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Theoretical and experimental analysis of turbidity fluctuations in the simultaneous measurement of particle size and aerosol concentration
J. Aerosol
Sci. Vol. 29, Suppl. 1, pp. S425-S426, 1998
8 1998 Published by Else&r Science Ltd. All rights reserved
Pergamon
Printed in Great Britain cml-8502/98 119.00 + 0.00
THEORETICAL AND EXPERIMENTAL ANALYSIS OF TURBIDITY FLUCTUATIONS IN THE SIMULTANEOUS MEASUREMENT OF PARTICLE SIZE AND AEROSOL CONCENTRATION E. G&in *, M. Attoui **. G. Grhan
***
E-Mail : [email protected], attoui@univ-parisl2,fr, [email protected] * LETIEF, ** LPATC, Umiversitt! Paris XII - Val de Marne, Avenue du GBnt!ral de Gaulle, 94010 Ckteil, France. *** LESP, URA CNRS 230- CORIA-INSA de Rouen - B.P.8- Place Emile Blondel, 76131, Rouen, France
INTRODUCTION
Aerosol concentration may be estimated by analysing signal fluctuations produced by statistical variations in the number of particles contained within a measurement volume (Kyle [1979]). Gregory [1985] applied this principle to turbidity measurements during his studies on aggregate formations in suspensions. This technique was shortly adapted by Altman [ 19941 and formed the basis of an apparatus capable of measuring the particle size for high concentration monodisperse aerosols (>104 particles / cm”). We will characterise the theoretical application limits of this method and study the influence of experimental constraints on the accuracy of particle sizing data obtained. THEORETICAL METHOD
The technique is based on transmittance mean and mean square (ms) value measurement (Figure 1). The BeerLambert relationship gives the transmittance of a slice of cc lengh, L, containing n particles : T = exp(-K,,L)
For non conherent and single scattering (T > 0,9), extinction coeffient (KXt) of a suspension is equal to the of each effective extinction cross section (C,,,). polydisperse flows containing N particleskms and with a distribution f(D), the extinction coefficient is written:: K,,
I
the a’ sum Photo&ie For Figure I : Optrcal set up size
= N. jf(d).C,(D).dD 0
It follows that : Lo T = exp f j f(D). C,,, (D). dD = exp(-n. A) with S as section of measurement
volume 1 0 The number of particles contained in the measurement volume, will follow Poisson distribution when the following conditions are obeyed (Kyle [ 19791): * no fluctuations in particle concentration occur in the aerosol flow * the apparatus response time (~1) is much smaller than the residence time of particles within the measurement volume (~2) * the time increment between two measurement (93) is greater than ‘~2. The transmittance is determined from the voltage outputs, U and IJo, produced by the detector regardless of whether or not the measurement volume contains a suspension. If there is no i
S426
Abstracts
of the 5th International
Aerosol Conference
1998
fluctuations in the particle size distribution of the aerosol, A is a constant and the voltage, U, is a random variable dependent on two independent variables, n and U0. If the variation coefficient of these variables are less than 10 %, the average voltage ouput, U, and its mean square value, ok, are a function of the means and means square value of U0 and n (Charlot [ 19781). The mean number of particles contained u < = -A -’ In(?) U0 In the case of log-normal size distributed dispersion coefficient (i.e. for monodisperse
in the measurement *+
:
-~~~.~~]~)(~-.~~(~]~~’
aerosols with median diameter D and negligible aerosols), A may be written as follows :
If the nature and shape of the particles is known, C&D) identify the median diameter. EXPERIMENTAL
volume is therefore
may be calculated
and it is possible to
SET UP
The experimental apparatus is a classical set-up used for turbidity measurements (figure 1). The light source is supplied by a laser diode with an emission wavelength of 670 nm. The detector is a photodiode having a response time of around 10 ns. Continuous and alternating signal outputs from the detector are measured individually by a 12 bit data acquisition card. The aerosol studied were generated by a MAGE (monodisperse Aerosol Generator) and injected as continuous F1gr1rr .? EKpermwlenlul ser.t!p flow into a measurement cell. The particle size and concentration were monitored using a condensation nucleus counter (CNC) and an aerosol particle sizer (APS). This data was then compared with experimental results for various size particle and aerosol concentration. RCfirences Altmann J., Rudolph A., Wessely B., (1994) Particle sizing of highly concentrated monodisperse aerosols, J. Aerosol Sci., Vol. 25, Suppl. 1, pp. S523-S524. Charlot G., (1978) Statistique appliquee a l’exploitation des mesures, Commissariat a 1’Energie Atomique, Mcr.sso~, Paris. Gregory J., (1985) Turbidity fluctuations in flowing suspensions, J. Colloid and interface Ski. Vol. 105, n”2, pp. 357-371. Kyle T. G., (1979) Determining particle concentration by statistics, J. Aerosol Sci., Vol. 10, pp. 87-93.
Sci. Vol. 29, Suppl. 1, pp. S425-S426, 1998
8 1998 Published by Else&r Science Ltd. All rights reserved
Pergamon
Printed in Great Britain cml-8502/98 119.00 + 0.00
THEORETICAL AND EXPERIMENTAL ANALYSIS OF TURBIDITY FLUCTUATIONS IN THE SIMULTANEOUS MEASUREMENT OF PARTICLE SIZE AND AEROSOL CONCENTRATION E. G&in *, M. Attoui **. G. Grhan
***
E-Mail : [email protected], attoui@univ-parisl2,fr, [email protected] * LETIEF, ** LPATC, Umiversitt! Paris XII - Val de Marne, Avenue du GBnt!ral de Gaulle, 94010 Ckteil, France. *** LESP, URA CNRS 230- CORIA-INSA de Rouen - B.P.8- Place Emile Blondel, 76131, Rouen, France
INTRODUCTION
Aerosol concentration may be estimated by analysing signal fluctuations produced by statistical variations in the number of particles contained within a measurement volume (Kyle [1979]). Gregory [1985] applied this principle to turbidity measurements during his studies on aggregate formations in suspensions. This technique was shortly adapted by Altman [ 19941 and formed the basis of an apparatus capable of measuring the particle size for high concentration monodisperse aerosols (>104 particles / cm”). We will characterise the theoretical application limits of this method and study the influence of experimental constraints on the accuracy of particle sizing data obtained. THEORETICAL METHOD
The technique is based on transmittance mean and mean square (ms) value measurement (Figure 1). The BeerLambert relationship gives the transmittance of a slice of cc lengh, L, containing n particles : T = exp(-K,,L)
For non conherent and single scattering (T > 0,9), extinction coeffient (KXt) of a suspension is equal to the of each effective extinction cross section (C,,,). polydisperse flows containing N particleskms and with a distribution f(D), the extinction coefficient is written:: K,,
I
the a’ sum Photo&ie For Figure I : Optrcal set up size
= N. jf(d).C,(D).dD 0
It follows that : Lo T = exp f j f(D). C,,, (D). dD = exp(-n. A) with S as section of measurement
volume 1 0 The number of particles contained in the measurement volume, will follow Poisson distribution when the following conditions are obeyed (Kyle [ 19791): * no fluctuations in particle concentration occur in the aerosol flow * the apparatus response time (~1) is much smaller than the residence time of particles within the measurement volume (~2) * the time increment between two measurement (93) is greater than ‘~2. The transmittance is determined from the voltage outputs, U and IJo, produced by the detector regardless of whether or not the measurement volume contains a suspension. If there is no i
S426
Abstracts
of the 5th International
Aerosol Conference
1998
fluctuations in the particle size distribution of the aerosol, A is a constant and the voltage, U, is a random variable dependent on two independent variables, n and U0. If the variation coefficient of these variables are less than 10 %, the average voltage ouput, U, and its mean square value, ok, are a function of the means and means square value of U0 and n (Charlot [ 19781). The mean number of particles contained u < = -A -’ In(?) U0 In the case of log-normal size distributed dispersion coefficient (i.e. for monodisperse
in the measurement *+
:
-~~~.~~]~)(~-.~~(~]~~’
aerosols with median diameter D and negligible aerosols), A may be written as follows :
If the nature and shape of the particles is known, C&D) identify the median diameter. EXPERIMENTAL
volume is therefore
may be calculated
and it is possible to
SET UP
The experimental apparatus is a classical set-up used for turbidity measurements (figure 1). The light source is supplied by a laser diode with an emission wavelength of 670 nm. The detector is a photodiode having a response time of around 10 ns. Continuous and alternating signal outputs from the detector are measured individually by a 12 bit data acquisition card. The aerosol studied were generated by a MAGE (monodisperse Aerosol Generator) and injected as continuous F1gr1rr .? EKpermwlenlul ser.t!p flow into a measurement cell. The particle size and concentration were monitored using a condensation nucleus counter (CNC) and an aerosol particle sizer (APS). This data was then compared with experimental results for various size particle and aerosol concentration. RCfirences Altmann J., Rudolph A., Wessely B., (1994) Particle sizing of highly concentrated monodisperse aerosols, J. Aerosol Sci., Vol. 25, Suppl. 1, pp. S523-S524. Charlot G., (1978) Statistique appliquee a l’exploitation des mesures, Commissariat a 1’Energie Atomique, Mcr.sso~, Paris. Gregory J., (1985) Turbidity fluctuations in flowing suspensions, J. Colloid and interface Ski. Vol. 105, n”2, pp. 357-371. Kyle T. G., (1979) Determining particle concentration by statistics, J. Aerosol Sci., Vol. 10, pp. 87-93.