United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S3-83-056 Aug. 1983
4>EPA Project Summary
Development of Instrumentation
for Monitoring Carbon
Fiber Emission
D. L Tague
This document reports the design of
an electrical instrument which utilizes a
variable capacitance in one leg of a
resistance-capacitance feedback net-
work to provide discriminatory informa-
tion regarding the air stream particulate
material. Sufficient testing was per-
formed on the breadboard to validate
system concept. The current instrument
counts fiber mass and indicates fiber
diameters to yield an approximate count
of individual fibers. Its operational range
has a lower limit of 104 fibers and an
upper limit internally restricted for this
breadboard stage of 2 x 108 fibers/m3.
The program scope did not allow com-
pletion of a prototype design; therefore,
emphasis is placed on recommenda-
tions to complete design efforts.
This Project Summary was developed
by EPA's Environmental Sciences
Research Laboratory, Research
Triangle Park, NC, to announce key
findings of the research project that is
fully documented in a separate report of
the same title (see Project Report order-
ing information at back).
Introduction
Previous studies have reached no
consensus on the most promising contin-
uous real-time carbon-fiber (CF) monitor
for targeted environments such as waste
incineration, manufacturing and
processing. Therefore, past CF activity
and instrument performance were
critically reviewed for concept
applicability, in light of recent advances in
electrical and electronic equipment that
significantly increase the capabilities and
signal-to-noise ratio (discriminative
sensitivity) of instrumentation. It was
determined that no one method could
meet all desired operational
parameters. Because of objections to
existing methods, two feasible design
options were considered:
a. Take advantage of the most sophis-
ticated electronic data reduction
schemes, advanced sampling, and
microprocessing capabilities to
develop a state-of-the-art sensing
device, or
b. Develop a new system to provide the
most information possible utilizing
the resources presently available.
The first option would involve extensive
research and development and material
cost, limiting efforts to electrical and
instrumentation schemes and thus
would yield an untried instrument. In the
best judgement of the scientists and
engineers involved, such a sophisticated
instrument would be expensive, delicate,
and require skilled operation, and yet still
possess objectionable features. The in-
strument could prove useful for test
facilities or to provide calibration and
baseline data, but it would not fulfill the
intent of the contract.
Therefore, the second design option
was chosen, thereby maximizing effort to
provide a practical instrument for its
function and operating environment. A
secondary objective was to demonstrate
that an inexpensive electronic circuit
could be developed which could provide
the needed information required by a
continuous CF monitor.
Following a review of existing CF
monitors, a monitor development
program based on the concept of a
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resistance-capacitance (R-C) feedback
network utilizing a variable capacitance
in one leg of two stable twin-T oscillators
was selected.
Instrument Description
The CF monitoring system is a
frequency shift device, whose sensor acts
as a variable capacitance in one leg of an
R-C feedback network. The frequency at
which a 180-degree phase shift takes
place across the R-C feedback network is
the frequency of oscillation. A reference
oscillator provides the second waveform,
thus two stable R-C (twin-T) oscillators'
waveforms are fed into a detector system
which outputs a difference frequency.
This difference frequency is fed into a
frequency to voltage (F/V) converter
which produces a direct continuous
voltage output that is proportional to the
capacitance variance (the fiber
concentration).
The reference oscillator differs from
the sampling oscillator which utilizes a
unique air dielectric capacitor in its
frequency feedback circuit (Figure 1). The
sampling unit consists of concentric rings
(plates) constructed to allow gaseous
flow through it. The conductive
impurities in the gaseous flow will
produce the above-mentioned phase shift.
The reference frequency is provided by an
identical oscillator which utilizes a fixed
capacitance.
The sensing capacitor was designed so
that it may be suspended in an incinerator
stack or vent ducting and the remaining
circuitry may then be more conveniently
situated. This feature decreased size
requirements (thus disturbance in
airflow, due to the location of the device
in the air stream).
Instrument Development and
Testing
Two discrete sequences of instrument
testing were conducted. Standard
development testing paralleled the
breadboard development and fabrication
tasks and was concluded by a pretest to
confirm concept design. The second
sequence tested the final breadboard to
establish the capabilities and limitations
of the instrument.
Figure 2 displays the initial chamber
used to test the CF monitor. A known
mass of fibers was placed in the chamber,
and a simple squirrel cage fan generated
fiber movement (chamber turbulence).
Knowing the internal volume of the
chamber and observing the distribution of
the fibers, gross measurements verified
previously made circuit performance
calculations.
Acid Resistant
Ceramic or
Quarts Spacers
(Plexiglas
Acceptable
for Test)
imimi
Space
Ceramic
Insulator
7 mil Copper
Electronics
Figure 1. Variable capacitor of carbon-fiber monitor.
The variable capacitor used as the
sensor was constructed of 7-mil copper
according to the dimensions provided in
Figure 1. The spacers were made from
Plexiglas because of ease in milling.
Alternate concentric circles were
electrically connected to provide the
plates comprising the capacitor.
Although the initial chamber was
modified into a recirculating system, its
inability to identify all particle loss and
failure to maintain defined CF-exposure
concentrations necessitated the system
be redesigned as a simple fall-through
chamber (Figure 3).
The carbon fibers used to test the
sensor were purchased from Union
Carbide Corporation and were chopped to
specific lengths prior to shipment. When
the fibers were received, electrostatic
bonds were established causing balls of
< 1020 fibers. Freeing single fibers was a
major problem, because testing depended
on the settling of discrete fibers at known
concentrations. The well-established CF
clumps proved resistant to wetting, and
several solvents were tried before the
proper mixture and concentrations were
discovered. To break up the clumps, fibers
were mechanically agitated in the solvent
and then dried in an antistatic atmos-
Sensor
Squirrel
Cage
Figure 2. Fan-blown test chamber.
phere, thus yielding the desired individual
fibers.
Following the separation treatment of
the fibers, controlled quantities were
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injected into the test chamber to begin
test procedure sequence. The sequence
began after establishing the CF loading,
initiating the chamber airflow, and
making final sensor adjustments. The
beginning of CF injection into the airflow
began a count of elapsed time. CF
injection ended 30 seconds later, airflow
was stopped at 45 seconds, and the tests
were discontinued when 300 seconds
elapsed. Immediately below the sensor
was a strip of sticky paper which sampled
a known area of the CF cloud while
allowing the fiber cloud to pass through
the capacitor undisturbed. All fibers
passing through the monitor were
collected on a glass fiber filter at the
bottom of the chamber. A shield around
the sensor caught all fibers not passing
through the monitor. Counts of collected
fibers enabled calculation of the fiber
cloud concentration with reasonable
accuracy. A gravimetric method was
utilized in early runs (corrected by
moisture curve analysis performed con-
currently with the test), but once fiber
counts established the negligible fiber
loss only the counts were performed. The
result was a uniform distribution with an
appropriate settling velocity which
permitted accurate CF concentration
measurements.
Each test run produced a fiber
concentration and distribution according
4 in. Square
Mess
Screen
Flat
Pull Out
j,\ Shelf
Smooth
Sleeve
Fitting
Figure 3.
Gravitational sett/ing test
chamber
to the output of (a) actual microscopic
counts, (b) an electronic count number, (c)
strip chart recording, and (d) a DC signal
stored by a magnetic tape recorder. The
strip chart readings and the fiber count
data were principally used to verify
instrument performance. Table 1 shows
the fiber count data obtained for runs 1
through 5.
Additional test runs were performed
with only an airstream or an injected
water mist, and with water mist and CF
present. No effect was seen by the
monitor; it performed normally in each
instance.
Results and Conclusions
Results of the instrument response to
changing fiber concentrations appear in
Figure 4 and show that the changing fiber
concentration response is linear for the
three lower concentration tests.
However, when coupled with the
changing fiber resistance effect a double
integral effect is seen at high
concentrations. This effect was seen
moving the high concentration point off
scale where electronic limitations inhibit
continued response.
Laboratory tests indicate an electronic
device which measures the capacitance
change resulting from the presence of
conductive carbon shunted across the
planes of a variable capacitor has
potential for further development.
The current breadboard instrument
counts fiber mass and indicates fiber
diameters to yield an approximate count
of individual fibers. Its operational range
has a lower limit of 104 fibers/m3andthe
Table 1. Fiber Distribution and Totals for Test Runs*
Run No.
< 1mm 2mm
3mm
4mm
5mm
Total
1
2
3
4
5
3.6
11.6
8.9
13.6
4.6
2.2
3.9
3.0
4.8
1.4
1.0
2.6
1.9
4.5
1.0
1.6
4.4
3.7
5.5
4.2
Negligible
Negligible
Negligible
8.4
22.4
17.5
28.4
11.2
* Fiber concentration for all lengths = 1O4 fibers/m?
• Experimental Test Results
O Projected Result
f 40
o
I
•s
• Limit of the Electronics
MAX
Due to AC
Figure 4.
10 20
Total Fibers fx 104 fibers/m2)
Carbon-fiber monitor output vs. fiber concentration.
3
30
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upper range is restricted to a maximum of
2 x 108 fibers/m3 by the internal
constraints placed in the system for
testing convenience.
D. L. Tague is with TRW/Systems Engineering and Development Divsion.
Redondo Beach. CA 90278.
William D. Conner is the EPA Project Officer (see below}.
The complete report, entitled "Development of Instrumentation for Monitoring
Carbon Fiber Emission." (Order No. PB 83-233 726; Cost: $11.50. subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
*U.l GOVERNMENT PRINTING OFFICE: 1983-S5J-017/7I51
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
USS ENVIR2PKOT£CTION AGENCY
CHICAGO IL 60604
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