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A combined DMA-CNC-test-system for commercial laser particle counters
J. Aerosol Scl., Vol. 20, No. 8, pp. 1529-1532, 1989. Printed In Great Britaln.
0021-8502/89 $3.00 + 0.00 Pergamon Press plc
A COMBINED DMA- CNC - TEST - SYSTEM FOR COMMERCIAL LASER PARTICLE COUNTERS J. Gebhart, P. Brand and C. Roth Gesellschaft fiir Strahlen- und Umweltforschung mbH Mtinchen Institvt fiir Biophysikalische Strahlenforschung, Paul - Ehrlich - Str. 20, D - 6000 Frankfurt/M, F.R.G.
INTRODUCTION Laser particle counters (LPC) with sample flow rates of 1 cfm ( = 28 1 min -1) and a sensitivity of 0.1 Inn or below are more and more used in clean room monitoring and high efficiency filter tests. A characteristic feature of such instruments is their counting efficiency towards the lower detection limit whereby the particle size with 50% counting efficiency is usually defined as the sensitivity drain of an instrument. Following previous studies (Gebhart et al., 1983; Gebhart and Roth, 1986) counting efficiency is the concentration c measured with the LPC under coincidence-free conditions in relation to the concentration co obtained by an independent reference method. According to this definition the measurement requires a monodisperse test aerosol with low number concentration (c < 5 crn-3) and a reference instrument which is more sensitive than the device undergoing the test. In former investigations a laser aerosol spectrometer (LASS) of the own laboratory (Roth and Gebhart, 1978) or a re-calibrated LPC - Model L A S - X of PMS (Wen and KasPer, 1986; Gebhart, 1989) - served as reference instrument. Due to a steady progress in the se~itivity of LPC's a condensation nucleus counter (CNC) with a sensitivity in the range of 0.01 Imx' seems to be a propriate reference instrument for future test-systems (l.,htimliki et al., 1989). When applying a ChIC to polystyrene latex (PSL) aerosols, however, some problems arise which are subjects of the present study.
PROPERTIES OF PSL-AEROSOLS Latex aerosols are generated by atomizing an aqueous suspension of PSL-particles with a jet nebulizer and drying the droplets in a stream of filtered air (Gebhart et al., 1980). A comparison of the total droplet concentration cT of a jet nebulizer with the concentration cA of the P S L particles demonstrates, that only about one per mille of the droplets contain particles whereas all other droplets remain empty and create residuals. For an upper droplet diameter of 10 pm and chemical impurities of about 0.1 ppm in the liquid (quality of the MiUi-Q Water Purification System) the diameter of such residuals should be below 0.05 pan. Such residuals are not detected by a LPC but a lot of them can be activated and counted in a CNC. Droplets containing more than one PSL-particle produce agglomerates of two or more uniform spheres. The amount of agglomerates in the PSL-aerosol depends on the droplet size and on the particle concentration c, in the liquid suspensions. Their fraction can be primarily reduced by diluting the liquid suspension with high purity water. Near threshold of a LPC agglomerates affect the measurement of counting efficiency, since they represent larger particles than the size stated.
DESIGN OF THE T E S T - S Y S T E M A schematic diagram of the D M A - C N C - T e s t - S y s t e m is shown in Fig. 1. Its main parts are a latex-generator, a differential mobility analyser (DMA, TSI Model 3071) and a condensation nucleus counter (CNC, TSI Model 3020). By passing the PSL-aerosol through the DMA with the voltage adjusted to the peak of the singlets it is expected that residuals and agglomerates are AS 20: 8-.~
1529
1530
J. GEBHART et a l .
SHEATH AIR |
FILTERED AIR '
MONODISPERSE
361mln" __
61:in"
/ niin"
AEROSOL
I~ ~ R A
___ I
i
PSL-
DMA
GENERATOR
TSI / 3071
I
EXCESS AEROSOL
•
POLYDISPERSE AEROSOL
(TEST PARTICLES)
ROOM 1 ~UR
(TEST PARTICLE8)
ROOM i AiR
kv..u,,)
:/ 7_¢
EXCESS AIR
.
-¢
;,,,+W,;}.
Fig. 1: Schematic diagram of the test-system. Version A; left: CNC and LPC suck aerosol from a common buffer volume. Version B; fight: CNC samples aerosol directly behind the DMA-output.
combinedDHA-CNC-test-system
A
1531
removed. In version A the CNC and the LPC suck aerosol at the same concentration level from a common buffer volume. In version B the CNC samples aerosol directly behind the monodisperse output of the DMA without any dilution. In this case the concentration measured with the CNC has to be divided by a constant dilution factor (given by the different flow rates) in order to obtain the reference concentration co. Experiments have been carded out with four LPC's. Their specifications as quoted by the manufacturers are: PMS Model PMS Model PMS Model I-Iiac/Royco
LAS-X HS-LAS p/.,PC-0710 Model 5109
: : : :
sensitivity sensitivity sensitivity sensitivity
0.09 0.065 0.07 0.09
pm; pro; lUn; pro;
0.01 0.01 1 1
cft rain -1 ch rain -1 cft rain -1 cft rain -I
EXPERIMENTAL RESULTS REMOVAL OF RESIDUALS: In case filtered air passed the DMA the background of system was below 80 counts per cft or 2.8 x 10-3 cm -3. In case PSL-aerosol passed the concentration measured with the CNC was in the average about 0.2 cm-3 higher indicated by the H S - L A S and the pLPC (see Fig. 2). This difference was measurable cm -3 but became negligible for higher concentrations.
clp/~m
CNC
HS - LAS
o
•
lil
uLPC -0710
~
A
[~I
\
0,079
0 , 1 1 7 0,15
n ~.o J ' -
0,087
FF-
z
Z loO"
/ / _ / 3w
/ A
/
~ft,
~~////7-
W 0
/
•
z
the t e s t the DMA than that below 1.~
O U LU -J U
~
//
Fr,'
/
0. .
.
.
.
l o -J-
i
J.o ° PARTICLE
.
.
.
I
.
HS - LAS" ~LPC-0710 CONCENTRATION
,,
lo ! /
cm - ~
Fig. 2: Particle concentration measured with the CNC versus particle concentration indicated by the HS - I.AS and the Id.PC - 0710. After extreme dilution of the PSL-suspension with high purity water and without using the DMA the H S - L A S counted only a few 0.087 pro-particles per cm 3 air whereas the CNC indicated concentrations between 430 and 670 cm -3. From these experimental findings it can be concluded that the filter effidency of the DMA for residuals was better than 99.9% but not 100%. REMOVAL OF AGGLOMERATES: In a study with the L A S - X as reference instrument the fraction of agglomerates in the monodisperse output of the DMA was decreased in comparison to the input for a particle diameter of 0.163 pm (Table 1). In an experimental run with the H S - L A S the amount of doublets, however, increased for 0.087 pro-particles after passage through the DMA.
1532
J. GEBHART et al.
without DMA part. concentr,
with DMA
f r a c t , of agglom, part. concentr,
f r a c t , of agglom.
cm-3
~
cm-a
16000
45
2200
13
8500
31
800
6
2200
12
300
1.6
Tab. 1: Number concentration and fraction of agglomerates of 0.163 ima-particles measured by a L A S - X with and without passing a DMA. Let Z I ( + ) be the electrical mobility of a singlet (indicated by the index) carrying one charge. Then the electrical mobilities Zn(+), Zn(+ +) etc. can be calculated in relation to ZI(+). These ratios Z,(+...)/Z~(+) depend on the slip-correction and on the dynamic shape factor of the particles. The calculations demonstrate that with careful adjustment of the voltage to the peak of the singlets and with a control of the flows through the DMA agglomerates should be removed by the DMA to a remarkable extent. COUNTING EFFICIENCY: The HS-I_AS recorded the whole peak of the singlets of the 0.087 pm-particies and classified most of the 0.074-particles in its lowest channel (0.065-0.07 pan). Using both the H S - L A S and the CNC as reference instruments the ~J.~PC-0710 showed 100% counting efficiency for particles of 0.087 pxn diameter. The amount of agglomerates during this experimental run was about 15%. From the coincidence curves of the Model 5109 of Hiac/Royco counting efficiencies of 58% for 0.106 p.m-particles and 42% for 0.087 ixm-particles could be derived. The measurements have been carried out with version A of the test-system. The contribution of the agglomerates was not investigated separately during this test but could not have been the dominating factor for the results. REFERENCES Gebhart, J., Heyder, J., Roth, C. and Stahlhofen, W. (1980): Herstellung und Eigenschaften von Latex-Aerosolen. Staub-Reinhalt. Luft 40, 1. Gebhart, J., Blankenberg, P., Bormann, S. and Roth, C. (1983): Vergleichsmessungen an optischen Partikelzalhlern. Staub- Reinhalt. Luft 43, 439. Gebhart, J. and Roth, C. (1986): Background noise and counting efficiency of single optical particle counters. In: Aerosols, Formation and Reactivity (Eds.: Schikarski, W., Fissan, H., Friedlander, S.) Pergamon Press, Oxford, New York, 607. Gebhart, J. and Anselm, A. (1987): Effect of particle shape on the response of single particle optical counters. Int. Syrup. on Optical Particle Sizing. May, 12-15; ROUEN, France. Gebhart, J. (1989): Funktionsweise und Eigenschaften optischer Partikelzahler. Technisches Messen tln 56, 192. Lehtlm~lki, M. and Keskinen, J. (1989): Experiments with a high sensitivity clean room particle counter. Journ. R 3 - Nordic 3, 17. Roth, C. and Gebhart, J. (1987): Rapid particle size analysis with an ultramicroscope. Microscopia Acta 81, 119. Wen, H.Y. and Kaspar, (3. (1986): Counting efficiencies of six commercial particle counters. J. Aerosol $ci. 17, 947.
0021-8502/89 $3.00 + 0.00 Pergamon Press plc
A COMBINED DMA- CNC - TEST - SYSTEM FOR COMMERCIAL LASER PARTICLE COUNTERS J. Gebhart, P. Brand and C. Roth Gesellschaft fiir Strahlen- und Umweltforschung mbH Mtinchen Institvt fiir Biophysikalische Strahlenforschung, Paul - Ehrlich - Str. 20, D - 6000 Frankfurt/M, F.R.G.
INTRODUCTION Laser particle counters (LPC) with sample flow rates of 1 cfm ( = 28 1 min -1) and a sensitivity of 0.1 Inn or below are more and more used in clean room monitoring and high efficiency filter tests. A characteristic feature of such instruments is their counting efficiency towards the lower detection limit whereby the particle size with 50% counting efficiency is usually defined as the sensitivity drain of an instrument. Following previous studies (Gebhart et al., 1983; Gebhart and Roth, 1986) counting efficiency is the concentration c measured with the LPC under coincidence-free conditions in relation to the concentration co obtained by an independent reference method. According to this definition the measurement requires a monodisperse test aerosol with low number concentration (c < 5 crn-3) and a reference instrument which is more sensitive than the device undergoing the test. In former investigations a laser aerosol spectrometer (LASS) of the own laboratory (Roth and Gebhart, 1978) or a re-calibrated LPC - Model L A S - X of PMS (Wen and KasPer, 1986; Gebhart, 1989) - served as reference instrument. Due to a steady progress in the se~itivity of LPC's a condensation nucleus counter (CNC) with a sensitivity in the range of 0.01 Imx' seems to be a propriate reference instrument for future test-systems (l.,htimliki et al., 1989). When applying a ChIC to polystyrene latex (PSL) aerosols, however, some problems arise which are subjects of the present study.
PROPERTIES OF PSL-AEROSOLS Latex aerosols are generated by atomizing an aqueous suspension of PSL-particles with a jet nebulizer and drying the droplets in a stream of filtered air (Gebhart et al., 1980). A comparison of the total droplet concentration cT of a jet nebulizer with the concentration cA of the P S L particles demonstrates, that only about one per mille of the droplets contain particles whereas all other droplets remain empty and create residuals. For an upper droplet diameter of 10 pm and chemical impurities of about 0.1 ppm in the liquid (quality of the MiUi-Q Water Purification System) the diameter of such residuals should be below 0.05 pan. Such residuals are not detected by a LPC but a lot of them can be activated and counted in a CNC. Droplets containing more than one PSL-particle produce agglomerates of two or more uniform spheres. The amount of agglomerates in the PSL-aerosol depends on the droplet size and on the particle concentration c, in the liquid suspensions. Their fraction can be primarily reduced by diluting the liquid suspension with high purity water. Near threshold of a LPC agglomerates affect the measurement of counting efficiency, since they represent larger particles than the size stated.
DESIGN OF THE T E S T - S Y S T E M A schematic diagram of the D M A - C N C - T e s t - S y s t e m is shown in Fig. 1. Its main parts are a latex-generator, a differential mobility analyser (DMA, TSI Model 3071) and a condensation nucleus counter (CNC, TSI Model 3020). By passing the PSL-aerosol through the DMA with the voltage adjusted to the peak of the singlets it is expected that residuals and agglomerates are AS 20: 8-.~
1529
1530
J. GEBHART et a l .
SHEATH AIR |
FILTERED AIR '
MONODISPERSE
361mln" __
61:in"
/ niin"
AEROSOL
I~ ~ R A
___ I
i
PSL-
DMA
GENERATOR
TSI / 3071
I
EXCESS AEROSOL
•
POLYDISPERSE AEROSOL
(TEST PARTICLES)
ROOM 1 ~UR
(TEST PARTICLE8)
ROOM i AiR
kv..u,,)
:/ 7_¢
EXCESS AIR
.
-¢
;,,,+W,;}.
Fig. 1: Schematic diagram of the test-system. Version A; left: CNC and LPC suck aerosol from a common buffer volume. Version B; fight: CNC samples aerosol directly behind the DMA-output.
combinedDHA-CNC-test-system
A
1531
removed. In version A the CNC and the LPC suck aerosol at the same concentration level from a common buffer volume. In version B the CNC samples aerosol directly behind the monodisperse output of the DMA without any dilution. In this case the concentration measured with the CNC has to be divided by a constant dilution factor (given by the different flow rates) in order to obtain the reference concentration co. Experiments have been carded out with four LPC's. Their specifications as quoted by the manufacturers are: PMS Model PMS Model PMS Model I-Iiac/Royco
LAS-X HS-LAS p/.,PC-0710 Model 5109
: : : :
sensitivity sensitivity sensitivity sensitivity
0.09 0.065 0.07 0.09
pm; pro; lUn; pro;
0.01 0.01 1 1
cft rain -1 ch rain -1 cft rain -1 cft rain -I
EXPERIMENTAL RESULTS REMOVAL OF RESIDUALS: In case filtered air passed the DMA the background of system was below 80 counts per cft or 2.8 x 10-3 cm -3. In case PSL-aerosol passed the concentration measured with the CNC was in the average about 0.2 cm-3 higher indicated by the H S - L A S and the pLPC (see Fig. 2). This difference was measurable cm -3 but became negligible for higher concentrations.
clp/~m
CNC
HS - LAS
o
•
lil
uLPC -0710
~
A
[~I
\
0,079
0 , 1 1 7 0,15
n ~.o J ' -
0,087
FF-
z
Z loO"
/ / _ / 3w
/ A
/
~ft,
~~////7-
W 0
/
•
z
the t e s t the DMA than that below 1.~
O U LU -J U
~
//
Fr,'
/
0. .
.
.
.
l o -J-
i
J.o ° PARTICLE
.
.
.
I
.
HS - LAS" ~LPC-0710 CONCENTRATION
,,
lo ! /
cm - ~
Fig. 2: Particle concentration measured with the CNC versus particle concentration indicated by the HS - I.AS and the Id.PC - 0710. After extreme dilution of the PSL-suspension with high purity water and without using the DMA the H S - L A S counted only a few 0.087 pro-particles per cm 3 air whereas the CNC indicated concentrations between 430 and 670 cm -3. From these experimental findings it can be concluded that the filter effidency of the DMA for residuals was better than 99.9% but not 100%. REMOVAL OF AGGLOMERATES: In a study with the L A S - X as reference instrument the fraction of agglomerates in the monodisperse output of the DMA was decreased in comparison to the input for a particle diameter of 0.163 pm (Table 1). In an experimental run with the H S - L A S the amount of doublets, however, increased for 0.087 pro-particles after passage through the DMA.
1532
J. GEBHART et al.
without DMA part. concentr,
with DMA
f r a c t , of agglom, part. concentr,
f r a c t , of agglom.
cm-3
~
cm-a
16000
45
2200
13
8500
31
800
6
2200
12
300
1.6
Tab. 1: Number concentration and fraction of agglomerates of 0.163 ima-particles measured by a L A S - X with and without passing a DMA. Let Z I ( + ) be the electrical mobility of a singlet (indicated by the index) carrying one charge. Then the electrical mobilities Zn(+), Zn(+ +) etc. can be calculated in relation to ZI(+). These ratios Z,(+...)/Z~(+) depend on the slip-correction and on the dynamic shape factor of the particles. The calculations demonstrate that with careful adjustment of the voltage to the peak of the singlets and with a control of the flows through the DMA agglomerates should be removed by the DMA to a remarkable extent. COUNTING EFFICIENCY: The HS-I_AS recorded the whole peak of the singlets of the 0.087 pm-particies and classified most of the 0.074-particles in its lowest channel (0.065-0.07 pan). Using both the H S - L A S and the CNC as reference instruments the ~J.~PC-0710 showed 100% counting efficiency for particles of 0.087 pxn diameter. The amount of agglomerates during this experimental run was about 15%. From the coincidence curves of the Model 5109 of Hiac/Royco counting efficiencies of 58% for 0.106 p.m-particles and 42% for 0.087 ixm-particles could be derived. The measurements have been carried out with version A of the test-system. The contribution of the agglomerates was not investigated separately during this test but could not have been the dominating factor for the results. REFERENCES Gebhart, J., Heyder, J., Roth, C. and Stahlhofen, W. (1980): Herstellung und Eigenschaften von Latex-Aerosolen. Staub-Reinhalt. Luft 40, 1. Gebhart, J., Blankenberg, P., Bormann, S. and Roth, C. (1983): Vergleichsmessungen an optischen Partikelzalhlern. Staub- Reinhalt. Luft 43, 439. Gebhart, J. and Roth, C. (1986): Background noise and counting efficiency of single optical particle counters. In: Aerosols, Formation and Reactivity (Eds.: Schikarski, W., Fissan, H., Friedlander, S.) Pergamon Press, Oxford, New York, 607. Gebhart, J. and Anselm, A. (1987): Effect of particle shape on the response of single particle optical counters. Int. Syrup. on Optical Particle Sizing. May, 12-15; ROUEN, France. Gebhart, J. (1989): Funktionsweise und Eigenschaften optischer Partikelzahler. Technisches Messen tln 56, 192. Lehtlm~lki, M. and Keskinen, J. (1989): Experiments with a high sensitivity clean room particle counter. Journ. R 3 - Nordic 3, 17. Roth, C. and Gebhart, J. (1987): Rapid particle size analysis with an ultramicroscope. Microscopia Acta 81, 119. Wen, H.Y. and Kaspar, (3. (1986): Counting efficiencies of six commercial particle counters. J. Aerosol $ci. 17, 947.