ISO 13317-5:2025
(Main)Determination of particle size distribution by gravitational liquid sedimentation methods — Part 5: Photosedimentation techniques
Determination of particle size distribution by gravitational liquid sedimentation methods — Part 5: Photosedimentation techniques
This document specifies principles and methods for the use of gravitational photosedimentation techniques for the characterization of dispersed phases of suspensions and emulsions. These techniques monitor the gravity-induced phase separation of particulate materials dispersed in liquids by recording photometric signals (i.e. intensity of transmitted or scattered light) as a function of either vertical position or measurement time, or both. This document does not cover particle migration by centrifugal, electric or magnetic forces, or sedimentation at high particle concentrations (e.g. zone sedimentation). Moreover, it does not cover the determination of properties other than sedimentation velocity and particle size (i.e. it does not cover particle concentration, particle shape, particle density, zeta-potential or apparent viscosity). Additionally, this document does not cover alternative techniques for gravitational sedimentation including balance based and X-ray based techniques. NOTE This document does not purport to address all the safety problems associated with its use.
Détermination de la distribution granulométrique par les méthodes de sédimentation par gravité dans un liquide — Partie 5: Techniques de photosédimentation
General Information
Standards Content (Sample)
International
Standard
ISO 13317-5
First edition
Determination of particle size
2025-01
distribution by gravitational liquid
sedimentation methods —
Part 5:
Photosedimentation techniques
Détermination de la distribution granulométrique par les
méthodes de sédimentation par gravité dans un liquide —
Partie 5: Techniques de photosédimentation
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 6
5 Measurement principle and instrumentation . 8
5.1 General measurement principle .8
5.2 Primary and derived measurement results .10
5.3 Instrumentation .11
6 Measurement data and calculation of distribution function .13
6.1 Primary and derived measurands . 13
6.2 Intrinsically measured distribution functions . 15
6.3 Conversion to volume-weighted distribution functions .17
6.4 Determination of the start position .18
6.5 Assumptions behind data analysis in photosedimentation . 20
6.5.1 Assumptions related to Stokes law . 20
6.5.2 Assumptions related to photometric particle quantification .21
6.6 Working range with respect to particle size and concentration .21
6.6.1 Limits defined by the applicability of Stokes law .21
6.6.2 Limits defined by the applicability of photometric detection . 22
7 Performing size analyses .24
7.1 General .24
7.2 Sampling .24
7.3 Dispersion process and primary sample preparation.24
7.4 Secondary sample preparation (sample conditioning) . 25
7.5 Instrument preparation . 25
7.6 Measurement . 26
7.7 Data analysis . 26
7.8 Reporting .27
8 System qualification and quality control .28
8.1 General remarks . 28
8.2 Reference materials . 29
8.3 Performance qualification. 30
8.4 Measurement uncertainty . 30
Annex A (informative) Measurement position .33
Annex B (informative) Calculation of number-weighted particle size distribution .37
Annex C (informative) Detailed multi-wavelength approach .40
Annex D (informative) Guide to uncertainty determination .42
Annex E (informative) Beyond velocity and size determination. 47
Bibliography .50
iii
Foreword
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The procedures used to develop this document and those intended for its further maintenance are described
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of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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This document was prepared by Technical Committee ISO/TC 24, Particle characterization including sieving,
Subcommittee SC 4, Particle characterization.
A list of all parts in the ISO 13317 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
Introduction
The principles of gravitational photosedimentation and its potential use for the granulometric
characterization of particle systems have been known for several decades. Recent developments in
optoelectronics and data processing have boosted the commercial success and popularity of this
measurement technique, which is currently employed in manifold academic and industrial applications.
This document is a part of the ISO 13317 series that provides a general overview on the principles, techniques,
methods and underlying physics of particle size analysis by gravitational sedimentation. Photosedimentation
employs photometric signals (i.e. transmitted, reflected or scattered light) in order to monitor the changes
in the local particle concentration, which arise by the downward or upward particle migration under
gravity (called hereafter sedimentation). The temporal or spatial functions of these signals can be directly
transformed to distributions of the sedimentation velocity, without referring to model assumptions or being
restricted by essential preconditions. Provided the applicability of Stokes’ law on particle mobility, one can
derive equivalent diameters from the sedimentation velocities (the Stokes diameter) and the corresponding
particle size distributions. Size fractions are then intrinsically weighted by photometric quantities (e.g. light
extinction or scattered light intensity), which is in contrast to the sedimentation techniques described in
ISO 13317-2, ISO 13317-3 and ISO 13317-4. However, conversion into volume-weighted distributions is often
an integrated part of signal processing, which employs established models for light-particle interactions. A
noteworthy feature of gravitational photosedimentation is its ability to finely resolve details in the particle
distribution functions. This is related to the physical fractionating of particle systems under gravity and
constitutes an advantage compared to spectroscopic ensemble techniques.
Gravitational photosedimentation facilitates the granulometric characterization of dispersed materials
of non-zero density contrast to the continuous phase, including solid particles and emulsion droplets.
The available measurement range depends on dispersed and continuous phase properties and typically
amounts to 200 nm to 100 μm for aqueous samples, whereas the sedimentation velocity can be quantified
for the range 0,6 µm/s to 10 mm/s. Also, the working range with regard to particle concentration is strongly
affected by material properties and by particle size, yet it is typically well below 1 vol%. The data analysis
relies on the assumption that all particles have the same density and comparable shape and do not undergo
chemical or physical change in the continuous phase.
In addition, photosedimentation techniques that monitor gravity-induced concentration changes along the
complete sample height, e.g. by position-scanning or time-resolved projection, facilitate the characterization
of dense dispersion beyond particle size, e.g. with respect to clarification, segregation, agglomeration,
consolidation and physico-chemical stability (see ISO/TR 13097). Gravitational photosedimentation
is equally applicable in determining particle density (see ISO 18747 series) as well as the formation of
sediments and cream layers.
v
International Standard ISO 13317-5:2025(en)
Determination of particle size distribution by gravitational
liquid sedimentation methods —
Part 5:
Photosedimentation techniques
1 Scope
This document specifies principles and methods for the use of gravitational photosedimentation techniques
for the characterization of dispersed phases of suspensions and emulsions. These techniques monitor the
gravity-induced phase separation of particulate materials dispersed in liquids by recording photometric
signals (i.e. intensity of transmitted or scattered light) as a function of either vertical position or
measurement time, or both.
This document does not cover particle migration by centrifugal, electric or magnetic forces, or sedimentation
at high particle concentrations (e.g. zone sedimentation). Moreover, it does not cover the determination of
properties other than sedimentation velocity and particle size (i.e. it does not cover particle concentration,
particle shape, particle density, zeta-potential or apparent viscosity).
Additionally, this document does not cover alternative techniques for gravitational sedimentation including
balance based and X-ray based techniques.
NOTE This document does not purport to address all the safety problems associated with its use.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content constitutes
requirements of this document. For dated references, only the edition cited applies. For undated references,
the latest edition of the referenced document (including any amendments) applies.
ISO 9276-1, Representation of results of particle size analysis — Part 1: Graphical representation
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
sedimentation
directional motion of particles (3.7) in a viscous liquid under the action of gravity or centrifugal fields
Note 1 to entry: For a positive density contrast (3.17), sedimentation occurs in the direction of gravitational
acceleration; it is counter directed to this acceleration for a negative density contrast.
Note 2 to entry: A downward motion under gravity is also called “settling” or “falling”.
Note 3 to entry: An upward motion under gravity is also called “creaming” (e.g. droplets) or more generally, “rising” or
“floating”.
[SOURCE: ISO 13317-1:2024, 3.1]
3.2
migration
directional motion of particles (3.7) in a viscous liquid under the action of a force field
Note 1 to entry: Migration in gravitational or centrifugal fields is called sedimentation (3.1).
[SOURCE: ISO 13317-1:2024, 3.2]
3.3
terminal sedimentation velocity
sedimentation (3.1) velocity in the case that gravity or centrifugal force is completely balanced by buoyancy
and drag force
[SOURCE: ISO 13317-1:2024, 3.3]
3.4
Stokes diameter
equivalent diameter of a sphere that has the same buoyant density (3.16) and terminal sedimentation velocity
(3.3) as the real particle in the same liquid under creeping flow (3.19) conditions
Note 1 to entry: The general rule that the buoyant density is used for calculating the Stokes diameter applies also to
coated particles or multiconstituent particles (such as droplets in multiple emulsions). The buoyant density can be
approximated with the skeleton density (3.14) for monoconstituent particles.
Note 2 to entry: For porous particles, it is common use to compute particle size based on the apparent particle density
(3.15). This approach considers the stagnant liquid in the open pores as intrinsic constituent of the dispersed phase.
Thus, the obtained size values are hydrodynamic equivalent diameters.
Note 3 to entry: For close-packed agglomerates (3.8) or aggregates, the buoyant density can be replaced by the
apparent particle density– with particle referring to the agglomerate or aggregate – in order to get the hydrodynamic
equivalent diameter.
[SOURCE: ISO 13317-1:2024, 3.4]
3.5
shape correction factor
ratio of the sedimentation velocity of a non-spherical particle to the one of a spherical particle of the same
volume and apparent density (3.15)
[SOURCE: ISO 13317-1:2024, 3.5]
3.6
hindrance function
ratio of the terminal sedimentation velocity (3.3) of a particle (3.7) placed in well-mixed dispersion divided by
its sedimentation velocity in an infinite vessel for the absence of other particles
[SOURCE: ISO 13317-1:2024, 3.6]
3.7
particle
minute piece of matter with defined physical boundaries
[SOURCE: ISO 26824:2022, 3.1.1, modified — Notes 1, 2 and 3 to entry have been deleted.]
3.8
agglomerate
cluster of particles (3.7) held together by weak or medium strong forces with an external surface area, which
is similar to the sum of the surface areas of the individual particles
Note 1 to entry: The forces acting between the constituent particles of an agglomerate are relatively weak. They result,
for example, from van der Waals attraction or simple physical entanglement.
Note 2 to entry: Agglomerates are also termed secondary particles and the original source particles are termed
primary particles.
[SOURCE: ISO 13317-1:2024, 3.8]
3.9
open pore
pore not totally enclosed by its walls and open to the surface either directly or by interconnecting with
other pores and therefore accessible to liquid
[SOURCE: ISO 15901-1:2016, 3.11, modified: “fluid” has been replaced with “liquid” in the definition.]
3.10
closed pore
pore totally enclosed by its walls and hence not interconnecting with other pores and not accessible to liquids
[SOURCE: ISO 15901-1:2016, 3.10, modified: “fluids” has been replaced with “liquids” in the definition.]
3.11
dynamic viscosity
measure of flow resistance for Newtonian liquids, calculated as the ratio of the shear stress to the rate of
shear for laminar flow exposed to a pre-set shear stress or strain
[SOURCE: ISO 13317-1:2024, 3.11]
3.12
apparent viscosity
measure of flow resistance for non-Newtonian liquids at a defined shear stress or strain, calculated as the
ratio of the shear stress to the shear rate
[SOURCE: ISO 13317-1:2024, 3.12]
3.13
true density of the dispersed phase
ratio of mass to volume for a body solely consisting of the dispersed phase without pores, voids, inclusions
or surface fissures
[SOURCE: ISO 13317-1:2024, 3.13]
3.14
skeleton density
ratio between sample mass and the volume of the sample including the volume of closed pores (3.10) (if
present) but excluding the volumes of open pores (3.9)
Note 1 to entry: The skeleton density refers to solid particles (3.7) and is determined for samples of dry powder.
[SOURCE: ISO 13317-1:2024, 3.14]
3.15
apparent particle density
effective particle density
ratio of mass to volume for a particle (3.7) including particulate inclusions, entrapped stagnant liquid and
gas in pores, voids and surface fissures as well as surfaces layers and coatings
Note 1 to entry: The apparent particle density is the density of a migrating entity and is calculated as the weighted
average of its constituents.
Note 2 to entry: The apparent particle density depends on wettability of open pores (3.9) and the kinetics of wetting or
replacement of pore liquid. Therefore, it is affected by sample preparation.
Note 3 to entry: The apparent particle density is not identical with the buoyant density (3.16). They deviate from each
other for porous particles and particle agglomerates (3.8) in particular.
[SOURCE: ISO 13317-1:2024, 3.15]
3.16
buoyant density
ratio of mass to volume for a particle (3.7) including particulate inclusions, liquid and gas in closed pores and
voids as well as surfaces layers and coatings, but excluding the liquid continuous phase that penetrates open
pores (3.9)
Note 1 to entry: The buoyant density equals the (hypothetical) density of the continuous phase for which the
gravitational force acting on the immersed particle is counterbalanced by buoyancy.
Note 2 to entry: The buoyant density of a particle can be experimentally determined (see ISO 18747-1 and ISO 18747-2
for more information)
Note 3 to entry: The buoyant density of monoconstituent particles can be approximated with their skeleton density (3.14).
Note 4 to entry: The buoyant density of multiconstituent particles (e.g. coated pigments, droplets of multiple
emulsions) can be approximated with the averaged skeleton densities of the single constituents.
Note 5 to entry: The buoyant density is affected by the adsorption of dissolved species at the particle surface and
therefore depends on the solvent and its composition.
Note 6 to entry: The buoyant density is not identical with the apparent particle density (3.15), particularly for porous
particles and particle agglomerates (3.8).
[SOURCE: ISO 13317-1:2024, 3.16]
3.17
density contrast
difference between the particle density and the density of the continuous phase
Note 1 to entry: For quantifying the density contrast, the buoyant (particle) density (3.16) is used, but for porous
particles, the apparent particle density (3.15) is more appropriate.
[SOURCE: ISO 13317-1:2024, 3.17]
3.18
particle Reynolds number
dimensionless parameter expressing the ratio of inertial to viscous forces within a fluid flowing past a
particle
Note 1 to entry: The particle Reynolds number is based on the volume equivalent diameter.
Note 2 to entry: In other contexts, the definition of the particle Reynolds number can refer to different equivalent
diameters or to the equivalent radii.
Note 3 to entry: The particle Reynolds number is a characteristic of the flow field and mobility of the particle.
[SOURCE: ISO 13317-1:2024, 3.18]
3.19
creeping flow
type of flow that is solely governed by viscous forces and not affected by inertial effects
Note 1 to entry: For moving particles (3.7) or for the flow past a particle, the creeping flow condition applies if the
particle Reynolds number (3.18) is well below 0,25.
[SOURCE: ISO 13317-1:2024, 3.19]
3.20
Brownian motion
random motion of particles (3.7) caused by collisions with the molecules or atoms of the surrounding
continuous phase
Note 1 to entry: The trajectory of Brownian motion is not differentiable.
Note 2 to entry: Brownian motion results on a macroscopic level in mass transport of the dispersed phase, e.g. in case
of diffusion, thermophoresis or photophoresis.
[SOURCE: ISO 13317-1:2024, 3.20]
3.21
lower size limit
size of the smallest particles that are detectable and with a diffusional particle flux that is negligible
compared to the sedimentational particle flux
Note 1 to entry: The ratio of sedimentational flux to diffusional flux (also called Péclet number, Pe) should be > 1.
[SOURCE: ISO 13317-1:2024, 3.21]
3.22
upper size limit
size of the largest particle that satisfies the condition of creeping flow (3.19) and of which the terminal
sedimentation velocity (3.3) is detectable
[SOURCE: ISO 13317-1:2024, 3.22]
3.23
type of quantity
specification of the physical property employed to quantify the individual particle (3.7) fractions
Note 1 to entry: The type of quantity is a cumulable property of single particles or disperse systems, such as number,
mass, intensity of scattered light (within the single scattering limit), light extinction (within Lambert-Beer-limit),
refractive index increment or X-ray attenuation.
Note 2 to entry: The type of quantity is indicated by a numerical or character subscript when symbolising the density
and cumulative function of a size distribution. Moreover, the subscript also specifies distribution parameters, such as
median, mean and modal values or any quantiles.
Note 3 to entry: The following conventions apply for the subscript of geometric or gravimetric properties:
number: subscript r = 0
length: subscript r = 1
area: subscript r = 2
volume or mass: subscript r = 3
Note 4 to entry: The following conventions apply for the subscript of physical properties:
light extinction: subscript toq = “ext”
light intensity: subscript toq = “int”
[SOURCE: ISO 13317-1:2024, 3.23]
3.24
sensitivity
change of instrument response with respect to changes in concentration or absolute quantity of particles
(3.7) in a specified size class
Note 1 to entry: A concentration or quantity can be given in relative or absolute values in dependence on the detection aim.
Note 2 to entry: Sensitivity depends on the type of quantity (3.23).
Note 3 to entry: Sensitivity is a function of size.
[SOURCE: ISO 13317-1:2024, 3.24]
3.25
limit of quantity detection
smallest quantity of specified particle size class for which the instrument response can be distinguished
from the background
Note 1 to entry: The limit of quantity detection depends on factors such as size range, precision, noise level, and
smoothing algorithms.
Note 2 to entry: The limit of quantity detection affects the lower size limit (3.21) and upper size limit (3.22).
[SOURCE: ISO 13317-1:2024, 3.25]
3.26
measurement uncertainty
uncertainty of measurement
parameter, associated with the result of a measurement that characterises the dispersion of the values that
can reasonably be attributed to the measurand
[SOURCE: ISO Guide 98-3:2008, 2.2.3, modified — Notes 1 to 3 to entry have been deleted and the term
“measurement uncertainty” has been added.]
4 Symbols and abbreviated terms
For the purposes of this document, the following symbols apply.
Ar Archimedes number dimensionless
b systematic deviation of measured value from true value varying
0,5 0,5
C transformation coefficient, see Formula (27) m ·s
C extinction cross section
ext
c concentration with respect to extensive property M varying
M
2 −1
D particle diffusion coefficient m ·s
p
E extinction
−2
g gravitational acceleration m·s
h sedimentation distance m
sed
I light intensity
k coverage factor dimensionless
K extinction efficiency
ext
−1
k Boltzmann constant J·K
B
L optical pathlength
Lj Ljaščenko number dimensionless
M extensive property indicating the amount of dispersed phase varying
m number of bias determinations dimensionless
N number of particles dimensionless
n number of replicate analyses dimensionless
Pe Péclet number dimensionless
Q cumulative function of distributed quantity, index “toq” indicates the type ofd imensionless
toq
quantity, in which the fractions are weighted
q density function of distributed quantity, index “toq” indicates the type of quantity,v arying
toq
in which the fractions are weighted
Re particle Reynolds number dimensionless
P
s standard deviation varying
T absolute temperature K
t time s
t time point of observation s
observ
t sedimentation time s
sed
U expanded uncertainty varying
u uncertainty varying
V measurement volume
meas
−1
v terminal sedimentation velocity m·s
sed
x particle size (equivalent diameter) m
x Stokes diameter m
Stokes
x volume equivalent diameter m
V
z Cartesian coordinate in vertical direction, vertical position m
−3
Δρ density contrast kg·m
δ thickness m
η viscosity of the continuous phase Pa·s
c
−3
ρ particle density kg·m
p
−3
ρ density of the continuous phase kg·m
c
φ volume fraction dimensionless
V
In addition, the following subindices are frequently employed.
app apparent
c combined
cr critical
ext extinction
int intensity
lab laboratory
max maximum
meas measurement
psca partial scattering
ref reference
rel relative
rep repeatability
sca scattering
Rw reproducibility
toq type of quantity
Moreover, this document uses the following abbreviated terms.
CRM certified reference material
ILC interlaboratory comparison
NIR near infrared radiation
QCM quality control material
RM reference material
RTM representative test material
UVA ultraviolet A radiation
5 Measurement principle and instrumentation
5.1 General measurement principle
Gravitational photosedimentation allows the characterization of liquid disperse systems based on their
phase separation under gravity. Unlike other sedimentation techniques, the phase separation is monitored
photometrically as the depletion or accumulation of particles at (a) defined vertical position(s) in the initially
[1]
well-mixed sample. The measurement can be conducted at a fixed position or a continuously varying one
(scanning mode) or even at multiple positions for the same measurement time, including the spatially resolved
measurement along the vertical axis. The observed quantity is the intensity of light transmitted through,
scattered by, or reflected at the sample. These quantities correlate with the local particle concentration
at the measurement position(s). Temporal and spatial changes in local particle concentration are entirely
attributed to gravitational sedimentation of the particles. The direction of sedimentary motion depends on
the density contrast (Δρ) between the dispersed and continuous phase (see 3.17); the corresponding particle
motion is either called settling and falling (Δρ > 0) or creaming, rising and floating (Δρ < 0).
NOTE This document refers to photometric measurement techniques that monitor the gradual depletion or
accumulation of particles in the continuous phase, but not to techniques that observe the growth of sediment or
cream layers by photometric means (see ISO 6344-3 and ISO 8486-2). Moreover, the operational method requires an
initially homogeneous dispersion sample [so called homogeneous-start mode (HSM)]; any approach starting with a
thin dispersion layer on top or beneath a particle-free liquid (so called line-start mode) is beyond the applicability of
this document.
Figure 1 illustrates the measurement principle for the photometric monitoring of transmitted light (for a
scattering configuration, see Figure 3) The example assumes a bi-disperse particle system having a positive
density contrast. At the beginning of the sedimentation analysis (t = 0), the sample is well-mixed, i.e. particles
of each size fractions are uniformly distributed along the vertical axis and the local particle concentration
is independent from position. However, gravity makes the particle settle downward with a size-dependent
velocity. This gives rise to four layers differing in particle concentration. A particle-free liquid layer at the
top (zone 1). In zone 2, the coarse particles settled out (segregation) within the time t and only the
observe
fine particles remain (c = c ). The particle content in the third layer does not change at all and composition
fine
and concentration are not position dependent and remain at its initial value (c = c = c + c ). The
initial fine coarse
sediments forms at the bottom (zone 4), where particles have a very high concentration. Composition is
not uniform. The sedimentation process is finished when all particles have settled and the formation of
the sediment is completed. Nevertheless, the analysis can be stopped when the last of the particles that
will migrate due to gravity have passed through the measurement zone [see Figure 1 a)]. In general,
measurements can be stopped when photometric signals have reached a stable value, which is associated
with the particle-free supernatant.
a) Homogenous mixing state and b) Formation of 4 zones and c) Evolution of the
corresponding concentration corresponding concentration photometric
profile at t = 0 profile at t concentration at the
observ
measurement position
Key
X photometric concentration
Y position
h sedimentation distance
sed
t time
t time point of observation
observ
1 zone 1 = particle-free supernatant
2 zone 2 = depleted dispersion phase (due to loss of coarse particles)
3 zone 3 = original dispersion phase
4 zone 4 = sediment
a
Sedimentation cell.
b
Light beam, with fixed or varying position.
c
Measurement zone.
d
Photometric sensor.
The grey area between the light beam and the sensor depicts the measurement zone.
SOURCE: Reproduced with the permission of LUM GmbH©.
Figure 1 — Phase separation due to sedimentation for a bi-disperse sample with positive density
contrast and its monitoring via light transmission
Within the scope of this document, the techniques should only be applied to dilute samples, in which
particles settle or rise independently from each other (i.e. no hydrodynamic hindrance, see Reference [2]
and ISO 13317-1). This condition offers a chance to quantify individual particle properties, such as size. Yet
in principle, photosedimentation techniques also allow the characterization of concentrated samples, e.g. for
the purpose of dispersion separation or stability analysis (see Annex E).
5.2 Primary and derived measurement results
The primary measurement data of photosedimentometer are optical signals from the sample contained in
a transparent measurement cell (e.g. cuvette). Such signals are either the intensities of transmitted light or
scattered light¸ which are frequently presented as normalized values (e.g. transmittance, relative scattering
intensity). In general, they may be registered either as functions of time at a (multiple) vertical cell position(s)
or at vertical position(s) at a fixed time(s). Several types of original measurement results are possible:
— time course of the optical signal at a constant vertical position of the sedimentation cell;
— time courses of the optical signal at few vertical positions of the sedimentation cell;
— time courses of the optical signal at different positions obtained by vertical scanning along the
sedimentation cell (allows reconstruction of vertical profiles if scanning rate is faster than rate of phase
separation);
— time-resolved vertical profiles of the optical signal along the measurement cell (i.e. simultaneous time
courses of the optical signals at multiple, finely resolved vertical positions).
Hence, there are two principal types of original measurement results: time courses for a specified position
or vertical profiles for a specified time [see Figure 2 a) and b)].
a) Primary measurement: b) Primary measurement: c) Derived result:
Optical signal vs. Optical signal along Particle concentration vs.
sedimentation time vertical position sedimentation velocity
Key
1 scattering signal
2 transmission signal
3 cumulated particle concentration
A’ time, at which the fastest particles have completely passed the vertical measurement position
B’ time, at which the slowest particles have left the zone above (Δρ > 0) or below (Δρ < 0) the measurement position
A” distance in the direction of sedimentation, below which the liquid is completely depleted of the fastest particles
B” distance in the direction of sedimentation, below which the liquid is free of particles
c particle concentration
h sedimentation distance
sed
S signal
t sedimentation time
sed
v sedimentation velocity
sed
The real signal shape of the schematic curves depends on several factors, including particle size distribution, particle
shape, wavelength or refractive indices.
Figure 2 — Primary and derived measurement signals of gravitational photosedimentation
The primary optical signals correlate with the particle concentration but are also affected by the optical
configuration of the instrument, the optical properties of the measurement cell and the optical properties
of the dispersion medium. The quantification of particle fractions, i.e. for photosedimentometer applications
within this document, requires a signal transformation to get rid of the signal contributions not caused by
particles (see 6.1).
— Transmitted signal intensity is first converted into sample transmission and finally into extinction.
— Scattered intensity is converted into relative excess scattering signals or excess Rayleigh ratio.
The transformed signals are considered to be proportional to the particle concentration, which is prerequisite
for the quantification of particle fractions. The evolution of signals with time or vertical position is due to the
sedimentational separation and segregation of the dispersed phase as shown in Figure 1. It reflects changes
in the composition of the dispersed phase and thus the quantities of particle fractions that have just left the
measurement zone. Those fractions migrate with a sedimentation velocity, which is identical to the ratio
of sedimentation distance to measurement time and which can be further transformed into an equivalent
diameter, the Stokes diameter (see 6.1). Hence, time-curves or profiles of transmitted or scattered signals
can be considered as scaled sum functions of the velocity or particle size distribution [see Figure 2 c)].
5.3 Instrumentation
Typical photosedimentometers essentially consist of a vertically aligned measurement cell (or sedimentation
column), a unit for adjusting to a constant temperature in the measurement cell (e.g. water bath, contact
cooling and heating), an illuminating light source or sources operating at different wavelengths, optical
detection system(s) and an electronic signal processing unit.
Key
a
Incident beam.
b
Transmission detector.
c
Scattering detector.
Figure 3 — Transmission and scattering setup
The most important feature for grouping photosedimentometer is whether they employ transmitted
[3],[4] [5]
light or scattered light for monitoring phase separation. The former has the advantage of a defined
measurement volume, which simplifies the “extraction” of particle-induced signal contributions. On the other
hand, it principally requires a certain minimum sample opacity and turbidity and allows the quantification
of separate particle fractions, i.e. for the determination of velocity distribution or particle size distribution,
only if opacity keeps below a critical threshold. Scattered light allows the monitoring of phase separation
for fully transparent samples (i.e. low particle concentrations) and, in the case of backscattered (reflected)
light, for opaque samples. However, the reliable quantification of separate particle fractions from scattered
light requires low optical concentrations.
NOTE 1 Photosedimentometers used for the quantification of separate particle fractions, i.e. for the determination
of the velocity distribution or the particle size distribution, are typically operated in transmission mode.
The optical detection system may be comprised of one (or more) detector(s) at fixed vertical position(s) or a
movable detector, which scans the sample along the vertical axis. The width of the illuminating light beam(s)
can be adjusted to ensure optimum signal-to-noise ratio. An alternative is spatially resolved detectors (e.g.
a linear CCD array), which ideally allow the simultaneous observation of concentration changes at each
[2]
vertical position (STEP Technology ). For this purpose, the measurement cell is illuminated with a two-
dimensional beam. In practice, each type of detection system can be attributed a certain vertical resolution
Δh, which affects the minimum resolution of the distribution functions (see Annex A).
Illumination in photosedimentometers employs radiation from the optical domain including visible light,
UVA and NIR:
— at one, fixed wavelength (or a very narrow part of the spectrum);
— at multiple, fixed wavelengths;
— with spectral resolution (i.e. for a large number of consecutive wavelengths).
Currently, the first two are the most frequently used.
The central part of a photosedimentometer is the measurement cell, in which the liquid dispersion sample is
placed for gravitational phase separation. Measurement cells for transmission measurements are typically
rectangular in cross-section with the light beam perpendicularly passing two parallel windows, whereas
scattering-based photosedimentometry is frequently performed on round cuvettes. The most important
characteristics are the height and the optical path (diameter) of the measurement cell. The height limits the
sedimentation distance and thus the maximum sedimentation velocity, which can be accessed for a given
time resolution. The optical path affects the overall turbidity of the sample and thus restricts the particle
concentration, at which the measurements must be run. In some cases, instruments employ different cells
(cell heights, path lengths, and material) to increase flexibility regarding measurable samples. In addition,
it also sets an upper limit for particle size because the photometric principle of particle quantification relies
on a large number of pa
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