&EFA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
EPA-600 2-80-053
March 1980
Research and Development
Design and
Performance of an
Aerosol Mass
Distribution Monitor
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-80-053
March 1980
DESIGN AND PERFORMANCE
OF AN AEROSOL MASS DISTRIBUTION MONITOR
by
W. Stober, F. J. Monig, H. Flachsbart and IT. Schwarzer
Fraunhofer Institute of Toxicology and Aerosol Research
5949 Grafschaft-Sauerland
and
4400 Miinster-Roxel
Federal Republic of Germany
Research Grant No. 803592
Project Officer
Jack Wagman
Director, Emissions Measurement and Characterization Division
Environmental Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ENVIRONMENTAL SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U. S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental
Sciences Research Laboratory, U. S. Environmental Protection
Agency, and approved for publication. Approval does not
signify that the contents necessarily reflect the views and
policies of the U. S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute
endorsement or recommendation for use.
ii
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ABSTRACT
The aerosol mass monitor has been built to measure the masses
of non-volatile aerosols in the range of 0,05 to 5 pm aero-
dynamic particle diameter. The instrument consists of a newly
designed spiral duct aerosol centrifuge equipped with highly
sensitive quartz sensors for in situ weighing of the deposited
aerosol masses. The instrument further includes a clean air
device for maintaining constant aerosol flow conditions, and
electronic parts for the operation of the quartz sensors.
The mass of aerosol deposited on the quartz crystals is auto-
matically measured by an electronic counter while the rotor of
the centrifuge continues spinning. The data are handled by a
microprocessor.
This report was submitted in fulfillment of Research Grant No.
803592 by the Fraunhofer Institute of Toxicology and Aerosol
Research under the sponsorship of the U. S. Environmental Pro-
tection Agency. This report covers the period from January 19,
1977, to April 1, 1979, and work was completed as of November 30,
1979.
iii
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CONTENTS
Abstract iii
1. Introduction 1
2. Summary 3
3. Recommendations 4
4. General Description of Instrument 5
4.1 Aerosol Centrifuge 5
4.1.1 Centrifuge Rotor 5
4.1.2 Driving Unit 10
4.1.3 Clean Air Unit 10
4.1.4 Method of Operation 15
4.2 Mass Measurement System 20
4.2.1 Quartz Sensors 20
4.2.2 Rotor Electronics 20
4.2.3 Inductive Power Supply 26
4.2.4 Control Electronics 26
4.2.5 Frequency Measurement . 29
4.2.6 Principle of Piezoelectric Microbalance . 30
4.2.7 Aerosol Concentration Measurement .... 31
4.3 Microprocessor 31
5. Handling of Quartz Inserts 34
6. Calibration 35
7. Instrument Operation 38
8. Instrument Performance 40
References 45
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1. INTRODUCTION
In most applied research projects and in many routine investi-
gations of.airborne material like ambient particulate air pollu-
tants, emission sources, mine dusts or other particulate matter
suspended in gases, the particle size distribution in the air-
borne state is the most important information to be acquired. In
the analysis and abatement of air pollution, the particle size
distribution determines the degree of penetration of aerosol
filters by the airborne material, influences visibility through
polluted air and affects the aerodynamic stability and life time
of aerosols, smog and other particulate clouds. With regard to
health hazards, the size distribution rules the respirability of
particulate air pollutants and, thus, governs the potential toxic
effects of airborne material.
Since most aspects of air pollution are related to the respective
masses of the pollutants, experimental data directly giving the
corresponding particle mass distribution of the suspended matter
would be most desirable. Thus, the most preferable measurement
is by gravimetric procedures. A number of instrument types, most
notably cascade impactors, comply with this requirement and
yield gravimetric information of size fractions of precipitated
airborne material. However, a number of practical drawbacks like
low sensitivity, wall losses, load limitation, and, most signifi-
cantly, batchwise operation requiring tedious sample weighing
limit the applicability of this device. Clearly, an instrument
operated as a continuous monitor of mass distributions of air-
borne particulate matter would be highly desirable. No such
instrument, however, is available as of now.
Therefore, the principal objective of this project was to design,
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build and test an instrument which continuously samples airborne
particulate matter, precipitates the particles under size separa-
tion and monitors the particle mass distribution of the deposit
without interrupting the sampling process.
A subordinate objective of this project is to facilitate the
collection of size-selective deposits of particulate matter of
very low airborne concentration for subsequent physical and
chemical analysis by X-ray fluorescence, microprobe, electron
microscopy, neutron activation, wet chemical processing, etc. In
this case, the monitoring system will be used to ensure adequate
sampling times by observing the build-up of the required amounts
of deposited mass.
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2. SUMMARY
Under the terms of the grant, a monitoring device for the
gravimetric determination of aerosol mass distribution was built.
The instrument combines the size-separating capability of a
spiral duct aerosol centrifuge with the mass sensor function of
a thin piezo-electric quartz crystal. The arrangement is to permit
telemetric read-outs during sampling and has a total of 13
size-selective sensors and one reference crystal.
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3. RECOMMENDATIONS
Till now the mass monitor only measures surface concentrations,
referred to equal length intervals of the channel, given by
the crystal diameter. In order to get the more interesting
aerosol mass concentrations and an aerosol mass spectrum
referred to equal particle diameter intervals further calcula-
tions are necessary. This can be done by another program for
the microprocessor and by connecting a plotter to it.
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4. GENERAL DESCRIPTION OF INSTRUMENT
The design concept of the projected particulate mass distribution
monitor is a combination of two recently developed technologies
in aeros.ol instrumentation:
1) aerodynamic aerosol size spectrometry in the spiral duct
of the rotor of a centrifuge (1,2,3,4),
2) electronic gravimetry of mass deposits by means of oscilla-
ting quartz crystal detectors (6,7,8,9) .
An overall view of the instrument is shown in Figure 1.
4-1 AEROSOL CENTRIFUGE
4.1.1 Centrifuge Rotor
The rotor consists of the spiral duct plate and the lid with the
inlet system. The rotor is mounted on the motor shaft and held
in place by one left-handed nut (Figure 2). The rotor contains
the spiral duct (length 151 cm, depth 4 cm), the laminator,
the aerosol inlet, and 2 filters; one at the entrance of the
laminator, and one at the end of the spiral duct, as shown in
Figure 3. In the outer wall of the duct, there are the 14 quartz
mounting inserts. All these parts can be easily removed. The
oscillators and some additional electronic circuits are mounted
on a print board at the bottom of the rotor. At this end the rotor
is closed by a disc of print board material (Figure 4) . In the
center of this disc one half of the core transformer for the
inductive power supply is mounted. The ring electrode in the
middle of the disc is part of a condenser, which couples the high
frequency signals of the quartz oscillators to the counter.
The spiral duct is closed by a lid which contains the inlet
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Figure 1 Complete mass monitor
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Left handet Nut
Spiral Duct Plate
Core Transformer
Figure 2 Vertical section of mounted centrifuge rotor
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r
Figure 3 Centrifuge rotor with spiral duct and quartz inserts,
filters, (1) and (4), Laminator, (2) and aerosol
inlet, (3)
E
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Figure 4 Rotor lower side
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system (Figure 5). A vertical section of the inlet system is
shown in Figure 6. With regard to rotation it consists of two
parts: one part is rotating within the bearing, the other one is
mounted on the outer ring of the bearing and does not rotate. In
the center of the unit there are two chambers; the upper one for
the incoming clean air and the lower one for the total flow. Both
of these chambers are tightened with 2 V-rings. The aerosol
enters the inlet system through the center bore'.
4.1.2 Driving Unit
The Heraeus-Christ driving unit contains a 650 Watt DC-Motor
which allows speed stabilized rotor spinning rates of up to
6OOO RPM. Standard rotor speed is, however, 3000 RPM.
After switching off the motor, it works like a generator, which
can brake the centrifuge rotor in a short time. The driving unit
further has a 150 Watt cooling system, which keeps the rotor-
temperature constant near 20 °C by means of convection. Constant
rotor temperature is important for the frequency stability of the
quartz crystals.
The driving unit has been modified with regard to rotor mounting,
higher stability of speed and temperature, and less electric
noise.
4.1.3 Clean Air Unit
For operating the quartz centrifuge, a device producing and
controlling clean air or carrier gas is necessary (Figure 7). The
gas flow of this unit is shown schematically in Figure 8. The
closed circuit gas flow is achieved by a magnetic piston pump, C,
which is controlled by a regulating transformer. The carrier
gas is drawn by the pump from the end of the spiral duct, F,
passing a 4-way-valve in position "2" and two silicagel columns, 2,
then passing a glassfiber filter,1, into the origin of the
spiral duct,G,via the centrifuge inlet system,H.
10
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Figure 5 View of closed rotor with inlet system and driving
unit
11
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Figure 6 Vertical section of inlet system
12
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»
Figure 7 Front panel of clean air device
13
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1 Filter
2 Silicagel column
3 Drying tube
I Flowmeter. total flow
n Flowmeter, aerosol flow
A Aerosol flow bleeding
B Carrier gas inlet
C Pump
E Needlevalve
F Total flow outlet
G Clean air inlet
H Inlet system
of centrifuge
I Rotor
VxX positii
4-way valve
Position 1
I
1
_ ^ m A
Al
\ \
14 fc
;i |
& K
ji ^
n
A yv
ME
3 U
I
Ev .
^\^^
G^^^
T^
H
Figure 8 Flow schematic of clean air device
14
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By regulating the needle valve at flow meter II, carrier gas
is bleeded from the closed circuit gas flow and replaced by aero-
sol containing gas thus establishing the desired sampling rate.
When aerosols with high humidity are to be sampled the drying
tubej3 (Figures 8 and 9), should be mounted to the aerosol
inlet of the centrifuge.
4.1.4 Method of Operation
While the rotor is spinning, clean air enters the duct through
the off-center air inlet, 1 (Figure 3) , and flows to the inserted
laminator, 2. The laminator has 11 thin brass foils mounted paral-
lel to the vertical walls of the rectangular duct (Figure 10).
In this section, the clean air is quickly stabilized and emerges
as a laminar flow from the downstream end. It then approaches
the exchangeable aerosol inlet, 3 (Figure 3), at the center of
the rotor. The aerosol enters the inlet section through the cen-
ter bore and is released into the duct as an air sheath parallel
to the inner wall of the duct, where it will be entrained into
the laminar flow. Figure 11 shows an aerosol inlet section which
is used in experiments with high aerosol flow rates. This parti-
cular design releases the aerosol through a set of short capil-
laries. For high-resolution tests, however, where reduced aerosol
flow rates are required, another design is employed which has a
narrow slit adjacent to the inner wall. This aerosol inlet is
shown in Figure 12.
When leaving the center of rotation, the aerosol particles are
subjected to the centrifugal forces of the spinning rotor and
start moving in a radial direction across the laminar air stream.
Their trajectories are determined by the operating conditions
of the centrifuge and the aerodynamic size of the particles. Thus,
while the air is flowing down the duct to the outlet, 4 (Figure 3) ,
the particles are settling, according to their size, in
different locations on the outer wall with the quartz inserts.
15
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Figure 9 Drying tube
16
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Figure 10 Laminator
17
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Figure 11 Aerosol inlet (low resolution)
18
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Figure 12 Aerosol inlet (high resolution)
19
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4.2 MASS MEASUREMENT SYSTEM
4.2.1 Quartz Sensors
The centrifuge rotor is equipped with 14 10-MHz-quartz crystals,
12 mm in diameter. The housings of these crystals are made of
stainless steel and have a cylindrical shape cut along the axis
by the spiral duct, as shown in Figure 13. Every crystal disc is
mounted on a high quality 4 pin quartz holder which is positioned
flush in a hole in the stainless steel insert. Two of the pins
are connected to the gold electrodes of the crystal and the 2
pin connector at the bottom of the insert.
4.2.2 Rotor Electronics
For operation every quartz crystal needs an electronic oscillator
circuit. The quartz oscillators are of the Pierce-Colpitts type
(Figure 14). They have 3 stages: oscillator, separator and output.
A photograph of a complete oscillator is shown in Figure 15. The
oscillator stage of every oscillator unit is permanently activa-
ted and all crystals are vibrating. Read-out selection of the
oscillators (5) is done one at a time by an electronic counter
(Figure 16). This counter has 14 outputs by which the second
and third stage of the chosen oscillator unit is switched on.
The 10 MHz output signal is controlled by an electronic switch,
which is part of every oscillator unit, and the HF output
amplifier which is connected to the ring condenser at the bottom
of the rotor. As the distance between the two ring electrodes
is 4 mm only the 1O MHz signal can easily be coupled to the non
rotating part of the ring condehsor which is shown in Figure 17.
Clock and reset pulses for the oscillator selector are generated
by two special band pass circuits which are activated via the
20
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Figure 13 Insert with quartz crystal mounted on 4 pin holder
21
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Quarz
10 MHz
2,7k 47k. =L=
Figure 14 Pierce Colpitts oscillator
22
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Figure 15 Oscillator construction
23
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10MHz Condensor
Figure 16 Rotor electronics
24
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Figure 17 Disk with ring condenser half (second ring),
core half and motor shaft
25
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core transformer when the operation frequency of the rotor
supply voltage is changed from 19 KHz to 17 KHz or 21 KHz,
respectively, for some milliseconds.
4.2.3 Inductive Power Supply
The power consumption of all rotor circuits is 1.5 watts only. So
it was possible to build an inductive power supply. It consists
of a transformer with two cup-shaped ferrit cores powered by an
alternating voltage of 19 KHz. One core is mounted at the bottom
of the rotor center (Figure 4) and the other one at the center
of a stationary disc below (Figure 17).
The airgap between the two cores is 1 mm. Power transmission to
the rotor doesn't depend on the spinning rate and allows the
operation of the rotor electronics during actual rotation and
standby as well. After passing the secondary rotor coil of the
core transformer, the AC-voltage is rectified and stabilized.
Figure 18 is a photograph of the rotor printboard with all rotor
circuits. The square parts are the quartz oscillators, which are
mounted in such a way that the distance to the quartz inserts is
as short as possible.
4.2.4 Control Electronics
This part has two selective counters: one for choosing the quartz
oscillators by means of 14 toggle switches on the front panel of
the control unit, and the other one for choosing the measuring
time (Figure 19). This is the time in which the 10 MHz signal of
the chosen oscillator can pass from the oscillator to the frequen-
cy counter. The measuring time can be set between 0.5 and
99 minutes.
The rotor-mounted oscillator selector is synchronized to the
selective quartz counter in the control electronics. This is
effected by a reset pulse (manually, or automatically after
reading quartz 14) which brings both circuits into start position.
26
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Figure 18 Rotor print board
27
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LF, Generators
220V
Power
Supply
(•),,
1
rillator Preselector
2
I I I 1 I I 1 1 I I 1
Control Unit
U
17KHz
2
19 KHz
3
21KHz
—
L
F.Amplf. Cof(
D>
Trar
10MHz Amplf.
I I
I l_
I I
I I
Printer
Micro-
process
'<
Counter
<
I
i; ^
1:
Condenser
Figure 19 Principle of control electronics
28
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If, for instance, oscillators 1 and 3 are selected (by means of
toggle switches 1 and 3 at the front panel) , the first clock
pulse connects oscillator 1 to the frequency counter. At the end
to the preset measuring time, the second clock pulse is generated
and, in the same way, effects oscillator 2. But, since this
oscillator has not been selected the next clock pulse is gene-
rated only a few milliseconds later and switches to oscillator 3.
The frequency of this oscillator will now be measured until, at
the end of the measuring time, oscillator 4 is connected to the
frequency counter. But this again is only done for some milli-
seconds as the clock frequency for non-chosen oscillators is
^ 3OO Hz. So the selective quartz-counter in the control electro-
nics and the oscillator selector in the rotor quickly and consec-
utively count to 14. Then a reset pulse is generated and the
cycle starts again.
The described clock and reset pulses are rectangular pulses. They
are sent to a voltage tuned low frequency oscillator (Figure 19)
which is normally generating 19 KHz for the inductive power
supply. During the clock pulse time this frequency is changed to
17 KHz; the corresponding value for the reset pulses is 21 KHz.
Then the low frequency is amplified in a power amplifier in
order to energize the rotor circuits via the two separated halves
and the airgap of the core transformer.
4.2.5 Frequency Measurement
After passing the ring condenser at the bottom of the rotor, the
signal of the selected quartz oscillator passes to a selective
10 MHz amplifier. This amplifier is necessary to get a good
signal amplitude for the frequency counter and to suppress
the noise of the driving unit. Frequency measurements are made
by means of a Ballentine frequency counter. It has a high
stability time base with temperature stability of better than
2=10~9.°C~1. Frequency measurements with a resolution of O-1 Hz
can be made within 1O seconds.
29
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However the shortest measuring time for a single quartz oscilla-
tor depends upon the control electronics and is at least 0.5
minute. Since there is no synchronisation between counter time
base and clock pulses of the control electronics, at least 2
complete frequency measurements of one oscillator are made within
O.5 minute. The last of these frequency values is stored in the
microprocessor. The shortest measuring time for a complete run of
13 deposit sensors and 1 reference crystal is 7 minutes.
4.2.6 Principle of Piezoelectric Microbalance
A quartz crystal disc, ground along appropriate crystallographic
planes (AT-guartz) and used as a component of an electronic
oscillator circuit, will vibrate with tangential amplitudes
along the face of the disc. The mechanical frequency of these
oscillations is determined by the material properties and the
dimensions of the crystal. In particular, the thickness of the
disc determines the individual resonance frequency of the
crystal.
For surface layers on the crystal, a theoretical relationship
A f = - -^— • ^ where f = f
d • p
predicts a frequency shift Af of the resonance frequency f as
an effect of a layer of total mass Am on the active surface
area F of the crystal of thickness d. As material constants, the
frequency constant N = 167 kHz -cm and the density p = 2,65 g'cm
of quartz also enter the equation. Thus, the surface
\ I /
concentration \f) _ ^m
f
is linearly related to the frequency shift Af by a proportionali
ty factor
1= -d2-P ' (2)
Cf N
which, for a given material, depends only upon the crystal
thickness d, and the surface concentration is
30
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= C^ • At (3)
For a 10 MHz AT-quartz of 0,O167 cm thickness
~- = 4.4-10'9 g-cm~2-Hz~1
. f
and a surface concentration
f= 4.4 ng • cm"2
will be caused by a frequency shift of Af = -1 Hz.
The factor •=- is the gravimetric constant. To measure the surface
f
concentration, only the shift Af of the oscillator frequency has
to be known. This frequency shift is the difference of two
absolute frequency values which have been measured by the electro-
nic counter in a given time interval.
4.2.7 Aerosol Concentration Measurement
In order to obtain aerosol mass concentrations and a mass spec-
trum instead of surface concentrations, further calculations are
necessary. Surface concentrations refer to equal length intervals
of the spiral duct, while size-related mass concentrations refer
to equal intervals of particle diameter. Therefore, a faktor
Kp for every crystal position must be calculated from the slope
of the (centrifuge) calibration curve. These values are shown
in table 1.
Thus the size-related mass concentration K belonging to one
crystal is
K = f- F - K • 1— (4)
ae
where F is an area represented by quartz diameter and channel
depth and V is the volume of the sampled aerosol.
ae
4-3. MICROPROCESSOR
All data, that are important for the calculations, are received
by the microprocessor MP from the control electronics and the
31
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TABLE 1. 1C, VALUES FOR CALCULATION OF AEROSOL CONCENTRATION
r
Quartz
Oscillator
02
03
04
05
06
07
08
09
10
11
12
13
14
Factor K for equal particle
diameter intervals of 1/u.m (i.e.,
diameter-
0.6
1 .4
2.9
5.2
7.8
12.8
25.5
49.2
76.9
133.0
293.6
402.6
521 .9
TABLE 2. MP PROGRAM COMBINATIONS
Mode
0
1
2
3
4
5
6
7
8
9
Program
DPI + QL
DFI + DF
DPI
DFI + Pause
QL
QL + Pause
DF
DF + Pause
DFI + QL + DF
DFI + QL + DF + Pause
32
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frequency counter. These data are: quartz number (2 digits),
quartz frequency (9 digits), measuring time (4 digits), and mode
of operation (1 digit) . Data transfer is made just before the
measuring time of an oscillator ends.
When the microprocessor is started, it first stores the absolute
resonance frequency of a chosen crystal and the printer prints the
crystal number followed by the frequency value in Hz, and
measuring time in minutes (referring to program start) . This is
repeated until the resonance frequencies of all chosen crystals
are printed. Then the MP "looks" at the position of the mode
switch, to decide which type of program is to be run. Table 2
shows the possible types of programs and program combinations.
Program "DFI": The MP compares the actual frequency value of a
crystal with the first stored value and calculates the difference
A f. This difference is an integral or cummulative value and,
therefore, gets the MP name "DFI". The unit of DFI is Hz. When
particles settle down on the crystal surface, the resonance
frequency drops. In this case, DFI is counted as a positive value.
If however the frequency rises, which can be caused by decreasing
humidity of the winnowing air, the DFI value decreases and the
printed value gets a minus sign by the MP, showing that this
is a false value.
Program "QL": In this program the surface concentration r> accord-
_ Q — 2
ing to Eq. 3 is calculated. The result has the units 1O g-cm ,
but only "NCR" is printed because of the limited character number
of one printer line. QL means "Quartz Layer" and is an MP name
for y?. If the DFI value was negative, the mass value gets an
interrogation mark for a negative mass value.
Program "DF": In contrast to DFI,DF is a differential value,
refering to the last frequency value.
33
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Program "Pause": If the number of printed data becomes too large,
a special pause program can be chosen in mode positions 3, 5, 7
or 9. The pause program automatically starts when the absolute
frequencies have been printed.
The cycle time of the pause program is calculated by multiplying
the measuring time in minutes with the constant 56 stored in the
MP. If for example the shortest measuring time of 0.5 minutes
is chosen the shortest cycle time of the pause program is 28
minutes. In the first quarter of this time all the 14 values
are printed, then calculating and printing is stopped for 21
minutes, and after this pause the cycle starts again. The pause
program works well when 1, 2, 4, 7, 8, or 14 crystals are
switched on; for other numbers the pause cycle time of 2
corresponding crystals no longer is a constant.
5. HANDLING OF QUARTZ INSERTS
Several measurements showed that, for particle diameters above
1 ym, a coating of the crystals is necessary in order to get a
good mechanical coupling of the deposited particles to the
crystal surface. Coating is done by applying some microliters
of an adhesive dissolved in cyclohexane to the cleaned crystal
surface and removing the excess with filter paper. After drying
the crystals in an oven at 80 °C for 3O minutes, the inserts can
be put into the grooves of the spiral duct.
The frequency shift caused by coating the crystals should be
2000 to 3000 Hz.
As the size tolerances of the inserts are very small, the walls
of the grooves should get a thin film of a vacuum grease
every now and then.
The useful range of frequency shift is 2OOO Hz for uncoated crys-
tals and about 5OOO Hz for coated ones. In order to obtain
34
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long operating times, coating of all crystals is possible.
When approaching the end of the useful range of frequency shift,
the inserts must be cleaned in an ultrasonic bath (20 to 40 KHz)
of cyclohexane for 1 to 3 minutes. After this they are carefully
rinsed with pure cyclohexane and ethanol and then blown dry with
compressed air.
If the crystals are very dirty, they can be cleaned by gently
wiping them with a cyclohexane soaked cotton swab.
However, care should be taken that the gold electrodes of the
crystals will not be damaged.
6. CALIBRATIONS
For calibration, a thin brass foil was inserted along the outer
channel wall, and the aerosol centrifuge was operated at the
standard spinning rate of 3OOO RPM, while the total flow was
— 1 ~1
11.5 L-min and the aerosol flow 1 L-min . Fluorescein served as
a test aerosol. At the locations of the crystals, small samples
were cut out of the foil, and the particle diameters were
measured by means of a scanning electron microscope. The result
is shown in table 3. The diagram, Figure 20, shows the cali-
bration curve with aerodynamic particle diameters for quartz
positions Q2 to Q14 plotted vs channel length. The accuracy of the
diameter values is estimated to be better than 5 percent.
A calibration of the quartz crystals is not necessary because the
surface concentration depends only on the gravimetric constant
4- and the shift in frequency Af, as given in Eq. 3. This implies
•P
tfiat the particles are evenly distributed over the crystal surface
and well coupled, which was check by some quantitative tests with
low (80 yg-m~3) and high (1 mg-ra~ ) airborne concentrations of
fluorescein aerosols. In these measurements, the frequency
shifts of the quartz sensors were found to be proportional to
the deposited masses on the crystals. Photometrically determined
mass values were within less than 15 percent of the mass values
35
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TABLE 3. CALIBRATION VALUES OF MASS MONITOR
Quartz
Crystal
01
02
03
04
05
06
07
08
09
10
11
" 12
13
14
Deposition
Length in cm
2.0
5.7
8.5
11.3
14.1
17.0
21.0
27.0
35.0
48.0
66. 0
89. 0
117.0
141.0
Aerodynamic
Diameter in
-
7.5
4.430
2.913
2.102
1 .656
1.250
0.922
0.691
0.505
0.359
O.213
0.125
0.065
36
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3000 rpm
Total flow 11.5liters/min
0.05 .1 .2 .3 .1* .5.6.7.8.91.0
Aerodynamic Diameter (jjm)
Figure 20 Centrifuge calibration curve
2.0 3.0 405.0 7.0
37
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derived from the frequency shifts of the crystals according to
Eq. 3.
7. INSTRUMENT OPERATION
1. The cleaned quartz crystals are inserted into the proper
grooves of the rotor duct. Then laminator and aerosol inlet
are inserted and the rotor is closed by the lid which con-
tains the inlet system with the air inlet (blue), the outlet
(red), and the aerosol entrance (Figure 5). Inlet and outlet
are coupled to corresponding connectors on the lid of the
driving unit.
2. Then, a connection to the clean air device (Figure 7) is made.
The aerosol entrance of the centrifuge is connected to
junction A, which allows the operation of the centrifuge
with particle-free dry air. At the beginning of a measurement,
the centrifuge should be run in this way for some time in
order to get constant quartz frequencies.
3. The cooling system and the electronics of the driving unit is
switched on by pressing button 1 (Figure 5). The switch may
remain in this standby position if the room temperature is
below the fixed value of 2O °C of the rotor. If this is not
the case, the cooling machine should only be switched on
when the lid of the driving unit is closed in order to avoid
condensation of water at the walls of the rotor chamber.
4. The centrifuge motor can only be started if the lid of the
driving unit is closed and the cooling machine is switched
on. Rotor starting is done by rotating switch 3 (Figure 5)
into position "infinite". In the other positions, the switch
works like an alarm clock which switches off the centrifuge
motor when the chosen time is over. Rotor speed is corrected
by dial 4. Adjustment of this dial is only necessary in
case of a new calibration of the rotor speed. The standard
38
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speed is 3OOO RPM which is read by the instrument shown on
top of the right side of Figure 5. The Rotor speed is well
stabilized and should be constant within 0.1 percent after
a warm-up time of 3O minutes.
5- Control electronics, frequency counter, printer and
inductive power supply are to be switched on.
6. Before the pump is turned on, the 4-way-valve (Figures 7
and 8) should be in standard position "2" which allows a
circulation of the particle free and dry air through the
spiral duct of the rotor. (Position "1" of the 4-way-valve
is only used for flushing the rotor with carrier gas if a
gas other than air is chosen.) Total flow (flowraeter I)
should be near 11 L-min~ , aerosol flow (flowmeter II) near
1 L-min . After a warm-up period of 3O minutes the total
flow is adjusted to the standard value of 11.5 L-min by
changing the pump supply voltage through a regulating trans-
former and dial D (Figure 7) while the needle valve E
(flowmeter I) is fully opened. The aerosol flow is" adjusted
to the standard value of 1.0 L-min~ by means of needle
valve E of flowmeter II.
7. The quartz 'crystals can be chosen by means of the toggle
switches at the control unit; measuring time and mode of
printing by means of the thumb-wheel switches. For a precise
frequency measurement the resolution must be switched to
0.1 Hz by pushing th'e left black and the adjacent white
button of the frequency counter. The sensitivity of the
counter must be switched to maximum by means of the sensiti-
vity knob.
8. In order to get printed values, the microprocessor must be
switched on. Program start is effected by pressing the MP
start button. By means of the mode switch, different programs
of the MP can be run as shown in table 2.
39
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9. After the initial frequencies of the selected crystals have been
printed and a repetition shows that they become constant,
sampling can be started. This is done by disconnecting the
aerosol entrance from junction A (Figure 7), and connecting
the tube to the aerosol source. Sampling will start immedi-
ately with the sampling rate set at flow meter II. If the
humidity of the aerosol is very high the drying tube
(Figure 9) should be used.
10. In order to stop aerosol sampling, the aerosol entrance
must be disconnected from the source and connected to junc-
tion A of the clean air device. Then, the centrifuge motor is
switched off by a rotation of dial 3 (Figure 5) into posi-
tion O. If dial 5 is in 0-position the centrifuge is not
v
braked, in the other positions braking takes place. Braking
becomes stronger the more clockwise the dial is rotated.
11. In standby operation, the pump, the frequency counter and
the inductive power supply should remain switched on in
order to get low frequency drifts.
The lid of the driving unit can only be opened when the
centrifuge rotor has stopped. Then, the lid opens by pushing
button 2 (Figure 5).
8. INSTRUMENT PERFORMANCE
Standard rotor speed of the centrifuge is 30OO RPM. A good reso-
lution of deposited material is obtained for particles of aerody-
namic diameters from 7.5 ym to 0.065 ym at a total flow of
11.5 L-min and an aerosol flow of 1 L-min~ .
The sensitivity of the mass monitor is given by the gravimetric
constant of the 1O MHz quartz crystals:
~ = 4.4 - 10~9 g • cnf2 • Hz~1 .
40
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The weighing accuracy depends on the quality of the particle
coupling to the crystal surface and on the frequency drifts
caused by changes in temperature and humidity. With regard to the
mass measurement of large solid particles there is a boundary
above particle diameters of 5 urn. Particles of 7.5 ym in diameter
are too large, to be well coupled to the surface of quartz Q2
although it is coated. However there are no problems with 7.5 ym
droplets, even with an uncoated quartz Q2.
Since the coupling problem is solved by coating the crystals (for
particle diameters of 1 ym to 5 ym) and the rotor temperature is
kept constant near 2O °C, the only significant disturbance of the
frequency is caused by changes in humidity. A humidity increase
of 1 percent causes a frequency drop of about 1 Hz. Changes of
humidity can be measured by means of quartz Q1 which is mounted
at the beginning of the spiral duct where no particles can be
deposited. In order to get good results, humidity should be kept
as constant as possible. This is done by drying the carrier gas in
the clean air device.
As the aerosol flow is nearly yy of the total flow, humidity
changes of the aerosol flow cause total humidity changes of
4-y only. For aerosols of high humidity, a drying tube should be
attached to the centrifuge aerosol inlet as shown in Figure 1.
Carrier gas drying is accomplished by two drying tubes, which may
serve for an operating time of some 12O hours. But during this
time, depending upon the humidity of the sampled aerosol the
effective humidity is slowly increasing and a corresponding
frequency drift is caused. Measurements showed that, after a
warm-up time of 1 hour, these frequency drifts(of newly inserted
and cleaned crystals) are <1 Hz per hour. But there are differences
in the drift values of different crystals which could be caused
by small differences in the mounting of the crystals to the 4 pins,
Assuming unwanted frequency drifts of 1 Hz per hour, frequency
shifts caused by particle deposition should be at least 2 or 3
41
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60
Af
Hz
50--221.5
1772
30
.10
265.8
P
132.9
20 88.6
t=8h
\ Fluorescein Concentration
V} ~80 pg-m'3
\
/
S \i
\ ° .' \
x / \
\ / •«
V /
x '
14 xx 13/ ,,'
-'''
0.05 0.1
11 10 9 8 7 °-«' 4 3 Quartz
I | | | li I Jy.l...
0.5 1.0 2 3456
Figure 21 Deposited masses of fluorescein
42
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0 0.5 1.0 1J5 2.0 2.5 3.0 3.5 4.0 45 5,0
Figure 22 Spectrum of fluorescein aerosol
43
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times higher. Thus, the measurement of an aerosol spectrum with
a concentration near 5O ug-m is possible.
The high sensitivity of the mass monitor is shown in the example
of sampling a fluorescein test aerosol with a concentration of
80 yg-m over a time period of 8 hours. During this time the
centrifuge was operated under standard conditions. Figure 21
shows the result: frequency shift Af, and corresponding, surface
concentration V^, calculated by the instrument microprocessor is
plotted vs the particle diameter D. Already after one hour of
operation a spectrum of surface concentrations can be seen, the
shape of which does not change very much after sampling times of
4 and 8 hours.
(These values have been measured by means of the German version
of the mass monitor, which has a smaller channel depth than the
EPA version, causing small differencies in the calibration
curve).
Figure 22 shows the airborne mass vs. particle size distribution
of the same fluorescein test aerosol as calculated by Eq. 5
and the Kj, values given in table 1. The second maximum at the Q4
quartz crystal near particle diameters of 3 ym is nearly lost in
Figure 22 because of the relatively small Kp values for larger
particles.
44
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REFERENCES
1. Stober, W. and H. Flachsbart, Environmental Science and
Technology, 3 128O (1959).
2. Stober, W. and H. Flachsbart, Journal of Aerosol Science,
2 1O3 (1971) -
3. Kops, J. , Hermans, L. , and Vate, J.F. van de, Journal of
Aerosol Science, (5 329 (1975).
4. Stober, W. in B.Y.H. Liu (Editor), Fine Particles,,
Academic Press 1976, p. 351.
5. Meinke, H. und F.W. Gundlach, Taschenbuch der Hochfrequenz-
technik, Springer-Verlag 1962, p. 142, 1112, 1163.
6. Sauerbrey, G.Z., Zeitschrift fur Physik, 155 206 (1959).
7. Chuan, R.L., Journal of Aerosol Science, j_ 111 (1970).
8. Olin, J.G. and G.J. Sem, Atmospheric Environment, 5_ 653
(1971) .
9. Carpenter, T.E. and D.L. Brenchley, American Industrial
Hygiene Association Journal, 33 503 (1972).
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
i. REPORT NO.
EPA-600/2-80-053
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
DESIGN AND PERFORMANCE OF AN AEROSOL
MASS DISTRIBUTION MONITOR
5. REPORT DATE
March 1980
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
W. Stober, F. J. Mbnig,
H. Flachsbart and N. Schwarzer
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Fraunhofer Institute of Toxicology and
Aerosol Research
Federal Replublic of Germany
10. PROGRAM ELEMENT NO.
1NE833D EB010 (FY-79)
11. CONTRACT/GRANT NO.
R803592
12. SPONSORING AGENCY NAME AND ADDRESS
13. TYPE OF REPORT AND PERIOD COVERED
Environmental Sciences Research Laboratory - RTF, NC
Office of Research and Development
U. S. Environmental Protection Agency
Research Triangle Park. N.C. 27711
Final 1977-1979
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
16. ABSTRACT
An aerosol mass monitor has been built to measure the masses of non-volatile
aerosols in the range of 0.05 to 5 ym aerodynamic particle diameter. The instrument
consists of a newly designed spiral duct aerosol centrifuge equipped with highly
sensitive quartz sensors for in situ weighing of the deposited aerosol masses. The
instrument further includes a clean air device for maintaining constant aerosol
flow conditions, and electronic parts for the operation of the quartz sensors. The
mass of aerosol deposited on the quartz crystals is automatically measured by an
electronic counter while the rotor of the centrifuge continues spinning. The data
are handled by a microprocessor.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b-IDENTIFIERS/OPEN ENDED TERMS
c. COS AT I Field/Group
Air pollution
Aerosols
Particle size
Centrifuges
Crystal detectors
Quartz
MLcrobalances
13B
07D
14B
09A
08G
14B
3. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
52
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
46
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