SIST ISO 13320-1:2002
(Main)Particle size analysis -- Laser diffraction methods -- Part 1: General principles
Particle size analysis -- Laser diffraction methods -- Part 1: General principles
Analyse granulométrique -- Méthodes par diffraction laser -- Partie 1: Principes généraux
La présente partie de l'ISO 13320 fournit des directives sur le mesurage des distributions granulométriques effectué dans tout système bi-phase, par exemple poudres, pulvérisateurs, aérosols, matières en suspension, émulsions, bulles de gaz dans des liquides, par l'analyse de leurs motifs de diffusion de la lumière angulaire. Elle ne traite pas des prescriptions spécifiques relatives au mesurage granulométrique de produits particuliers. La présente partie de l'ISO 13320 s'applique aux particules dont la taille est comprise dans une plage approximative de 0,1 µm à 3 mm.Pour les particules non sphériques, le modèle optique de cette technique suppose que les particules sont sphériques; on obtient ainsi une distribution granulométrique équivalente à celle des particules sphériques. La distribution granulométrique obtenue peut être différente de celles obtenues avec les méthodes fondées sur d'autres principes physiques (par exemple sédimentation, tamisage).
Sejalna analiza - Metoda z lasersko difrakcijo - 1. del: Splošna načela
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INTERNATIONAL ISO
STANDARD 13320-1
First edition
1999-11-01
Particle size analysis — Laser diffraction
methods —
Part 1:
General principles
Analyse granulométrique — Méthodes par diffraction laser —
Partie 1: Principes généraux
A
Reference number
ISO 13320-1:1999(E)
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ISO 13320-1:1999(E)
Contents
1 Scope .1
2 Normative reference .1
3 Terms, definitions and symbols.1
3.1 Terms and definitions .1
3.2 Symbols.3
4 Principle.4
5 Laser diffraction instrument .4
6 Operational procedures .6
6.1 Requirements.6
6.2 Sample inspection, preparation, dispersion and concentration.7
6.3 Measurement.9
6.4 Repeatability.11
6.5 Accuracy.11
6.6 Error sources; diagnosis .12
6.7 Resolution; sensitivity .14
7 Reporting of results.14
Annex A (informative) Theoretical background of laser diffraction.16
Annex B (informative) Recommendations for instrument specifications .25
Annex C (informative) Dispersion liquids for the laser diffraction method.28
Annex D (informative) Refractive index for various liquids and solids .29
Bibliography.34
© ISO 1999
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
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© ISO
ISO 13320-1:1999(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 13320-1 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other
sizing methods, Subcommittee SC 4, Sizing by methods other than sieving.
ISO 13320 consists of the following parts, under the general title Particle size analysis — Laser diffraction methods:
Part 1: General principles
Part 2: Validation of inversion procedures
Annexes A to E of this part of ISO 13320 are for information only.
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© ISO
ISO 13320-1:1999(E)
Introduction
Laser diffraction methods are nowadays widely used for particle sizing in many different applications. The success
of the technique is based on the fact that it can be applied to various kinds of particulate systems, is fast and can be
automated and that a variety of commercial instruments is available. Nevertheless, the proper use of the instrument
and the interpretation of the results require the necessary caution.
Therefore, there is a need for establishing an International Standard for particle size analysis by laser diffraction
methods. Its purpose is to provide a methodology for adequate quality control in particle size analysis.
Historically, the laser diffraction technique started by taking only scattering at small angles into consideration and,
thus, has been known by the following names:
Fraunhofer diffraction;
(near-) forward light scattering;
low-angle laser light scattering (LALLS).
However, the technique has been broadened to include light scattering in a wider angular range and application of
the Mie theory in addition to approximating theories such as Fraunhofer and anomalous diffraction.
The laser diffraction technique is based on the phenomenon that particles scatter light in all directions with an
intensity pattern that is dependent on particle size. All present instruments assume a spherical shape for the
particles. Figure 1 illustrates the characteristics of single particle scattering patterns: alternation of high and low
intensities, with patterns that extend for smaller particles to wider angles than for larger particles [2-7, 10, 15 in the
bibliography].
Within certain limits the scattering pattern of an ensemble of particles is identical to the sum of the individual
scattering patterns of all particles present. By using an optical model to compute scattering patterns for unit volumes
of particles in selected size classes and a mathematical deconvolution procedure, a volumetric particle size
distribution is calculated, the scattering pattern of which fits best with the measured pattern (see also annex A).
a) b)
Figure 1 — Scattering pattern for two spherical particles: the particle generating pattern a) is twice as large
as the one generating pattern b)
A typical laser diffraction instrument consists of a light beam (usually a laser), a particulate dispersing device, a
detector for measuring the scattering pattern and a computer for both control of the instrument and calculation of the
particle size distribution. Note that the laser diffraction technique cannot distinguish between scattering by single
particles and scattering by clusters of primary particles forming an agglomerate or an aggregate. Usually, the
resulting particle size for agglomerates is related to the cluster size, but sometimes the size of the primary particles
is reflected in the particle size distribution as well. As most particulate samples contain agglomerates or aggregates
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© ISO
ISO 13320-1:1999(E)
and one is generally interested in the size distribution of the primary particles, the clusters are usually dispersed into
primary particles before measurement.
Historically, instruments only used scattering angles smaller than 14°, which limited the application to a lower size of
about 1 mm. The reason for this limitation is that smaller particles show most of their distinctive scattering at larger
angles (see also annex A). Many recent instruments allow measurement at larger scattering angles, some up to
about 150°, for example through application of a converging beam, more or larger lenses, a second laser beam or
more detectors. Thus, smaller particles down to about 0,1 mm can be sized. Some instruments incorporate
additional information from scattering intensities and intensity differences at various wavelengths and polarization
planes in order to improve the characterization of particle sizes in the submicrometre range.
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INTERNATIONAL STANDARD © ISO ISO 13320-1:1999(E)
Particle size analysis — Laser diffraction methods —
Part 1:
General principles
1 Scope
This part of ISO 13320 provides guidance on the measurement of size distributions of particles in any two-phase
system, for example powders, sprays, aerosols, suspensions, emulsions and gas bubbles in liquids, through
analysis of their angular light scattering patterns. It does not address the specific requirements of particle size
measurement of specific products. This part of ISO 13320 is applicable to particle sizes ranging from approximately
0,1 mm to 3 mm.
For non-spherical particles, an equivalent-sphere size distribution is obtained because the technique uses the
assumption of spherical particles in its optical model. The resulting particle size distribution may be different from
those obtained by methods based on other physical principles (e.g. sedimentation, sieving).
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this part of ISO 13320. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 13320 are encouraged to investigate the
possibility of applying the most recent edition of the normative document indicated below. For undated references,
the lates edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO 9276-1:1990, Representation of results of particle size analysis — Part 1: Graphical representation.
3 Terms, definitions and symbols
For the purposes of this part of ISO 13320, the following terms, definitions and symbols apply.
3.1 Terms and definitions
3.1.1
absorption
reduction of intensity of a light beam traversing a medium through energy conversion in the medium
3.1.2
coefficient of variation
relative measure (%) for precision: standard deviation divided by mean value of population and multiplied by 100
(for normal distributions of data the median is equal to the mean)
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ISO 13320-1:1999(E)
3.1.3
complex refractive index
N
p
refractive index of a particle, consisting of a real and an imaginary (absorption) part
N = n - ik
p p p
3.1.4
relative refractive index
m
complex refractive index of a particle, relative to that of the medium
m = N /n
p m
3.1.5
deconvolution
mathematical procedure whereby the size distribution of a particle ensemble is inferred from measurements of their
scattering pattern
3.1.6
diffraction
spreading of light around the contour of a particle beyond the limits of its geometrical shadow with a small deviation
from rectilinear propagation
3.1.7
extinction
attenuation of a light beam traversing a medium through absorption and scattering
3.1.8
model matrix
matrix containing light scattering vectors for unit volumes of different size classes, scaled to the detector’s
geometry, as derived from model computation
3.1.9
multiple scattering
subsequent scattering of light at more than one particle, causing a scattering pattern that is no longer the sum of the
patterns from all individual particles (in contrast to single scattering)
3.1.10
obscuration
optical concentration
percentage or fraction of incident light that is attenuated due to extinction (scattering and/or absorption) by the
particles
3.1.11
optical model
theoretical model used for computing the model matrix for optically homogeneous spheres with, if necessary, a
specified complex refractive index, e.g. Fraunhofer diffraction, anomalous diffraction, Mie scattering
3.1.12
reflection
return of radiation by a surface, without change in wavelength
3.1.13
refraction
change of the direction of propagation of light determined by change in the velocity of propagation in passing from
one medium to another; in accordance with Snell’s law
n sin Q = n sin Q
m m p p
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ISO 13320-1:1999(E)
3.1.14
scattering
general term describing the change in propagation of light at the interface of two media
3.1.15
scattering pattern
angular or spatial pattern of light intensities [I(q) and I(r) respectively] originating from scattering, or the related
energy values taking into account the sensitivity and the geometry of the detector elements
3.1.16
single scattering
scattering whereby the contribution of a single member of a particle population to the scattering pattern of the entire
population is independent of the other members of the population
3.1.17
width of normal size distribution
standard deviation (absolute value) or coefficient of variation (relative percentage) of the size distribution
NOTE For normal distributions about 95 % of the population falls within ± 2 standard deviations from the mean value and
about 99,7 % within ± 3 standard deviations from the mean value.
3.2 Symbols
c volumetric particulate concentration, %
f focal length of lens, mm
I(q) angular intensity distribution of light scattered by particles (scattering pattern)
I(r) spatial intensity distribution of light scattered by particles on the detector elements (measured scattering
pattern by detector)
i indication for imaginary part of refractive index
i photocurrent of detector element n, mA
n
k wave number: 2p/l
k imaginary (absorption) part of particle’s refractive index
p
l illuminated path length containing particles, mm
L vector of photocurrents (i , i ,., i )
1 2 n
m relative, complex refractive index of particle to medium
n real part of refractive index of medium
m
n real part of refractive index of particle
p
N complex refractive index of a particle
p
r radial distance from focal point in focal plane, mm
n velocity of particles in dry disperser
x particle diameter, mm
x median particle diameter, mm; here used on a volumetric basis, i.e. 50 % by volume of the particles is
50
smaller than this diameter and 50 % is larger
x particle diameter corresponding to 10 % of the cumulative undersize distribution (here by volume), mm
10
3
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ISO 13320-1:1999(E)
particle diameter corresponding to 90 % of the cumulative undersize distribution (here by volume), mm
x
90
a dimensionless size parameter: px/l
q scattering angle with respect to forward direction
q angle with respect to perpendicular at boundary for a light beam in medium (as used in Snell's law; see
m
refraction)
q angle with respect to perpendicular at boundary for a light beam in particle (as used in Snell's law; see
p
refraction)
l wavelength of illuminating light source in medium (i.e. liquid or gas/air), nm
w rotational velocity of particles in dry disperser
4 Principle
A representative sample, dispersed at an adequate concentration in a suitable liquid or gas, is passed through the
beam of a monochromatic light source, usually a laser. The light scattered by the particles at various angles is
measured by a multi-element detector and numerical values relating to the scattering pattern are then recorded for
subsequent analysis. These numerical scattering values are then transformed, using an appropriate optical model
and mathematical procedure, to yield the proportion of total volume to a discrete number of size classes forming a
volumetric particle size distribution.
5 Laser diffraction instrument
A typical set-up for a laser diffraction instrument is given in figure 2.
Key
1 Obscuration detector 7 Light source laser
2 Scattered beam 8 Beam processing unit
3 Direct beam 9 Working distance of lens 4
4 Fourier lens 10 Multi-element detector
5 Scattered light not collected by lens 4 11 Focal distance of lens 4
6 Particle ensemble
Figure 2 — Example of the set-up of a laser diffraction instrument
In the conventional set-up, a light source (typically a laser) is used to generate a monochromatic, coherent, parallel
beam. This is followed by a beam processing unit, usually a beam expander with integrated filter, producing an
extended and nearly ideal beam to illuminate the dispersed particles.
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ISO 13320-1:1999(E)
A representative sample, dispersed at an adequate concentration is passed through the light beam in a measuring
zone by a transporting medium (gas or liquid); this measuring zone should be within the working distance of the lens
used. Sometimes, the particle stream in a process is illuminated directly by the laser beam for measurement, as in
the case of sprays, aerosols and air bubbles in liquids. In other cases (such as emulsions, pastes and powders),
representative samples can be dispersed in suitable liquids (see annex C). Often dispersants (wetting agents;
stabilizers) and/or mechanical forces (agitation; ultrasonication) are applied for deagglomeration of particles and
stabilization of the dispersion. For these liquid dispersions a recirculating system is most commonly used, consisting
of an optical measuring cell, a dispersion bath usually equipped with stirrer and ultrasonic elements, a pump and
tubing.
Dry powders can also be converted into aerosols through application of dry powder dispersers, which apply
mechanical forces for deagglomeration. Here a dosing device feeds the disperser with a constant mass flow of
sample. The disperser uses the energy of a compressed gas or the differential pressure to a vacuum to disperse
the particles. It outputs an aerosol that is blown through the measuring zone, usually into the inlet of a vacuum pipe
that collects the particles.
There are two positions in which the particles can enter the laser beam. In the conventional case the particles enter
the parallel beam before and within the working distance of the collecting lens [see Figure 3 a)]. In the so-called
reversed Fourier optics case the particles are entered behind the collecting lens and, thus, in a converging beam
[see Figure 3 b)].
The advantage of the conventional set-up is that a reasonable path length for the sample is allowed within the
working distance of the lens. The second set-up allows only small path lengths but enables measurement of
scattered light at larger angles, which is useful when submicrometre particles are present.
The interaction of the incident light beam and the ensemble of dispersed particles results in a scattering pattern with
different light intensities at various angles (see annex A for theoretical background of laser diffraction). The total
angular intensity distribution I(q), consisting of both direct and scattered light, is then focused by a positive lens or
an ensemble of lenses onto a multi-element detector. The lens(es) provide(s) for a scattering pattern which, within
limits, is not dependent upon the location of the particles in the light beam. So, the continuous angular intensity
distribution I(q) is converted into a discrete spatial intensity distribution I(r) on a set of detector elements.
It is assumed that the recorded scattering pattern of the particle ensemble is identical to the sum of the patterns
from all individual single scattering particles presented in random relative positions. Note that only a limited angular
range of scattered light is collected by the lens(es) and, thus, by the detector.
The detector generally consists of a number of photodiodes; some instruments apply one photodiode in combination
with moving slits. The photodiodes convert the spatial intensity distribution I(r) into a set of photocurrents i .
n
Subsequent electronics then convert and digitize the photocurrents into a set of intensity or energy vectors L ,
n
representing the scattering pattern. A central element measures the intensity of the non-scattered light and, thus,
with a calculation, provides a measure of optical concentration or obscuration. Some instruments provide special
geometries of the central element in order to automatically re-centre or re-focus the detector by moving the detector
or the lens. It is desirable that the detector elements are positioned so as to prevent the light reflected from the
surface from re-traversing the optical system.
A computer controls the measurement and is used for storage and manipulation of the detected signals, for storage
and/or calculation of a proper form of the optical model (usually as a model matrix containing light scattering vectors
per unit of volume per size class, scaled to the detector's geometry and sensitivity) and calculation of the particle
size distribution (see annex A for theoretical background of laser diffraction). Also it may provide automated
instrument operation.
Several significant differences exist, both in hardware and software, not only between instruments from different
manufacturers but also between different types from one company. The instrument specifications should give
adequate information for proper judgement of these differences. In annex B recommendations are presented for the
specifications of laser diffraction instruments.
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© ISO
ISO 13320-1:1999(E)
Key
1 Detector 4 Working distance
2 Fourier lens 5 Focal distance
3 Particle ensemble
a) Conventional set-up: particles are in parallel beam before and within working distance of lens
Key
1 Detector
2 Flow through cuvette
3 Particle
b) Reverse Fourier set-up: particles are in converging beam between lens and detector
Figure 3 — Set-ups of laser diffraction instruments
6 Operational procedures
6.1 Requirements
6.1.1 Instrument location
The instrument should be located in a clean environment that is free from excessive electrical noise, mechanical
vibration, and temperature fluctuations and is out of direct sunlight. The operating area should be well ventilated.
The instrument should either contain a rigid internal optical bench or be installed on a rigid table or bench to avoid
realignment of the optical system at frequent intervals.
WARNING — The radiation of instruments equipped with a low power laser can cause permanent eye
damage. Never look into the direct path of the laser beam or its reflections. Avoid cutting the laser beam
with reflecting surfaces. Observe the local laser radiation safety regulations.
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© ISO
ISO 13320-1:1999(E)
6.1.2 Dispersion liquids
Any optically transparent liquid of known refractive index may be used. Thus, a variety of liquids is available for
preparation of liquid dispersions of powders. Annex C provides requirements for the dispersion liquids.
If an organic liquid is used for dispersion, observe the local health and safety regulations. Use a cover for the
ultrasonic bath when using liquids with a high vapour pressure in order to prevent the formation of hazardous
vapour concentrations above the bath and/or the generation of low-temperature zones with fluctuating refractive
indices in the fluid by evaporation.
6.1.3 Dispersion gases
For dry dispersion and spray applications a compressed gas is sometimes used. If used, it is essential that it is free
from oil, water and particles. To achieve this, a dryer with a filter is required. Any vacuum unit should be located
apart from the measurement zone, so that the output of the hot air does not reach the measuring zone. Draught
should be avoided in order to avoid unstable particulate streams.
6.2 Sample inspection, preparation, dispersion and concentration
6.2.1 Sample inspection
Inspect the material to be analysed, visually or with the aid of a microscope, firstly to estimate its size range and
particle shape and later to check whether the particles have been dispersed adequately.
The size distribution measured in a sample is only valid for a batch of material if the sample is representative for
that batch and has been dispersed adequately.
6.2.2 Preparation
For dry powders, prepare a representative sample of suitable volume for the measurement by an adequate sample
splitting technique, for instance a rotating riffler. If very small samples are required, or in the case of wet powders, it
is also possible to take fractional samples out of a well-mixed sample paste. The consistency of the paste then
avoids segregation errors. The pastes are formed by adding dispersant to the sample drop by drop while mixing it
with a spatula. As long as the mixture forms lumps, single drops should be added while continuing the mixing after
each drop. A good consistency for the paste is one like honey or toothpaste. If the paste becomes too fluid by
mistake, it shall not be used, and a new preparation should be initiated.
If the maximum size exceeds the measuring range, remove the material that is too coarse, e.g. by presieving. In this
case determine and report the amount/percentage removed.
Sprays, aerosols and gas bubbles in liquid should be measured directly, provided that their concentration is at an
adequate level (see 6.2.3 and 6.2.4), since sampling or dilution is generally impossible without altering the particle
size distribution.
6.2.3 Dispersion
6.2.3.1 Dry powders can be dispersed either in air or in liquid. The dispersion procedure shall be adjusted to the
purpose of the measurement, e.g. it has to be decided whether agglomerates should be detected or broken down to
the primary particles.
6.2.3.2 An adequate dry disperser should be applied; here, generally compressed air or vacuum is applied for
dispersion by shear stress with the assistance of mechanical de-agglomeration by particle-particle or particle-wall
collisions (see figure 4). For dry dispersion, the complete fractional sample shall be used for the measurement. Note
that the use of large sample quantities can overcome the poor statistical representation of coarse particles in a wide
size distribution. It is necessary to check that comminution of the particles does not occur and conversely that a
good dispersion has been achieved. This is usually done by direct comparison of dry dispersion with a liquid one:
ideally, the results should be the same. Another possibility for checking the degree of dispersion or comminution is
by changing the dispersing energy (e.g. the primary air pressure) and monitoring the change of the size distribution.
Usually upon increasing the dispersing energy the amount of fines is increased at first, due to improved dispersion,
until a plateau is reached, where the size distribution is nearly constant with increasing energy. At still higher
energies the amount of fines may rise again as a result of comminution. On some occasions, agglomeration has
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© ISO
ISO 13320-1:1999(E)
been found at high flow rates through a cascade. The centre of the plateau defines the optimum dispersing energy.
Note, however, that a plateau is not always found (for instance for highly aggregated or fragile particles).
a) Velocity gradients caused by shear stress
b) Particle to particle collisions
c) Particle to wall collisions
Figure 4 — Processes involved for dry dispersion of powders
6.2.3.3 For the preparation of liquid dispersions a variety of liquids is available. Annex C presents requirements
and some advice. Generally, pasting, stirring and ultrasonication can be used to facilitate proper dispersion of
particles in the liquid. A preliminary check on the dispersion quality can be made by visual/microscopic inspection of
the suspension. Also, it is possible to perform some measurements of the suspension in the laser diffraction
instrument, with intermediate ultrasonication: the measured size distribution should not change significantly if the
sample is well dispersed and the particles are neither fragile nor soluble.
The minimum volume of sample, required for repeatable measurement, increases as the width of the size
distribution becomes greater in order to allow a sufficient number of large particles to be present. Accordingly, the
volume of the dispersion fluid required to suspend these samples also increases if the limits of optical concentration
are to be observed.
For example, for a sample with particles in the approximate size range of 2 mm to 200 mm, a sample volume of at
least 0,3 ml is needed. Thi
...
SLOVENSKI STANDARD
SIST ISO 13320-1:2002
01-januar-2002
6HMDOQDDQDOL]D0HWRGD]ODVHUVNRGLIUDNFLMRGHO 6SORãQDQDþHOD
Particle size analysis -- Laser diffraction methods -- Part 1: General principles
Analyse granulométrique -- Méthodes par diffraction laser -- Partie 1: Principes généraux
Ta slovenski standard je istoveten z: ISO 13320-1:1999
ICS:
19.120 Analiza velikosti delcev. Particle size analysis. Sieving
Sejanje
SIST ISO 13320-1:2002 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO 13320-1:2002
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SIST ISO 13320-1:2002
INTERNATIONAL ISO
STANDARD 13320-1
First edition
1999-11-01
Particle size analysis — Laser diffraction
methods —
Part 1:
General principles
Analyse granulométrique — Méthodes par diffraction laser —
Partie 1: Principes généraux
A
Reference number
ISO 13320-1:1999(E)
---------------------- Page: 3 ----------------------
SIST ISO 13320-1:2002
ISO 13320-1:1999(E)
Contents
1 Scope .1
2 Normative reference .1
3 Terms, definitions and symbols.1
3.1 Terms and definitions .1
3.2 Symbols.3
4 Principle.4
5 Laser diffraction instrument .4
6 Operational procedures .6
6.1 Requirements.6
6.2 Sample inspection, preparation, dispersion and concentration.7
6.3 Measurement.9
6.4 Repeatability.11
6.5 Accuracy.11
6.6 Error sources; diagnosis .12
6.7 Resolution; sensitivity .14
7 Reporting of results.14
Annex A (informative) Theoretical background of laser diffraction.16
Annex B (informative) Recommendations for instrument specifications .25
Annex C (informative) Dispersion liquids for the laser diffraction method.28
Annex D (informative) Refractive index for various liquids and solids .29
Bibliography.34
© ISO 1999
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
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SIST ISO 13320-1:2002
© ISO
ISO 13320-1:1999(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
Draft International Standards adopted by the technical committees are circulated to the member bodies for voting.
Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.
International Standard ISO 13320-1 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other
sizing methods, Subcommittee SC 4, Sizing by methods other than sieving.
ISO 13320 consists of the following parts, under the general title Particle size analysis — Laser diffraction methods:
Part 1: General principles
Part 2: Validation of inversion procedures
Annexes A to E of this part of ISO 13320 are for information only.
iii
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SIST ISO 13320-1:2002
© ISO
ISO 13320-1:1999(E)
Introduction
Laser diffraction methods are nowadays widely used for particle sizing in many different applications. The success
of the technique is based on the fact that it can be applied to various kinds of particulate systems, is fast and can be
automated and that a variety of commercial instruments is available. Nevertheless, the proper use of the instrument
and the interpretation of the results require the necessary caution.
Therefore, there is a need for establishing an International Standard for particle size analysis by laser diffraction
methods. Its purpose is to provide a methodology for adequate quality control in particle size analysis.
Historically, the laser diffraction technique started by taking only scattering at small angles into consideration and,
thus, has been known by the following names:
Fraunhofer diffraction;
(near-) forward light scattering;
low-angle laser light scattering (LALLS).
However, the technique has been broadened to include light scattering in a wider angular range and application of
the Mie theory in addition to approximating theories such as Fraunhofer and anomalous diffraction.
The laser diffraction technique is based on the phenomenon that particles scatter light in all directions with an
intensity pattern that is dependent on particle size. All present instruments assume a spherical shape for the
particles. Figure 1 illustrates the characteristics of single particle scattering patterns: alternation of high and low
intensities, with patterns that extend for smaller particles to wider angles than for larger particles [2-7, 10, 15 in the
bibliography].
Within certain limits the scattering pattern of an ensemble of particles is identical to the sum of the individual
scattering patterns of all particles present. By using an optical model to compute scattering patterns for unit volumes
of particles in selected size classes and a mathematical deconvolution procedure, a volumetric particle size
distribution is calculated, the scattering pattern of which fits best with the measured pattern (see also annex A).
a) b)
Figure 1 — Scattering pattern for two spherical particles: the particle generating pattern a) is twice as large
as the one generating pattern b)
A typical laser diffraction instrument consists of a light beam (usually a laser), a particulate dispersing device, a
detector for measuring the scattering pattern and a computer for both control of the instrument and calculation of the
particle size distribution. Note that the laser diffraction technique cannot distinguish between scattering by single
particles and scattering by clusters of primary particles forming an agglomerate or an aggregate. Usually, the
resulting particle size for agglomerates is related to the cluster size, but sometimes the size of the primary particles
is reflected in the particle size distribution as well. As most particulate samples contain agglomerates or aggregates
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and one is generally interested in the size distribution of the primary particles, the clusters are usually dispersed into
primary particles before measurement.
Historically, instruments only used scattering angles smaller than 14°, which limited the application to a lower size of
about 1 mm. The reason for this limitation is that smaller particles show most of their distinctive scattering at larger
angles (see also annex A). Many recent instruments allow measurement at larger scattering angles, some up to
about 150°, for example through application of a converging beam, more or larger lenses, a second laser beam or
more detectors. Thus, smaller particles down to about 0,1 mm can be sized. Some instruments incorporate
additional information from scattering intensities and intensity differences at various wavelengths and polarization
planes in order to improve the characterization of particle sizes in the submicrometre range.
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SIST ISO 13320-1:2002
INTERNATIONAL STANDARD © ISO ISO 13320-1:1999(E)
Particle size analysis — Laser diffraction methods —
Part 1:
General principles
1 Scope
This part of ISO 13320 provides guidance on the measurement of size distributions of particles in any two-phase
system, for example powders, sprays, aerosols, suspensions, emulsions and gas bubbles in liquids, through
analysis of their angular light scattering patterns. It does not address the specific requirements of particle size
measurement of specific products. This part of ISO 13320 is applicable to particle sizes ranging from approximately
0,1 mm to 3 mm.
For non-spherical particles, an equivalent-sphere size distribution is obtained because the technique uses the
assumption of spherical particles in its optical model. The resulting particle size distribution may be different from
those obtained by methods based on other physical principles (e.g. sedimentation, sieving).
2 Normative reference
The following normative document contains provisions which, through reference in this text, constitute provisions of
this part of ISO 13320. For dated references, subsequent amendments to, or revisions of, any of these publications
do not apply. However, parties to agreements based on this part of ISO 13320 are encouraged to investigate the
possibility of applying the most recent edition of the normative document indicated below. For undated references,
the lates edition of the normative document referred to applies. Members of ISO and IEC maintain registers of
currently valid International Standards.
ISO 9276-1:1990, Representation of results of particle size analysis — Part 1: Graphical representation.
3 Terms, definitions and symbols
For the purposes of this part of ISO 13320, the following terms, definitions and symbols apply.
3.1 Terms and definitions
3.1.1
absorption
reduction of intensity of a light beam traversing a medium through energy conversion in the medium
3.1.2
coefficient of variation
relative measure (%) for precision: standard deviation divided by mean value of population and multiplied by 100
(for normal distributions of data the median is equal to the mean)
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3.1.3
complex refractive index
N
p
refractive index of a particle, consisting of a real and an imaginary (absorption) part
N = n - ik
p p p
3.1.4
relative refractive index
m
complex refractive index of a particle, relative to that of the medium
m = N /n
p m
3.1.5
deconvolution
mathematical procedure whereby the size distribution of a particle ensemble is inferred from measurements of their
scattering pattern
3.1.6
diffraction
spreading of light around the contour of a particle beyond the limits of its geometrical shadow with a small deviation
from rectilinear propagation
3.1.7
extinction
attenuation of a light beam traversing a medium through absorption and scattering
3.1.8
model matrix
matrix containing light scattering vectors for unit volumes of different size classes, scaled to the detector’s
geometry, as derived from model computation
3.1.9
multiple scattering
subsequent scattering of light at more than one particle, causing a scattering pattern that is no longer the sum of the
patterns from all individual particles (in contrast to single scattering)
3.1.10
obscuration
optical concentration
percentage or fraction of incident light that is attenuated due to extinction (scattering and/or absorption) by the
particles
3.1.11
optical model
theoretical model used for computing the model matrix for optically homogeneous spheres with, if necessary, a
specified complex refractive index, e.g. Fraunhofer diffraction, anomalous diffraction, Mie scattering
3.1.12
reflection
return of radiation by a surface, without change in wavelength
3.1.13
refraction
change of the direction of propagation of light determined by change in the velocity of propagation in passing from
one medium to another; in accordance with Snell’s law
n sin Q = n sin Q
m m p p
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3.1.14
scattering
general term describing the change in propagation of light at the interface of two media
3.1.15
scattering pattern
angular or spatial pattern of light intensities [I(q) and I(r) respectively] originating from scattering, or the related
energy values taking into account the sensitivity and the geometry of the detector elements
3.1.16
single scattering
scattering whereby the contribution of a single member of a particle population to the scattering pattern of the entire
population is independent of the other members of the population
3.1.17
width of normal size distribution
standard deviation (absolute value) or coefficient of variation (relative percentage) of the size distribution
NOTE For normal distributions about 95 % of the population falls within ± 2 standard deviations from the mean value and
about 99,7 % within ± 3 standard deviations from the mean value.
3.2 Symbols
c volumetric particulate concentration, %
f focal length of lens, mm
I(q) angular intensity distribution of light scattered by particles (scattering pattern)
I(r) spatial intensity distribution of light scattered by particles on the detector elements (measured scattering
pattern by detector)
i indication for imaginary part of refractive index
i photocurrent of detector element n, mA
n
k wave number: 2p/l
k imaginary (absorption) part of particle’s refractive index
p
l illuminated path length containing particles, mm
L vector of photocurrents (i , i ,., i )
1 2 n
m relative, complex refractive index of particle to medium
n real part of refractive index of medium
m
n real part of refractive index of particle
p
N complex refractive index of a particle
p
r radial distance from focal point in focal plane, mm
n velocity of particles in dry disperser
x particle diameter, mm
x median particle diameter, mm; here used on a volumetric basis, i.e. 50 % by volume of the particles is
50
smaller than this diameter and 50 % is larger
x particle diameter corresponding to 10 % of the cumulative undersize distribution (here by volume), mm
10
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particle diameter corresponding to 90 % of the cumulative undersize distribution (here by volume), mm
x
90
a dimensionless size parameter: px/l
q scattering angle with respect to forward direction
q angle with respect to perpendicular at boundary for a light beam in medium (as used in Snell's law; see
m
refraction)
q angle with respect to perpendicular at boundary for a light beam in particle (as used in Snell's law; see
p
refraction)
l wavelength of illuminating light source in medium (i.e. liquid or gas/air), nm
w rotational velocity of particles in dry disperser
4 Principle
A representative sample, dispersed at an adequate concentration in a suitable liquid or gas, is passed through the
beam of a monochromatic light source, usually a laser. The light scattered by the particles at various angles is
measured by a multi-element detector and numerical values relating to the scattering pattern are then recorded for
subsequent analysis. These numerical scattering values are then transformed, using an appropriate optical model
and mathematical procedure, to yield the proportion of total volume to a discrete number of size classes forming a
volumetric particle size distribution.
5 Laser diffraction instrument
A typical set-up for a laser diffraction instrument is given in figure 2.
Key
1 Obscuration detector 7 Light source laser
2 Scattered beam 8 Beam processing unit
3 Direct beam 9 Working distance of lens 4
4 Fourier lens 10 Multi-element detector
5 Scattered light not collected by lens 4 11 Focal distance of lens 4
6 Particle ensemble
Figure 2 — Example of the set-up of a laser diffraction instrument
In the conventional set-up, a light source (typically a laser) is used to generate a monochromatic, coherent, parallel
beam. This is followed by a beam processing unit, usually a beam expander with integrated filter, producing an
extended and nearly ideal beam to illuminate the dispersed particles.
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A representative sample, dispersed at an adequate concentration is passed through the light beam in a measuring
zone by a transporting medium (gas or liquid); this measuring zone should be within the working distance of the lens
used. Sometimes, the particle stream in a process is illuminated directly by the laser beam for measurement, as in
the case of sprays, aerosols and air bubbles in liquids. In other cases (such as emulsions, pastes and powders),
representative samples can be dispersed in suitable liquids (see annex C). Often dispersants (wetting agents;
stabilizers) and/or mechanical forces (agitation; ultrasonication) are applied for deagglomeration of particles and
stabilization of the dispersion. For these liquid dispersions a recirculating system is most commonly used, consisting
of an optical measuring cell, a dispersion bath usually equipped with stirrer and ultrasonic elements, a pump and
tubing.
Dry powders can also be converted into aerosols through application of dry powder dispersers, which apply
mechanical forces for deagglomeration. Here a dosing device feeds the disperser with a constant mass flow of
sample. The disperser uses the energy of a compressed gas or the differential pressure to a vacuum to disperse
the particles. It outputs an aerosol that is blown through the measuring zone, usually into the inlet of a vacuum pipe
that collects the particles.
There are two positions in which the particles can enter the laser beam. In the conventional case the particles enter
the parallel beam before and within the working distance of the collecting lens [see Figure 3 a)]. In the so-called
reversed Fourier optics case the particles are entered behind the collecting lens and, thus, in a converging beam
[see Figure 3 b)].
The advantage of the conventional set-up is that a reasonable path length for the sample is allowed within the
working distance of the lens. The second set-up allows only small path lengths but enables measurement of
scattered light at larger angles, which is useful when submicrometre particles are present.
The interaction of the incident light beam and the ensemble of dispersed particles results in a scattering pattern with
different light intensities at various angles (see annex A for theoretical background of laser diffraction). The total
angular intensity distribution I(q), consisting of both direct and scattered light, is then focused by a positive lens or
an ensemble of lenses onto a multi-element detector. The lens(es) provide(s) for a scattering pattern which, within
limits, is not dependent upon the location of the particles in the light beam. So, the continuous angular intensity
distribution I(q) is converted into a discrete spatial intensity distribution I(r) on a set of detector elements.
It is assumed that the recorded scattering pattern of the particle ensemble is identical to the sum of the patterns
from all individual single scattering particles presented in random relative positions. Note that only a limited angular
range of scattered light is collected by the lens(es) and, thus, by the detector.
The detector generally consists of a number of photodiodes; some instruments apply one photodiode in combination
with moving slits. The photodiodes convert the spatial intensity distribution I(r) into a set of photocurrents i .
n
Subsequent electronics then convert and digitize the photocurrents into a set of intensity or energy vectors L ,
n
representing the scattering pattern. A central element measures the intensity of the non-scattered light and, thus,
with a calculation, provides a measure of optical concentration or obscuration. Some instruments provide special
geometries of the central element in order to automatically re-centre or re-focus the detector by moving the detector
or the lens. It is desirable that the detector elements are positioned so as to prevent the light reflected from the
surface from re-traversing the optical system.
A computer controls the measurement and is used for storage and manipulation of the detected signals, for storage
and/or calculation of a proper form of the optical model (usually as a model matrix containing light scattering vectors
per unit of volume per size class, scaled to the detector's geometry and sensitivity) and calculation of the particle
size distribution (see annex A for theoretical background of laser diffraction). Also it may provide automated
instrument operation.
Several significant differences exist, both in hardware and software, not only between instruments from different
manufacturers but also between different types from one company. The instrument specifications should give
adequate information for proper judgement of these differences. In annex B recommendations are presented for the
specifications of laser diffraction instruments.
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Key
1 Detector 4 Working distance
2 Fourier lens 5 Focal distance
3 Particle ensemble
a) Conventional set-up: particles are in parallel beam before and within working distance of lens
Key
1 Detector
2 Flow through cuvette
3 Particle
b) Reverse Fourier set-up: particles are in converging beam between lens and detector
Figure 3 — Set-ups of laser diffraction instruments
6 Operational procedures
6.1 Requirements
6.1.1 Instrument location
The instrument should be located in a clean environment that is free from excessive electrical noise, mechanical
vibration, and temperature fluctuations and is out of direct sunlight. The operating area should be well ventilated.
The instrument should either contain a rigid internal optical bench or be installed on a rigid table or bench to avoid
realignment of the optical system at frequent intervals.
WARNING — The radiation of instruments equipped with a low power laser can cause permanent eye
damage. Never look into the direct path of the laser beam or its reflections. Avoid cutting the laser beam
with reflecting surfaces. Observe the local laser radiation safety regulations.
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6.1.2 Dispersion liquids
Any optically transparent liquid of known refractive index may be used. Thus, a variety of liquids is available for
preparation of liquid dispersions of powders. Annex C provides requirements for the dispersion liquids.
If an organic liquid is used for dispersion, observe the local health and safety regulations. Use a cover for the
ultrasonic bath when using liquids with a high vapour pressure in order to prevent the formation of hazardous
vapour concentrations above the bath and/or the generation of low-temperature zones with fluctuating refractive
indices in the fluid by evaporation.
6.1.3 Dispersion gases
For dry dispersion and spray applications a compressed gas is sometimes used. If used, it is essential that it is free
from oil, water and particles. To achieve this, a dryer with a filter is required. Any vacuum unit should be located
apart from the measurement zone, so that the output of the hot air does not reach the measuring zone. Draught
should be avoided in order to avoid unstable particulate streams.
6.2 Sample inspection, preparation, dispersion and concentration
6.2.1 Sample inspection
Inspect the material to be analysed, visually or with the aid of a microscope, firstly to estimate its size range and
particle shape and later to check whether the particles have been dispersed adequately.
The size distribution measured in a sample is only valid for a batch of material if the sample is representative for
that batch and has been dispersed adequately.
6.2.2 Preparation
For dry powders, prepare a representative sample of suitable volume for the measurement by an adequate sample
splitting technique, for instance a rotating riffler. If very small samples are required, or in the case of wet powders, it
is also possible to take fractional samples out of a well-mixed sample paste. The consistency of the paste then
avoids segregation errors. The pastes are formed by adding dispersant to the sample drop by drop while mixing it
with a spatula. As long as the mixture forms lumps, single drops should be added while continuing the mixing after
each drop. A good consistency for the paste is one like honey or toothpaste. If the paste becomes too fluid by
mistake, it shall not be used, and a new preparation should be initiated.
If the maximum size exceeds the measuring range, remove the material that is too coarse, e.g. by presieving. In this
case determine and report the amount/percentage removed.
Sprays, aerosols and gas bubbles in liquid should be measured directly, provided that their concentration is at an
adequate level (see 6.2.3 and 6.2.4), since sampling or dilution is generally impossible without altering the particle
size distribution.
6.2.3 Dispersion
6.2.3.1 Dry powders can be dispersed either in air or in liquid. The dispersion procedure shall be adjusted to the
purpose of the measurement, e.g. it has to be decided whether agglomerates should be detected or broken down to
the primary particles.
6.2.3.2 An adequate dry disperser should be applied; here, generally compressed air or vacuum is applied for
dispersion by shear stress with the assistance of mechanical de-agglomeration by particle-particle or particle-wall
collisions (see figure 4). For dry dispersion, the complete fractional sample shall be used for the measurement. Note
that the use of large sample quantities can overcome the poor statistical representation of coarse particles in a wide
size distribution. It is necessary to check that comminution of the particles does not occur and conversely that a
good dispersion has been achieved. This is usually done by direct comparison of dry dispersion with a liquid one:
ideally, the results should be the same. Another possibility for checking the degree of dispersion or comminution is
by changing the dispersing energy (e.g. the primary air pressure) and monitoring the change of the size distribution.
Usually upon increasing the dispersing energy the amount of fines is increased at first, due to improved dispersion,
until a plateau is reached, where the size distribution is nearly constant with increasing energy. At still higher
energies the amount of fines may rise again as a result of comminution. On some occasions, agglomeration has
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been found at high flow rates through a cascade. The centre of the plateau defines the optimum dispersing energy.
Note, however, that a plateau is not always found (for instance for highly aggregated or fragile particles).
a) Velocity gradients caused by shear stress
b) Particle to particle collisions
c) Particle to wall collisions
Figure 4 — Processes involved for dry dispersion of powders
6.2.3.3 For the preparation of liquid dispers
...
NORME ISO
INTERNATIONALE 13320-1
Première édition
1999-11-01
Analyse granulométrique — Méthodes par
diffraction laser —
Partie 1:
Principes généraux
Particle size analysis — Laser diffraction methods —
Part 1: General principles
A
Numéro de référence
ISO 13320-1:1999(F)
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ISO 13320-1:1999(F)
Sommaire
1 Domaine d’application .1
2 Référence normative .1
3 Termes, définitions et symboles .1
3.1 Termes et définitions.1
3.2 Symboles.3
4 Principe.4
5 Instrument de diffraction laser .4
6 Modes opératoires de fonctionnement.6
6.1 Prescriptions.7
6.2 Contrôle, préparation, dispersion et concentration de l'échantillon.7
6.3 Mesurage .10
6.4 Répétabilité.12
6.5 Exactitude.12
6.6 Sources d'erreurs; diagnostics .13
6.7 Résolution; sensibilité .15
7 Rapport des résultats.15
Annexe A (informative) Arrière-plan théorique de la diffraction laser .17
Annexe B (informative) Recommandations pour les spécifications des instruments.27
Annexe C (informative) Liquides de dispersion utilisés pour la méthode de diffraction laser .30
Annexe D (informative) Indice de réfraction de divers liquides et solides.31
Bibliographie.36
© ISO 1999
Droits de reproduction réservés. Sauf prescription différente, aucune partie de cette publication ne peut être reproduite ni utilisée sous quelque
forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'éditeur.
Organisation internationale de normalisation
Case postale 56 • CH-1211 Genève 20 • Suisse
Internet iso@iso.ch
Imprimé en Suisse
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Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiée aux
comités techniques de l'ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du comité
technique créé à cet effet. Les organisations internationales, gouvernementales et non gouvernementales, en
liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec la Commission
électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI, Partie 3.
Les projets de Normes internationales adoptés par les comités techniques sont soumis aux comités membres pour
vote. Leur publication comme Normes internationales requiert l'approbation de 75 % au moins des comités
membres votants.
La Norme internationale ISO 13320 a été élaborée par le comité technique ISO /TC 24, Tamis, tamisage, et autres
méthodes de séparation granulométrique, sous-comité SC 4, Granulométrie par procédés autres que tamisage.
L'ISO 13320 comprend les parties suivantes, présentées sous le titre général Analyse granulométrique —
Méthodes par diffraction laser:
Partie 1: Principes généraux
Partie 2: Validation de procédures d'inversion
Les annexes A à D de la présente partie de l'ISO 13320 sont données uniquement à titre d'information.
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Introduction
Aujourd'hui, les méthodes par diffraction laser sont largement utilisées pour différentes applications granulo-
métriques. Le succès de cette technique est fondé sur le fait qu'elle peut être appliquée à de nombreux types de
systèmes particulaires, qu'elle est rapide et peut être automatisée, et qu'en outre, de nombreux instruments sont
commercialisés. Néanmoins, il convient de prendre toutes les précautions nécessaires lors de l'utilisation de
l'instrument et de l'interprétation des résultats.
Il y a donc lieu d'établir une Norme internationale sur l'analyse granulométrique par des méthodes de diffraction
laser. L'objectif de cette norme est de fournir une méthodologie relative au contrôle qualité des analyses
granulométriques.
À l'origine, la technique de diffraction laser consistait à ne prendre en considération que la diffusion de la lumière à
petits angles. Cette technique a ainsi pris les noms suivants:
la diffraction de Fraunhofer;
la diffusion de la lumière vers l'avant;
la diffusion de la lumière laser à angle plat.
Toutefois, la technique s'est beaucoup développée en intégrant maintenant la diffusion de la lumière à des angles
plus grands, ainsi que l'application de la théorie de Mie, qui vient s'ajouter aux théories plus approximatives telles
que la diffraction de Fraunhofer ou la diffraction anormale.
La technique de diffraction laser est fondée sur le phénomène selon lequel les particules diffusent de la lumière
dans toutes les directions, le type d'intensité dépendant de la dimension des particules. Tous les instruments
actuellement disponibles partent du principe que les particules sont sphériques. La Figure 1 illustre les
caractéristiques des motifs de diffusion d'une particule unique: alternance d'intensités faibles et élevées, les
particules plus petites s'étendant selon des angles plus grands que les particules plus grandes [2-7, 10, 15 dans la
Bibliographie].
Dans certaines limites, le motif de diffusion d'un ensemble de particules est identique à la somme des motifs de
diffusion de toutes les particules présentes. On calcule la distribution granulométrique volumétrique en utilisant un
modèle optique pour calculer les motifs de diffusion pour les volumes d'unité de particules de catégories de tailles
sélectionnées et une procédure de déconvolution mathématique; le motif de diffusion de cette distribution est le plus
proche du modèle mesuré (voir aussi l'annexe A).
a) b)
Figure 1 — Motif de diffusion de deux particules sphériques: la particule qui génère le modèle (a) est deux
fois plus grande que celle qui génère le modèle (b)
Un instrument de diffraction laser type est constitué de différents éléments: un faisceau lumineux (généralement un
laser), un dispositif de dispersion particulaire, un détecteur permettant de mesurer le motif de diffusion et un
ordinateur pour, d'une part, commander l'instrument, et d'autre part calculer la distribution granulométrique. À noter
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que la technique de diffraction laser ne distingue pas la diffusion par particules uniques et la diffusion par groupes
de particules primaires formant un agglomérat ou un agrégat. Généralement, le résultat de la taille des particules
d'un agglomérat correspond à la taille des particules regroupées, mais dans certains cas, la distribution
granulométrique indique également la taille des particules primaires. Étant donné que la plupart des échantillons
contiennent des agglomérats ou des agrégats et que l'étude concerne généralement la distribution granulométrique
des particules primaires, les groupes sont généralement répartis en particules primaires avant d'être mesurés.
À l'origine, les instruments prenaient uniquement en compte les angles de diffusion inférieurs à 14°, ce qui limitait
l'application à des particules supérieures à environ 1 mm, les particules plus petites développant la plus grande
partie de leur diffusion distinctive à des angles plus grands (voir aussi l'annexe A). Les instruments plus récents
permettent, pour beaucoup, d'effectuer des mesurages à des angles de diffusion plus grands, certains allant jusqu'à
des angles d'environ 150°; c'est le cas notamment lors de l'utilisation d'un faisceau convergent, d'un plus grand
nombre d'objectifs ou d'objectifs plus grands, d'un second faisceau laser ou d'un plus grand nombre de détecteurs.
Ainsi, les particules plus petites (pouvant descendre jusqu'à environ 0,1 mm) peuvent être mesurées. Certains
instruments contiennent des informations supplémentaires données par les intensités de diffusion et les différences
d'intensité à différentes longueurs d'onde et différents plans de polarisation, afin d'améliorer la caractérisation
granulométrique des particules dans la plage sous-micrométrique.
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Analyse granulométrique — Méthodes par diffraction laser —
Partie 1:
Principes généraux
1 Domaine d’application
La présente partie de l'ISO 13320 fournit des directives sur le mesurage des distributions granulométriques effectué
dans tout système bi-phase, par exemple poudres, pulvérisateurs, aérosols, matières en suspension, émulsions,
bulles de gaz dans des liquides, par l'analyse de leurs motifs de diffusion de la lumière angulaire. Elle ne traite pas
des prescriptions spécifiques relatives au mesurage granulométrique de produits particuliers. La présente partie de
l'ISO 13320 s'applique aux particules dont la taille est comprise dans une plage approximative de 0,1 mm à 3 mm.
Pour les particules non sphériques, le modèle optique de cette technique suppose que les particules sont
sphériques; on obtient ainsi une distribution granulométrique équivalente à celle des particules sphériques. La
distribution granulométrique obtenue peut être différente de celles obtenues avec les méthodes fondées sur
d’autres principes physiques (par exemple sédimentation, tamisage).
2 Référence normative
Le document normatif suivant contient des dispositions qui, par suite de la référence qui y est faite, constituent des
dispositions valables pour la présente partie de l’ISO 13320. Pour les références datées, les amendements
ultérieurs ou les révisions de ces publications ne s’appliquent pas. Toutefois, les parties prenantes aux accords
fondés sur la présente partie de l’ISO 13320 sont invitées à rechercher la possibilité d'appliquer l'édition la plus
récente du document normatif indiqué ci-après. Pour les références non datées, la dernière édition du document
normatif en référence s’applique. Les membres de l'ISO et de la CEI possèdent le registre des Normes
internationales en vigueur.
ISO 9276-1:1990, Représentation de données obtenues par analyse granulométrique — Partie 1: Représentation
graphique.
3 Termes, définitions et symboles
Pour les besoins de la présente partie de l'ISO 13320, les termes, définitions et symboles suivants s'appliquent.
3.1 Termes et définitions
3.1.1
absorption
diminution de l'intensité d'un faisceau lumineux traversant un milieu par conversion de l'énergie dans le milieu
3.1.2
coefficient de variation
mesure relative (%) de la précision: écart-type divisé par la valeur moyenne de la population et multiplié par 100
(pour les distributions normales de données, la médiane est égale à la moyenne)
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ISO 13320-1:1999(F)
3.1.3
indice de réfraction complexe
N
p
indice de réfraction d'une particule, constitué d'une partie réelle et d'une partie imaginaire (absorption):
N = n - ik
p p p
3.1.4
indice de réfraction relatif
m
indice de réfraction complexe d’une particule, rapporté à celui du milieu:
m = N /n
p m
3.1.5
déconvolution
procédure mathématique selon laquelle la distribution granulométrique d'un ensemble de particules est déduite des
mesurages du motif de diffusion
3.1.6
diffraction
étalement de la lumière sur les contours d'une particule, en deçà de son ombre géométrique, avec un faible écart
par rapport à la propagation rectiligne
3.1.7
extinction
atténuation du faisceau lumineux traversant un milieu par absorption et diffusion
3.1.8
matrice de motif
matrice contenant les vecteurs de diffusion de lumière pour des volumes d'unité de différentes classes de tailles,
mis à l'échelle par rapport à la géométrie du détecteur, telle qu'elle est dérivée du calcul du motif
3.1.9
diffusion multiple
diffusion ultérieure de la lumière sur plusieurs particules, entraînant la formation d'un motif de diffusion qui ne
correspond plus à la somme des motifs de toutes les particules individuelles (par opposition à la diffusion unique)
3.1.10
obscurcissement
concentration optique
pourcentage ou fraction de la lumière incidente atténuée en raison de l'extinction (diffusion et/ou absorption) par les
particules
3.1.11
modèle optique
modèle théorique utilisé pour calculer la matrice du modèle pour des sphères optiquement homogènes avec, si
nécessaire, un indice de réfraction complexe spécifié: par exemple la diffraction de Fraunhofer, la diffraction
anormale, la diffusion de Mie
3.1.12
réflexion
renvoi d’une radiation par une surface, sans modification de la longueur d'onde
3.1.13
réfraction
changement de direction de propagation de la lumière, déterminée par le changement de vitesse de propagation en
passant d'un milieu à un autre, conformément à la loi de Snell:
nnsinQQ= sin
m mp p
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3.1.14
diffusion
terme général décrivant le changement de propagation de lumière à l'interface de deux milieux
3.1.15
motif de diffusion
motif angulaire ou spatial des intensités lumineuses [I(q) et I(r) respectivement] dont l'origine est la diffusion, ou
valeur de l'énergie liée, en tenant compte de la sensibilité et de la géométrie des éléments du détecteur
3.1.16
diffusion unique
diffusion par laquelle la contribution d'un membre unique de la population de particules au motif de diffusion de la
population entière est indépendante des autres membres de la population
3.1.17
amplitude de la distribution granulométrique normale
écart-type (valeur absolue) ou coefficient de variation (pourcentage relatif) de la distribution granulométrique
NOTE Dans le cas des distributions normales, environ 95 % de la population tombe dans l’intervalle de – 2 écarts-types
par rapport à la valeur moyenne, et environ 99,7 % de – 3 écarts-types de la valeur moyenne
3.2 Symboles
c Concentration particulaire volumétrique, en pour-cent
f Distance focale de l'objectif, en millimètres
I(q) Distribution de l'intensité angulaire de la lumière diffusée par des particules (motif de diffusion)
I(r) Distribution de l'intensité spatiale de la lumière diffusée par des particules sur les éléments du détecteur
(motif de diffusion mesuré par le détecteur)
i Indication de la partie imaginaire de l'indice de réfraction
i Courant photoélectrique de l'élément du détecteur n, en microampères
n
k Répétence, égale à 2 p/l
k Partie imaginaire (absorption) de l'indice de réfraction des particules
p
l Longueur de chemin illuminé contenant les particules, en millimètres
L Vecteur de courants photoélectriques (i , i , ., i )
1 2 n
m Indice de réfraction complexe relatif d'une particule au milieu
n Partie réelle de l'indice de réfraction du milieu
m
n Partie réelle de l'indice de réfraction des particules
p
N Indice de réfraction complexe d’une particule
p
r Distance radiale du point focal dans le plan focal, en micromètres
n Vitesse des particules dans un dispositif de dispersion à sec
x Diamètre de la particule, en micromètres
x Diamètre de la particule médiane, en micromètres, utilisée ici sur une base volumétrique, c'est-à-dire
50
que, par volume, 50 % des particules sont inférieures au diamètre, et 50 % sont supérieures
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Diamètre de la particule correspondant à 10 % de la distribution cumulative des tamisats (ici, par
x
10
volume), en micromètres
x Diamètre de la particule correspondant à 90 % de la distribution cumulative des tamisats (ici, par
90
volume), en micromètres
aParamètre de taille sans dimension, égal à px/l
qAngle de diffusion par rapport à la direction vers l'avant, en degrés
qAngle limité, par rapport à la perpendiculaire, du faisceau lumineux dans le milieu (tel qu'il est utilisé dans
m
la loi de Snell; voir réfraction), en degrés
qAngle limité, par rapport à la perpendiculaire, du faisceau lumineux de la particule (tel qu'il est utilisé dans
p
la loi de Snell; voir réfraction), en degrés
lLongueur d'onde de la source lumineuse d'éclairage dans le milieu (c'est-à-dire liquide ou gaz/air), en
nanomètres
wVitesse de rotation des particules dans le dispositif de dispersion à sec
4 Principe
Un échantillon représentatif, dispersé à une concentration adéquate dans un liquide ou un gaz adapté, passe à
travers le faisceau d'une source lumineuse monochromatique, généralement un laser. La lumière diffusée par les
particules à divers angles est mesurée par un détecteur multi canaux, et les valeurs numériques liées au motif de
diffusion sont alors enregistrées pour être ensuite analysées. Ces valeurs numériques de diffusion sont ensuite
transformées à l'aide d'un modèle optique approprié et suivant une procédure mathématique, de façon à répartir la
proportion du volume total dans un nombre discret de catégories de tailles formant une distribution granulométrique
volumétrique.
5 Instrument de diffraction laser
La Figure 2 présente le schéma de montage d'un instrument de diffraction laser type.
Légende
1 Détecteur d'obscurcissement 7 Laser de source lumineuse
2 Faisceau diffusé 8 Unité de traitement du faisceau
3 Faisceau direct 9 Distance de travail de l'objectif 4
4 Objectif de Fourier 10 Détecteur multi-éléments
5 Lumière diffusée non collectée par l'objectif 4 11 Distance focale de l'objectif 4
6 Ensemble de particules
Figure 2 — Exemple de montage d'un instrument de diffraction laser
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Dans le cas d'un montage conventionnel, une source lumineuse (généralement un laser) est utilisée pour générer
un faisceau monochromatique, cohérent et parallèle. Cette source lumineuse est suivie d'une unité de traitement du
faisceau, généralement un dispositif d'expansion de faisceau à filtre intégré, qui produit un faisceau étalé et presque
idéal pour illuminer les particules dispersées.
Un échantillon représentatif, dispersé selon une concentration adéquate, est transporté par le biais d'un moyen de
transport (gaz ou liquide) à travers le faisceau lumineux, dans une zone de mesure; il convient que la zone de
mesure soit située dans la distance de travail de l'objectif utilisé. Dans certains cas, le flux de particules en
traitement est illuminé directement par le faisceau laser pour être mesuré; c'est le cas notamment des
pulvérisateurs, aérosols ou des bulles d'air dans les liquides. Dans d'autres cas (émulsions, pâtes, poudres, etc.),
les échantillons représentatifs peuvent être dispersés dans des liquides appropriés (voir l'annexe C). Souvent, des
agents dispersants (agents humidifiants, stabilisateurs) et/ou des forces mécaniques (agitation, ultrasons) sont
appliqués pour désagglomération des particules et stabilisation de la dispersion. Un système de recirculation est
généralement utilisé dans le cas de la dispersion par un liquide. Ce système est constitué d'une cellule de mesure
optique, d'un bain de dispersion généralement muni d'un agitateur et d'éléments à ultrasons, d'une pompe et de
tubes.
Les poudres sèches peuvent également être transformées en aérosols grâce à l'application de dispositifs de
dispersion des poudres sèches, qui appliquent des forces mécaniques permettant la désagglomération. Le principe
est le suivant: un dispositif de dosage alimente le dispositif de dispersion selon un flux constant d'échantillon. Le
dispositif de dispersion utilise l'énergie d'un gaz comprimé ou la différence de pression par rapport au vide pour
disperser les particules. Cette opération conduit à la formation d'un aérosol qui est soufflé à travers la zone de
mesure, généralement à l'entrée du vide qui récupère les particules.
Les particules peuvent traverser le faisceau laser de deux façons. Dans le cas conventionnel, les particules entrent
dans le faisceau lumineux avant, et se maintiennent dans la distance de travail de l'objectif [voir Figure 3 a)]. Dans
le cas de l'analyse de Fourier inverse, les particules entrent derrière l'objectif et, par conséquent, dans un faisceau
convergent [voir Figure 3 b)].
L'avantage du montage conventionnel est qu'une longueur raisonnable de chemin est autorisée pour l'échantillon, à
l'intérieur de la distance de travail de l'objectif. Le second montage autorise uniquement les petits parcours
optiques, mais permet de mesurer la lumière dispersée à de grands angles, ce qui est très utile dans le cas de la
présence de particules submicrométriques.
L'interaction du faisceau lumineux incident et de l'ensemble des particules dispersées forme un motif de dispersion
avec différentes intensités lumineuses, à divers angles (voir l'annexe A pour obtenir les arrière-plans théoriques de
la diffraction laser). La distribution totale de l'intensité angulaire I(q), constituée d'une lumière directe et d'une
lumière diffusée, subit ensuite une mise au point par un objectif positif ou un ensemble d'objectifs d'un détecteur
multi-éléments. Le ou les objectifs fournissent un motif de dispersion qui, jusqu'à une certaine limite, ne dépend pas
de l'emplacement des particules dans le faisceau lumineux. La distribution continue de l'intensité angulaire I(q) est
donc convertie en une distribution discrète de l'intensité spatiale I(r) sur un ensemble d'éléments du détecteur.
On suppose que le motif de diffusion enregistré de l'ensemble de particules est identique à la somme des motifs de
toutes les particules individuelles de diffusion présentées dans des positions relatives aléatoires. Il est à noter que
le ou les objectifs (donc le détecteur) ne rassemblent qu'une plage angulaire limitée de lumière diffusée.
Généralement, le détecteur est constitué de plusieurs photodiodes. Certains instruments associent une photodiode
à des fentes mobiles. Les photodiodes transforment ensuite la distribution spatiale de l'intensité I(r) en un ensemble
de courants photoélectriques i . Le dispositif électronique qui en résulte transforme et numérise ensuite les
n
courants photoélectriques en un ensemble de vecteurs d'intensité ou d'énergie L , qui représentent le motif de
n
diffusion. Un élément central mesure l'intensité de la lumière non diffusée et fournit ainsi, grâce à un calcul, la
mesure de la concentration ou de l'obscurcissement optique. Certains instruments fournissent des géométries
spéciales de l'instrument central pour recentrer ou régler le détecteur ou l'objectif. Il est préférable que les éléments
du détecteur soient positionnés de façon à empêcher la lumière reflétée de la surface de retraverser le système
optique.
Un ordinateur commande le mesurage; cet ordinateur permet de stocker et de manipuler les signaux détectés pour
stocker et/ou calculer la forme correcte du modèle optique (généralement sous la forme d'une matrice de modèle
contenant des vecteurs de diffusion lumineuse par unité de volume par catégorie de taille, mis à l'échelle par
rapport à la géométrie et à la sensibilité du détecteur). L'ordinateur est également utilisé pour calculer la distribution
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granulométrique (voir l'annexe A pour les arrière-plans théoriques de la diffraction laser). Il peut également
automatiser le fonctionnement de l'instrument.
Les instruments des différents fabricants, mais également les différents types de produits conçus par une même
société présentent des différences significatives tant au niveau matériel que logiciel. Il convient que les
spécifications portées sur l'instrument permettent de juger correctement ces différences. L'annexe B donne les
recommandations relatives aux spécifications des instruments de diffraction laser.
Légende
1 Détecteur 4 Distance de travail
2 Objectif de Fourier 5 Distance focale
3 Ensemble de particules
a) Montage conventionnel: les particules entrent au préalable dans le faisceau parallèle et restent dans la distance
de travail de l'objectif
Légende
1 Détecteur
2 Flux au travers d'une cuvette
3 Particule
b) Montage de Fourier inverse: les particules se trouvent dans un faisceau convergent entre l'objectif et le détecteur
Figure 3 — Montage d'un instrument de diffraction laser
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6 Modes opératoires de fonctionnement
6.1 Prescriptions
6.1.1 Emplacement de l'instrument
Il est recommandé d'installer l'instrument dans un endroit propre, exempt de tout bruit électrique excessif, vibration
mécanique, fluctuation de température, et dans un endroit non exposé à la lumière directe du soleil. Il convient de
bien aérer la zone de fonctionnement. Il est recommandé d'utiliser un instrument muni d'un banc optique interne, ou
d'installer l'instrument sur une table ou un banc rigide, pour éviter le réalignement fréquent du système optique.
AVERTISSEMENT — La radiation des instruments équipés d'un laser à faible puissance peut provoquer
des dommages visuels irréversibles. Ne jamais regarder directement dans le champ d'un faisceau laser ou
de ses réflexions. Eviter de couper le faisceau laser avec des surfaces réfléchissantes. Observer les règles
de sécurité locales en matière de radiation laser.
6.1.2 Liquides de dispersion
Tout liquide optique transparent d'indice de réfraction connu peut être utilisé. De nombreux liquides permettent donc
de disperser une poudre. L'annexe C fournit des informations relatives aux exigences des liquides de dispersion.
Dans le cas de l'utilisation d'un liquide organique, observer les règles de santé et de sécurité locales. Dans le cas
de l'utilisation de liquides ayant une pression de vapeur élevée, utiliser un couvercle pour bain d'ultrasons, afin
d'éviter la formation de concentrations de vapeur dangereuses au-dessus du bain et/ou la génération de zones à
basse température avec indices de réfraction fluctuant dans le liquide par évaporation.
6.1.3 Gaz de dispersion
Un gaz comprimé est parfois utilisé dans le cas d'un pulvérisateur ou d'une dispersion à sec. Si un tel gaz est
utilisé, il est essentiel qu'il soit exempt d'huile, d'eau et de particules, cette propreté devant être atteinte grâce à
l'utilisation d'un séchoir muni d'un filtre. Il convient de placer tout vide à l'extérieur de la zone de mesurage, de façon
que la sortie d'air chaud n'atteigne pas la zone de mesure. Il convient d'éviter tout courant d'air pour éviter la
formation de faisceaux particulaires instables.
6.2 Contrôle, préparation, dispersion et concentration de l'échantillon
6.2.1 Contrôle de l’échantillon
Effectuer un contrôle, visuel ou à l'aide d'un microscope, du matériel à analyser, pour d'une part évaluer la plage de
tailles et la forme des particules, et d'autre part pour vérifier que les particules ont été correctement dispersées.
La distribution granulométrique mesurée sur un échantillon ne peut être valide pour un lot de matériau que si
l'échantillon est représentatif de ce lot et qu'il a été correctement dispersé.
6.2.2 Préparation
Dans le cas de poudres sèches, préparer un volume suffisant d'échantillon représentatif pour pouvoir être mesuré
en utilisant une technique de fractionnement adéquate de l'échantillon, par exemple un sablier rotatif. Dans le cas
de l'utilisation d'échantillons très petits ou de poudre humide, il est également possible de prélever des fractions
d'échantillons d'une pâte bien mélangée. La consistance de la pâte évite ainsi les erreurs de séparation. La
formation d'une pâte s'effectue en ajoutant goutte à goutte des agents dispersants à l'échantillon et en mélangeant
à l'aide d'une spatule. Il convient d'ajouter des goutte
...
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