EPA-650/2-73-027
October 1973 ENVIRONMENTAL PROTECTION TECHNOLOGY SERIES
I
1
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EPA-650/2-73-027
AN INSTRUMENT
FOR SIMULTANEOUS MONITORING
NOx AND S02
IN STATIONARY SOURCES
by
Huel C. Tucker and Joseph Cheng
Monsanto Research Corporation
Dayton Laboratory
Dayton, Ohio 45407
Contract No. 68-02-0554
Program Element No. 1A1010
EPA Project Officer: Fred Jaye
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, N.C. 27711
Prepared for
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
October 1973
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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.
ii
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ABSTRACT
A Monsanto Model 3^09 Chemilumlnescent Ambient Air Monitor
was converted to a two-channel configuration for simulta-
neously monitoring NOX and S02 in stack gases. Channel
separation was obtained by means of narrow-band optical
filters. The analog sample-hold part of the automatic
zero circuit was replaced by a digital memory circuit. A
permeation dryer was included to remove water from the
sample. The atomic oxygen source was changed to an ozone
generator-thermal decomposition configuration, which failed
to provide sufficient oxygen for S02 or N02 analysis. A
microwave oxygen generator provided adequate quantities of
oxygen but was rejected because of its unreliability.
iii
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CONTENTS
Page
ABSTRACT ill
CONTENTS iv
FIGURES v
TABLES vi
ACKNOWLEDGEMENTS vii
SECTIONS
I. INTRODUCTION 1
II. RESULTS OF EXPERIMENTAL WORK 3
III- CONCLUSIONS 5
IV. TECHNICAL DISCUSSION 7
Design 7
Sampling System 9
Oxygen Generators 11
Reaction Chamber 12
Pneumatic Circuit 12
Electronic Modifications 16
Programmer 16
Amplifier 20
Connecting Wiring 22
RESULTS OF EXPERIMENTAL WORK 22
Permeation Dryer 22
Oxygen Generator 22
APPENDIX 33
iv
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FIGURES
No. Page
1 Sample Probe Schematic 10
2 Ozone Generator Photograph 13
3 Decomposition Chamber Photograph 13
4 Reaction Cell Photograph 14
5 Detector Assembly 14
6 Pneumatic Schematic 15
7 Clark Circuit Schematic 17
8 Logic Circuit Schematic 18
9 Solenoid Driver Circuit Schematic 19
10 Amplifier Circuit Schematic 21
11 Response to NO and N02 with Ozone Generator 24
12 Schematic for Microwave Excitation 29
13 Response to S02 With Microwave Excitation 30
14 Front Panel Layout 31
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TABLES
No. Page
1 Response to NO, S02 With Ozone 25
2 NO Response With Ozone 26
3 Response to N02 27
4 Response to S02 With Microwave 28
vi
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ACKNOWLEDGEMENTS
Significant contributions were made to this project by
G. W. Wooten, Dr. A. D. Snyder, C. M. Ellas, and D. J. David
of Monsanto Research Corporation and Dr. F. J. Jaye of EPA.
vii
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SECTION I
INTRODUCTION
Three major air pollutants are NO, N02 and S02. Each may be
monitored with available commercial instruments. However,
no one instrument is capable of analyzing for all three
simultaneously. Simultaneous monitoring of all three
pollutants requires two or three instruments depending on
whether one of them has the capability to directly monitor
NOX, which is the sum of the NO and N02 present. Since NO
and N02 are usually lumped as NOX, the Environmental Pro-
tection Agency (EPA) deemed it to be desirable to develop
an instrument to simultaneously monitor S02 and NOX for coal-
fired stack emission monitoring.
Previous work in the area of chemiluminescence indicated
that the reaction of nitric oxide with ozone could be used
in relatively straightforward equipment to measure ambient
air levels of NO and N02. There are a number of N02-to-NO
converters in use with such ambient air equipment.
Work in the stationary source area was somewhat complicated
by the presence of side reactions that can create or destroy
NO or N02 in these converters.
The work by G. W. Wooten using microwave-produced atomic
oxygen instead of ozone indicated that the reaction
N02 + 0 >• NO + 02
could be used instead of an external converter to monitor
total NOX. Analogous S02 chemiluminescent reactions could
also be used to measure S02 at the same time.
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The potential desirability of a dual S02-NOX monitor for
certain stationary sources prompted this contract effort.
The modification of an experimental chemiluminescence in-
strument which was designed for low-level NOX analysis was
decided upon as the most expedient approach to the design
of such a dual analyzer. The design goal was to develop an
instrument for continuously and simultaneously monitoring
NOX and S02 at high levels at remote locations with minimal
attention from the operator.
Evaluation of the original instrument showed that the analog
sample hold circuits used for zero drift correction were not
adequate over long time periods. With the recent advances
in digital integrated circuitry in terms of functions and
cost, we decided to rework the auto-zero circuitry to use
digital components.
Concurrently with the initiation of this program, a sample
drying system based on selective water permeation became
commercially available. Since removal of condensable water
was required for the proper operation of the modified in-
strument, we undertook to incorporate this new dryer into
the sampling system.
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SECTION II
RESULTS OF EXPERIMENTAL WORK
The ability of the permeation dryer to selectively remove
water from a sample stream was confirmed. However, no
effort was made to establish the operating parameters and
their limitations.
The automatic zero circuit employing a digital memory was
successfully demonstrated. This technique extends the
usefulness of all-electronic automatic zero circuitry to
include applications requiring memory retention times
measured in days or even weeks as opposed to the 10 to 15
minute maximum memory retention time of analog memories.
All attempts to simultaneously measure NOX and S02 using
an ozone generator with a thermal decomposition chamber to
generate atomic oxygen failed. The response of the instrument
to NO was adequate and relatively easy to achieve. The
response to N02 was inadequate for a measuring circuit having
no background noise compensation. N02 response was difficult
to obtain and required that all parameters be optimized. No
response to S02 was observed when generating oxygen by this
method. It was not established if the response existed or
if it was merely hidden in the noise of the system. It was
established that the response to NO was destroyed by the
Introduction of small amounts of S02.
The previous work where response to S02 and N02 was reported
was confirmed by a test using a microwave generator and a
cavity to generate atomic oxygen. Again, the response to
NO was easy to obtain. An adequate response to S02 was
obtained only by careful and methodical optimization of all
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parameters of the reaction chamber. No effort was made to
explain the effects of the various parameters.
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SECTION III
CONCLUSIONS
A chemilumlnescence instrument based on atomic oxygen to
simultaneously monitor stationary emission sources for NOX
and S02 is impractical with the present state of the art.
Further clarification of the requirements of the chem-
iluminescence reaction of atomic oxygen with various com-
pounds such as N02 and S02 is required before any serious
consideration can be given to further development of such
an instrument.
Although the instrument can be made to function adequately
over a short time period by using microwave excitation to
generate atomic oxygen, the mechanical instability of the
microwave cavity-to-oxygen line system, the projected short
life of the microwave tube, and the uncertainties related to
maintaining a plasma discharge over a long period of time
make that approach unacceptable for a continuous monitor.
Either the ozone generator/thermal decomposition method for
making atomic oxygen failed to produce sufficient oxygen, or
an unknown reaction between S02 and another compound present
had a quenching effect on the chemiluminescence reaction.
The saturation effect observed with N02 indicates that the
NO measurement can be made in the absence of S02 by using a
second photomultiplier to compensate for background variations
by limiting the flow of the sample into the reaction chamber
to keep it within the linear range.
The successful application of a digital memory to an automatic
zero circuit represents a significant advance in the state
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of the art pertaining to programmed field instruments. It
can replace high maintenance servo systems in almost any
application where the inherent drift of an analog memory is
unacceptable. It may also be useful as a long-term sample-
and-hold amplifier.
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SECTION IV
TECHNICAL DISCUSSION
DESIGN
The overall design goals were outlined by parts A and C of
the following Scope of Work statement, included as Exhibit
A in the contract:
"A. The contractor shall modify the existing Monsanto
Model 3409 ambient air NOX monitor to simultaneously
measure stationary source S02 and NOX emissions in
the following ranges:
S02 - 0-200 ppm, 0-500, 0-1000, 0-3000 ppm
NOV - 0-200 ppm, 0-500, 0-1000, 0-3000 ppm
A.
The unit shall have equivalent responses to NO and
N02 or mixtures thereof without the use of a con-
version device.
B. Tasks required in this conversion include, but may not
be limited to:
1. Replacement of differential signal processing
circuits and replacement with two independent
channels including independent automatic zero
correction circuits and manual span adjustment
for both channels.
2. Removal of discharge atomic oxygen generator and
replacement with ultraviolet ozone generator with
small thermal decomposition furnace downstream
(1" diameter, 2" long, 800°C) or microwave genera-
tor.
3. Replacement of reaction chamber with two port
chamber for simultaneous S02 and NOX measurements.
4. Replacement of optical filter on second photo-
multiplier with filter centered at 3510 A, % width
50 A.
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5. The unit shall retain the internal automatic zero
correction and calibration functions.
C. The contractor shall provide a sampling system suitable
for operation at a coal-fired steam electrical power
plant. This sytem shall include particulate removal
and other sample preconditioning as required by the
covered prototype.
D. The contractor shall calibrate the instrument for S02,
NO and N02 in the specified ranges and establish inter-
ference ratios for 500 ppm CO, 3% 02, 100 ppm C2H6, IQ%
H20, 15% C02, 100 ppm N20, and 500 ppm NH3.
E. The converted monitor and sample system shall be deliver-
able. The contractor shall document and blueprint all
changes. The contractor shall provide fifty (50) copies
of a design and operation manual and a set of engineer-
ing drawings which will be sufficient to enable the
manufacture of items of equipment furnished under this
contract (other than components or items of standard
commercial design or items fabricated heretofore) by a
firm skilled in the art of manufacturing items of equip-
ment of the general type and character of the items or
equipment furnished under this contract or a set of flow
sheets and engineering drawings which will be sufficient
to enable performance of any process developed under
this contract by a firm skilled in the art of practicing
processes of the general type and character of good
process. Such set or sets of drawings and flow sheets
shall be reproducible copies incorporating all changes
made in the equipment or process in the form in which
it was delivered to the Government."
Parts B, D and E in the above statement pertain to more
specific details to be included in the overall design. Design
decisions were based on this statement but were influenced
by additional information supplied by the contract monitor.
The design goal of up to 3000 ppm of each NOX and S02 was
modified to 3500 ppm sum of NOY and S02 on the assurance of
J\,
the monitor that that capability would be sufficient.
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Sampling System
The requirements of the sampling system were set forth in
parts C and D of the Scope of Work statement, above. Further
clarification by the contract monitor established the need
for removing much of the water from the sample at the point
of exit from the stack to prevent condensation as the water-
laden sample gas cooled enroute to the analyzer. Because of
the highly reactive nature of N02 and S02, any condensation
in the sample system would have a drastic effect on these
analyses. At the recommendation of the monitor, a permeation
dryer was obtained and tested for its effects on S02 and N02.
Simple test procedures were used where 3502 ppm S02 and
11350 ppm N02 were passed through the dryer and analyzed to
determine any changes in the concentrations. There was no
detectable change in S02 and no more than 2% change in the
concentration of N02 where the water content of the drying
gas was held low. However, it was found that where the
water content of the drying air was high, the effect on the
concentrations of the S02 and N02 at the exit of the dryer
was drastic in that much of the S02 or N02 was not passed
by the dryer.
A sampling probe was designed utilizing a Model PD-500-40
permeation dryer purchased from Permapure Products, Inc. With
reference to the schematic shown in Figure 1, the sample gas
is pulled through the permeation tube via a coarse filter at
the end of a 6 ft long, 1/2 inch stainless steel pipe designed
to be inserted into a stack. After exiting the dryer, the
dry gas is routed through flexible tubing to the analyzer.
Dry air, obtained from a bottled supply or through a dryer,
is pulled through the vacuum compartment via a valve, VI, by
pump, PI, and discharged through a solenoid valve, SV2, to
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Dry Air Inlet
Filter
1/2",6'Long S.S. Pipe/
Flexible Tubing to Analyzer
Wooden Enclosure
Vent
Figure 1. Sample Probe Schematic
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atmosphere. During the "zero" and "standard" cycles, where
a bottled zero or standard gas is being accepted by the
analyzer, the tubing is blocked and SV2 closes, SV1 and SV3
open, routing the effluent from pump, PI, in a reverse
direction through the permeation tube, probe, and filter.
This procedure should remove some particles collected on the
filter, thereby minimizing the maintenance required for
cleaning it.
Oxygen Generator
The design of the oxygen generator was the most critical
phase of the project. The two approaches that were con-
sidered feasible were (1) the direct disassociation of 02
using microwave power and (2) the indirect method of generat-
ing ozone and thermally decomposing it. A survey of the
ozone generators available revealed that only the first
method could produce sufficient concentrations of atomic
oxygen to accommodate the high concentrations of pollutants
expected. However, agreement was reached with the contract
monitor that, considering the present state of the art, the
solution of the problems to be incurred in the use of a
microwave generator in an instrument of this type where
continuous, unattended operation was required was beyond
the scope of this project.
An investigation revealed that commercial ozone generators
were available. Previous work reported that linear results
could be obtained by the ozone/thermal decomposition methods
to well over 100 ppm NO in N2. The reported signal level
X
at 100 ppm was adequate to provide a readout for the instru-
ment. Therefore, it was decided to design the oxygen
generator using the ozone/thermal decomposition method, and
11
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to dilute the sample gas with air to reduce the pollutant
concentration to an acceptable level. An Orec Model 03V5-OM
ozonator was purchased and a thermal decomposition chamber
designed to provide atomic oxygen. The generator is shown
in Figures 2 and 3.
Reaction Chamber
The reaction chamber was designed to mix the sample stream
with a stream containing the atomic oxygen. It provides
observation ports at opposite ends for two photomultipliers.
There is a relatively long path for the mixed stream to
exit to the vacuum pump as shown in Figures 4 and 6. The
chamber is housed in a light-tight box as shown in Figure 5.
The flow rates of both sample and oxygen/ozone are controlled
by stainless steel needle valves to provide the proper mixing,
A by-pass valve provides control of the pressure in the
chamber.
Pneumatic Circuit
The plumbing for the instrument is shown by the schematic in
Figure 6. All fittings, tubing, valves, etc., are stainless
steel, glass, quartz, or inert plastic. The pump, P2, pulls
the sample and the diluent air past the inlet to the reactor.
The mix of air and sample is controlled by valves V5 and V2,
respectively. The flow of the diluted sample into the vacuum
reaction chamber is controlled by V6. Valves V8 and V9 are
manipulated to obtain optimum flow of oxygen through the
ozone generator and to maintain near atmospheric pressure in
the ozone generator. Atomic oxygen is generated as the
ozone passes through the heated quartz tube. The chem-
iluminescence occurs within the inner tube as the two gas
12
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Figure 2. Ozone Generator
Figure 3. Thermal Decomposition Chamber
13
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Figure
Reaction Chamber
Figure 5. Detector Assembly
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VJ1
Air Inlet—
V-5
Standard_
Gas Inlet
Sample
Inlet
Zero Gas
Inlet -
FI-2
PI
(\
V-6
Fl-l
Optical Filter-1
I
PMT-1
Reaction
Chamber Optical Filter-2
-^—
Vent
iip- v-9
I
Exhaust
V-8 0- Supply
Figure 6. Pneumatic Schematic
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streams mix. The mixture remains within the field of both
photomultipliers as it passes from the point of entry at the
center through the inner tube to either end and back to the
exit point at the center through the outer tube. The pressure
can be controlled by the by-pass around the vacuum pump, P3.
Electronic Modification
The electronic circuit modifications were divided into three
parts. First, the programmer modifications required some
circuit changes but no major design. Second, the amplifier
circuits required a complete redesign. Third, the connecting
wiring required partial redesign.
Programmer - The programmer consists of a clock circuit
(Figure 7), a logic circuit (Figure 8), and a solenoid driver
circuit (Figure 9). The clock consists of a 100 kHz oscil-
lator and a five-decade counter circuit for generating a one-
second clock pulse. A second counter circuit is programmable
for a divide by 1-99 counter to establish the basic time
unit for the pin board programmer on the front panel. As
wired, the basic time unit is 60 seconds. Thus, a function
programmed to remain on 15 time units will actually have a
900 second duration. The programmer is designed to reset
itself at a programmable time. The auto zero cycle can be
programmed to occur at any time within the interval of the
total cycle and to have any desired duration. The electronic
auto zero function is on only during the last time unit
(60 seconds) of the auto zero cycle. The standard gas cycle
can be programmed to occur at the beginning of each.Nth cycle
where N is a number greater than 1. A typical program would
have a auto zero cycle of 10 time units (600 sec.) occurring
16
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CLOCK IOAIO 1
CLOCK BOARD 2
Vcc Ground
I.CNo. Pin No. Pin No.
1
7
8
9
10
11
12
13
I
14
14
14
14
14
14
14
SP MO A
SN7440N
SN7440N
SN7440N
SN7400N
SN7404N
SN 7402 N
SN7400N
Figure 7. Clark Circuit Schematic
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co
SN 74.04 N
SN74.WN
SN 74LM N
SN74IBN
SN74.10N
SN7400N
SN7400N
SN7CON
Figure 8. Logic Circuit Schematic
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•H4V
H
VO
+24V
Figure 9. Solenoid Driver Circuit Schematic
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each cycle of 2^0 time units (4 hours) with a 10 time unit
(600 sec.) standard cycle each 6 cycles (24 hours).
Amplifier - The amplifier circuit was completely redesigned
and new printed circuits made. Identical channels were
provided for each of two photomultipliers. Because the
intervals between the auto zero cycles were long, a digital
memory circuit was incorporated to prevent drift normally
associated with capacitor leakage in analog memory circuits.
The circuit is shown in Figure 10. The amplifier consists
of three stages. The first is a LM308A operational amplifier
with adjustable gain and a front panel zero adjustment. The
second stage is a LM301A with a front panel gain adjust and
a zero control input from the auto zero circuit. The third
stage is a LM201A amplifier with a switchable gain on the
front panel and a zero adjustment on the printed circuit
board. The output of the second stage also goes to the input
of a LM201A acting as a comparator such that its output always
has the opposite polarity of the second-stage amplifier and
is much greater in magnitude. Therefore, when the second-
stage amplifier output is not zero, the output of the com-
parator is large. The comparator drives a Schmitt trigger
(SNT^ISN) which in turn drives a logic circuit controlling a
two-stage, binary up-down counter whose digital output drives
a digital-to-analog (D-A) converter (DAC372-8). Upon being
enabled by a signal from the programmer or a switch on the
front panel, a 100 pulse per second clock signal is routed
through the logic circuit to the up-down counter, which
counts up or down depending on the polarity of the comparator.
The analog output of the D-A converter is fed to one input
of the second stage LM301A such that the LM301A output is
always driven toward zero. The system responds much as a
slightly oscillatory servo system in that the LM301A output
20
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»ONT PANEL
L C
470 <
Figure 10. Amplifier Circuit Schematic
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oscillates ± one bit around zero. By making one bit in-
significantly small, the output of the LM301A can be forced
to zero for all practical purposes. Upon removal of the auto
zero signal, the counter is disabled, but it retains its
digital output. Therefore, the analog zeroing signal from
the D-A converter is maintained until the circuit is again
activated.
Connecting Wiring - The connecting wiring was modified as
required. A new harness was made for the front and rear
interconnections and the mother board was cut and patched
where necessary. The new wiring diagram is included in
Figure 10.
EXPERIMENTAL WORK
Permeation Dryer
The ability of the Model PD-500-40 Perma Pure dryer to pass
S02 and N02 unchanged was determined. Water vapor, N02 and
S02 at various known levels of concentration were introduced
into the sample inlet of the dryer while the outlet was
monitored to detect losses in transient. It was found that
significant losses were incurred unless the drying air in
the outer shell was much dryer than the sample. Almost total
loss of N02 was experienced under wet air conditions. How-
ever, no detectable change in S02 and only a small 1-2% change
in N02 was found when dry air was used.
Oxygen Generator
The Orec Model 03V5-OM Ozonator failed to operate satisfac-
torily within the recommended input current range of 0-150 mA
22
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but did function at 175-250 mA. Sustained operation at that
level produced no detectable ill effects. It was found that
the pressure affected the operation of the unit. Optimum
performances seemed to occur at 0-2 psig.
The modification of the monitor was delayed for some time
awaiting delivery of the optical filters. The 6500 ± 20&
filter was received and installed in mid-November 1972. A
test was run to test the response to NO with the results
shown in Figure 11. No effort was made to calibrate the
instrument at that time since the 3400& filter had not been
delivered.
In view of the considerable delay in the delivery of the
3400& filter, a Fish-Schurman UG-1 filter was ordered and
received late in January 1973. After demonstrating that
linear results could be produced by microwave excitation to
produce atomic oxygen, an effort was again made to obtain
positive results using the ozone/thermal decomposition method.
Again, no detectable response was obtained for S02. The
response to N02 is shown in Figure 11.
Although the UGI filter was inferior to the T-5, Infrared
Industries filter,, it was useful in evaluating the performance
of the instrument. On February 1, 1973 an effort to detect
S02 failed. The gain of the amplifiers was increased to a
point where the noise level was detectable and the results
tabulated in Table 1 were obtained.
As a result of the failure to detect S02, a test was devised
using a microwave generator to generate atomic oxygen as
shown in Figure 12. The results of tests are shown in
Figure 13.
23
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50
Qi
8
CT>
TO
30
20
=3
o
10
0
1550 ppm NO, 02 Flow: 200 ccm
1550 ppm NO, 02 Flow: 300 ccm
1550 ppm NO, 02 Flow: 500 ccm
4350 ppm N02, 02 Flow: 200 ccm
4350 ppm N02, 02 Flow: 300 ccm
4350 ppm N02, 02 Flow: 500 ccm
0
5 10 15
Sample Flow Rate, cc/min
20
Figure 11. Response to NO and N02 with Ozone Generator
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Table 1. RESPONSE TO NO, S02 WITH OZONE
Conditions:
Reaction Chamber Pressure:
Ozone Generator Current:
02 Flow Rate:
Decomposition Heater Temperature:
Samples: 1550 ppm NO in N2
280 ppm S02 in N2
1050 ppm S02 in N2
1-5 torr
250 mA
200 cc/min
800 - 900°C
NOX Filter:
S0? Filter:
6500 ± 50)
3^00 ± 50J
•J-J-^GJ. . JTUU J. ~j\jn.
(S02 filter0transmission is appreciable
above 6000A)
NOX photomultiplier: EMI 9781-B
S02 photomultiplier: RCA Ip 28
NOX gain:
S02 gain:
0.50
maximum
State of
Sample Input
Ozone Gen. off
No sample
Ozone Gen. on
No sample
Ambient Air
NO
S02 (280 ppm)
S02 (1050 ppm)
S02 (1050 ppm) + NO
NO
S02 (1050 ppm) + NO
S02 (1050 ppm) + NO
NOX Output,
mV
(1000 mV fs)
± 5
5 ± 5
30 ± 10
550 ± 10
30 ± 10
30 ± 10
30 ± 10
250
30 ± 10
30 ± 10
Flow Rate
of Sample,
cc/m
0
0
100
100
100
100
50 ea.
50
50 ea.
50 NO, 5 S02
S02 Output
mV
± 15
5 ± 15
20 ± 20
110 ± 20
20 ± 20
20 ± 20
30 ± 20
50 ± 20
30 ± 20
30 ± 20
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Table 2.
NO RESPONSE WITH OZONE
Conditions:
Ozone Generator Current: 225 mA
Ozone Generator Chamber Pressure;
Decomposition Heater Volts: 70
Sample: 1550 ppm NO in N2
Span Gain: 4.00
Ozone Generator Input: 02
Range Switch: 0-3000 ppm
1-5 psig
Reaction Chamber
Pressure,
torr
1.2
1.2
1.2
1.2
1.2
1.3
1.3
1.3
1.3
1.3
1.5
1.5
1.5
1.5
1.5
°2
Flow Rate,
cc/min
200
200
200
200
200
300
300
300
300
300
500
500
500
500
500
Sample
Flow Rate,
cc/min
0
25
50
75
100
0
25
50
75
100
0
25
50
75
100
Output, %
(full scale)
0
20
30
40
50
0
23
36
42
53
0
19
28
35
44
26
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Table 3-
RESPONSE TO N02
Conditions:
Ozone Generator Current: 225 mA
Ozone Generator Chamber Pressure:
Decomposition Heater Volts: 90
Sample - 1*350 ppm N02 in N2
Span Gain - 4.00
Ozone Generator Input - 02
Range Switch 0 - 3000 ppm
1.5 psig
Reaction Chamber
Pressure,
torr
0
1-3
1.3
1.3
1.3
1.3
1.3
1.3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
02
Flow Rate,
cc/min
200
200
200
200
200
200
200
300
300
300
300
300
300
300
500
500
500
500
500
500
500
Sample
Flow Rate,
cc/min
0
5
10
25
50
75
100
0
5
10
25
50
75
100
0
5
10
25
50
75
100
Output, %
(full scale)
0
5
7
8
8
8
8
0
6
9
12
12
12
12
0
5
8
13
15
15
16
27
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Table 4. RESPONSE TO S02 WITH MICROWAVE
Conditions:
Sample: 3500 ppm S02 in N2
Span Gain: 3.10
Range: 0-1000 ppm
Microwave
Power,
watts
50
50
50
75
75
75
02 Flow,
cc/min
120
153
225
120
153
255
Sample Flow
to Reactor,
cc/min
0
5
10
20
0
5
10
20
0
5
10
20
0
5
10
20
0
5
10
20
0
5
10
20
Output
Reading,
% Full Scale
0
18
28
43
0
14
27
44
0
10
22
45
0
21
32
49
0
17
30
50
0
12
25
50
28
-------
ro
vo
Optical
Supply Cylinders,
^Pressure Regulators
and Indicators
To Vacuum Pump
Flow Indicators
SO;
NO
Figure 12. Schematic for Microwave Excitation
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50
O)
0
• 120 ccm 02, 50 watts
* 153 ccm 02, 50 watts
° 255 ccm 02, 50 watts
* 120 ccm 02, 75 watts
153 ccm 02, 75 watts
• 255 ccm 02, 75 watts
5 10 15 20
Sample Flow Rate, cc/min
Figure 13. Response to S02 With Microwave Excitation
30
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Zero
Sample
Standard
aooo
Range
A3
OA1
A2
*-
A1
„„
-*
Standard |™
Auto Zero S™
Recycle
Cycles of
Std. Gas
0123456789
oooooooooo
oooooooooo
oooooooooo
oooooooooo
oooooooooo
oooooooooo
Hundreds
0123456789
oooooooooo
oooooooooo
oooooooooo
oooooooooo
oooooooooo
oooooooooo
Tens
0123456781
oooooooooo
ooooo 0,0 o o o
oooooooooo
oooooooooo
oooooooooo
oooooooooo
Units
'0'
Range
»•»
A
DA1
\ /
*~
A'
~" *
24 v
Meter
Auto
S0
Program
Auto
Meter
Reset
NOX
Figure
Front Control Panel
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Further experiments results in configurations which produced
linear results for S02 and N02 when using microwave excitation.
Table 4 shows the effect of oxygen flow rate on the response
to both S02 and NO.
32
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APPENDIX
OPERATING INSTRUCTIONS
PROGRAMMING
Programming of the instrument is accomplished by means of
the recessed pin board matrix switch on the front panel.
See Figure 14. The five upper rows of pin holes control the
normal cycle program except that the standard cycle occurs
only as programmed by the lower row of pins.
The third row sets the start of the auto zero cycle and
the fourth row sets the end of auto zero cycle. The fifth
row sets the time at which the program cycle resets itself
and starts over.
The first row sets the start of the standard cycle and the
second row sets the end. However, the standard cycle only
occurs as often as programmed by the lower row of pins. For
instance, a pin in the sixth position of the lower row causes
the standard cycle to occur once each sixth program cycle.
The format for programming is reasonably straightforward.
The basic time unit is one minute, and there are three
decades on each row such that program cycles can be up to
998 minutes.
One peculiarity of programming is that the actual programmed
time is one time unit less than the program on the board.
Therefore, a one-hour time required a 061 pin setting.
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AMPLIFIER
Each channel amplifier consists of a printed circuit board
and various resistors and potentiometers on the front panel.
See Figure 10. Each P.O. board contains three integrated
circuit analog amplifiers for signal conditioning and one
I.C. analog amplifier acting as a comparator which, together
with various logic and counting and a digital to analog con-
verter, comprises an automatic zero circuit.
The first amplifier stage, a high-input impedance LM308A,
accepts the current signal from the anode of a photomultiplier
tube. The gain this stage is set by a potentiometer on the
P.C. board, such that its output is approximately five volts
for a 3000 ppm sample. A ten-turn, 5K potentiometer on the
front panel provides a suppression current for nulling any
background current from the P.M.T. The second stage is a
LM301A, the gain of which can be adjusted by the ten-turn,
100K span potentiometer on the front panel. The output of
this stage is adjusted to provide five volts at its output,
for a 3000 ppm sample. Zero adjustment of this stage comes
from the output of a digital-to-analog converter in the
automatic zero section. The third stage accepts the signal
from the output of the second stage. This signal is amplified
by an amount determined by the range switch on the front panel
to provide one volt at the output for a sample containing
the full scale concentration set on the range switch. Zero
adjustment is made by means of a 20K trim potentiometer on
the P.C. board.
A LM201 amplifier accepts the output of the second stage
and generates a large output voltage of the opposite polar-
ity. That output is accepted by a Schmitt trigger logic gate
-------
which provides a switching action with very small hysteresis.
The gate output routes a clock pulse to one of two inputs of
an up/down counter, depending on the polarity of the second
stage output, through another set of gates which turns the
clock pulse train on or off, depending on the state of the
command signal which is received through a switch on the
front panel. That switch controls the auto zero circuit or
connects it to the programmer which turns it on at the end
of the auto zero cycle. A push-button switch resets the
counters to mid range. The binary output of the counter is
accepted by a DAC 372-8, eight bit digital-to-analog converter
which supplies a zero signal to the input of the second stage.
The action of the total circuit is such that when the auto
zero circuit is on, the counter counts in a direction, up or
down, to make the D-A converter output change in a direction
to cause the second state output to decrease. Upon crossing
zero, that output causes the count direction to change so
that the circuit oscillates one to two bits around zero. By
making the effect of the D-A converter output on the second
state output small enough, that oscillation is insignificant.
Thus, the circuit operates very much as a slightly oscillatory
servo system. When the auto zero is turned off, the counter
and D-A converter remain in their last state, thereby re-
taining the suppression current to the second amplifier
stage input.
AUTOMATIC OPERATION
In the automatic mode, the programmer transmits logic level
signals to the solenoid driver and automatic zero circuits.
These signals are derived from the clock counters through
decode and latching circuits, such that the automatic zero
mode and the standard mode are programmable, as is the
35
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frequency of the standard mode and the period of a program
cycle. The logic is such that the sample mode is on at all
times when neither the zero nor the standard mode is on.
In the sample mode, SV4, the sample solenoid, is activated
allowing the sample to be drawn through the instrument, see
Figure 6. The dryer vent valve, SV2, is also activated, see
Figure 1, allowing the drying air in the sample conditioning
probe to vent to atmosphere. A part of the gas passing
through the instrument is bled through a needle valve, V6,
into the reaction chamber where the pollutant reacts with
atomic oxygen entering from the opposite side. If necessary,
needle valve V5 may be adjusted to provide the required
dilution of the sample by entering clean air into the stream.
Atomic oxygen is generated by passing molecular oxygen
through an ozone generator at near atmosphere pressure and
thermally decomposing the ozone as it passes through a heated
zone at 800 - 900°C. The flow rate of the ozone/oxygen
mixture is set by needle valve V9, which bleeds it into the
vacuum chamber ahead of the heated zone. The mixture con-
taining ozone and molecular, as well as atomic, oxygen then
passes into the reaction chamber where it mixes with the
sample and passes in either direction down the inner tube of
the reaction chamber to the end and back to the middle where
it exits to the vacuum pump, P3.
Upon initiation of a zero cycle, the dryer vent solenoid
valve and the sample solenoid valve are deactivated, thereby
blocking entrance of the sample to the instrument and blocking
the exit of the drying air from the permeation dryer in the
sample probe. Simultaneously SV1 and SV3 activate allowing
an increased flow of drying air to be pumped in a reverse
direction through the permeation tube and input filter to
36
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vent into the stack. Also SV6 activates allowing a clean
dry air flow into the instrument. Needle valve, V4, is
adjusted to maintain the same flow through the instrument as
was present during the sample mode of operation. During the
last minute of the automatic zero mode, the electronic auto
zero circuit is activated causing the output of the amplifier
to read zero. That level of suppression is then maintained
until the next automatic zero mode.
Upon initiation of the standard mode, the sequence is
identical to that described for the automatic zero mode,
except that SV5 is activated, rather than SV6, allowing a
standard gas of known concentration to enter the instrument
and the electronic automatic zero circuit is not activated.
Since the composition of the standard gas is known, span
adjustments may be made to compensate for any changes since
the last such adjustment, or the changes may be noted or
displayed on a recorder for later computations.
ELECTRICAL
Upon receipt of the instrument, it should be checked for
damage in shipping. Check all printed circuit cards and
connectors to see that they are seated well. Connect the
probe to the monitor by means of the cable with ampherol
connectors furnished. Plug all A.C. line plugs into the
power distribution panel in the top rear of the monitor. If
independent on-off control is desired, use plugs with
switches adjacent. Supply 115V, 60 Hertz, to the male
receptacle at the rear. Turn on all applicable power
switches.
37
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PNEUMATIC
Connect sample line between the probe and the monitor.
Connect zero gas, standard gas, and oxygen lines to the
monitor and to the appropriate bottles with regulators.
Adjust oxygen flow to 100 CC/min. Connect vacuum pump to
outlet at rear of instrument and turn on.
ELECTRONIC
Adjust Powerstat at rear of monitor for 225 milliamperes
ozone generator current. Allow 15 minutes for thermal
decomposition heater to stabilize. Set up desired program
and reset programmer by pushing the reset switch.
INITIAL CALIBRATIONS
Provide a calibration sample of known composition and set
range switch to appropriate range. Set up a long standard
mode period following an automatic zero mode. Allow a
sufficient response time (5 to 10 minutes); then switch the
meter to read up-scale on Al and adjust the trim potent-
iometers on the P.C. boards to 5 volts per 3000 ppm of the
pollutants. For example, 1500 ppm should provide 2.5 volts
at Al. The meters are calibrated for approximately 10 volts,
full scale. Next, switch to A3 and adjust the span potent-
iometers on the front panel to provide the correct readings
on the meters. Re-program for the desired mode of operation
and reset the programmer.
38
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