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
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Fees Paid
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Penalty for Private Use $300
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