Final Report
AN INSTRUMENT FOR MEASURING
TRACE QUANTITIES OF OXIDES OF NITROGEN IN AIR
Authors
James C. Mueller
Huel C. Tucker
Anatol Wojtowicz
George W. Wooten
Contract No. CPA 70—70
For
Environmental Protection Agency
Methods Development Section
Research Triangle Park
North Carolina 27711
MONSANTO RESEARCH CORPORATION
A SUBSIDIARY OF MONSANTO COMPANY
DAYTON
LABORATORY
« DAYTON, OHIO 4S4OT
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AN INSTRUMENT FOR MEASURING TRACE
QUANTITIES OP OXIDES OF NITROGEN IN AIR
Final Report
20 June 1970 to 19 July 1971
Authors
James C. Mueller
Huel C. Tucker
Anatol Wojtowicz
George W. Wooten
For
Environmental Protection Agency
Methods Development Section
Research Triangle Park
Raleigh, North Carolina
Attn: Robert K. Stevens, Project Officer
Contract No. CPA 70-70
MONSANTO RESEARCH CORPORATION
DAYTON LABORATORY
Dayton, Ohio 45^07
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FOREWORD
This Final Technical Report and Operating Manual, prepared
by Monsanto Research Corporation under contract CPA 70-70,
Project Number 6774, entitled "Oxides of Nitrogen Analyzer"
covers work performed at the Dayton Laboratory of Monsanto
Research Corporation and was sponsored by Environmental Protection
Agency (formerly National Air Pollution Control Administration),
Durham, North Carolina, Robert K. Stevens (Raleigh, N. C. ) was
the Project Officer.
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ABSTRACT
An instrument for the measurement of NOx was developed
based on the chemiluminescent reaction of NO and N02 with atomic
oxygen. The detectable limit is less than 20 ppb. Atomic oxygen
was generated by an electrical discharge in an oxygen-argon
atmosphere at one torr pressure. A pair of photomultiplier tubes
connected in a bridge circuit configuration measures the chemilu-
mines cent emission in the presence of appreciable background light
from the discharge. Automatic background correction is periodically
initiated by a digital programmer.
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SECTION
1
2
3
4
5
TABLE OF CONTENTS
PAGE
INTRODUCTION
1
SUMMARY
CONCLUSIONS AND RECOMMENDATIONS
2
TECHNICAL DISCUSSION
4
4.1
4.2
4.3
4.4
Reactor/Generator
Electronic Measuring
Programmer
Flow System
Circuit
5
6
7
8
OPERATING PROCEDURES
5.1
5.2
5.3
Programmer
Start-Up Procedure
Calibration
16
19
20
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FIGURE
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
LIST OF FIGURES
Block Diagram of Programmer
Schematic Diagram of Flow System
Sketch of Discharge Tube
Sketch of Reaction Chamber
Calibration Curve for Sample Flow
Calibration Curve for Argon-Oxygen Flow
Calibration Curve for Diluent Air Flow
Calibration Curve for Standard NO Flow
Photo
Photo
Photo
Instrument - Front
Instrument - Control Panel
of NOx
of NOx
of NOx
Instrument
- Side View with Side
Panel Removed (Left)
- Side View with Side
Panel Removed (Right)
Photo of NOx Instrument
Sketch of Control Panel
Calibration Curve
Schematic Diagram, Differential Amplifier
Schematic Diagram, Integrator
Typical Recorder Trace
Schematic Diagram, Auto Zero
Schematic Diagram, Read-Out Amplifier
Schematic Diagram, Standard Light
Schematic Diagram, Discharge High Voltage Supply
Schematic Diagram, Discharge Voltage Control
Schematic Diagram, Solenoid Drivers
Schematic Diagram, Front Panel Circuits
Schematic Diagram, Monitor Meter Circuit
Schematic Diagram, Solenoid and Lamp Circuits
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9
11
12
13
14
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
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TABLE
1
NO Concentration
LIST OF TABLES
PAGE
42
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ACKNOWLEDGEMENTS
The number of people who contributed significantly to this
project is too great to allow naming all of them. However,
special recognition goes to Rumult lItis who was the initial
Project Leader, G. W. Wooten on whose work the instrument was
based, and Dr. A. D. Snyder whose advice and assistance was
invaluable. Also, technicians Charles Elias, Patrick Rice and
Larry DuFou~ contributed many ideas as well as their diligent
efforts to the project.
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1.
INTRODUCTION
A large amount of research has gone into the development
of techniques for measuring small amounts of pollutants in the
atmosphere. The justification for that research is the need for
sensitive instruments for monitoring air pollution in order to
establish some degree of control over it when required. In
order to obtain meaningful measurements of air pollution, it is
usually necessary to move the instrument to the locality being
monitored. In the case where continuous monitoring is required,
portable field mounted instruments are essential.
The toxic character of the oxides of Nitrogen and their
generation in commonly used equipment such as automobiles make
them important pollutants in the atmosphere, especially in
industrial and densely populated areas. The detection of these
oxides, NO and N02, in quantities as low as few parts per
billion in air is required for adequate monitoring in some
localities. Trace quantities of NO and N02 have been detected
in laboratory studies by measuring the chemiluminescence of
their reaction with atomic oxygen.
This report describes an instrument based on the chemilum-
inescent reaction of NO and N02 with atomic oxygen. It is
designed for field operation and is capable of continuously
monitoring NOx down to less than 20 ppb concentration. Its
read-out system is compatible with most commonly used data
acquisition systems.
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2.
SUMMARY
The NOx instrument can be conveniently divided into four
sections:
(1)
(2 )
(3)
(4)
The reactor/generator system.
The flow system.
The electronics system.
The cabinet.
The reactor/generator assembly includes an oxygen generator,
a reactor chamber, a dual photomultiplier detector and two high
voltage power supplies. Atomic oxygen is generated by a high
voltage discharge in an atmosphere of 85% argon and 15% oxygen
at one torr pressure. The atomic oxygen enters a reaction
chamber where it reacts with NO and/or N02 which enters via a
sample inlet. One photomultiplier monitors the light emitted
by the atomic oxygen stream before it mixes with the sample
stream and another monitors the chemiluminescent reaction in the
chamber. '
The flow system consists of the vacuum system and of the
plumbing and pressure controls required to maintain the correct
flow rates into the reaction chamber. The flow rate of the
sample is set by means of an orifice between the vacuum chamber
and the inlet port which is at atmospheric pressure. The flow
of clean diluent air which is used to zero the instrument is
set to the sample flow rate by adjusting the upstream pressure
of another orifice into the reaction chamber. Still another
orifice provides a means for adding a small amount of a known
concentration of NO or N02 to the diluent air at a predetermined
rate. Stream switching is accomplished by means of solenoid
valves.
The electronics system is composed of the programmer, the
measuring and read-out circuits and the various power supplies
which are required.
The digital programmer is comprised of a crystal controlled
oscillator, counters, logic components and driven modules. The
programmer performs all the stream and eletrical switching
necessary for automatic zeroing and for span checking as well as
for controlling the integration cycle of the measuring circuit.
The switching functions are programmed by means of a peg board
type matrix switch on the front panel.
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A differential amplifier in the measuring circuit accepts
the signals from the two photomultipliers and transmits the
amplified difference signal through a filter to an integrator
which repetitively integrates, holds, then resets to a pre-
determined value. The integrator output is transferred to a
memory amplifier in a manner that the output of the memory
amplifier is updated during each "Hold" period of the integration
cycle. During the automatic zero cycle, the output of the
integrator is switched to memory amplifier which supplies a
signal to the "Initial Condition" input of the integrator such
that the integral of any background signal is applied to the
integrator as an initial condition. During the "Measure" cycle,
the integrator output starts at the initial condition such that
the final output is proportional to the chemiluminescence seen by
the photomultipliers. The output is transferred to a read-out
memory amplifier which is calibrated to read one volt full scale
for any range.
The cabinet is a standard relay rack type enclosure with
removable back and sides for easy access to the plumbing and
assemblies inside. The programmer and measuring circuits are
mounted on printed circuit cards to provide easily accessible
modules. The electronics circuits are mounted on a slide assembly
to facilitate check-out and maintenance procedures.
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3.
CONCLUSIONS AND RECOMMENDATIONS
The pulsed discharge method of generating atomic oxygen was
abandoned because there was not sufficient time to develop the
more sophisticated circuits necessary for that type of operation.
Although the pulse technique requires further circuit development,
it is superior to the D.C. discharge method iri an instrument of
this type which must operate for long periods virtually unattended.
The low duty cycle of the discharge pulse can add several multiples
to the effective lifetime of the discharge electrodes.
The rather elaborate digital programmer used in this instru-
ment is the result of the original intent to provide a pulsed
discharged oxygen generator. Adequate programming for the D.C.
discharge instrument can be accomplished by electro-mechanical
means or by a hybrid analog-digital programmer with a consider-
able reduction of costs.
Much of the measuring circuit for the NOx instrument was
designed under the restriction that integration of the signal
was required for the purpose of filtering. That concept should
be further investigated to determine if conventional filtering
means are adequate. A considerable reduction in complexity and
cost of the programmer and the auto-zero circuits as well as the
measuring circuit can be accomplished if conventional filtering
is adequate.
It is recommended that an electrodeless microwave discharge
method for generating oxygen be substituted for the D.C. discharge
system now used. The resulting stablization of the constantly
changing background and sensitivity caused by the degradation of
the electrodes with time should reduce the maintenance to an
accepuable level.
The analog automatic zero circuit is limited to application
where an auto-zero cycle can be tolerated relatively often. For
instance, the best all-electronic analog memory circuit can be
relied on no more than 15-20 minutes in a normal uncontrolled
environment and even less if the relative humidity is high. By
comparison, a servo or. a digital memory can retain its value
indefinitely. It is suggested that a servo type auto-zero
circuit is adaptable to this type of instrument and should be
considered in any new instrumentation where the auto-zero cycle
can be limited to twice per hour or less ~nd where no digital
signal is available.
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4.
TECHNICAL DISCUSSION
4.1
REACTOR/GENERATOR
The research work done under contract No. CPA-22-69-8,
"Feasibility Study for the Development of a Multifunctional
Emission Detector," wherein a microwave discharge was used to
produce atomic oxygen was reviewed. Because the microwave gener-
ators available did not appear adaptable to a portable 'instrument,
alternate methods for the generation of atomic oxygen were studied
(thermal, photolysis, radialysis, neutron bombardment, and
electric discharge). The electric discharge appeared most approp-
riate for miniaturization.
A pulse technique with a low duty cycle was selected as a
means for the electric discharge to minimize the problems due to
heating and to extend the life of the discharge electrodes. A
high voltage power supply capable of producing variable pulse
rates and widths was designed to provide current regulation during
the discharge.
Efforts to derive a stable signal from the pulsed discharge
failed. Although the pulse appeared to be relatively stable,
the signal appeared to be too unstable for sensitive measurements.
As a result, the programmer was simplified to provide a D.C. ,.
discharge which resulted in a marked increase in the stability of
the chemiluminescence signal.
It was found that the background light varied with the
energy in the discharge. As a result, the discharge power supply
was redesigned to provide a constant current to the discharge
over a wide range of conditions.
The geometry of the reactor was optimized to improve
sensitivity and signal-to-background ratio. Forty-nine experi-
ments were run and tabulated as 02 flow, Ar flow, diluent air
flow, and pressure were varied. Optimum performance was calculated
to occur when the following conditions were met.
02 flow
Ar flow
Diluent
- 15 cc/minute
- 85 cc/minute
Air - 50 cc/minute
System pressure - one torr
These conditions resulted in a background of 70 n.a. and a
signal level of 12-15 n.a. of PMT current for 500 parts per
billion.
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The initial development was done using a single photomulti-
plier to sense the light emitted from the reaction tube. As the
work progressed, it became evident that the background light from
the generator and other possible sources was much greater than
the light emerging from the reaction of small concentrations of
NOx with the atomic oxygen. A special photomultiplier tube with
an extended response curve and a band pass filter improved the
signal-to-background ratio. However, the initial difficulty in
obtaining those tubes resulted in the selection of a more common
variety of photomultiplier tube.
In order to further improve the performance of the detector,
the reactor tube was modified to accept two photomultiplier tubes.
One PMT senses the light just upstream from the sample injection
point and the other senses the light in the reaction tube which
includes both background and signal. In that geometry, the
difference between the two PMT currents, properly balanced, is
proportional to the chemiluminescence in the reactor tube and is
relatively independent of background light.
4.2
ELECTRONICS MEASURING CIRCUIT
The initial concept of the measuring circuit included pro-
visions for a low level signal superimposed on a relatively large
pulse. These concepts were partially retained when the system
was converted to a D.C. discharge in that a large amount of gain
(400) was provided in the differential amplifier. Provisions were
made to balance the PMT outputs to compensate for differences
in the PMT's and in the optical characteristics of the two areas
in the reaction tube. System stability was improved markedly by
increasing the input resistors by a decade to one megohm and
decreasing the gain of the differential amplifier by a factor of
20. Further improvement was obtained by installing 0.01 micro-
farad capacitors across the input resistors.
A filter amplifier accepts the output of the differential
amplifier and provides a somewhat cleaner signal to a variable
gain amplifier. Switches on the front panel change the input
and feedback resistors of the variable gain amplifier such that
the gain changes by factors of two. Two switches provide four
gains or ranges.
A commercial integrator with internal semi-conductor
switching for on-off and reset functions and an inital condition
input accepts the output of the variable gain amplifier. The
integrator is programmed to continuously repeat the sequence,
reset one second - integrate "on" ten seconds - "hold" one second.
A programmed switch connects the output of the integrator to the
input of another programmed switch during each "hold" period of
the integrator. The second switch either connects the integrator
to the read-out memory amplifier or the auto-zero integrator
input depending on the mode of operation.
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When in a "measure" mode, either "sample" or "standard", the
output memory amplifier is updated during each "hold" period of
the integrator. In the auto-zero mode, the second switch connects
the integrator output to the input of the auto-zero which has a
time constant of one. Thus, the auto-zero integrator output
changes by the precise amount of the integrator output during the
one second "hold" period. The integrator then resets to a new
initial condition such that if the integrator input remains
constant, the output of the integrator at the end of the next
"integrate" period is zero. Thus, 1f only diluent "clean" air
enters the instrument during auto-zero cycle, any offset in the
system ahead of the integrator is automatically compensated for
by adjusting the initial condition of the integrator. Since the
auto-zero integrator holds its output, the compensation 1s
retained from one auto-zero cycle until it is updated in the
ne xt .
The output of the read-out memory amplifier 1s calibrated
to read 0-1 volt for full scale on any range. That output is
brough to a connector for use with a data acquisition system.
A recorder output is brought to a connector for continuous
monitoring. A meter on the front panel provides continuous
indication of the NOx concentration.
4.3
PROGRAMMER
The digital programmer consists of four sections: clock,
integrator programmer, sample programmer and standard gas
programmer.
The clock is comprised of a 100 KHz crystal controlled
oscillator and a five decade counter which provides a pulse train
at precisely one pulse per second.
The integrator programmer provides a fixed program which is
continuously repeated except that it can be interrupted at any
time 'by the reset switch on the front panel. The programmer is
stopped as long as the reset switch is depressed, but it auto-
matically starts a new cycle when the switch is released. The
programmer provides three time intervals during which it transmits
commands to the measuring circuit. During the first interval of
one second, a signal is transmitted to the "reset" input of the
integrator, thereby resetting the integrator to whatever voltage
is present at its "initial condition~ input. During the second
interval of ten seconds, a signal is transmitted to the "integrate"
input of the integrator causing it to integrate its voltage. In
the third interval of one second, the integrator receives no
signal cuasing it to hold the output it had at the end of the second
interval. However, a signal is transmitted to logic circuit
which updates either the output memory or the auto-zero memory
depending on the state of the sample programmer.
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The preceding sequence is repeated once each twelve seconds so
that either the auto-zero memory on the read-out memory is
updated five times per minute.
The sample programmer is advanced one count for each twelve
second cycle of the integrator programmer. The normal state of
the sample programmer is the sampling mode in which a signal is
transmitted to the sample valve solenoid to open that valve and
another signal switches "on" the discharge power supply. Two
additional modes can be programmed by means of a "peg board"
matrix switch on the control panel. "Standard light" and auto-
zero" start and stop functions are programmed for any desired
time which is measured in integrator cycle, or twelve second
units.
In the "standard light" mode, the discharge power supply is
turned off and a pair of light emitting diodes are turned on. The
out put memory continues to be updated five times per minute.
In the "auto-zero" mode, the discharge voltage is turned on,
the sample valve is closed, the diluent air valve is open and the
integrator output is switched to the auto-zero circuit where it
updates the auto-zero memory once each twelve seconds.
The sample cycle is terminated and another began at any time
up to 999 integrator cycles, or 1998 minutes, by programming the
recycle point on the matrix switch.
The standard gas programmer is advanced one count for each
recycle of the sample programmer. Its function is to provide a
calibration check for the instrument. At the end of thirty-two
recycles of the sample cycle, the standard gas mode is initiated
wherein the sample valve is closed and the diluent air valve and
a "standard NO" valve are opened to provide a sample with a
known concentration of pollutant. The number of sample cycles
that occur before the "standard NO" cycle terminates is programm-
able 'on the matrix switch. The following "standard" cycle opens
the standard N02 valve rather than the NO valve. Thus, the
standard cycle alternates NO standard gas and N02 standard gas.
4.4
NOy FLOW SYSTEM - GENERAL DESCRIPTION
The flow portion of the instrument consists essentially of
the sample, diluent, argon-oxygen and NO-N02 gas flow sub systems.
Appropriate system pressure in the discharge and reaction chambers
is maintained with a vacuum pump. A simplified schematic
illustration of the instrument flow system is shown in Figure 1.
The instrument is packaged into a standard 22 inch wide by 28
inches high relay rack cabinet. The vacuum pump is attached to
the system through a port on the back side of the cabinet.
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Amplifier
Control Panel Swlldt
.
!:
0
Z
1/1
»
Z
-t
0
~
m
1/1
"'
»
~ \D
(')
:I:
(')
0
~
'tJ
0
~
»
-t
0
Z
.
Gain
Switch
Divide lIy
105.
Divi>llelly 12
Pragrammer
Frant Panel
pegllaard Switch
{LED f On
Off
Auto Zero On
Off
Recycle
Fig.l
Block Diogrom
NOx Progrommer
& Readout Circuits
SPST
Switch
ON
1-11 sec.
0- 11_.
11-12Iec.
Caunter -
Pragrammer
A
SPOT
Switch
Number of Std. Cyclel
Front ....., I'wgItoard Switch
Counter -
Programmer
8
Sample
Sol-HI
LED ami Dilchar.. Voltage
Diluent Solenoid
NO Std. Solenoid
N~ Std. Sol-'d
'OR~
Gate
Flip Flap,
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The outside air sample is drawn through the system at the
sampling port (BH-2) and through the normally closed solenoid
valves (SV-2 and SV-3) and filter (F-2). The volumetric flow
rate of the air sample is controlled by the critical flow orifice
(CFO-2). A flow calibration curve for this orifice is illustrated
in Figure 2. Since the sample is drawn at the atmospheric
pressure, the sample flow rate is essentially fixed at 54 cc/min-
ute. The sample then enters the reactor chamber and is discharged
through the vacuum pump (VP).
The ultra clean diluent air enters the system through the
storage tank (TK-2) port (BH-l), normally closed solenoid valve
(SV-l). The diluent air pressure is regulated by the pressure
regulator on the storage tank as well as by diluent air regulator
(PR-l). The diluent air flow rate is controlled by the critical
flow orifice (CFO-l) using zero air is illustrated in Figure 3.
A 15% mixture of oxygen in Argon is utilized in this instru-
ment for the discharge tube gas flow. The oxygen-argon gas
mixture is released from a storage cylinder (TK-l). It flows
through the port (BH-5), and the normally closed solenoid valve
(SV-6). The gas pressure is regulated by the pressure regulator
(PR-3) and the gas flow rate is controlled by the critical flow
orifice (CFO-4). A flow rate calibration for this orifice with
15% oxygen, 85% argon mixture is illustrated in Figure 4. The
oxygen-argon mixture enters the discharge tube and the reaction
chamber. It is exhausted from the system with the other gases
through the vacuum pump (VP).
Two ports are provided for admitting either NO or N02
calibration gases. One port is designed to utilize calibration
gas from a pressurized cylinder. The calibration gas is released
from the cylinder (TK-3) and admitted to the port (BH-3) pressure
regulator (PR-2) filter (F-2) and critical flow orifice (CFO-3),
and through the normally closed solenoid valve (sv-4). The flow
rate of calibration gas is controlled by the pressure regulator
(PR-2) and the quartz-tube critical flow orifice (CFO-3). A
flow rate calibration for the critical flow orifice is illustrated
in Figure 5. From the critical flow orifice the calibration gas
enters the reaction chamber and is expelled by the vacuum pump
(VP). Another port (BH-4) can be utilized for admitting cali-
bration gases by a permeation tube. In this case, the permeation
tube is connected to the permeation port (BH-4). Operation of
the permeation port is controlled by the solenoid valve (SV-5).
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SV6
CF04 IH5
I
fiisatAI"- - - - I DISCHARGE
F4
I "'I. I SVI GAS.
I I
I I CFOI IHI
L__- ---_J I
I
I I FI SV2 DILUENT
I lIMn I ZERO AIR
I CF02
I PMn [t I I SAMPLE
I I I
L- - - - J F2 IH4
VG . PEIMiATION
,
IH3
'3 STANDARD
Fig.2
Flow System Schematic Diagram
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Outlet to Reaction Chamber
Negative Electrode
Argon Oxygen Inlet
',\
, . .
~/
Positive Electrode
Fig- 3 Discharge Tube
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Reference PMT
I
I J"
Inlet from Discharge Tube
\
I
..- ~----_. ---
Sample Iniection
..- - - -" -_.._.~.
Fig." Reaction Chamber
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170
lS0
130
A
z 110
~
~
~
90
50
o
S
10
lS
20
2S
30
PSIG ~
Fig. S Sample Flow (CF02) Calibration Curve
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The vacuum range of 1 Torr in the discharge tube and reaction
chambers is maintained with Welsh Model 1402 vacuum pump. Vacuum
pressure is monitored with Leybold Hareaus Model TM201S vacuum
gage. Pressure in the system can be controlled by either intro-
ducing outside air into the system through the bleed valve (BV)
or by proper adjustment of the system shut-off valve (SOV).
Toggle valve (SRV) is used to quickly expose the system to the
atmospheric pressure.
The heat generated by the reactor assembly is dissipated with
convective cooling provided by a blower. The blower provides a
positive pressure inside the instrument cabinet and thus helps
to keep dust from entering the instrument enclosure.
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5.
OPERATING PROCEDURE
5.1
PROGRAMMER
The control logic for the NO/NOx instrument consists of four
sections: Reset and Clocks logic, Integrator Programmer, Sample
Programmer, and Standard Programmer- Of these, only the sample
programmer and the standard programmer are programmed by the
operator. However, some knowledge of the integrator programmer
operation will aid the operator in programming the other two
programmers.
The integrator programmer controls the operation of an
electronic integrator whose output is a function of the NO/NOx
concentration in the gas being sampled by the instrument. The
programmer first resets the integrator to an initial condition
voltage, it then causes the integrator to integrate for ten
seconds a signal proportional to NO/NOx concentration. The
programmer then stops integration, the integrator holds the
result, and the integration result is used to update either the
system's read-out memory or its auto-zero memory. One or the
other of these memories is updated every twelve seconds. The
sample programmer is advanced one count for easy cycle of the
integrator programmer i.e., once every twelve seconds.
The standard programmer (which is actually a sub unit of
the sample programmer) is advanced one count for each cycle of
the sample programmer. The controls for the sample and standard
programmers are; 1) Program reset pushbutton which when depressed
resets all programmers to zero and when released allows the
program to start. 2) A programming matrix board which allows
the operator to select start and stop times for the progammable
functions.
The upper five rows (horizontal) of the matrix are used
to control the sample programmer. These rows represent functions
and are labeled: STD light: Start - Stop, Auto-Zero:
Start - Stop and Recyle. The columns (vertical) of the matrix
represent time and are labeled: HUNDREDS (0 through 9), TENS
(0 through-gy-and UNITS (0 through 9). A unit of time represented
by the columns is twelve seconds (for the sample programmer
functions). The bottom row of the UNITS holes only is used to
control the standard programmer. This row is labeled number of
cycles of STD gas. These holes represent cycles of the sample
programmer and thus have a time value dependent on the cycle
length for which the sample programmer is set.
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An example will illustrate the programming procedure.
Suppose it is desired to "look at" standard light for two minutes,
and then to have a five minute auto zero cycle followed by a five
minute analysis of a sample gas. Programming in the NO/NOx system
is done on an "actual time minus one" basis. That is, to program
for actual time "t", the programming pins are placed in holes
corresponding to t-l units. In this system 000 is programmed
999 (there are no negative numbers) time 002 is programmed 001,
time 999 is programmed 998, and so on.
For the example program, standard light is to start at (t)
000 and stop at (t) 010, and elapsed time of (000-010) times
12 equals 120 seconds equals two minutes. Auto-zero is to start
at (t) 010 and stop at (t) 035, and elapsed time for auto-zero
of five minutes. Sample measurement is to start at (t) 035 and
stop at (t) 060. Programming pins are placed as shown on the
chart.
Function
Hundreds
Tens Pin Units Pin
9 9
o 9
o 9
3 4
5 9
STD Light Start-----------9
Stop------------O
Auto-Zero Start-----------O
Stop------------O
Recycle---------------O
Note that the sample measurement time is programmed indirectly.
During the program cycle, sample gas is automatically on when
STD light and Auto-Zero are both off. Therefore, in the example
program, when Auto-Zero goes of at (t) 035, sample gas is turned
on and being analyzed from (t) 035 through the end of the cycle
at (t) 060.
The example program would cause the NO/NOx instrument to
act as follows:
At the release of the Program Reset button the discharge
high voltage is off, the sample gas valve is open, the
diluent STD NO and STD N02 valves are closed and the
integrator begins its one second reset period. After
one second, the integrator begins its ten second
integration period. The integrator is now integrating
the "signal" caused by the Standard Light. At the end
of the integration period, the integrator holds the
integration result for one second while the read-out
memory is updated with this result. The integrator
is then reset and the reset-integrate-hold and update
cycle begins again.
17
. MONSANTO RESEARCH CORPORATION.
-------
After ten such cycles the Standard Light is turned
off, the sample gas valve is closed, the diluent
valve is opened and the discharge high voltage is
turned on. The integrator resets for one second
and begins its ten second integration period. Now
the signal integrated is due to NO/NOx in the diluent
and discharge gases. This time the integrated r
result is used to update the Auto-Zero memory. The
Auto-Zero memory voltage is used as the initial
condition voltage which is set into the integrator
during its reset period. This voltage is of a polarity
such that at the next reset period the integrator
is set to an initial condition voltage opposite in
polarity and approximately equal in amplitude to
the result obtained by the previous integration.
Thus, the result of the next integration will be
close to zero. Assuming the same integration input
signal, after several reset-integrated-hold and
update cycles the integrated result will be very
near zero volts. In this manner the "background"
signal produced by NO/NOx present in the diluent and
discharge gases is cancelled.
Note, however, that the read-out memory is not changed
during the auto-zero process but retains the last
result obtained prior to starting auto-zero. After
twenty-five l2-second cycles of reset-integrate-hold
and update auto-zero memory, the program begins the
sample measurement at (t) 035 then the sample gas
valve opens and the diluent gas valve closes. The
integrator continues its cycle as before except that
now it resets to an initial condition voltage equal
to that held in the auto-zero memory (which will not
be updated during this portion of the program).
During the integrate periods the integrator will be
integrating a signal proportional to the NO/NOx
concen-tration in the sample gas. During each hold-
update period the read-out memory will be updated.
Thus, every 12 seconds the system read-out will
receive new information as to the NO/NOx concentra-
tion in the sample gas. At (t) 060 after twenty-five
l2-second integrator programmer cycles the sample
programmer will recycle and the Standard Light mode
will be started again. As was mentioned before the
Standard Programmer merely modifies the sample program
at the proper time. The Standard Programmer logic
counts cycles of the sample program and after thirty-
two cycles causes the sample program to be modified
to a standard program. When this happens, standard
gas plus diluent is substituted whenever sample gas
is called for by the program.
18
. MONSANTO RESEARCH CORPORATION.
-------
The operator may select the number of cycles (minimum
one cycle, maximum ten cycles) that the sample gas/
diluent mixture is analyzed. This is done by placing
a pin in the UNITS holes of the No. of cycles of
STD gas row. The same t-l rule used on the Sample
Programmer applies here. Therefore, to program three
sample programmer cycles of standard gas measure-
ment one places a pin in Units two of No. of cycles
of STD gas row.
The logic of the Standard Programmer causes the
standard gas selected each time the standard
program is entered to alternate between standard
NO and standard N02.
If in our example program a pin is placed in Unit
two of No. of cycles of STD gas row, then after 32
cycles of the sample program (6 hours and 24 minutes
after starting program) a standard cycle will begin.
The Standard Light and Auto-Zero portions of the
program will be unchanged. When Auto-Zero ends,
however, the sample gas valve will remain closed, th
the diluent valve will remain open and the standard
NO valve will open. .
The standard cycle will repeat three times so that
36 minutes after entering the standard measurement
mode the program will return to the sample mode.
After another 32 cycles in the sample mode, the
program will again enter the standard mode for
three cycles. This time, however, the standard
gas selected will be N02 and the NO valve will remain
closed with the N02 valve opening instead.
5.2
START-UP PROCEDURE
After the instrument has been checked for obvious damage,
the vacuum system should be connected and with all input valves
closed, the vacuum chamber should be pumped down.
To insure proper out gasing, the initial pump down period
should be several hours, preferrably overnight. During the pump
down period, the oxygen~argon, diluent air, and standard NO
bottles may be connected to their respective fitting at the rear
of the instrument. After the vacuum chamber is evacuated to well
below one Torr, the pressure is adjusted to one Torr by means of
the bleed valve (BV-l). The power should be turned on and the
instrument programmed for the auto-zero mode. (See programming
instructions.) The argon-oxygen mixture valve can then be
opened and the flow set by means of the inlet pressure (PR-3).
After a two minute delay, the discharge voltage is automatically
turned on and the electric discharge initiated.
19
. MONSANTO RESEARCH CORPORATION.
-------
The monitor meter should indicate low in the green band when
the discharge is properly adjusted. The discharge power supply is
current regulated and can be adjusted by means of the potentio-
meter, R4, in the discharge supply module. The diluent air can
then be turned on and its flow adjusted. In all probability, the
read-out circuit will be considerably unbalanced initially- At
least one day is required for the entire system to stabilize
sufficiently to provide significant readings. When in the auto-
zero mode, the output of the integrator (point AA on the mother
board) should approach zero at the end of each integration cycle,
then reset to some level representing the unbalance of the back-
ground. As the system becomes stable, that unbalance becomes
small. After sufficient time, the instrument can be programmed
for a five minute read-out cycle followed by a five minute auto-
zero cycle. It should be insured that the air entering the
sample inlet is essentially free of NOx. Therefore, any offset
in the sample, after the initial switching transient decays, is
the result of a difference in flow of the dilUent and the sample.
That difference can be adjusted out carefully adjusting the flow
of the diluent pressure by means of SV-l.
After thirty-two cycles (auto-zero, read-out) a mixture of
Standard gas and diluent air is substituted for the sample, by
adjusting the flow of the standard gas (PR-2) and knowing its
composition, the calibration can be checked. Some offset may
result from the flow change and must be accounted for. Since
the NOx instrument contains an all electronic auto-zero circuit,
the memory circuits can not be depended on for more than approx-
imately 15 minutes, worst case, or one hour, typical. The
frequency of the auto-zero cycle should be determined by the
limitations of the memory devices. Differences in the photo
multiplier outputs can be balanced out by means of the potentio-
meter, R4 on the differential amplifier board.
The instrument is calibrated to read out 500 ppb on 1000 ppb
depending on the position of the range switch on the front panel.
The ranges may be extended by a range multiplier switch on the
front panel by factors of 2 or 4. Thus, the 500 ppb range can be
extended to 1000 ppb or 2000 ppb and the 1000 ppb range can be
extended to 2000 ppb or 4000 ppb. Each range is based on one
volt full scale output. The recorder switch on the front panel
matches the recorder full scale span to one volt. Thus, it is
set at 10 m.v. for use with a 10 m.v. recorder. If the cali-
bration should require adjusting, the three potentiometers on
the "Gain" amplifier must be adjusted separately to provide the
correct (one volt full scale) output when their respective
positions on the range multiplier are selected.
20
. MONSANTO RESEARCH CORPORATION.
-------
80
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Fig.6 Argon Oxygen Flow (CF04) Calibration Curve
21
. MONSANTO RESEARCH CORPORATION.
-------
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. MONSANTO RESEARCH CORPORATION.
-------
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. MONSANTO RESEARCH CORPORATION.
-------
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Photo of NOx Instrument - Control Panel
25
. MONSANTO RESEARCH CORPORATION.
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Figure 11.
Photo of NOx Instrument - Side View with Side
Panel Removed (Left)
26
. MONSANTO RESEARCH CORPORATION.
-------
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40
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and
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Tab Ie 1
Tabulation of NO Concentration
Dil. Flow 11 PPM PPB NO
PSIG NO Flow 11(NO)(1000)
:to.3 PSIA cc/min. :t.Ol 53+NO
3 17.7 :t.3 0.73 :t.03 150 :t6
4.5 19.2 :t.3 0.83 :t.03 170 :t6
6.0 20.7 :t.3 0.93 :t.03 190 :t6
7.5 22.2 :t.3 1.06 :t.04 216 :t 8
9.0 23.7 :t.3 1.19 :t.04 242 :t8
10.5 25 . 2 :t.. 3 1. 33 :t. 04 270 :t 8
12.0 '26.7 :t.3 1.48 t.04 299 :t 8
13.5 28.2 :t.3 1.65 :!::.04 332 :t 8
15.0 29.7 :t.3 1.82 :t.05 365 :tl0
42
. MONSANTO RESE.ARCH CORPORATION.
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