DSVELOPiMENT OF PORTABLE ELECTROCHEMICAL TRANSDUCERS
FOR
THE DETECTION OF SULFUR DIOXIDE AND OXIDES OF NITROGEN
CONTRACT NUMBER
CPA 22-69-118
FINAL REPORT
PERIOD COVERED: JUNE 1969 TO AUGUST 1970
AUTHORS
RAMESH CHAND ROLAND V. MARCOTE
SEPTEMBER, 1970
PREPARED FOR
DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
DURHAM, NORTH CAROLINA
DYNASCIEKCES CORPORATION
INSTRUMENT SYSTEMS DIVISION
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NSTRii^EHT SYSTEM
CORPORATION
TABLE OF CONTENTS
Section Page
1.0 INTRODUCTION . . . . 1
2.0 GENERAL DISCUSSION . . 2
2.1 Sweep Voltaimietry 2
2.1.1 Instrumentation. . . . '. 2
2.1.2 Experimental Procedure 5
2.2 The Electrochemical Transducer. 5
3.0 EXPERIMENTAL 11
3.1 Sv/eep Voltammctry Studies 11 .
3.2 Experimental S02 and NOX Transducers 17
3.2.1 Evaluation of Semipermeable Membranes 17
3.2.2 Transducers Employing Aqueous
Sulfuric Acid Electrolyte. 19
3.2.3 Transducers Employing KPF6 in
Propylene Carbonate as Electrolyte 20
3.2.4 Transducers Employing Dryed KPFg and
Cl^COOH in Propylene Carbonate as Electrolyte. . 22
3.2.5 Transducers Employing Other Electrolytes 23
3.3 Selected NOX and S02 Transducers 25
3.4 Sulfur Dioxide Scrubbing Systems 26
3.4.1 Electro-oxidation Systems 26
3.4.2 Wet Scrubbing Systems 27
4.0 PROTOTYPE MONITORS PERFORMANCE DATA 30
4.1 Sensitivity 30
4.1.1 Sensitivity and Operational Temperature 30
4.1.2 Sensitivity Ranges 30
4.2 Selectivity 30
4.3 Response and Recovery Times 34
4.4 Stability of NOX and S02 Prototype Monitors 34
4.5 Transducers Life Data 38
4.6 Temperature Control and Compensation 38
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RPORATIOIM
i
LIST OF FIGURES - \
*
- ' ' - s
Figures . Page j
.••''' ^
1 Oxidation of N02, NO, and S02> in l.ON HUSO, . |
at a Gold Sensing Electrode 2 • j
2 Cyclic Voltammetric Instrumentation 4 i
3 Sweep Voltammetric Cell 6 !
4 Electrochemical Transducer (Schematic) 8 ,
i
5 Potenti.ostatic Control of an Electrochemical Transducer . . 9
6 Linear Response of an NOX Prototype Monitor 31 :
7 Characteristic Response of Monitor SS-330 35
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ORPQRATIQN
LIST OF TABLES
Table" • Page
1 Sweep Voltaimnetry Evaluations 12
2 Sweep Voltammetry Evaluations .... 13
3 Sweep Voltammetric Evaluations of
Potential Interferences 15
4 Sweep Voltammetric Evaluations of
Potential Interferences 16
5 Selective Membranes 18
6 Evaluation of NOV Transducers Employing KPF.--PC .... 21
x o
7 Evaluation of Various Organic Electrolytes.. 24
8 The Effect of Temperature Upon Transducers
Sensitivity 32
9 Evaluation of Various Interferences . . 33
10 Stability of NO and S00 Prototype Monitors 37
X £*
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ORPO RATION
t> .
"l.O INTRODUCTION
o ' '
This is the Final Report on the Dept. of Health, Education and Welfare
Contract CPA-22-69-118, covering the period from June 1969 to August 1970.
The objective of this effort was to develop and provide portable electro-
chemical transducers for the detection of sulfur dioxide and oxides of
nitrogen in the industrial stack.
The v?ork carried out involved the screening of different aqueous as well
as nonaqueous electrolytes for the purpose of developing selective sulfur
dioxide and oxides of nitrogen transducers. Though many organic electrolytes
were found to be feasible based on the sweep voltammetry studies, they were
all abandoned from consideration in the final design because the transducers
employing these' electrolytes gave very limited useful life, slow response
time and erratic behavior.
A dilute solution of sulfuric acid (IN H2SO^) was selected as the electrolyte
for both sulfur dioxide and oxides of nitrogen transducers.
The selectivity of the sulfur dioxide transducer was obtained by subjecting
its sensing electrode to a fixed potential and by the use of polyethylene
as the gas permeation membrane. The total interference from 1000 ppm NO
and 100 ppm N0£ was found to be about 5% full scale on a 0-1000 ppm S02
transducer.
The selectivity of the oxides of nitrogen transducer could not be obtained in
the presence of sulfur dioxide and a scrubber was designed to remove S0?
without appreciable affecting NOV concentration. The scrubber consisting of
X
aqueous solution of I^CO-j, KHCO^j KNOo and KNO~ was found to absorb less
than 1% of 2000 ppm of NO and less than 20% of 100 ppm N02- However,
considering the low concentration of NOo in the stack, the overall effect
on the NO concentration is insignificant.
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SYSTEMS Bi
a2.0 .-GENERAL DISCUSSION ..___. •'.."'
' • •
2.1 JWEEP VQLTAMMETRY " . " •
The oxidation and reduction potentials of different electroactive gases _
in different aqueous and nonaqueous electrolytes were determined using-
sweep voltammetry . This technique, which permits the scanning of an entire
voltage range in a matter of seconds, not only shows the potentials at
which reactions occur, but also their rates and efficiencies in terms of
current generated. Moreover, information regarding electrolyte stability
and electrode kinetics also result from sweep voltammetric studies. The
existence of an electrode reaction is indicated by a peak in the voltage-
current curve traced out by an x-y recorder. The potential at which the .
peak occurs is related to the specific electrode reaction, whereas the psak
height (current) is proportional to the concentration and diffusion coefficient
of the reacting species. The efficiency of the electrode reaction, in terms
of electrochemical reversibility, is also reflected in the peak height and
the shift of peak potential. The purpose of this study was to find an
electrolyte which showed selective oxidation peaks for either SO, or NO , or
£* ft.
which showed their respective oxidation peaks at substantially different
values. It was understood at this stage that both NOX and S02 being highly
F;'
electroactive are likely to offer the most interference with each other. As K
an example, Figure 1 shows the voltammograms for S02, NO and N02 obtained in \
IN H2SO/,. electrolyte. • "
2.1.1 INSTRUMENTATION
The instrumentation employed for the cyclic voltammetric measurements is
schematically shown in Figure 2. Basic to the method is a potentiostat
which maintains the voltage between the reference-working electrode couple
at a value equal to the sum of the input reference voltages represented
by the dc reference and function generator values. The potentiostat
accomplishes this by controlling the current between the working and counter-
electrodes. The potentiostat senses the difference between the reference-
working electrode voltages and the sum of the reference input voltages by.
means of a high gain amplifier system, and feeds the output through the
working electrode - counterelectrode couple. This amplifier has high input
and low output impadance, and a rise-time less than 10 l sec. The system
permits the measurement of current-voltage-time relationships under
2
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POTENTIOSTAT
[REFERENCE
VOLTAGE
REFERENCE
ELECTRODE
WORKING
ELECTROD
FUNCTION
GENERATOR f
POWER ' '
AMPLIFIER
•PC-Y RECORDER
COUNTERELECTRODE
.CELL'
-------
well-defined conditions. No voltage change is possible at the reference
.electrode, and the measurements reflect only the voltage changes at the
working electrode-electrolyte interface independent.of ohmic losses through
the electrolyte and polarization at the counterelectrode.
The potentiostat was.designed and built at this laboratory. Employing solid
state operational amplifiers and a Harrison type 6824A current amplifier,
the instrument is capable of an output of ±50 volts at 1.2 amp. The function
generator is a Hewlett Packard Model 3300, with a Hewlett Packard Model
3302A Trigger, phase lock plug-in unit.
The two-chamber sweep voltammetry cell (Figure 3) employs three electrodes. j
These electrodes are the working, counter and reference electrodes. The »
two chambers are connected to each other by an electrolyte bridge. The J
working or sensing electrode was a 1.27cm x 0.127cm piece of gold mesh 99.9% .>
pure. The counterelectrode was a platinum (99.9%) mesh, and the reference jj
electrode was cadmium/cadmium fluoride (Cd/CdF2) obtained by direct fluorination i>
of 99.9% pure cadmium wire. ji
2.1.2 EXPERIMENTAL PROCEDURE . |(
The sweep voltammetry cell was cleaned and dried. The gold electrode was . jj
cleaned by immersion in hot 50% aqueous HN03 followed by water rinse, immersion jj
in hot 20% aqueous KOH, water rinse, another brief immersion in the 50% HNO_3 j
and rinsed with distilled water. This electrode was further rinsed with the
electrolyte.
The Cd/CdF7 reference electrode was cleaned by brief immersion in diluted
HN03 followed by rinses with distilled water and the electrolyte. The
platinum counterelectrode was anealed at a flame before using it.
Individual gases were bubbled through the electrolyte for three minutes,
and sweep curves were recorded on the third sweep cycle. The scanning
range was -0.5 volts to -KL.25 volts at a sweeping rate of 40 mv/second.
2.2
The electrochemical transducer is a sealed faradaic device in which the
electro-oxidation or electro-reduction of absorbed gas molecules at a
sensing electrode results in a current directly proportional to the partial
pressure of the gas.
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WORKING ELECTRODE LEAD
REFERENCE ELECTRODE
\
GAS INLET TUBE
(TEFLON)
<™»GAS INLET
PORT .
-COUNTER -
ELECTRODE
LEAD
-WORKING
ELECTRODE
PLATINUM
GAUZE COUNTER-
ELECTRODE
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BJlillll
CORPOfiATION
INSTRUMENT SYSTEMS DiViSiO;]
' Figure 4 shows a schematic drawing of an electrochemical transducer. The
pollutant molecules diffuse through the semi-permeable membrane, dissolve
in the film of electrolyte, and diffuse to the surface of the sensing electrode
where they undergo electro-oxidation or electro-reduction, depending upon the
nature of the gas molecules and the transducer operational mode.
The counterelectrode material has an oxidation potential higher than that of
the pollutant. When the two electrodes are joined together externally
through a small resistor, the sensing electrode which is a polarizable
electrode assumes the potential of the counterelectrode less the iR drop
across the resistor. The pollutant molecules, upon reaching the sensing electrode
encounter a potential higher than required for their electro-oxidation. The
net result is lowering of the potential of the sensing electrode proportional
to the concentration of the pollutant molecules and equivalent release of
electrons from the sensing to the counter electrode. Electrochemically,
while the pollutant is being oxidized at the sensing electrode, a corresponding
reduction reaction occurs at the counterelectrode. Because the current is
diffusion limited, the resulting electrical current is directly proportional
to the pollutant partial pressure. The magnitude of the concentration gradient
at the sensing electrode is directly proportional to the gas concentration in
the electrolyte layer (Pick's law of diffusion), and therefore to the partial
pressure of the pollutant in the sample. The diffusion current is expressed
by:
nFADC
x e_
Where i is the current in amps, n is the number of exchanged electrons par
mole of pollutant, F is the Faraday constant (96500 coulombs), D is the
f\
diffusion coefficient of the gas in cm /sec., C is the concentration of
the gas dissolved in the electrolyte layer (moles per cm-*) and £ is the
thickness of the diffusion layer in cms.
The potential exerted upon the gold electrode by the counterelectrode was
regulated by use of potentiostatic control. This is schematically shown
in Figure 5. A stable D.C. power source, such as a mercury battery across
a potentiometer was used to lower or increase the effective potential on
the sensing electrode. We have used a lead dioxide counterelectrode for
the sensors employing aqueous sulfuric acid electrolytes and a cadmium
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THIN FILM ELECTROLYTE
SENSING ELECTRODE
/
''/.
BULK ELECTROLYTE
77T7// '777/T7
/ / / / / // / f / < / //
COUNTERELECTRODE / //
TO
AMP
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FIGURE 5 SCHEMATIC SH.Ovr.tHG POTEHTIOSTATIC CONTROL OF ELECTRO-
/' ". . . CHEMICAL TRANSDUCER TO OBTAIN S02 SELECTIVITY
SENSING ELECTRODE
OUNTER- ELECTRODE
M.35 VOLTS
TRANSDUCER
-o
-o
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CORPORATION
The lead dioxide electrodes were made by the anodic electroplating on
platinum mesh (52 mesh size, Englehard Industries). The plating solution
consisted of 350 gms/liter of Pb(NO~)0 and 2 gms/liter of Cu(NOo)9% H.O.
3 f. ~> / 2.
The plating was carred out at 15 amp/ft current density and at a solution
temperature of 120°F.
The cadmium fluoride electrodes were prepared by mixing 85% CdF9 (Research
Inorganic, 98%), 5% carbon (Research Inorganic, 99.9997=) and 10% polypropylene
poxtfder (Hercules Co.). About 6 gms of this mixture was evenly spread on
both sides of a platinum mesh and pressed to 3,000 psig for three minutes
at 110°C.
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CORPORATION
,3.0 EXPERIMENTAL
3.1
In the evaluation of sweep voltanmograms the most useful parameters are the
potential and current flow values at which the oxidation and reduction of
the gas being studies occurs. • These values are generally referred to as the
peak potentials. The peak potential values represent the minimum voltage
required to obtain the maximum current f'low.
The primary objective of these sweep voltammetry studies was the discovery
of electrolytes with selective response for NOX and S0?. With this purpose
in mind, a number of electrolytic systems were characterized. The oxidation
and reduction potentials of NO, N0?, S0_ and other electroactive species
which could interfere with NOX and S02 were obtained. Tables 1 and 2 show
the oxidation and reduction,potential of various gaseous pollutants in each
of the electrolytes studied.
The selective oxidation or reduction of a pollutant in the presence of other
electroactive speci.es is obtained if either its oxidation potential is lower,
or its reduction potential is higher than those of the interfering gases •
(Stockholm Convention). The electro-oxidation of sulfur dioxide in the i:
I '
presence of oxygen and oxides of nitrogen clearly exemplifies the selective '• *
i !
oxidation of pollutants. The electrochemical half-cell reactions, in ij
IN H^SO,, for these gases are shown below. The potential values (E°) S
i
throughout this report are in volts relative to the normal hydrogen v
electrode (NHE)
p
298° ' Volts
•p
& -~00 »
S02+2H20 —* S04 44H+42e~ 40.17
N024H 0 —»? NO "+2H+4e~ 40.80
N0421I20 -^» NO "44li+4-3e~ 40.96
, 4 {
i
}
The sweep voltammetry studies indicated the existence of several electrolytes j.
potentially suitable for the selective sensing of NO and SO . The most £
x i £
promising systems were for SO- IN 1I_SO,, and for NO undistilled propylene ?
carbonate saturated with undried KPF, . *'
• l>
?
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• TABLE 1
SWEEP VOLTAMMETRY EVALUATIONS
Electrolyte
IN H SO, aqueous
IN H_SO, aqueous
Saturated with Tri-
nitrobenzene
7.8 N H2S04
36 N H2S04 '
Distilled propylene
Carbonate -0.75M
Dry KPF,
Undistilled propylene
carbonate -0 .7511 Undried
KPF,
o
Undistilled propylene
carbonate -0.75M Undried
KPF, +10% H00
D /
Peak Potentials, Volts (NKE)
OXIDATION
NO
0.94
1.20
0.94
1.14
0.96
N0£
1.10
1.30
0.97'
1.25
1 . 20 j
0.67
1.55
0.66
1.07
0.85
1.35
1.10
1.70
0.70
1.26
1.00
1.50
1.03
so2
0.85
1.16
1.00
NH3
NPO
NPO
NPO
!
0.75
1.55
0.78
•1.20
1.30
1.09
NPO
-0.17 '
NPO
NPO
REDUCTION
NO
NPO
NPO
NPO
0.60
-
0.56
. ' 0.75
NPO
N02
NPO
NPO
NPO
0.90
0.56
0.70
0.83
so2
NPO
NPO
NPO
NPO
-0.50
NPO
NPO
NH3
0.45
0.50
l.OS
NPO
-0.50
-0.85
0.15
-0.83
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SWEEP VOLTAMMETRY EVALUATIONS
Undistilled propylene
carbonate saturated
with undried KPF,
Distilled gaircna-
butyrolactone -0.75M
dry KPFg
Distilled gamma-
butyrolactone -0.75M
undried KPF-
5
Distilled gamma -
butyro lac tone -0.75M
undried KPFg +10% H.,0
N,N-Dimethyl Formamide
-0.75M dry XPFg
Distilled Formamide
-0.75M dry KPFg
Distilled Formamide
-0.75M undried KPF,
+10% H20
Glycerol - H»0 (1:1)
saturated wiuh KPF-
6
Peak Potentials, Volts (NKE)
OXIDATION
NO
0.86
0.77
1.07
0.98
1.00
0.77
0.97
0.95
1.18
0.87
NPO
K02
0.72
0.77
0.94
0.99
1.00
1,25
1.03
0.70
1.04
so2
1.25
0.88
0.30
1.00
0.87
0.55
1.02
0.90
0.90
0.65
NH3
-0.11
NPO
NPO
NPO
NPO
NPO
NPO
NPO
REDUCTION
NO
0.73
0.41
0.98
NPO
NPO
NPO
NPO
0.62
N°2
0.55
0.75
0.52
0.82
0.85
1.08
NPO
NPO
NPO
so2
-0.44
-0.43
-0.50
NPO
-0.30
NPO
NPO
-0.45
NPO
NPO
NPO
NPO
NPO .
. NPO
NPO
NPO.
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CORPORATION
• Sulfur dioxide in IN aqueous H_SO, has an oxidation peak at 0.85 volts. »
This potential is lower than those of NO and N0_. Both, NO and N02> have
two oxidation peaks at 0.94 and 1.20, and at 1.10 and 1.30 respectively.
Therefore, IN aqueous HJ50, can be employed for the selective sensing of
S02 (Figure 1). . ' '
In 7.8 N aqueous H_SO, and 36 N H?SO, nitric oxide undergoes electro-
oxidation at a lover potential than sulfur dioxide and nitrogen dioxide.
These electrolytes could be utilized in an NO monitor, but they are not
suitable for use in either NO or S00 monitors.
Jt ^
Nitric oxide, in N -H«SO, aqueous electrolyte saturated with trinitrobenzene,
was found to have an oxidation potential of 0.94 V. Another oxidation peak
was observed at: 1.14V. Nitrogen dioxide showed an oxidation potential of
0.97V and sulfur dioxide of 1.16. This electrolyte is thus of possible use
in an NO sensor. A moderate oxidation maximum current flow for 00 was
X . £.
observed at 1.4V. Three large reduction peaks were observed at 0,0 volts
for II , 0 and CO (Tables 3 and 4).
The 0.75M KP1;' in undistillecl f ormamide ,with 10% H-O by weight had a
potential use in an S0? monitor. Sulfur dioxide in this electrolyte is
oxidized at a lower potential (0.85V) than NO and N0?. Formic acid and
H«S are oxidized at 0.85V and 0.35V respectively and thus they would
interfere. Small reduction maximums were measured at -1.0V for formalde- •
hyde, H90, 0., and CO. A moderate current flow was observed for !!„ at
£• £• £.,
-1.0V. Large reduction maximums were measured for C09 and E S at -1.0
volts and for S00 at -0.6V. The 0.75M dryed KPF, in distilled formamide
3 6
also was found to be suitable for the selective sensing of S09 (Table 2).
Gamma-butyrolactone (distilled) - 0.75M dryed KPF,. electrolyte appeared
to be useful for an N02 monitor. The addition of 10% of HO by weight to
this electrolyte causes a reversion of its selectivity. In other words,
it becomes a selective electrolyte for S09.
The N, N-dimethyl formamide - 0.75M KPF- electrolyte was found to be selective
for sulfur dioxide, S09 (0.55V) being oxidized below NO and NO.. A small
^ ^
oxidation peak for formic acid was observed at 1.24V, and two large peak
potentials for H?S at -0.21 volts and 1.40 volts were observed. Formic acid
reduces at -0.80 volts giving a large current flow. Sulfur trioxide and
H«S gave large reduction peaks at -0.937 and -0.3V respectively (Table 3).
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TABLE 3
SWEEP VOLTAMMETRIC EVALUATION OF POTENTIAL INTERFERENCES
Electrolyte
Forrnamide (undistilled)
-0.75M KPF, +107. (by
weight) HO
& -Butyrolactone (Distilled)
-0.75HXPF, +107, (by -weight)
H20
Propylsne Carbonate
(undistilled) saturated
with KPF,
IN K2S04 saturated with
Trinitrobenzene
Spectro-grade N,N-Dimethyl
Forrnamide -0.75M KPF,
6
Peak Potentials, Volts (NEE)
OXIDATION
YJL~~'
0.35
NPO
-0.21
1.40
NPO
NPO
o? | so3 co
NPO
NPO
1.19 NPO
NPO NPO
NPO 1 NPO j NPO NPO
! !
NPO
-1.40
NPO
. NPO
NPO NPO
' ' REDUCTION
E2S
-1.0
NPO
-0.93
H2°
-1.0
NPO
NPO
NPO
°2
-1.0
0.0
NPO
0.0
v*PO
so3
-0.6
NPO
-0.8
-0.3
,co
-1.0
NPO
NPO
0.0
NPO
-------
TABLE 4
SWEEP VOLTAMMETRIC EVALUATION OF POTENTIAL INTERFERENCES
Electrolyte
IN H.SO^ saturated with
Trinitrobenzene
Spectro-Grade N,N- Dimethyl
Formsraide - 0.75M KPF,
0
Formamide (Undistilled)
-0.75M KPF6 +10% (by
weight) H20
<£ -Butyrolactone (Distilled)
-0.75M KPF, +10% (by weight)
H20
Propylene Carbonate (Undis-
tilled) saturated with KPF,
0
Peak Potentials, Volts (NKE)
OXIDATION
N2
NPO
NPO
+1.8
NPO
NPO
co2
—
NPO:.
NPO
NPO
NPO
HCOOK
— —
1.24
0.85
NPO
NPO
H£CO
—
NFO
NFO
NPO
NPO
H2
NPO
NPO
NPO
NPO
NPO
REDUCTION
N2
NPO
NFO
-0.5
-1.5
NPO
co2
—
NPO
-1.0
NPO -
NPO
HCOOH
•* •
-0.80
NPO
0.0
-1.0
H2CO
—
NPO
-1.0
NPO
NPO
H2 .
0.0
NPO
-1.0
-0.75
NPO
-------
fffiWiffliK
CORPORATION
,'Voltammo grams of undis tilled propylene carbonate saturated with KPF, showed
*
the possibility of utilizing this electrolyte in NO transducers, both NO
X • •
(0.86V) and N02 (0.72V) being oxidized at a lower potential than S02 (1.25V).
Sweep voltammetric curves of C02, H-, H2S, H_0, 0 ,.. SO-, CO, formic acid and
formaldehyde indicated that these gases would not interfere with the oxidation
of NO and N0. •
3.2 EXPERIMENTAL SO AND NO TRANSDUCERS
3.2.1 EVALUATION OF SEMIPERMEABLE MEMBRANES
The concept of selective permeation, which dates back to the nineteenth
century, is based on the phenomenon that gases diffuse at a different rate
through certain polymeric films. A membrane or polymeric film, through
which a gas diffuses at a rate several times larger in order of magnitude
than another gas, is said to be selective for the former.
From the above discussion, the importance of semipermeable membranes in
the development of S09 and NO selective sensors seems evident. Electro-
£• X
chemical transducers were assembled employing different semipermeable
membranes, PbO_ and Au electrodes, and IN aqueous H_SO, electrolyte. The
net selectivity (potentiostatic control was not employed) of these permeation
membranes toward N02, NO and SO- are shown in Table 5.
These studies revealed the existence of several semipermeable membranes
highly suitable for SO- transducers, they are: MEM 213, polyethylene,
cellophane, 'poly-vinylchloride and j>ellicon. Unfortunately not a single
NO selective membrane was found.
x
The polyvinylchloride and cellophane membranes are absolutely impermeable
to NO and N0_, but they present severe drawbacks. The polyvinylchloride
membrane has a sluggish response. The cellophane membrane is prepared by
imbibing the natural polymer, cellulose, in glycerol, glucose or polyhydric
alcohol. These compounds, which act as plasticizers, were found to slowly
hydrolyze in acidic solutions. Once the plasticizer is totally removed, this
membrane becomes brittle and loses its selectivity towards S0_.
The pellicon membrane, which responds almost equally to NO- and S0?, could
be employed in either S0« or NO- transducers. The utilization of this
membrane would require potentiostatic control in an SO- transducer and would
require an SO- scrubber in an NO- sensor. Pellicon hydrolyses in diluted
-------
• . TABLE 5 . •
• SELECTIVE MEMBRAKES
MEMBRANES'
Teflon, 1/4 mil
MEM. 213, 1.2 mils
Polyethylene, 1.2 rails .
Polyethylene, 0.3 mils
Polyvinylchloride, 1.5 mils
Tedlar, 1.2 mile
Saran Wrap, 1 mil
Pellicon, 0.5 mils
Zitex 12-137B, 2 mils
Cellophane, 1 mil. .
Polyethylene and Pelicon
Double Membrane, 1.5 mils
Selectivity Ratios
so2
0.83
1.00
1.00
1.00
1.00
0.0
0.0
0.94
1.00
1.00
1.00
NO
0.73
0.08
0.08
0.14
0.0
0.0
0.0
0.0
0.30
0.0
0.0
K02
1.0
0.39
0.03
0.0
0.0
0.0
0.0
1.0
0.69
0.0
0.0
V- "\
|
-------
'JORPORATION
/solutions of strong acids (H-SO , ) , but it is very stable in diluted and
even reasonably concentrated solutions of weak acids (CHiGOCl!) . Transducers
J
employing IN acetic acid electrolyte were assembled and tested. It was
found that IN CH^COOH can successfully replace the IN H-SO, electrolyte.
' -J £. H-
The polyethylene and MEM 213 (a silicone-polycarbonate copolymer) membranes
are the most suitable for S0_ transducers. MEM 213 responds to S0_ at a
slower rate than the polyethylene membrane, and its sensitivity is temperature
dependent. Sensors employing polyethylene membranes require potentiostatic
control. By adjusting the applied potential upon the gold sensing electrode
to 0.550V, the N0_ interference is totally eliminated. The interference
from NO cannot, at this potential, be eliminated, 'but it is very small
(2 to 5%). At an applied potential of 0.533V the NC>2 is reduced and its
interference is of the order of -2 to -5%, while that of the NO is 0.4 to
1%.
3.2.2 TRANSDUCERS EMPLOYING AQUEOUS SULFURIC ACID ELECTROLYTE
- 1 - 2
Transducers employing 10 N, 10 N, IN, 25% and 50% aqueous II SO, were
f-~ M*
assembled and tested. In all cases Pb09 counter and Au sensing electrodes
were used. No substantial differences -- selectivity, response and
response time wise -- were observed for S0?, N0_ and NO in the first four
aqueous H9SO. electrolytes. But in the fifth one, 50% aqueous H SO. ,
£> *\ • £ T-
sluggish responses were observed.
Electrolytes composed of IN H9SO, with various additives were tested with
£. *"T
the objective of improving the N09:NO ratio, the selectivity for NO , and
£•* • 2C
the response time. Attempts were made to reduce the S0? solubility by
employing the principle of "common ion effect". The ideal ion for this
purpose would have been SO,-, because it affects the first order reaction:
Unfortunately, this cannot be accomplished because, if S0_ was added to
the electrolyte, it would also undergo electro-oxidation. Therefore,
efforts were made to reduce the S0? solubility by employing the same
principle, common ion effect, upon the second order reaction.
= + 4ll+ + 2e~
This was done by employing electrolytes composed of K0SO. or Na SO in
. 2424
-------
CORPORATION
IN II' SO. at concentration ranging from 5% to saturation. The addition of
sulfate ion did not have any effect upon the sensors selectivity.
Electrolytes composed of IN H^SO, saturated with three nitroderivates of
benzene, C,HCN00, D-C..H, (N0_)0 and C,H0(N00) 0 were evaluated. The C,HCNO '
O _> 2 OH/i^ O J £. O • O J /.
and the CfiH.>(N(V)- exhibited neither positive, nor'negative effects upon
selectivities, sensitivities and response times. The IN H9SO. saturated
£. ^T
with p-C,H. (NO,.") „ electrolyte showed a positive effect upon the response
O *T £* f~
times of oxides of nitrogen, and made the NO^rNO selectivity ratio almost
equal to one.
Polyalcohols are known to inhibit the oxidation of sulfur dioxide. For
this reason, sensors utilizing 5% and 10% of glycerol and mannitol in
IN H-SO^ respectively were evaluated. Glycerol showed a medium detrimental
effect upon the S0~ and NO- responses, and a very large negative effect
on the NO response.
3.2.3 TRANSDUCERS EMPLOYING KPF6 IN PROPYLENE CARBONATE AS ELECTROLYTE
Transducers utilizing various concentrations of potassium hexafluorophosphate
in propylene carbonate, CdF? counterelectrode and Au sensing electrode
were assembled and tested. 'Dr3' gaseous mixtures of 2000 pprn NO, 2000 ppm
NO- and 2000 ppm SO., were used in the evaluation of these sensors. The
range of applied voltage was +1.45V to -1-1.60V anodic to the CdF~ counter-
electrode .
The experimental work performed employing these transducers corroborated
the results obtained from the sweep voltam.aie.tric studies, i.e., electrolytes
composed of PC-KPF, are selective for NO . No interferences from CO ,
O X tL
formic acid, formaldehyde, II , HpS, 0?, CO and NH~ were observed. A small
response to SO^ was observed. Table 6 shows the selectivity ratios of
these sensors under various conditions, for S0_, NO and N0_.
The effect of electrolyte additives to the electrolyte comprising 0.25M KPF,
in propylene carbonate were evaluated. The addition of 1.3% (by volume)
nitrobenzene and 10% HO to the 0.25X KPF in P.C. resulted in a faster
response (2.5 times faster) to N0_. But the backgi-ound current was increased
from 7.8uA to 84.5uA for nitrogen, and the response to NO and NO decreased
from 23uA to 2.5uA approximately. The addition of 1.3% HK03 to the 0.25M KPF,
in P.C., 1.3% nitrobenzene, and 10% II 0 electrolyte, raised the background
current to approximately 125 uA and the NO and NO response time increased to
-------
TABLE 6
. EVALUATION OF NO- TRANSDUCERS EMPLOYING K?F,-PC
x 6
ELECTROLYTE
Saturated KPFg in Propylene
Carbonate
Saturated KPF, in Propylene
Carbonate
0.5M KPF, in Propylene
Carbonate (160°F)
0.25H KPF in Propylene
Carbonate
MEMBRANE
1/4 mil
Teflon
1/4 mil
Teflon
1/4 mil
Teflon
1/4 mil
Teflon
POTENTIAL
+1.560
+1.45
+1.53
+1.59
CURRENT FLOWS (ua)/2000 ppai
N2
11.0
1.36
27.8
7.8
so2
12.65
1.4
27.8.
7.83
NO
23.85
10.3
83.2
21.8
NO 2
27.30
17.2
60.4
23.9
SELECTIVITY • '
RATIOS
S°2
9.S
0.46
0
.21
NO
76.7
100
100
87.0
N02
100
92.5
58.9
-------
CORPORATION
.Electrolytic systems employing KPF, in P.C., despite their selectivity for
NO , present severe drawbacks, the most important being response and recovery
2t
times. The average time to reach 90% response was 5 minutes for NO and 30/40
minutes for NO . The average time required to reach 90% recovery was 1 hour
and 1.. 3 hours respectively. These sensors also require long periods of time
to achieve stable responses. This time may vary from a few hours to several
days. .The other drawback of this electrolyte is that it loses its selectivity
for NO when H90 is present.
X £+
3.2.4 f^. ^COOH IN PROPYLENE CARBONATE
A single experiment performed utilizing P.C. saturated with KPF, and doped
o
with CELCOOH revealed a system with apparently large potentialities. The
J
addition of CH'COOH caused a reversion of the NO-NO selectivity, i.e., the
systems became moire sensitive to NO than to NO,.,. It also results in improved
response and recovery times.
A considerable effort was placed upon the development of an NO sensor
JC
utilizing the KPFg-PC-CH COOH electrolytic system. In the course of these
studies, Pt and Au sensing electrodes and Ag?S, CdF~ and PbO,, counterelectrodes
were evaluated. Approximately thirty sensors using four different concentrations
of CiUCOOH in 0.75M KPF in propylene carbonate were exhaustively tested. The
acetic acid concentrations were: three drops per 30 mis. of electrolyte,
2% by weight, 5% by weight and 10% by weight. Previous findings, i.e.,
reversed NO,:NO ratio and faster response times, were corroborated; but
the number of difficulties encountered were sizable. Among them ware the
following:
a. Response times two to three times slower than in NTUSO, electrolyte
sensors .
b. Large zero and span drift.
c. Background current twice as large as the span current for 1000 ppm
of NO.
d. Sensor exhibited erratic behavior.
e. None of the sensors assembled were operational for periods longer
than two weeks. No reasons for this behavior were observed.
f. Sensors were very noisy.
-------
CORPORATION
•Due to time shortage, the study of the KPF^-PC-CH^CQOH electrolytic
system was abandoned. ' • .
3.2.5 TRANSDUCERS EMPLOYING OTHER ELECTROLYTES
Table 7 summarizes the electrolytic systems studied in this phase. The
transducers, in most cases, had gold sensing and cadmium fluoride counter-
electrodes. Gaseous dry mixtures, in the 2000 ppm range, of NO, N0_ and S0_
were employed.
Sweep voltammotrie studies indicated the possibility of employing N, N dimethyl
formamide, 0.75M KPF, electrolyte in SO, sensors. Sensors were evaluated
at applied anodic to counterelectrode voltages of 4-0.75V, +1.00V, +1.10V
and -1-1.20 volts; but no S0? selectivity was observed. Apparently, the
oxidation peak values for NO, NO, and S0? differ but their oxidation curves
overlap. Nitrogen dioxide was reduced at voltages of +1.10V or lower. At
1.10V the current flows were 30ua for 1987 ppm NO, 26 ua for 2054 ppm S02
and 24 ua for N?.
Transducers were assembled using the electrolyte gamma-butyrolactone 0.75M
KPFg +10% H2<3. Teflon. (1/4 mil), pellicon (0.5 mil), polyethylene (1 mil),
and MEM 213 (1 mil) were used as membrane materials. Nitrogen dioxide
was reduced and nitrogen oxide was oxidized at potentials lower than 0.93V.
This makes the electrolyte unsuitable for use in NO sensors.
Different permeation membranes were used in the evaluation of gamma-
butyrolactone -0.75M KPF, +10% H_0 electrolyte. The pellicon membrane
dissolved in the electrolyte. The silicone membrane became brittle and
disintegrated. A transducer assembled using a 1/4 mil teflon membrane .
performed well but there was too much interference from nitric oxide.
Another transducer using 1 mil polyethylene as a membrane showed good
selectivity towards S0? in the presence of NO and N0?. However, rapid
loss of electrolyte during life data evaluations at 160°F resulted in
salt crystallization under the 0.2-0.5 mil polyethylene membrane causing
the transducer to become inoperative.
Transducers were also assembled utilizing 0.375M KPF, gamma-butyrolactone
•1-5% 11JD as electrolyte. Decreasing the salt concentration reduced
crystallisation as electrolyte was lost, but the response current and
response rate to NO were significantly decreased. The use of 0.2-0.5 mil
X •
polyethylene, 2 mil polyvinyl chloride membranes, and 1/2 mil teflon
-------
TABLE 7
EVALUATION OF VARIOUS ORGANIC ELECTROLYTES
Electrolyte
Formaruide -0.75M KPF, +
10% H20
N, N- Dimethyl Formamide
-0.75M KPF,
D
Propylene Carbonate
Saturated with- KPF,
o
% Butyrolactone -0.75M
KPF6 4-10% H20
.375M KPF, in "o Bu ty-
ro lactone-^5% H£0 *
.375H KPF, in ^ Buty-
rolactone +5% EJD •>''>'
Transducer
so2
so2
NO
X
so2
so2
S02
Potential
+1.05
•fl.lOv
+1 . 60v
-rO.92
+0.92
+0.92
so2- .
+100%
+100%
0.48%
+100%
63.5%
100%
NO
0
+300%
+100%
+12.3%
100%
0
N02
+0.935%
Large Reduction
92.5%
-3%
-5.15%
0
-•> With I/2m Teflon membrane
-------
U irillWl. ^OiitiMiit'l!:^
COKPGRATIOiM
membranes did not substantially affect the entent of the response to
S02*
In the evaluations of transducers using formamide -0.75M KPF,. 4-10% H_0,
the voltage applied to the gold working electrode was varied from -KL.OOV
to 41.10V to achieve a large response to S09 without interference from NO
and N02< At -1-1. 05V the current flow was 61.5 ua for 1941 ppm SO , 33 ua
for 2011 ppm NO, 33.6 ua for 2410 ppm N02, and 33 ua for dry nitrogen. The
interference was 0% for KO and 0.90% for NO . At applied voltages of +1.00V
and balow, N0? is reduced, but at potentials of -i-1.05 and above it is
oxidized. This electrolyte was considered unsuitable because of the large
background current .
3.3 SELECTED NO AND S00 TRANSDUCERS
" X 2. ,-.— ———
The transducer utilizing l.ON HJSO, electrolyte, gold sensing and lead
dioxide counterelcctrodes, and a 1 mil polyethylene membrane was selected
for employment in the S0~ prototype monitor. The transducer utilizing
aqueous l.ON H^SO, was selected over the transducer utilizing formamide
-0.75M KPF,. -M0% HJD primarily because of the latter 's small response
to SOy in comparison to background current. The transducer utilizing gamma -
butyrolactone -0.75M KPF, -!-10% H?0 was unsatisfactory primarily due to
the rapid loss of electrolyte and the crystallization of salt. The 1 mil
polyethylene membrane material was selected over other materials because
of its stability, selectivity for S0« and permeation characteristics.
Sweep voltaianetry studied revealed the existence of only one electrolyte,
KPF,. in propylene carbonate, suitable for use in NO selective transducers.
6 l L J ' x .
Further evaluation of this electrolyte, with and without additives
employing the actual electrochemical sensors, showed the existence of severe
drawbacks. The overcoming of the problems encountered with this transducer
would most likely require a considerable length of time. Studies performed
employing a wide variety of simepermeable membranes did not uncover the
existence of NO selective membranes. Therefore, because of lack of alterna
x '
tives, the NO transducer to be used in the prototype monitor will employ
X
IN II^SO, , gold sensing electrode, lead dioxide counterelcctrodes and 0.25
mils teflon semipcrmeable membrane. This sensor is not selective for NO
• *••
ancl therefore a scrubber for the removal of S0_ is a necessary part of the
NO sensing system. :
X •
-------
COHPORAYICKO
, '3.4 SULFUR DIOXIDE .SCRUBBING SYSTEMS
Once the necessity of an SO scrubber was established, various SO scrubbing
systems were designed and evaluated. Scrubbing is a basic chemical engineering
unit process which consists in the absorption of gases-" in liquid or solid
phases. The process is favorably affected by turbulence, high inter- 1
facial surface areas, and high diffusion coef f iciencies . In order to have jj
efficient absorption the gas must either react irreversibly with, or be f
very soluble in the absorption liquid or solid. The studied scrubbing systems I
may be grouped in tv?o categories, electro-oxidation and wet scrubbing. The I
I
latter clascif ication, wet scrubbing, can also be divided into two sub-
groups, oxidants and alkaline absorbants .
v
3. A.I ELECTRO- OXIDATION SYSTEMS I
The possibility of scrubbing SO by electro-oxidation was considered because I
sulfur dioxide and nitrogen oxides can be readily oxidized to sulfuric acid \
V
and nitric acid respectively. i
The reactions take place on a solid electrode and the principles involved jj
are similar to the gas diffusion fuel cells. Porous electrodes are used to C
fi
provide large contact areas between the gaseous fuel (in this case, pollutants), f:
the liquid electrolyte and the solid electrode. However, there are some
important differences. Hydrogen as a fuel in a typical H?-0 fuel cell requires
a platinum surface which needs to be rejuvenated often. On the other hand, we
have abserved that NO and S09 may be readily electro-oxidized on a variety
X £»
of surfaces including porous nickel, carbon, gold and platinum. Also, unlike
in a fuel cell, the cathode may be contained as an integral part of the system.
Thus, in place of the 0_ electrode, we could have a strong oxidizing lead
•
dioxide electrode. The reaction taking place at the lead dioxide electrode
is given below: . .
Pb02 H- SO^," + 4H+ + 2e - PbSO^ + 2^0 1.685 Volts
The reaction product, PbSO,, can be converted back to Pb09 in a fashion
similar to that of a lead-acid battery.
-------
CORPORATION
. '3.4 -SULFUR DIOXIDE SCRUBBING SYSTEMS
Once the necessity of an S0» scrubber was established, various SO. scrubbing
systems were designed and evaluated. Scrubbing is a .basic chemical engineering
unit process which consists in the absorption of gases"' in liquid or solid
phases. The process is favorably affected by turbulence, high inter-
facial surface areas, and high diffusion coefficiencies . In order to have
efficient absorption the gas must either react irreversibly with, or be
very soluble in the absorption liquid or solid. The studied scrubbing systems
may be grouped in two categories, electro- oxidation and wet scrubbing. The
latter classification, wet scrubbing, can also be divided into two sub-
groups, oxidants and alkaline absorbants .
3.4.1 ELECTRO- OXIDATION SYSTEMS
The possibility of scrubbing S0_ by electro-oxidation was considered because
sulfur dioxide and nitrogen oxides can be readily oxidized to sulfuric acid
and nitric acid respectively .
The reactions take place on a solid electrode and the principles involved 5
are similar to the gas diffusion fuel cells. Porous electrodes are used to [..
I;
provide large contact areas between the gaseous fuel (in this case, pollutants), i
$
the liquid electrolyte and the solid electrode. However, there are some ||
important differences. Hydrogen as a fuel in a typical H^-O fuel cell requires I
t
a platinum surface which needs to be rejuvenated often. On the other hand, we \
have abserved that NO and SO- may be readily electro-oxidized on a variety |
X £•* t
of surfaces including porous nickel, carbon, gold and platinum. Also, unlike i
in a fuel cell, the cathode may be contained as an integral part of the system. jj
Thus, in place of the 0_ electrode, we could have a strong oxidizing lead i.
dioxide electrode. The reaction taking place at the lead dioxide electrode ' ' [<
£
is given below: - ?
Pb02 H- SO, -f 4H -h 2e = PbSO, + 21^0 1.685 Volts
The reaction product, PbSO, , can be converted back to PbO in a fashion
similar to that of a lead-acid battery.
-------
FTiP'Irl'lfi : ; ______ .JEE^^Hiii^EiSi
CORPOKATIOPJ
The electro-oxidation cell was assembled using a porous nickel screen as
the electrode. The electrode was made hydrophobic on one side by a thin
teflon coating. The cathode used, was a lead dioxide electrode obtained
from a lead-acid battery. Dilute sulfuric acid was used as the electrolyte.
The cell was assembled such that the gas sample could be passed over the
hydrophobic side of the electrode and not come into direct contact with the
electrolyte. Applied potentials were varied between 0.0V and 0.4V. Sulfur
dioxide was most efficiently scrubbed at an applied voltage of 0.0V. Oxides
of nitrogen (NO and N09) were also electro-oxidized at this potential. The
S09 removal efficiency was of the order of 60-70 percent. This electro-
chemical oxidation system could most likely be made more efficient and
selective. The system could be made selective by employing a more selective
membrane, by the addition of nitrate and nitrite ions, or by finding a mean
to keep the potential difference bet\?een the electrodes selective and efficient
for S0~. The complete development of this scrubbing system would require an
effort beyond the scope of the original contract.
3.4.2 WET SCRUBBING SYSTEMS
3.4.2.1 The __Fe (OH).- iJKaNO3_OxMSH1L.S_cr
The ferric ion selectively oxidizes SO,, in the presence of oxides of
nitrogen according to the following half-cell reactions:
+3 - +2
Fe -f e -:.--—>- Fe 0.77
NO + 2H20 ^r±: NO " 4- 4H+ + 3e" 0.96
N02 -I- HO ^=ZZ N03" + 2H"'" + e- 0.80
S0? + 2H20 ?L=.-£. S04"2 + 411* + 2e" 0.17
The absorption and ulterior oxidation of S0? occurs according to the
following reactions;
-I- 2Fe (OH)3 ^rz±: 2Fe (OH)2
H- Fe (Oil) =ivrr±: . 2\i, + Fe
-------
Y-w; IKSTRIIM SYSTEMS BSVISiCfi
tiil!:© __. __ .——-—— —
COKPORATSON
'The overall reaction can be. summarized by:
2Fe(QU), -I- SO 5> 2LI 0 + FeSO. -I- Fe(OH),
J £» • £• r - ^£
The addition of NaNO~ is necessary because of the large solubility of N0~.
Sodium nitrate and nitrite decrease the solubility of N0? by means of the
"common ion effect" principle.. A large nitrate ion concentration causes
the NOp solvation reaction to shift to the left; hence causing the N0?
solubility to decrease accordingly with:
.: : J> + j> „ H+shift
H,0: -I- ;N-N ;s=5r 1L.O -N + NO -£——!? HONO + UNO
. 0 D ^0 .
The scrubbing solution was prepared by slowly adding Fe (SO,)„ (100 g) to
a solution containing KaOlI and 250 g of NaNO~ in 400 mis of water. The
scrubber container was a standard, glass bubbler with 1/2-inch diameter
disperser. This scrubbing solution removed 2% of 2000 ppm KO, 30% of
257 ppm NO and was 100% efficient in the removal of 2000 ppm SO at a
flow rate of 2 SCFH. The major drawback of this scrubbing solution is
its bulkiness, i.e, for one week operation at 2 SCFH of 2000 ppm SO ,
400 g of Fe (SO,)0 in two liters of water would be necessary.
3.4.2.2 The_H 0 7^NaNO jOx idant JSc^uJbb ing_Svj;J: £m
/C. £. J)
This sc7:ubbing solution was composed of 250g of NaNO», 320 mis of II 0 and
80 mis of 30% HO. Sodium nitrate is also used in this solution to reduce
the solubility of N0_. The gaseous mixtures employed in the evaluation of
this scruber were 2000 ppm NO, 2000 ppm SO and 257 ppm NO . The SO
removal was 100%, the NO's 4% and the N0» removal 12%. The main drawback
of the H_0?-NaNO_ scrubber is that its decomposition would be catalyzed
by organics present in the stack gas.
3.4.2.3 lhe_Na CO«1NaNO»_.Alkaline Scrubber
2 ~> ~ ~~>
A scrubbing solution containing 112g of Na CO and 320g of NaNO,, in 500 mis
of water was evaluated employing 2000 ppm NO, 1049 ppm NO , 100 ppm NO and
2000 ppm S0». The SO was totally removed. This scrubber also absorbed 2%
of the NO, 13.7% of the 100 ppm NO and 18% of the 1049 ppm NO . It must b
f. 2
noted that this scrubber does not oxidize the S0_, but merely neutralizes
the sulfurous acid 'formed by the dissolution of S0? in water.
-------
-h
Na + OH" -h NaHCO,
NalIC0
-I- Oil
H2co3 ^
H20 + CO
H- 2XaOH
2H0
This scrubber was selected, with some modifications, for use with the NO
prototyps monitors.
3.4.2.3 The_KrCOoHKHGO[,-KNOoiKNO A Ucalina_Scrubber
This scrubbing solution is basically like the aforementioned one. The
NaJX),. was substituted by K CO,, because of the latters larger solubility;
this will extend the useful life of the scrubber. In addition, the solution
also contains KKCO^ for the. purpose, of reducing the absorption of the C0_
contained in stack gases. Because experimental data showed that the addition
of KNO,, reduces the absorption of N0_, this chemical is also contained in
the scrubbing solution. The composition of the scrubber, par 100 gr of I
water, is; 10.6 grs. KHC03> 75.7 grs. K^O , 3.2 grs. KNO , and 8.17 grs. KNO . '
Laboratory tests have shows that this scrubber will remove 2000 ppm S0_n_
completely while absorbing less than 1% of 2000 ppm NO and less than 20% of
100 ppm N0?. Because of the low NO concentration (with respect to NO) in
stack gases, the overall effect on the NO concentration is insignificant.
-------
' ' . iriSTjUJ?,;Ef!T SYSTEMS DP«'ISiD:i
i ._.--. ....._ -i— r-i-_ ._ .. . -._... -••_-.. .. .. .-— ___ -.-u.r--.f-TT - - — axu- _LI.--.- • i~ ' . ..... " ' -------------- --------- - — ~»...a-mJ»..i».MD«mi^.. ••.-ma .• -*...s^.i ..• ......... aiia.im^
-ORPORATIOIM
4.0 PROTOTYPE MONITORS PERFOR^IAHCE DATA
The S02 and NO. transducers response is linear throughout the 0 to 5000 ppm
concentration range. Figure 6 shows a plotting of NO concentrations versus
X
the sensor's output and versus the amplified output, thereby illustrating the
linear response of an NX- 130 transducer. Similar results are obtained with
S0_ transducers.
4.1.1 SENSITIVITY AND OPERATIONAL TEMPERATURES
Table 8 shows the effect of temperature upon the sensitivity of NO and SO
• X
transducers, for various pollutant concentrations, and at three operational
temperatures -- 70°F, 120°F and 160°F. The values shown in Table 8 are
mean valuar, obtained! from long- exposure data of a large number of transducers
It is interesting to note that although the background (zero) currents
increase at a rate larger than those of the spaa currents, the transducers
sensitivity increases, with increasing temperatures.
4.1.2 SENSITIVITY RANGES
Originally the NO and S00 prototype analysers were to have three ranges of
x /
sensitivity: 0-50, 0-1000 and 0-3000 parts per million. This is equivalent
to span ratios of 1:30:60. Two analyzers, SS-330 and NX- 130, capable of
sensing currents of the order of 150ua, without saturating the operational
amplifier, and having the aforementioned span ratios were evaluated. It
was found that at the lower range (0-50 ppm) the instruments zero and span
pots, had poor resolution. It was therefore, necessary to increase the lower
range from 0-50 ppm -to 0-100 ppm, the new span ratios being 1:10:30.
4.2 .iELj^rVTrY
The effect of various potential interferences was determined employing the
final version of the S0~ and N0_ prototype analyzers (Table 9). The
selectivity ratios of various permeation membranes for NO, NO and S0_ ware
shown in Table 5.
Four different pollutants were found to interfere with the selective sensing
-------
0
1000
2000
3000
4000
5000
PPM NO
Figure 6. Response of KO Prototype-Monitor
X
-------
TABLE 8
TIB! EFFECT OF TEMPERATURE UPON TRANSDUCERS SENSITIVITY
STACK
MONITOR
SS-330
NX- 130
POLLUTANT
CONCENTRATION
20 ppm SO „
.'.
300 ppm SO
1000 ppm SO
2000 ppm SO
N2
50 ppm NO „
2000 ppm NO
CURRT
70°F
0.15
1.75
6.95
15.8
0.03
0.7
2.84
1000 ppm NO | 12.10
2000 ppm NO
N2
23.0
0.03
:NT, .MIC ROAM
120°F
__
7.3
25
48
1.2
--
21.4
44 . 1
1.0
PERES _
160°F
. __
13
45
95
8- '-
1.8
6,5
32.7
64.0
5.5
-------
'TABLE 9
EVALUATION OF VARIOUS INTERFERENCES
MONITOR
NX130
SS330
.
CO
121ppm
0.0
0.0
266ppm
45.0 '
30.2
lOOppm
0.0
0.0
]
Q
2
217.
0.0
0.0
INTERFERENCES PERCENT*
HCOOH ; H CO
600ppni ; 500?pn:
0.0
0.0
0.0
0.0
C°2
m-
,
0.0
0.0
* /
548ppm
2.00
3.34
E2
0.0
0.0
•
.
03 ! 10%
SOOppsd : H,0
£
0.0
0.0
0.0
0.0
* •
Monitor
J NX130 respect 1000 ppn NO at 70°F
^ SS330 respect 1000 ppm S0? st 70°F
-------
^ J5SI5HIE!!
CqitPORATIOM
control (Figure 5) the interference from 300 ppm N09 is almost wholly
eliminated (0-0.5%), and the interference from 2000 ppm NO reduced to 7%.
The interference from 548 ppm SO- and 266 ppm E9S were found to be 3.37o
and 30.2% respectively. All of the aforementioned degrees of interference
are expressed with respect to 1000 ppm of S09. Among these interferences
only that of II93 may be said to be significant. Should it become desirable,
the H9S 'can be readily and selectively absorbed in a column containing silica
gel impregnated with Ag SO.. Ammonia interferes with the selective absorption
(1)
-of 1I_S from SO -H S gaseous mixtures. ' It is important to bear in mind
that 11 S-SQ mixtures are not stable because of the following probable
reactions:
US 4- 5SO 4- HO rrr——>• 2S 0 "2 4- 411*
8H2S -I- 4S02 -rs=±^ SO -I- 4lI20 4- Polythionic Acids
The NO (KX-130) analyzeris, despite of the results shown in Table 9, free
of interference from S0», 11 ~S and S0_ because this analyzer employs a scrubber
which absorbs SO,., SO,, and H^S with an efficiency of 100%.
4. 3 RESPONSE AHD RECOVERY TIMES
Figures 7 and 8 illustrate the response and recovery times characteristic
of S00 (Model SS-330) and NO (Model NX-130) respectively. The S09 transducer
£•• X ' f~
has a response time of 90% of full scale (F.S.) in less than forty-eight
seconds and a recovery time of 90% of F.S. in less than eighteen seconds.
The NO electrochemical transducers have a response time of 90% F.S. in
x •
five to twelve seconds for NO, and thirty to forty seconds for NO,.,. The
recovery times (90% F.S.) for these two pollutants are twenty-four and
forty seconds respectively.
4-4 ST^BILITY_pj;_KO AMD .-SO PROTOTYPE JHNITORS
The zero and span drifts per day of NO and SO selective analyzers were
measured at temperatures of 40°F, 100°F and 160°F (Table 10). The values
shown in this table were obtained under the most adverse conditions, i.e.,
no temperature control was employed. As it may be expected, the instrument's
drift increases with increasing temperature. Still, even at 160°F the daily
(1)
-------
FIGURE 7
CHARACTERISTIC RESPONSE OF MONITOR SS-330
TO VARIOUS SO CONCENTRATIONS
CHART SPEED: 1 INCH/HOUR
29 PPM
SO,,
II'
10 PPM
2000 PPM
2
.LOW.: LOW „_ .....HIGH.
A.P.M.
RANGE
-------
FIGURE 8
CHARACTERISTIC RESPONSE OF MONITOR NX-130
TO NO AND NO,,
2000
KO
HIGH
2000 PPM
•NO,,
CHART SPEED: 1 INCH/HOUR
20 PPM
NO
LOW
LOW
A.P.M.HANG IS
-------
TABLE 10
STABILITY OF NO^ AND S02 PROTOTYPE MONITORS
:-: "." " ~~ ' '
MONITOR
NX- 130 .
•
SS-330
TEMPERATURE
"F
40
100
160
40
100
160
±%, CURRENI
ZERO
0.15
0.50
0.9
0.23
0.61
0.95
DRIFT /DAY *
SPAN
0 . 24
0.51
1.3
0.5
0.61
1.2
'Relative to 340 PPM SO -Full Scale, in the instruments' most
sensitive range.
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CORPORATION
drifts do not exceed ±1.3%. At temperatures below 110°F. (the most widely
vised temperature) the instruments' drift is well below ±17o of full scale per
day. With the use of temperature control, the span and zero drifts are well
below ±0.5% F.S. per day.
4.5 TRWSDUCERS LIFE DATA
The transducer's life is a direct function of the diffusion rate of the
electrolyte through the permeation membrane, and therefore also a function
of the pollutants' flow rate through the sensor. The same liquid electrolyte
diffuses at different rates through different'membrane materials. The
evaporation rate of IN H-SO^ electrolyte through 0.25 mil Teflon and one
mil polyethylene membranes were determined at two different temperatures,
70°F and 160°F. The electrolyte losses were found to be 0.04 grs/day at
70°F, and 0.25 grs/day at 160°F for the polyethylene membrane. Through the .
0.25 mil Teflon membrane, the losses were 0.26 grs/day at 70°F and 1.86
grs/day at 160°F. The gas flow through the cells was one S.C.F.H. It is
necessary to emphasize that thase. values were obtained utilizing dry
gases. Under standard conditions, the losses of electrolyte would be
substantially smaller.
4•6 TEMPERATURE COMPENSATION AND CONTROL
Because of the large effect of temperature upon the sensors output (Section
4.1.1), the necessity of temperature compensation or control is evident.
Attempts were made to temperature compensate the sensors output by means of
a thermistor and associated electronics. Various different thermistors,
positioned in the top half of the sensors, were evaluated. The best results
obtained were with GA45P1 (50K, Fenwal Electronics Inc.) thermistor. This
thermistor and associated electronics were found to effectively compensate
(±87, of full scale) for temperature changes within the 40°F to 120°F range.
Since larger accuracy and stability were desirable, a temperature controlled f
K
environment for the sensor was designed. The heater and sensor assembly are f
discussed in detail in the Appendix. The heater and associated electronics :
effectively control (±0.5CF) the temperature of the sensor within the 40° to |
140°F range. This small temperature variation (±0.5°F) to which the sensor is is
3
subject to, has no significant influence upon the sensors output. /
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COPPOrtATION
APPENDIX
Figure 1A shows a front view of the newly developed - second generation -
Dyriasciences Air Pollution Monitor. Front, top and side schematic views
of the. Monitor are shown in Drawing No. 620432. Four of these instruments
(two NX-130 and'two SS-330) were supplied to H.E.W. as part of the Contract
CPA-22-69-118. The monitors are compact and portable, and they were
designed to continuously measure the concentration of different pollutants
in a gas stream. The pollutants that can be measured are sulfur dioxide
and oxides of nitrogen. The outstanding .feature of the monitor is that any
one of the six sensors developed by Dynasciences can be incorporated in the
same monitor. _ The model numbers, ranges, and the recommended applications
of the different sensors are as follows:
MODKL
1. NX--130 SP
2. SS-330 SP
GAS MEASURED
""Total oxides of
nitrogen
Sulfur dioxide
.RANGES
0-100, 0-1000
and 0-3000 ppm
0-100, 0-1000
and 0-3000 ppm
APPLICATION
Industrial stacks,
automobile exhaust,
process control,
development studies
Industrial stacks,
process control,
development studies
"Requires scrubber if S0? present.
Table 1A shows the specifications for Monitors NX- 130 and SS-330.
Each sensor is a sealed unit and no addition of any electrolyte is ever
required. The sensor operating life may be as long as one year or more
depending upon the condition of the sample. Generally spsaking, hot, dry
and dirty samples will shorten the sensor life. The sensor may be easily
replaced with a new sensor as described below. Drawing 672020 and 672021
show the design and specifications of the transducers polypropylene body.
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WIJASOTS
AIR POLLUTION MONITOR
BAMPLB FLOW
40 . EQ/ BO
• •'• -.., . mo
OF RANGE
SYSTEM HEATER ZEHO RANOB SPAN
POWER POWER „__
Figure 1 -A.
Figure 2-A.
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COHPORATIOW
TABLE 1A
MONITORS SS-330 AND MX- 3.30 SPECIFICATIONS
SENSITIVITY RANGES;
DETECTION LIMIT:
RESPONSE:
RESPONSE AND RECOVERY TIMES;
STABILITY:
SPECIFICITY:
TEMPERATURE CONTROL:
OUTPUT:
SENSOR LIFE:
POWER SUPPLY:
MONITOR WEIGHT:
FLOWRATE:
0-100,0-1000 and 0-3000
parts per million
•2% of full scale
Linear response over the entire
range of the pollutant. Overall
accuracy: ±2% full scale .with the
use of integral meter, ±1% full
scale with the use of external
potentiometrie recorder
907o of full scale in less than
60 seconds
Better than ±1% of full scale
per day
No response to N2, 0~> CO, COo,
water vapor or hydrocarbons
About 120°F ±1%F, adjustable
(see Text)
Meter readout and 0-10 mv output
for recorder
One year at 70°F, moist gases
105 to 125V 60 Hz 5 VA required for
powering solid state amplifier or
12 V.D.C.
Less than 15 Ibs.
Keep the flow rate within the
green indicator on the flow metc-.r
-------
A| -5.EE DON f^JtfM IOAU&70 \-'ff
,"ArL-t*i
VIEW B' B
OPTION CHART
-I SENSOR AMD HEATER (MODEL £OO)
-Z. SENSOR AND HEATER (MODEL4OO)
-3 SENSOR MODEL 2OO
-4 LESS SENSOR AND HEATER(5TOCK OMLY)
-5 LESS SENSORCSTOCK ONLY>
COVER MOUKITING
SCREWS-2 PX.ACE.SIREF)
S2 REOO
lEO'APART
TMRO \TEM IA\
-CHASSIS MOUMT
SCREWS-2 PLACESCREF)
FOP PARTS LIST SEE PL620432
VIEW A-A
mcr tan I uso> o
u
HR»J ft-18-fo
f nppnDATinM '
MON ITOR,POLLUTION,
AIR
-------
•®-\ K-®«- ;
A PARTS LISTS FOR DASM NUMBERS NOT DRAWN; INrORMATION
^"^ FOR REF ONLY.
/§> ARROWS INDICATE DIRECTION OF FLOW
^ HEM 16 IS REPLACEMENT LAMP FOR ITEMS 135118.
A, ITEM 45 IS 7/16 LONG AND CENTERED ON'S' POSITION.
P^ITEM 143 USED w ITH HEATER,
ITEM 88USEDVW/O HEATER
-------
,,
cor:rjor.ATioi\!
• SE-Ng Ojj, REPLA CEMENT
"
The instrument is designed to be maintenance free. Tha sensor must never
be taken apart. A fault in the sensor usually shows up as slower response .
time and/or inability to calibrate the instrument.
The faulty sensor may be easily replaced with a new sensor in the manner
described below. The newly replaced sensor may take 6 to 8 hours to
stabilize before it is ready for use. Before replacing the sensor, make
sure that the slide switch located near the battery on the chassis is in
the proper position. The use of models Nil- 210 and NR-230 for NO- monitoring
requires the slide switch moved towards the front of the monitor. All other
models require slide, switch in the opposite direction as marked.
The procedure . for sensor replacement is as follows:
Step 1 - Disconnect 115V input power cord.
Step 2 - Remove (2) screws, Item 65, and remove cover/rear
panel assembly.
Step 3 - Disconnect (2) tube fittings, Item 152.
Step l\ - Unplug electrical leads running from sensor to chassis.
Step 5 - Remove insulator, Item 130.
Step 6 - Remove (2) screws, Item 150, which hold cover, Item 141,
to enclosure, Item 140.
Step 7 - Remove cover/sensor assembly 602580 and replace with
new assembly.
Step 8 - Reverse Steps 1 through 7.
NOTE: Fittings, Item 152, shall be firmly Jhand tightened only.
HEATER
A heater assembly with the appropriate electronics is provided on all the
monitors. The purpose of the heater, as it was previously stated, is to
maintain a constant temperature environment for the sensor, thereby improving
the stability and accuracy of the system. Figures 2A and 3A show "the sensor
and heater enclosure in different stages of assembly. Drawing 620432
schematically shows the sensor and heater assembly.
A-5 1
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; . ______ inSTngOJT$Vg£gD;yi?g
COJU'ORATIOW
5 "The heater can be switched on by pushing the HEATER POWER switch on the front
panel. . About 3 1/2 to 4 hours are required for the sensor to come to equilibrium
at a factory-set temperature of about 120°F. Laboratory tests have shown that
heating the sensor at temperatures higher than 120°F result in shorting its
operating life. Continuous heating to 160°F for 4 hours resulted in permanent
damage to the sensor. The sensor may be operated at 140°F for a short
duration of up to 8 hours at a time, without damaging the sensor. To readjust
the temperature set-point, proceed as follows:
Remove the printed circuit board from its receptacle.
Using an Ohmmeter, check the resistance between Points A and B
(see Drawing No. 602517, Printed Wiring Assembly) and adjust
pot R41 to obtain 25K ohm resistance.
Install the printed circuit board back in its receptacle.
For the applications where the ambient sample and room temperatures are
fairly constant or where the monitor is required to be. used for a spot test,
the heater is not required, thus avoiding the delay for the sensor to come
to equilibrium.
The heater is proportionally controlled thus eliminating the possible inter-
ference from an On/Off action. The HEATER red light on the front panel
indicates the level of power to the heater. In tha beginning when the
HEATER POWER is switched on, the HEATER light is very bright. However,
within the first two hours it starts dimming and stays fairly dim for the
continuous operation. CAUTI.ON - If the HEATER light remains bright for more
than two hours, turn the power off and check for possible malfunction in the
heater circuit. The two white leads from the heater should be firmly plugged
in the white jacks on the chassis which supply the power to the heater. Check
the resistance across the two green leads after removing them from the chassis.
These loads are joined to a thermistor which should read about 100 Kohrn
resistance at room temperature and about 45 Kohm resistance at the controlled
temperature of 120°F. A broken thermistor would show an infinite resistance
avid would keep the heater power on all the time.
-------
s , i • • . •
The meter reading is directly proportional to the concentration of the
pollutant. In addition, a 10 mv recorder output is provided in the back
of • the monitor for continuous recording. The output jack is available on
the back of the instrument next to the sample fitting.
115/230 volts and 100 volts, 50 or CO Hz powered from the three-prong
receptacle on the back of the monitor. Also a 12 V.D.C. capability is
provided. The 12 V.D.C. input jack is provided on the back of the instru-
ment next to the fuse.
A size "D" mercury battery is used to bias the sensor. Its voltage at all ' I
times should remain 1.35V ±10%. The battery voltage may be checked by *
holding the RANGE switch on the front panel to the BATT TEST position. [
Replace the battery if the output on the meter shows less than 90%. full •, L
scale deflection. . f
?'
I:
.SCRUBBER I'
For the applications where the analysis of NO using NX-130 is desired in JT
the presence of SO,., a liquid scrubber is provided for the removal of SO >
before the sample is passed to the monitor. The scrubbing solution is
especially prepared to remove SO- without affecting the NO concentration.
A cartridge containing glass wool is attached on the exit of the scrubber
to trap axiy liquid carry-over.
SAMPLE _ C01TDI TIONIJgG AND gRESSURE AD.JITS THEM TS ,
The gas sample should be free of suspended material and the moisture
content of the sample should be such that it does not condense in the sensor.
Except for this restriction, it is actually beneficial in terms of life-
time of the sensor to use moist gases or gases with a dew point slightly
below the operating temperature of the sensor (relative humidity of 40 to
80% is recommended). The temperature of the sample should be kept less
than 100°1;1 and preferably at room temperature. Higher sample temperatures
to 140°F may be used for a short duration of up to 8 hours. Longer exposure
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CORPORATION
be ;passed over the sensor by connecting it to the input 1/4" polypropylene
i » -
tube fitting provided at the right side when facing the back of the instru-
ment. The sample passes through the sensor, flowmeter and exits from the
fitting on the left side which should be connected to the vent line. The
instrument reading is independent of the flow rate. However, it is suggested
that to get the optimum response time and a long operating life, a flow rate
of 0.5 to 2.0 SCi7H be kept steady throughout the run. If the flow indicator
is kept in the blue area, the flow rate will be satisfactory.
The sample should be preferably "pushed" rather than "pulled" through the
sensor. If it is desired to pull the sample by creating a suction at the
exit, care must be taken to assure that the vacuum created in the
sensor is not more than 6" of H90.
i ' £
The sensor is designed for operation at atmospheric pressure. It can be .
operated "in line4." at pressures slightly above or below atmospheric, but
it is of u^ino£t_im]>Drtance that any pressure changes occur slowly in order
to prevent damage to the sensor. j
!
Since the sensor measures the partial pressure of the gas being monitored, |
it must be realized that a correction should be made if the sensor has been 5
calibrated at one pressure and is used at another. For example, calibration
of the sensor at 740 mm Hg of ambient atmospheric pressure and 1009 ppm of
NO gas results in a full scale meter reading equivalent to 740 mm Hg.
J\ "•
If the total "in line" pressure is changed to say 700 mm Hg, and the ratio
of molecules of NO gas to total gas is kept constant at 1000 ppm, then
the new reading will by ~ — of full scale.
The instrument may be calibrated in any RANGE position. However, for
maximum accuracy, calibrate in the RANGE most commonly used. Turn on
power to the instrument by pushing in the SYSTEM POWER switch. Set the
RANGE switch to the desired position. Turn the SPAN knob completely
clockwise. Wail about 15 minutes before proceeding with the calibration.
If the heater is used, wait 4 hours before, proceeding. Flow the zero gas
(N9 or NO , and SO, free air) through the instrument and wait for 5 minutes
*~ 5t /-
Then bring the reading to zero by adjusting the ZERO knob. Lock the ZERO
knob. Pass the calibration gas over the sensor. Wait: 5 minutes. Adjust
the SPAN knob so that the meter reading corresponds to the concentration
-------
rill ____ l?ISTnijLM^5fE^
COFIPOKATIOW
in the calibrated sample. Lock the SPAN knob.
The instrument is designed to be essentially drift free and will retain
its calibration for a long time. However, for optimum results, calibrate
the instrument at regular intervals.
TjlE ELECTRONIC PACKAGE
The following material was prepared by Mr. J. W. Schmidt, Supervisor of
Laboratory Instruments.
During system operation the cell output current, is converted to a proportional
voltage by the Current Amplifier. This voltage signal is then scaled by
the span control and amplified by the scaling amplifier. The scaling amplifier
gain is determined by the range control (High - Keel - Low) which provides a
proper output for full scale meter deflection and recorder output.
.The current amplifier input is provided by the cell (MTJ.) and bias network
composed of resistors RGO through RG4 and battery (B) . The function of the
bias network is to provide the proper cell bias for system application
required.
The front panel zero control circuit consists of pot R66, resistors R3, R4
and R5 and 6.2V zeners VR, and VR?. This provides a ± current to offset
residual cell currents.
Q, , R- and R are creating a short circuit to cell currents when the power
is in the off state. This maintains the cell in a ready state for rapid
stabilization.
The current amplifier is basically a low noise FET input amplifier which
utilizes the ua 741 for high open loop gain and complimentary output
drivers Q. and Gv for linearity. -Potentiometer R,„ and resistors R and
*~r *J i. -J i.\J
R.„ are used to zero the amplifier during initial alignment. The amplifier
gain is determined by RT, , RI and R . R and R are used to modify the
temperature characteristic of thermistor RT . RT and R are physically
located in the cell to obtain temperature tracking gain which will provide
the best system accuracy during heater warm-up. The basic gain equation
is:
Vo = Iin RF
-------
UNLESS OTHEHWISE SPCCIFIED
INTPR DWG P£R MIL-STU-100
OiM.AflE IN INCHES
DO NOT SC*t£ DRAWING
DIM. TO BE MLT AFTER FINISH
STANDARD HOLES PFR AHD1038
HSTWUIKT SlTTtWS BYIPOW CHATSWOB'
SCHEMATIC D1AGE.A/Y\
MOX MOMITOE.
kPPROVED HR.W 4 - 19 - TO
4i.
G6024&
-------
II ^^.ZVPO
7.FOR ASSY SEE DWG N0.62043E.
^^COMPONENTS IN THIS AREA DELETED ON UNITS W/0 HEATER.
C23 IS REPLACED WITH IOO MF,^^5^25 v CAPACITOR.
^^P3 WILL MATE WITH 0 3, J4, J5,J £,J~7 OR J8 DEPENDING ON
APPLICATION.
GROUND POINT AT PINS C.D.&E OFCONMECTORSOKI
CHASSIS ADJACENT TO PIN C .
--... -.. GROUND POINT AT PINS 5 S. 10 OF TRANSFORMER
AND ON CHASSIS ADJACENT TO TRANSFORMER.
" RT'S>RT£ ARE LOCATED irl CELL MT I. RT3& RT<5
MAY BE REQUIRED FOR TEMPERATURE COMPENSATION ON
UNITS Y7/O HEATER. OPTION POR MOUNTING THEM ON
CIRCUIT BOARD SHALL BE PROVIDED.
2. CaPACITANCt VAl_UE6 AKE IMjjF, IO'/., IOO V
I. ftiliToK VALUES AE6 IN OHM1,±l?i, Vtl<*jf/
NOTES: UIILESS OTMESWISE SPECIFIED
-------
Rl RT]
lin, :•.;• the cell, current Rf = R, 4- «~T~i7^"~
' ' J. L KI T Jxi,
The scaling ampli.fj.er AR_ is an inverting operational amplifier with four
selectable gain settings determined by the range switch. The basic gain
equiation is:
. Rf .
• - Av ~ — -
Rin
where Rf is R 0 for all ranges and Rin composed of one of the series
resistors R.(7> R-.O or R,, and the equivalent source resistances. This
source resistance is the effective resistance of the span not R,0 and
1 68 ,
any parallel resistance such as R, , or R, ,_• I
J 16 45
I
To maintain an accurate raster reading, the meter is electrically contained '.
in the feedback loop and the system is therefore immuned to changes in the \
t
mater resistance as a function of time and temperature. t
Overload protection for the circuit is provided by the diode network of
CR.. through CR and R9r|. This limits the amplifier to ±1.4V. • F
The recorder output is developed by the resistive divider R ' and R9o- f,
rf— <— t*J I
This provides a lOmv full scale output with a low output impedance . £
Initial ^ero adjust for AR0 is set by R n . • i;
/L /.L v:
(.'
Temperature control of the cell heater is determined by resistive bridge ij
amplifier AR_ and associated driver transistors R through R . |
i
The resistive bridge is composed of resistors R/0> R/?> ^YiV ^9 (thermistor) [
and potentiometer R , , . Bridge unbalance is sensed by. amplifier AR which
proportionally changes the output voltage to the heater. At 25CC the
bridge unbalance causes pin 3 to be positive with respsct to pin 2. The
output at pin 6 is then at a maximum positive voltage. This voltage is
buffered by emitter followers Q., and Q which control the high current
/, O
pass transistors Q . As the heat increases, the thermistor RT (located
on the cell) v?j'.ll continue to decrease in valxie until the bridge becomes
balanced. The high gain of the amplifier maintains the heater at a high
input current for fast warm-up and then maintains a proportional control
within a few tenths of a degree.
Circuit protection is obtained by the heater power switch S_ and diodes
A-11
CR and CR. . .
-------
C O ft ij ORATION
The heater light DS3 is directly in parallel with the heater blankets
ana 'has a brilliance which is proportionate to the applied voltage.
Monitor pov?er can be provided by cither 115 VAC line voltage or +12 TOC.
The -I-12VDC operation is dependent-on the PSI DC to DC converter. This is_
a special module which provides ±22V of partially filter voltage in
parallel with the rectified AC line voltage.
The filtered DC voltages are regulated by integrated circuit regulators
Z. and Z . The regulated ±15 VDC is used to virtually eliminate the
effects of line voltage and. frequency. They also provide short circuit
protection to preve.nt propagation failures. Initial adjustment is set
by potentiometer R _ for the H-15VDC and R 3 for the -15VDC.
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