Counting efficiency and sizing characteristics of optical particle counters

The Eleventh A n n u a l Conference of the Association for Aerosol Research COUNTING

EFFICIENCY

AND SIZING CHARACTERISTICS PARTICLE COUNTERS

J. GEBHART*, P. BLANKENBERG'I" a n d C.

OF

279 OPTICAL

ROTH*

*Gesellschaft for Strahlen- und Umweltforschung, 6000 Ffm. and "tTrox GmbH, Filtertechnik. 4133 Neukirchen-Vluyn, FR.G Thirteen optical particle counters, including ten types of instruments from five different manufacturers were investigated experimentally, with respect to their counting efficiency and classification properties. Tests were carried out with monodisperse aerosols of polystyrene spheres (refractive index: 1.59) and of oil droplets consisting of di-2ethylhexyl-sebacate (refractive index: 1.45). The aerosol generators used for the investigations have been described elsewhere (Stahlhofen et al. 1975a; Gebhart et al. 1980). The polystyrene latex aerosol additionally passed an aerosol neutralizer (Model 3054 TSI) at the outlet of the generator. In a few experiments with 2.02'am particles the neutralizer was removed in order to study the effect of electrostatic charges on particle losses in the sampling tubes. No discharge was required for the oil droplets produced by condensation of vapour. For the characterization of the test aerosols, independent m e t h o d s like electron microscopy and sedimentation cell analysis were applied (Stahlhofen et al., 1975b). During an experimental run several instruments simultaneously sampled aerosol from a 3001. tank which was continuously supplied with a constant stream of aerosol and diluting air. To establish uniformity of particle n u m b e r concentration the aerosol was well stirred with a fan inside the tank. Two aerosol size spectrometers of the laboratory (Gebhart et al., 1976; Roth et al., 1978) which were checked independently with an electrostatic aerosol sampler (Model 3100 TSI), served as reference instruments for the actual n u m b e r concentration in the tank. The instruments investigated are listed in Table 1. They differ in the angular range of scattered light, the kind of illumination (laser or white light), the sensitivity and the sample flow rate. For the evaluation o f the sizing characteristics, the cumulative size distributions of the test aerosols were compared to those measured with the instruments. The count median diameter derived from these cumulative size spectra was taken as a measure o f the sizing accuracy. In five instruments using white light illumination, all measured diameters o f the polystyrene particles were shifted to smaller sizes by more than 20°~o. To correct this systematic error the linear amplification has to be changed. All other instruments indicated the diameters o f the polystyrene spheres within an accuracy of 10 ° o with the exception o f the 0.95 # m particles. Due to an a m b i g u o u s part in the calibration curve all instruments with near forward scattering ( 10-30 °) classified those particles by about a factor 1.5 too small. Oil droplets with refractive index 1.45 were measured by about a factor 1.25 too small with instruments including 90 ° scattering. Particle counters with near forward scattering indicated 3.7 # m droplets about 40 °~o too large and 1.75 'am droplets about 60 oo too small. These deviations are in agreement with theory of light scattering on small spherical particles and are unavoidable errors of the measuring principle as long as the calibration standard differs in refractive index from the aerosol to be investigated. N u m b e r concentration c of the aerosol, sample flow rate V and counting rate N of a particle counter are connected by the relation: N =cV.

(1t

Equation (1) assumes that each particle contained in the sample flow produces a single count. In practice, however, counting losses due to coincidences occur which can be estimated by the equation (van der Meulen et al., 1980): C - - = e -'~v'" Ck

(21

where c is the measured and c~ the corrected n u m b e r concentration and t, the recovery time necessary between two successive counting events. For an estimation of the coincidence losses an experimental pulse time r was determined for each instrument by connecting the output of the counter with an oscilloscope. This measured time ~ may not be identical with the recovery time t, but can be considered as a useful approximation. For an evaluation of the counting efficiency q the measured and coincidence corrected concentration ck was related to the concentration Co obtained by the reference methods according to: Ck

~/= - - .

(3)

Co

By this definition, counting efficiency is not affected by coincidence losses and thus reflects a specific property o f an instrument. Replacing the time tr by the pulse time r and subgtituting equation (3) into equation (2) finally leads to: c

_ e-'l'c' v,.

(4)

~1Co Equation (4) was applied to calculate the counting efficiency t/from the measured quantities c, Co and r at a given flow rate F. The concentration c o was chosen in such a way that the coincidence correction did not exceed 2000 . Results are presented in Table 1. In the submieron range the counting efficiency of most instruments decreases with decreasing particle diameter so that the lower detection limit is a gradual transition rather than a sharp step. In the hypermicron size range, satisfactory results are obtained. However, for particle diameters above 2'am, losses in the sampling system of the counter may again reduce the counting efficiency especially if instruments with low sampling rates are used in connection with plastic tubes and charged particles (see Royco 236). Servicing of an instrument by the manufacturer Isee Kratel Partoskop A, A/R) improved the counting efficiency of 0.47 ,am particles considerably but did not remove the low counting efficiency of 0.327 ,am particles. For all ten instruments using white light the counting efficiency of 0.327,am particles remained below 0.2 at flow rates of 2800cmS/min. For the Laserinstruments, on the other hand, a counting efficiency of about 1 was achieved in the whole submicron range. In the particle counter HC 15. the sensing volume is formed by optical means only without using an aerosol

30

120 120 170'

I0

35 35 30

Royco 4102

Royco 226 Royco 236 R0yco 1000

He Ne Laser

white

white

white

0.12 0.12 0.3

0.3

0.3

0.5

0.3 i0.5}

280 300 300 28,000

40 9 9 8

280 2800-

2800

25 30

28,0OO

25

25

280

25

* Results obtained after servicing. t Valid for charged particles sampled through a 3 m m plastic tube o f about 4 0 c m length.

311

10

Royco 227

30

10

Royco 247

white

2800

30

25

10

0.3 t0.5J

2800

25

28,000

20 2800 280

280

30

25 30

2800

28,000

20

25

5.6 28,000 500

~ 10 4,5 ~ 15

Kratel Part A (5 ehannelsl

white

0.3 (0.5)

V (cm3/min)

280

3(1

white

0.3 (0.5)

0,3 0.3 0.5

r (#s)

dmin (urn)

35

10

3(1

white

white white white

Illumination

(32 channels)

Part A

Kratel

Kratel Part A/R 15 ehannelst

10

I0

Kratel Part A/R

(32 channels)

90 range 15 105 90 range

H C 15 Climet CI-208 C DAP-Test 2000

30

Scattering angle

Type of instrument

--

1.03 1.04 -

,

•

-

•-

0.99 0;98 0.93

1.02 0.9 I 1:06

1.30

1.01 0.62

0.92

O. 16

0.66

2.29

0.76

1.6

1.06

0.178

0.12

0.68

O. 18

O. 10

0.15

0.72

0.21

0.03 0.04

0.25 1.40

-

1.09

1.07

0.76

-0.21

1.06 0.64 0.085

0.033 O.15

--

0.052

0.03 0.065

-0.02

-

-

0.46 0.17

*~

-

-

0.163

1.03 1.05 0.95

1.47

0.98

1.05

0.71t

1.39 0.89

1.10

0.99

1.58

0.78

0.90

120

0.65

1,20

1.78

0.89

1.01

0.76 1.06

2.02

0.62

2.63

0.71

1.3

3.17

0.71

0.76

0.72 Z87

0.44

1,21

1.63

0.82

0.37 0.64

!.51 0.82 0.89

C o u n t i n g efficiency for particle (diameter d/#m) 0.327 0.47 0.95

Table I General specifications and measured counting efficiencies o f the optical particle counters involved in the p r o g r a m

O

¢_.

¢:z.

E

B¸

u

>

The Eleventh Annual Conference of the Association for Aerosol Research

281

nozzle, consequently, the sample flow rate cannot be defined exactly. Whereas small particles are only counted when they pass the central region of the stop, bigger particles can also give rise to countable pulses at the periphery of the imaging system even if these peripheral light flashes are partially shadowed. Thus the effective sample flow rate depends on particle size. The bigger the particles the more peripheral light flashes contribute to a measured cumulative number distribution (Helsper et al., 1980). A method to overcome this problem has been discussed in a recent paper (Umhauer, 1982). According to equation (1) errors in the sample flow rate directly influence the measured number concentration. Therefore variations of the counting efficiency in the micron range of the instruments may also be caused by incorrect sample flow rates. This was the case in about half of the instruments investigated. Only with three instruments could the sample flow rate be observed and adjusted on the panel plate. Instruments which could be operated at different flow rates changed their counting efficiency by more than a factor of two when they were switched over. This was mainly due to inaccuracies of the sample flow rates. But even after an adjustment of the sample flow rates with an external flow meter the counting efficiency at 280 cm3/min remained in the average about 50 ~, higher than at 2800cm3/min flow rate (see Kratel Partoskop A, A/R; Royco 4102). Observations of the pulse shape of these instruments on an oscilloscope showed that at 280 cm3/min flow rate a lot of stray particles having pulse times up to 150 #sec passed the sensing volume. Since pulse times o f this magnitude exceed the recovery time of the electronics more than one count can originate from a single stray particle. The existence of stray particles themselves may be explained by unstable flow conditions at 280cm3/min flow rate. REFERENCES Gebhart, J., Heyder, J., Roth, C. and Stahlhofen, W. (1976) Fine Particles (Edited by Liu, B. Y. H.) p. 793. Academic Press, New York. Gebhart, J., Heyder, J., Roth, C, and Stahlhofen, W. (1980) Staub-Reinholt. Luft 40, I. Helsper, C. and FiBan, H. J. (1980) 8th Annual Meetin 9 Gesellschaftfiir Aerosolforsehuno, Schmallenberg, F.R.G., 22 24 October 1980. van der Meulen, A., Plomp, A., Oeseburg, F., Buringh, E., yon Aalst, R. M. and Hoevers, W. [1980) Atmos. Envir. 14, 495. Roth, C. and Gebhart, J. (1978) Microsc. Acta 81, 119. Stahlhofen, W., GebharL J., Heyder, J. and Roth, C. (1975a) J. Aerosol SCL 6, 161. Stahlhofen, W., Armbruster, L., Gebhart, J. and Grein, E. (1975b) Atmos. Envir. 9, 851. Umhauer, H. (1982J lOth Ann. Meeting Gesellschqftfiir Aerosolforsehuno, Bologna, Italy, 14-17 September 1982.

A TEST PARTICLE

OF AN INVERSION SIZE DISTRIBUTION

METHOD FROM

FOR DETERMINATION OF LIGHT SCATTERING PATTERN

J. M. MAKYNEN Technical Research Centre of Finland, Nuclear Engineering Laboratory, P.O. Box 169. Helsinki. Finland INTRODUCTION The scattering of light from a particle cloud depends among other factors on the particle size. It is possible to determine the aerosol size distribution from the scattering pattern using so-ealled inversion methods (Swithenbank et al., 1977; Felton, 1978: Orr, 1978: Bayvel and Jones, 1981; Thomalla and Quenzel, t982). In the inversion problem one must determine fix) when the function g(y) in the integral equation (1) is known. g(y)=

K ( y , xI f ( x l d x . (1) a Here g(y) is a result of a measurement from the instrument, f(x) is a particle size distribution and K(y, x) is the response function of the instrument or the so-called kernel function of the measuring integral. In this work g()9 and f(x) are the scattered light intensity in the angle y and an unknown particle size distribution, respectively. K( y, x J is a scattering efficiency kernel. Integral equation (1) was approximated by the matrix equation ~! = A .1. Matrix A was calculated numerically using 100 logarithmically distributed particle size points in the number size distribution. Corresponding density value points were connected with straight lines instead o f the normally used stepwise bar graph distribution. The matrix elements a u were calculated as presented by Twomey (1977). Measured values were simulated using the Mie-scattering theory..1was calculated using Kapadia's modification (Kapadia, 1980~ from Twomey's nonlinear iteration algorithm. The iteration result number p was calculated from the previous iteration result p - 1 using algorithm

I ~, = t ~ -~

,,,,:Y'

Z

i

%here

,,.:

.J= 1 . . . .

s

(2~

i=l

. p

,~

l

=,~

d Y" a,j./j p

1

j=l

This algorithm also automatically eliminated negative inversion results. Similar algorithms have been suggested for inxerting diffusion bauery IKapadia, 19:~0~ and impactor data (Neumann and Dirnagl, 1983). RESULTS The accurac) of the method was tested using different refractive indices for the kernels and the analysed particles