2
optical thickness.
The most desirable methods for calibrating the sensor appears to
be the use of field data or to use an artificial stack. For SC>2 optical thick-
nesses greater than 5000 ppm-m, laboratory measurements generally give
repeatable results if the temperatures and concentrations of test gas mix-
tures are carefully controlled.
*
In attempting to simulate the theoretically predicted optimum gas
cell parameters, it was discovered that for UQJ = 10 and ur = 1. 8 cm-atm
of SO2, respectively, the throughput (and signal) of the sensor was reduced
by 70 percent (see Figure A-2) and the aperture adjustment did not have
enough travel to balance the sensor.
43
-------
N .'
E-
5!
s ~~T
FIG. A-2. Theoretical calculation and measurement of N« -diluted
pressurized to one atm for SO« sensor.
44
-------
Within the limits of the sensor's balancing adjustment and the rc-
pcatablc signals generated using the calibration cell, the apparent optimum
cell parameters were determined and are summarized below.
Ref erence
Parameter Upper Cell, AV2 Lower Cell, AVj
Chopping frequency 40 Hz 100 Hz 40 and 100 Hz
Length 10 cm 10 cm 23 cm
SO2 Concentration 17. 2 % 47. 2 % 2. 2 %
Optical thickness 2. 0 atm-cm 5. 0 atm-cm 0. 5 atm-cm
Transmissivity 0. 66 0. 44 0. 88
Note, the sensor's construction is such that the two 10 cm long
specifying cells and the 23 cm long reference cell actually give effective
u0's that are the sum of the SC«2 pressure in the specifying cell times 10 cm
plus the SO2 pressure in the reference cell times 13.
45
-------
TECHNICAL REPORT DATA
(/'lease read /HSl/uctions on the reverse he jure completing)
1. RtPOHT NO. 2.
EPA-650-2-75-041
4. TITLE ANDSUBTITLE
Infrared Sensor for Remote Monitoring c
/. AUTMOH(S)
E. R. Bartle and E. A. Meckstroth
9. PERFORMING ORGANIZATION NAME AND ADDRESS
JRB Associates, Division of SAI
1200 Prospect Street
P. O. Box 2351
La Jolla, CA 92037
3. RECIPIENT'S ACCESSIOI*NO.
5. REPORT DATE
May 1975
'f SOn 6. PERFORMING ORGANIZATION CODE
fu
8. PERFORMING ORGANIZATION HEPOFU NO.
10. PROGRAM ELEMENT NO.
1AA010
11. CONTRACT/GRANT NO.
68-02-1208
12. SPONSORING AGENCY NAME AND ADDRESS 13. TYPE OF REPORT AND PERIOD COVERED
National Environmental Research Center Final Report
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A prototype passive infrared sensor for the measurement of sulfur dioxide
emissions from stationary sources is described. The infrared radiation
emitted by gases in a plume originating from smokestacks may be detected,
and from this the 803 concentration in the plume may be determined. In
general, the radiation received by the sensor is a function of the intervening
and background atmosphere. Thus, the problem of quantitative measurements
Is generally complex. A technique is described, based upon the principle of
Gas Filter Correlation, which minimizes these effects.
This report presents a detailed description of the sensor, it's specifications,
and performance characteristics. The basic unit is battery operated and
weighs only 10 kgms; thus, it is readily portable. It's sensitivity is presently
limited to about 70 ppm-m for source plume temperatures of 270 C and about
290 ppm-m for temperatures of 170 C, but this can be improved.
The results of field testing at both oil and coal-burning power plants are com-
pared with extractive sample data. In general, the remote measurements
agree with the extractive data within + 25 percent over SO% concentrations
ranging from 150 ppm to 1300 ppm from slant ranges of 130 to 400 m.
This report is submitted in partial fulfillment of contract number 68-02-1208
by JRB Associates, a division of Science Applications, Inc. under the spon-
sorship of the Environmental Protection Agency.
17. KEY WORDS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
1. Remote Sensing
2. SCvj Measurements
3. Gas Filter Correlation
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