A method for number count efficiency checks of particle counter based on
corresponding particle mass collected in parallel
Field of the invention:
The present invention relates to a process for the number count
efficiency/calibration checks of particle counters.
The present invention particularly relates to a process for the precise and accurate
number count measurements of suspended particles in air because of their several
impacts and applications including health and climate effects.
More particularly, the present invention relates to a process of calibration checks
of the counters that is based on a primary method which generates data traceable
to SI unit.
Background of the invention:
In aerosol research, particle size measurement and corresponding particle number
concentration measurement both are equally important. Aerosol particle size and
number concentration relate with aerosol mass, which is an important criteria in
the detection of pollutants of the ambient air quality standards of many countries.
Also, particle number concentration has recently featured in vehicle emission
legislation. In particle research, particle size measurements are carried out using
differential mobility analyzer (DMA) based on particle electrical mobility, which
has been considered as one of the largely employed techniques. For DMA
calibration, known size particle standards are widely used. Whereas, particle
number concentration is generally measured using particle counters.
In general, particle counters are of three types: (1) Faraday Cup Electrometer
(FCE), in which particles are charged singly first, and then they are counted by
number charges. However, it is assumed that all particles get single charge. In this
manner charging of bigger sized particles is difficult, (2) Condensation Particle
Counter (CPC), in which particles (3nm above) undergo condensational growth
on a working liquid and are detected/counted by light scattering technique, (3)
Optical Particle Counter (OPC), which counts the particles by the measurement of
scattered light of individual particles. The OPC technique is only applicable for
the particles of above -200 nm size.
Condensation particle counter works on the particle growth followed by detection
by optical principle for counting the particles. For precise and accurate
measurement, it is important that CPC should be well calibrated (J. Yli-Ojanpera,
H. Sakurai, K. Iida, J. M. Makela, K. Ehara, and J. Keskinen,Comparison of three
particle number concentration calibration standards through calibration of a single
CPC in a wide particle size range, Aerosol Sci. Tech., 46:11, 1163-1173, 2012). It
is also important to note that FCE works based on a primary technique, so
generally used as a reference counter in the calibration of other counters.
However, this counter applicable only for ultrafine particle size range (R.
Hogstrom, P. Quincey, D. Sarantaridis, F. Liiond, A. Nowak, F. Riccobono, T.
Tuch, H. Sakurai, M. Owen, M. Hcinonen, J. Keskinen, and J. Yli-Ojanpera, First
comprehensive inter-comparison ofaerosol electrometers for particle sizes upto
200nm tod concentration range 1000cm"3 to 17000cm"3, Metrologia, 51, 293-303,
2014).
A few methods are available for the calibration of individual CPC (R. A. Fletcher,
G. W. Mulholland, M. R. Winchester, R. L. King, and D. B. Klinedinst,
Calibration of a condensation particle counter using a NIST traceable method,
Aerosol Sci. Tech., 43, 425-441, 2009) that used FCE as a reference counter to
calibrate CPC. They have first calibrated the electrometer using NIST electrical
and flow standard traceable to SI, and then used this electrometer for calibrating
the CPC. Because FCE can work precisely within ultrafine range of the particles
of < 200 nm size, this calibration can be applicable only for these sized particles.
Similarly, a patent has been granted (M. Pourprix, Process and apparatus for
calibrating a particle counter, U S Patent 5150036, September 22, 1992) on the
calibration of a counter using particle charging technique. This process comprises
the steps of forming a vector gas flow by an aerosol of particles of the same grain
size, developing ions in the vector gas with both sign by a bipolar charger, certain
particles being electrically charged to a stationary charge state in which the
distribution of the number of charges fixed to each particle follows a Gaussian
law (Gunn or Boltzmann), passing the charge aerosol into a mobility selector to
attract the charged particles to electrodes therein and classify them as a function
of the numbers p of their elementary electrical charges e and allowing the
electrically neutral particles to escape, collecting the neutral particles and passing
the neutral particles into the particle counter to be calibrated, the particles counter
displaying a value N'O; counting the values of Np and Np+lof the number of
particles of charges pe and (P+l)e fixed by the selector and calculating by the
formula, in which the number NO representing the number of neutral particles
supplied to the counter and comparing N'O with Np. An apparatus using above
process to separately pick-up the charged particles with a view to count the
numbers Np and Np+1 contains a particle counter in which particles of the same
grain size are produced comprising a cylindrical case containing an annular
cylindrical bipolar charge space and a mobility selector linked with the charge
space and are superimposed manner within the cylindrical case.
This suggests that this process needs a specific apparatus to collect charged
particles pe and (P+l)c and then count corresponding number Np and Np+1
particles, respectively. This information needs to be fed to a formula to count NO
and then compare with N'O, which is the number counts obtained from the
counter under the. test. Because of the need of specific designed apparatus, thio
process is not flexible to use it for inter-comparison studies. Also only fine
particles generally carry pe or (P+l)e charge and bigger size particles generally
get multiple charge. Therefore, size range has been a limitation of this process.
In another study by B. Giechaskiel, X. Wang, H.-G. Horn, J. Spielvogel, C.
Gerhart, J. Southgate, L. Jing, M. Kasper, Y. Drossinos and A. Krasenbrink,
Calibration of condensation particle counters for legislated vehicle number
emission measurements, Aerosol Sci. Tech., 43, 1164-1173, 2009 both, an
electrometer and a reference CPC have been used for calibration of a CPC,
wherein several issues are highlighted. These were measurement issues including
calibration against an electrometer or a reference CPC, the effect of multiply
charged particles on counting efficiencies, stability, repeatability, reproducibility
and comparability of CPCs and electrometers of different manufacturers were also
investigated.
These calibration processes/methods are based on FCE, which work with a
defined particle size (inlet) range. Also a great difficulty is being realized while
organizing an inter-comparison among different national metrology institutes
(NMIs) about particle count efficiency of the CPCs. For inter-comparison of
counter efficiency, counters from different laboratories need to send at one place,
where single particle source can be used to simultaneously feed aerosol particles
in equivalent number/volume to each counter, and thus the counter efficiencies of
the counters can be compared with the reference counter/s. This approach is not
very convenient as shipping of a CPC involves/causes several issues.
i.
In this invention, an approach (method and equation) for calibration checks of
counter has been developed in which a known material (solid or in liquid form)
needs to send to all participant laboratories. The participant laboratories can
generate particles from this material (e.g., inorganic salt) using atomizer, and
introduce them to differential mobility analyzer (DMA) for particle sizing. Then
particle stream was sent to the counter (device under test) and in parallel to a filter
fixed in cartridge/or quart crystal microbalance (QCM) with the same flow rate.
Based on the mass deposited on filter, particle size distribution and effective
density, particles volume can be calculated. Then these filter/QCM based volume
are compared with particle counter volume for counter's counting efficiency
checks. Because this approach is based on the gravimetric mass measurement
(primary technique), particle size is not a limit unlike the electrometer approach.
Objectives of the invention
Main object of the present invention is to develop a method for particle counter
efficiency checks.
Another object of the present invention is to provide a method for particle counter
efficiency checks that can be useful at laboratory scale (especially at national
metrology institutes (NMIs)).
Yet another object is to provide a method wherein the method should be useful for
inter-comparison studies in which particle counters from different laboratory
should not be needed to ship at a place.
Yet another object is to provide a method wherein the method should be not
limited to the particle size range, and can be applied over desired particle size
range measures.
Yet another object is to provide a method wherein the method should be based on
a primary method so the results obtained would be SI traceable.
Yet another object is to provide a method wherein the method should be simple,
precise and accurate.
Summary of the invention
Accordingly, a present invention is for a method for number count efficiency checks
of particle counter which comprises;
a. An aerosol generation system;
b. Two diffusion dryers in series for drying the generated aerosols (>5% RH).
These aerosols then pass through the impactor, neutralizer, and DMA for size
segregation;
c. Aerosol stream then divided into two equal flow sub-steams, where one steam
sent to CPC for number count measurement, and other one to the filter
unit/QCM for corresponding mass measurement;
d. Particle volume derived from the CPC data is compared with the particle
volume which is calculated based on the mass data obtained from filter
unit/QCM using the salt density.
In an embodiment, aerosol particle can be generated by using salts of known density
(purity > 99.95%) such as ammonium sulphate, ammonium nitrate or sodium
chloride and is aerosolized with particle free water (e.g. MilliQ water of >18MQ cm)
and particle tree gas (e.g. 5N nitrogen gas).
In yet embodiment, a setup of instruments comprises;
a) An air filtration unit (TSI 3074B) wherein particle free air exit at a defined
pressure (1.0 - 1.2 kg/cm ).
b) An aerosolization unit (TSI 3076) wherein aerosolization done from the salt
solution.
c) Two diffusion dryers (silica gel, TSI 3062) are used for drying the aerosol
stream to get aerosol stream with less than 5%.relative humidity at exit.
d) A radioactive source ( Kr) is used as neutralizer to maintain the neutral
charge on the aerosol stream.
e) An electrostatic classifier (TSI 3080) is used to maintain the sheath flow at a
fixed flow rate (6 1pm).
f) A differential mobility analyzer (TSI 3081) is used for particle size
segregation based on particles electrical mobility.
g) A condensation particle counter (TSI 3788) is used to count the particles in a
unit volume based on their condensational growth followed by the detection
using incident laser light scattering technique.
h) A filter unit includes SS holder and filter (48 mm quartz, PTFE and glass
filter) wherein particles were collected on the filter. After sampling, filter was
conditioned for weighing.
i) An electrostatic neutralizer (Matter Toledo, PRX U SET Small) is used to
remove the charge effects on the filter during weighing process.
j) Microbalance (Matter Toledo, MT5/AX105) was used to measure the mass of
particles collected on the filter.Alternatively, a quartz crystal microbalance
(California Measurements Inc., PC-2) is used (instead of filter unit) to obtain
the mass of the aerosols real time.
In further embodiment, counting efficiency of the particle counter is checked by
comparing counter derived volume of the particles and filter/QCM mass derived
volume which comprises;
a) An impactor placed at the inlet of classifier which allows fine particles to be
entered to neutralizer and then to DMA;
b) Fine particle size range at impactor exit depends on the sampling flow rate;
c) DMA segregates these particles in different size bins depending on the voltage
which is applied to DMA rod;
d) Particles in a defined size bin are counted in CPC;
e) Particle size and number distribution scans are obtained for a certain period
(e.g. 30 min, 60 min, 90 min, 120 min) and volume of the particles of one
scan and then for total period is calculated;
f) Simultaneously particles are collected on the filter at the downstream of
DMA;
g) By weighing of the filter mass before and after sampling, mass of the
corresponding particles is determined;
h) Then using the salt material density (P(NH4)2S04 = 1-77 g/cm3, pNaci = 2.17
g/cm , and PNH4N03 =1-72 g/cm ), volume of the particles is calculated.;
i) Alternatively, corresponding particle mass can be obtained from QCM
measurement. Then volume of particles is calculated.
In yet another embodiment, method can be applied for inter-comparison studies of
counter's efficiency checks which comprises:
a. The standard materials, salts (of known density) in solid form or in
solution form of known strength can be transfer to different laboratory
or national metrology institutes (NMIs).
b. NMIs can use a similar setup to check the counting efficiency of their
counters.
c. The results of particle volume derived from counter can be checked
against the corresponding volume derived from filter mass/QCM
measurement.
In yet another embodiment, CPC manufacturer and different service providers (those
used CPC) can use this method for CPC counting efficiency/calibration checks.
In yet another embodiment, this method can be applied for a wide particle size range
needed for the calibration of a counter by applying different voltage to DMA rod,
DMA segregates inlet particles in different size bins, e.g. 10 nm to <800 nm.
This method is simple, precise and accurate, and applicable at laboratory level
without a need to establish any additional expensive setup and instrumentation.
Description of the figures:
Figurela. Experimental setup
Figurelb. Experimental setup when QCM is operated in place of filter holder
Figure 2. Plot showing the uncertainty of CPC based particle volume versus
filter base volume
Figure 3. Plot showing the uncertainty of filter based volume versus CPC
based particle volume
Figure 4. Setup for flow optimization (see Table 4 for data recorded at points:
A, B, C,D and E)
Figure 5. Comparison between CPC derived particle volume and
corresponding QCM derived particle volume. For particle generation, different
standard materials (sodium chloride, ammonium sulfate, ammonium nitrate)
are used.
Detailed description of the invention
This method can be used for a wide particle size range needed for the calibration
of a counter. For example, for CPC the range should be from 10 nm to <800 nm.
This is an advantage over electrometer calibration method. In this method, CPC
calibration check is based on the comparison of particle volumes which are
derived from filter/quartz crystal microbalance (QCM) mass and CPC counts.
In this process, we used scanning mobility particle seizer (SMPS) consisting of a
DMA (TSI 3081) and CPC (TSI 3788). The experimental setup used for the
purpose is shown in Figure 1. Dried inorganic salt particles were introduced to
SMPS with a sample flow rate of 0.6 1pm and sheath flow rate of 6 1pm. Particles
of size range 14 - 615 nm were segregated by DMA and number concentration
was measured by CPC, which was operated with a flow rate of 0.3 1pm. Also,
after DMA a quartz filter (48 mm, which was baked at 450 °C, conditioned and
weighted) or a quartz crystal microbalance (QCM) was placed to collected
particles with a flow rate of 0.3 1pm simultaneously to measure the mass of the
particles downstream to DMA. After a certain period of particle sampling, the
filter was conditioned and weighed or in case of QCM, mass concentration was
directly obtained.
Using CPC particle count data, CPC derived particle volume can be calculated as:
V^vx^ld/xn, -(i)
where, V is the total volume of the size distribution in cm , v is the flow volume
(calculated from CPC inlet flow and the time of flow) enters to CPC, dPj and n, are
the particle size and number concentration in bin i.
Based on the particle mass collected on the filter, and using density of particle
material (e.g., ammonium sulphate), the volume of particles can be calculated as:
where, p is the density of particle material (e.g. in case of ammonium sulphate,
1.77 g/cm3), m is the mass of the particles collected on the filter/by QCM.
Novelty, inventive steps and utility
The novelty of the present invention for calibration check of a condensation
particle counter (CPC) is that the improved method is based on the comparison of
particle volumes measured by counter counts and by filter mass simultaneously.
In the improved method particles are generated by high purity salt (whose density
is known) solution and particle volume is calculated (i) by particle counter data
(ii) by particle mass collected on a filter, and then compared these two volumes to
check the calibration status of a counter. This method is different from prior art
method wherein (i) no need of standard particles (ii) no need of reference counter
(Faraday Cup Electrometer); and (iii) no particle size constrains for calibration
check size range. Moreover, these novelties makes this method more suitable/easy
to use by any laboratory without the need of any specific setup in comparison to
the prior art methods, thus can be used for inter-comparison study purposes.
The novelty of the process of the present invention for the calibration of the
particle counter has been achieved by the non-obvious inventive steps of:
(i) This method can be used for a wide particle size range needed for the
calibration of a counter.
(ii) In this method, standard size particles, such as standard polystyrene latex
particles are not needed. Instead, salt material such as ammonium
sulphate, ammonium nitrate or sodium chloride can be used for making
their solution and this solution can be aerosolized for generation of wide
particle size range. These salts are easy to ship, stable for a longer
duration of time, cost effective and easily available.
(iii) Generated particle are sent to DMA to get particles with their known sizes
at the downstream of the DMA. After this (at the downstream of the
DMA) aerosol stream is divided into two sub streams: (a) counter under
test is connected, (b) a filter is placed to collect particles simultaneously.
Particle volumes derived from both (a) and (b) are compared for counter
calibration checks. Method is based on a primary method (traceable to SI).
(iv) Therefore, no need of any reference counter for calibration checks as
needed in prior art methods.
(v) Filters used for the collection of particles are stable regarding their mass
measurement under controlled humidity condition for the whole duration
of experiment.
(vi) QCM can additionally be used at the downstream of the DMA to get the
mass concentration to validate the process real time.
Utility: The invented method for counting efficiency checks of particle counter
being simple and relatively cost efficient (based on instruments which are readily
available in a particle research laboratory), can be applied at on laboratory scale
facilities. The invented method can be useful for inter-comparison studies in
which the need of shipping of the particle counters from different laboratory to a
particular laboratory is not required. The method is based on a primary method so
that obtained results are SI traceable. This method can be used for different
national metrology institutes (NMIs) on the globe for inter-comparison studies of
CPC counting efficiency. In addition to this, CPC manufacturer and different
service providers (those used CPC) can also use this method for CPC counting
efficiency/calibration checks.
1. Brief description of the drawings:
Method involves a setup of instruments consisting of atomizer (aerosol generation
system), diffusion dryers, impactor, neutralizer, differential mobility analyser
(DMA), filter holder/quartz crystal micro balance (Figure la and b). Apart from
10
the particle counter, these instruments are used for testing the counting efficiency
as well.
2. Examples:
(1) Generated ammonium sulphate particles are introduced to DMA followed by
CPC and filter. CPC based volume calculated from Equation 1, and compared
with the corresponding filter based volume, which is calculated using
Equation 2. Table 1 shows the data obtained by both of the methods. Figure 2
illustrates comparison between both the data, i.e. filter based particle volume
versus CPC based particle volume. The uncertainty of diffusion loss
correction along the SMPS (DMA + CPC) channel and the particle charge
correction are incorporated to estimate the CPC derived particle volume.
Similarly, uncertainty components due to balance, relative humidity,
temperature and air buoyancy are considered in filter based particle volume
calculation.
Theoretically filter derived volume can be the reference volume and a
correction in CPC based particle volume/counts can be applied based on the
comparison.
For different sampling time duration, data obtained are shown in Table 2, and
pictorially in Figure 3.
Table 1: Particle volume calculated from both filter and CPC
CPC based particle volume, cm3
xlO"5
1.38
1.39
1.39
1.39
1.38
1.37
1.39
1.38
1.39
1.38*10 5±9.49xl0"8
3
Filter based particle volume, cm
xlO"5
1.17
1.28
1.39
1.47
1.21
1.58
1.28
1.51
1.62
1.39xl0"5 ±1.65xl(r6
Table 2: Particle volume calculated from both filter and CPC
11
Sample set
1.
2.
3.
Average
Std. dev.
Average ±
Uncertainty
CPC based particle
volume, cm3
3.57xl0"uy
3.69xl0"uy
3.46x10"uy
3.57xl0'uy
1.13xl0"10
3.57xl0"uy±
2.53xl0"10
Filter based particle
volume, cm3
3.39xl(ruy
4.52x10"uy
3.39xl0-uy
3.77xl0"uy
6.53xl010
3.77xl0"uy±
7.94X1010
(2) Calibration of differential mobility analyzer (DMA): SI traceable particle
standards are used for DMA calibration. Uncertainty was calculated, and
summary of the data is given in Table 3.
(3) Table 3. DMA calibration results
Reference Polystyrene latex sphere (PSL)- 80nm
DMA calibration - Statistical analysis
Average
geometric
diameter
(nm)
74.9±0.31
Average
mean
diameter
(nm)
75.58±0.35
Repeatability
(nm)
0.38
Repetability
(nm)
0.42
Reproducibility
(nm)
0.36±0.005
Reproducibility
(nm)
0.41
Average
geometric
standard
deviation
(nm)
1.14
Average
mode
diameter
(nm)
78.02±1.39
Repeatability
(nm)
0.006
Repetability
(nm)
1.39
Reproducibility
(nm)
0.01
Reproducibility
(nm)
1.38
(4) Flow through DMA, CPC and filter holder optimization: Flow of the system
was optimized and results are shown in Table 4 and Figure 4.
Table 4. Flow optimization data (see Figure 4 for experimental setup)
Flow test Reference flow meter - Gillian Gilibrator
(SI traceable, max flow rate: 6 1pm)
Flow test
(Duration in h)
Test 1(3)
Inlet dryer
(1pm)
(A)
0.587±0.008
Bypass inlet
CPC(lpm)
(B)
0.306±0.005
Inlet CPC
(1pm)
(C)
0.280±0.006
Outlet filter
holder (1pm)
(D)
0.306±0.005
Sheath
Flow(lpm)
(E)
6
12
Test 2(6)
Test 3(9)
0.578±0.008
0.572±0.004
0.279±0.003
0.275±0.002
0.321±0.004
0.298±0.004
0.279±0.003
0.275±0.002
6
6
(5) Example of filter weighing (effect of environmental conditions): Related
results are shown in Tables 5-6.
Table 5. Effect of environmental conditions on filter weighing.
yZ{ Filter weight (by microbalance, lug, 5g)
PTFE filter
/ (sampling
( duration in h)
Aluminium curl
j Filterl (3)
\ Filter2 (1 .5)
( Filter3(0.5)
( Filter4 (Blank)
r Room condition
Before sampling
weighing, mg
(25/11/2013,11:30
AM)
383.643
328.429
298.834
309.965
315.201
Before
weighing
T=22.71
°C
RH=45.0
%
P=989
hPa
After
weighing
T=21.99
°C
RH=47.6
%
P=989
hPa
After sampling
weighing, mg
(25/11/2013,
06:00 PM)
383.651
328.228
298.542
310.028
314.805
Before
weighing
T-22.34
°C
RH=44.3
%
P=986.5
hPa
After
weighing
T=23.01
°C
RH=48;4
%
P=986.2
hPa
After
sampling
weighing,
mg
(26/11/2013,
11:10 AM)
(Repeat)
383.654
328.405
298.759
310.056
314.842
Post
weighing
T=22.70
°C
RH=44.6
%
P=989.4
hPa
Table 6. Use of neutralizer and its effect on weighing
Test
1
2
3
4
5
Weight of PTFE-
0186280,g
Neutralizer
used
0.15252
0.15252
0.15252
0.15250
0.15250
Neutralizer
not used
0.15249
0.15252
0.15252
0.15250
0.15250
Weight of PTFE-
0186278,g
Neutralizer
used
0.14900
0.14901
0.14900
0.14900
0.14900
Neutralizer
not used
0.14900
0.14901
0.14900
0.14900
0.14900
T
CO
20.8
21.2
21.6
21.6
21.7
RH
(%)
58.1
57.5
55.4
55.4
54.7
Example of QCM measurement: Quartz crystal microbalance (QCM) is operated in
place of filter holder. The particle mass obtained from CPC and QCM is compared
and applied for calibration check of CPC. Results are shown in Table 7, Figure 5.
Uncertainty components considered are given in Table 8.
Table 7. Results of QCM derived particle volume and comparison of corresponding CPC
derived particle volume.
Inorganic
salts
particles
(NH4)2S04
NaCl
NH4NO3
QCM
derived
volume
(cm3)
1.31 x 10"uli
2.18x 10"us
1.03x 10~U8
Expanded
uncertainty
(IUp)
(cm3)
1.06* 10"uy
2.09x 10"uy
3.77x 10"uy
CPC
derived
volume
(cm3)
1.29x 10"os
2.06x 10"us
1.03x 10"U8
Expanded
uncertainty
(UM„)
(cm3)
6.76x 10"1U
1.34x 10"oy
1.29x 1Q-Uy
Table 8. Uncertainty components considered in CPC and QCM
measurements.
Uncertainty component: CPC
Charge Correction
Diffusion Correction
CPC Inlet Flow Rate
CPC bypass flow rate
Uncertainty component: QCM
Crystal frequency
QCM inlet flow rate
Advantages:
(i) The invented method for counting efficiency checks of particle counter is
simple and relatively cost efficient (based on instruments which are readily
available in a particle research laboratory), so can be applied at on
laboratory scale facilities.
(ii) The invented method can be useful for inter-comparison studies in which
particle counters from different laboratory should not be needed to ship in
a particular laboratory.
(iii) The method is based on a primary method so that obtained results are SI
traceable.
1 A
(iv) This method can be used for different national metrology institutes (NMIs)
on the globe for inter-comparison studies of CPC counting efficiency. In
addition to this, CPC manufacturer and different service providers (those
used CPC) can also use this method for CPC counting
efficiency/calibration checks.
(v) This method can be used for a wide particle size range needed for the
calibration of a counter. For example for CPC the range should be from 10
nm to <800 nm.
(vi) In this method, standard size particles, such as standard polystyrene latex
particles are not needed. Instead, salt material such as ammonium sulphate,
ammonium nitrate or sodium chloride can be used for making their
solution and this solution can be aerosolized for generation of wide
particle size range. These salts are easy to ship, stable for a longer duration
of time, cost effective and easily available.
(vii) Filter used at the downstream of the DMA is well placed to avoid any
diffusion loss.
(viii) Filter used for the collection of particles are stable regarding their mass
measurement under controlled humidity condition for the whole duration
of experiment.
(ix) QCM can additionally be used at the downstream of the DMA to get the
mass concentration to validate the process real time.

We Claim:
1. A method for number count efficiency checks of particle counter which comprises;
a. An aerosol generation system;
b; Two diffusion dryers in scries for drying the generated aerosols (>5% RH).
These aerosols then pass through the impactor, neutralizer, and DMA for size
segregation;
c. Aerosol stream then divided into two equal flow sub-steams, where one steam
sent to CPC for number count measurement, and other one to the filter
unit/QCM for corresponding mass measurement;
d. Particle volume derived from the CPC data is compared with the particle
volume which is calculated based on the mass data obtained from filter
unit/QCM using the salt density.
2. The method of claim 1 wherein aerosol particle can be generated by using salts of
known density (purity > 99.95%) such as ammonium sulphate, ammonium nitrate or
sodium chloride and is aerosolized with particle free water (e.g. MilliQ water of
>18MQ cm) and particle free gas (e.g. 5N nitrogen gas).
3. The method as claimed in claim 1 wherein a setup of instruments comprises;
a. An air filtration unit (TSI 3074B) wherein particle free air exit at a defined
pressure (1.0- 1.2 kg/cm2).
b. An aerosolization unit (TSI 3076) wherein aerosolization done from the salt
solution.
c. Two diffusion dryers (silica gel, TSI 3062) are used for drying the aerosol
stream to get aerosol stream with less than 5% relative humidity at exit.
O f
d. A radioactive source ( Kr) is used as neutralizer to maintain the neutral
charge on the aerosol stream.
e. An electrostatic classifier (TSI 3080) is used to maintain the sheath flow at a
fixed flow rate (6 1pm).
f. A differential mobility analyzer (TSI 3081) is used for particle size
segregation based on particles electrical mobility.
g. A condensation particle counter (TSI 3788) is used to count the particles in a
unit volume based on their condensational growth followed by the detection
using incident laser light scattering technique,
h. A filter unit includes SS holder and filter (48 mm quartz, PTFE and glass
filter) wherein particles were collected on the filter. After sampling, filter was
conditioned for weighing,
i. An electrostatic neutral izer (Matter Toledo, PRX U SET Small) is used to
remove the charge effects on the filter during weighing process,
j. Microbalance (Matter Toledo, MT5/AX105) was used to measure the mass of
particles collected on the filter.Alternatively, a. quartz crystal microbalance
(California Measurements Inc., PC-2) is used (Instead of filter unit) to obtain
the mass of the aerosols real time.
4. The method of claim 1 wherein counting efficiency, of the particle counter is checked
by comparing counter derived volume of the particles and filter/QCM mass derived
volume which comprises;
a. An impactor placed at the inlet of classifier which allows fine particles to be
entered to neutralizer and then to DMA;
b. Fine particle size range at impactor exit depends on the sampling flow rate;
c. DMA segregates these particles in different size bins depending on the voltage
which is applied to DMA rod;
d. Particles in a defined size bin are counted in CPC;
e. Particle size and number distribution scans are obtained for a certain period
(e.g. 30 min, 60 min, 90 min, 120 min) and volume of the particles of one
scan and then for total period is calculated;
f. Simultaneously particles are collected on the filter at the downstream of
DMA;
g. By weighing of the filter mass before and after sampling, mass of the
corresponding particles is determined;
h. Then using the salt material density (P(NH4)2S04 = 1-77 g/cm , pNaci = 2.17
g/cm , and PNH4N03 = 1-72 g/cm ), volume of the particles is calculated.;
i. Alternatively, corresponding particle mass can be obtained from QCM
measurement. Then volume of particles is calculated.
5. The method as claimed in claim 1 wherein method can be applied for intercomparison
studies of counter's efficiency checks which comprises:
a. The standard materials, salts (of known density.) in solid form or in solution
form of known strength can be transfer to different laboratory or national
metrology institutes (NMIs).
b. NMIs can use a similar setup to check the counting efficiency of their
counters.
c. The results of particle volume derived from counter can be checked against
the corresponding volume derived from filter mass/QCM measurement.
6. The method as claimed in claim 1 wherein CPC manufacturer and different service
providers (those used CPC) can use this method for CPC counting
efficiency/calibration checks.
7. The method as claimed in claim 1 wherein, this method can be applied for a wide
particle size range; needed for the calibration of a counter by applying different
voltage to DMA rod, DMA segregates inlet particles in different size bins, e.g. 10
nm to <800 nm.