ISO 13319:2007

Determination of particle size distributions - Electrical sensing zone method

ISO 13319:2007 gives guidance on the measurement of the size distribution of particles dispersed in an electrolyte solution using the electrical sensing zone method. The method measures pulse heights and their relationship to particle volumes or diameters, and it reports in the range from approximately 0,4 micrometres to approximately 1 200 micrometres. It does not address the specific requirements of the measurement of specific materials. However, guidance on the measurements of conducting materials such as porous materials and metal powders is given.

Détermination des répartitions granulométriques — Méthode de la zone de détection électrique

General Information

• Status: Withdrawn
• Publication Date: 01-Jul-2007
• Withdrawal Date: 01-Jul-2007
• ICS: 19.120 - Particle size analysis. Sieving
• Technical Committee: ISO/TC 24/SC 4 - Particle characterization
• Drafting Committee: ISO/TC 24/SC 4/WG 5 - Liquid displacement methods
• Current Stage: 9599 - Withdrawal of International Standard
• Start Date: 18-Mar-2021
• Completion Date: 13-Dec-2025

Relations

• Revised: ISO 13319-1:2021 - Determination of particle size distribution - Electrical sensing zone method - Part 1: Aperture/orifice tube method
• Effective Date: 10-Dec-2016
• Revises: ISO 13319:2000 - Determination of particle size distributions - Electrical sensing zone method
• Effective Date: 15-Apr-2008

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 13319
Second edition
2007-07-01
Corrected version
2007-09-01
Determination of particle size
distributions — Electrical sensing zone
method
Détermination des répartitions granulométriques — Méthode de la zone
de détection électrique
Reference number
©
ISO 2007
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©  ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
1 Scope .1
2 Normative references .1
3 Terms and definitions .1
4 Symbols .2
5 Principle.3
6 General operation .4
6.1 Response.4
6.2 Size limits .4
6.3 Effect of coincident particle passage .4
6.4 Dead time.5
7 Repeatability of counts .6
8 Operational procedures .6
8.1 Instrument location.6
8.2 Linearity of the aperture/amplifier system .6
8.3 Linearity of the counting system .7
8.4 Choice of electrolyte solution .7
8.5 Preparation of electrolyte solution .7
8.6 Recommended sampling, sample splitting, sample preparation and dispersion .8
8.7 Choice of aperture(s) and analysis volume(s).9
8.8 Clearing an aperture blockage .10
8.9 Stability of dispersion .10
8.10 Calibration .11
9 Analysis .11
10 Calculation of results .12
11 Instrument qualification .12
11.1 General.12
11.2 Report .12
Annex A (informative) Calibration for the measurement of porous and conductive particles .13
Annex B (informative) Technique using two (or more) apertures.16
Annex C (informative) Chi-squared test of the correctness of instrument operation or sample
preparation .18
Annex D (informative) Table of materials and electrolyte solutions.20
Annex E (informative) Mass integration method.30
Annex F (informative) Calibration and control of frequently used apertures .36
Bibliography .37

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 2.
The main task of technical committees is to prepare International Standards. 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.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 13319 was prepared by Technical Committee ISO/TC 24, Sieves, sieving, and other sizing methods,
Subcommittee SC 4, Sizing by methods other than sieving.
This second edition cancels and replaces the first edition (ISO 13319:2000), which has been technically
revised.
This corrected version of ISO 13319:2007 incorporates the following corrections:
[13] [11], [12]
— in 8.10.1, paragraph 2, line 2, “ ” has been deleted, and “ ” inserted;
— in E.2.4, the factor “K ” has been added to the right hand side of Equation (E.1).
d a
iv © ISO 2007 – All rights reserved

INTERNATIONAL STANDARD ISO 13319:2007(E)

Determination of particle size distributions — Electrical sensing
zone method
1 Scope
This International Standard gives guidance on the measurement of the size distribution of particles dispersed
in an electrolyte solution using the electrical sensing zone method. The method measures pulse heights and
their relationship to particle volumes or diameters, and it reports in the range from approximately 0,4 µm to
approximately 1 200 µm. It does not address the specific requirements of the measurement of specific
materials. However, guidance on the measurements of conducting materials such as porous materials and
metal powders is given.
2 Normative references
The following referenced documents are indispensable for the application 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 787-10, General methods of test for pigments and extenders — Part 10: Determination of density —
Pyknometer method
ISO 9276-2:2001, Representation of results of particle size analysis — Part 2: Calculation of average
particle sizes/diameters and moments from particle size distributions
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
dead time
time during which the electronics are not able to detect particles due to the signal processing of a previous
pulse
3.2
aperture
small-diameter hole through which suspension is drawn
3.3
sensing zone
volume of electrolyte solution within, and around, the aperture in which a particle is detected
3.3
sampling volume
volume of suspension that is analysed
3.4
channel
size interval
3.5
envelope size
external size of a particle as seen in a microscope
3.6
envelope volume
volume of the envelope given by the three-dimensional boundary of the particle to the surrounding medium
4 Symbols
For the purposes of this document, the following symbols apply.
A amplitude of the most frequent pulse
p
A amplitude of the electrical pulse generated by an arbitrary particle
x
d modal diameter of a certified particle size reference material
p
d mean particle diameter of a size interval or channel
d particle diameter at the lower boundary of a size interval or channel
L
d particle diameter at the upper boundary of a size interval or channel
U
D aperture diameter
K calibration constant of diameter
d
K mean calibration constant of diameter
d
K arbitrary calibration constant of diameter
d a
m mass of sample
∆N number of counts in a size interval i
i
V volume of electrolyte solution in which a mass, m, is dispersed
T
V analysis volume
m
V arithmetic mean volume for a particular size interval i
i
V volume of the particle obtained from a threshold or channel boundary
i
x diameter of a sphere with volume equivalent to that of the particle
xx,,x values of x corresponding to the 50 %, 10 % and 90 % points of the cumulative percent
50 10 90
undersize distributions
ρ mass of the particles per volume of the electrolyte displaced
σ standard deviation of mean calibration constant
K
d
2 © ISO 2007 – All rights reserved

5 Principle
A dilute suspension of particles dispersed in an electrolyte solution is stirred to provide a homogeneous
mixture and is drawn through an aperture in an insulating wall. A current applied across two electrodes,
placed on each side of the aperture, enables the particles to be sensed by the electrical impedance changes
as they pass through the aperture. The particle pulses thus generated are amplified and counted, and the
pulse height is analysed. After employing a calibration factor, a distribution of the number of particles against
the volume-equivalent diameter is obtained. This distribution is usually converted to percentage by mass
versus particle size, where the size parameter is expressed as the diameter of a sphere of volume and density
equal to that of the particle. See Figure 1.

Key
1 volumetric metering device 7 output
2 valve 8 stirred suspension of particles in electrolyte solution
3 pulse amplifier 9 aperture
4 oscilloscope pulse display 10 counter start/stop triggered by the volumetric device
5 counting circuit 11 electrodes
6 pulse-height analyser
Figure 1 — Diagram illustrating the principle of the electrical sensing zone method
6 General operation
6.1 Response
The response (i.e. the electrical pulse generated when a particle passes through the aperture) has been found
both experimentally and theoretically to be proportional to the particle volume if the particles are
[1]-[3]
spherical .This has also been shown to be true for particles of other shapes; however, the constant of
[4]
proportionality (i.e. the instrument’s calibration constant) may be different . In general, particles should have
a low conductivity with respect to the electrolyte solution, but particles with high conductivity can be measured
[5] [6] [7], [8]
e.g. metals , carbon , silicon and many types of cells and organisms, such as blood cells . For porous
[9], [10]
particles, the response may vary with the porosity . Recommendations for the measurement of
conducting particles and porous particles are given in Annex A.
As the response is proportional to the volume of particles, the pulse amplitude provides a relative scale of
particle volumes. By calibration, this scale may be converted to spherical diameter. The calibration constant
based on diameter may be calculated by Equation (1):
d
p
K = (1)
d
A
p
The size, x, of any particle can be calculated by Equation (2):
xK=⋅ A (2)
dx
6.2 Size limits
The lower size limit of the electrical sensing zone method is generally considered to be restricted only by
thermal and electronic noise. It is normally stated to be about 0,6 µm but, under favourable conditions, 0,4 µm
is possible. There is no theoretical upper size limit, and for particles having a density similar to that of the
electrolyte solution, the largest aperture available (normally 2 000 µm) may be used. The practical upper size
limit is about 1 200 µm, limited by particle density. In order to increase the possibility of keeping the particles
in homogeneous suspension, the viscosity and the density of the electrolyte solution may be increased, for
example by addition of glycerol or sucrose. The homogeneity may be checked by repeated analyses at a
range of stirrer speeds. The results of this should be compared to establish the lowest speed at which
recovery of the largest particles is maintained.
The size range for a single aperture is related to the aperture diameter, D. The response has been found to
depend linearly in volume on D, within about 5 % under optimum conditions, over a range from 0,015 D to
0,8 D (i.e. 1,5 µm to 80 µm for a 100 µm aperture) although the aperture may become prone to blockage at
particle sizes below the maximum size where the particles are non-spherical. In practice, the limitation of
thermal and electronic noise and the physical limitation of non-spherical particles passing through the aperture
usually restricts the operating range to 2 % to 60 % of the aperture size. This size range can be extended by
using two or more apertures (see Annex B). In practice, this procedure can be avoided by the careful selection
of the diameter of one aperture, to achieve an acceptable range.
6.3 Effect of coincident particle passage
Ideal data would result if particles traversed the aperture singly, when each particle would produce a single
pulse. When two or more particles arrive in the sensing zone together, the resulting pulse will be complex.
Either a single large pulse will be obtained, resulting in a loss of count and effectively registering a single
larger particle, or the count will be correct but the reported size of each will be increased, or some particles will
not be counted. These effects will distort the size distribution obtained but can be minimized by using low
concentrations. Table 1 shows counts per millilitre for the coincidence probability to be 5 %.
4 © ISO 2007 – All rights reserved

Table 1 — Counts for 5 % coincidence probability for typical aperture diameters
Aperture diameter Analysis Count for 5 %
a b
D
volume coincidence
V N
m
µm
ml
1 000 2 80
560 2 455
400 2 1 250
280 2 3 645
200 2 10 000
140 2 29 150
100 0,5 20 000
70 0,5 58 500
50 0,05 16 000
30 0,05 74 000
20 0,05 250 000
a
For other sampling volumes, use pro rata values.
41× 0 V
m
b
Calculated using the equation N =
D
Counts per millilitre should always be less than these quoted values. Since particle size distributions should
not be a function of concentration, the effect of coincidence can be tested by obtaining a distribution at one
concentration and comparing it with that obtained when the concentration is halved. In such a test, repeat
such dilutions until the reduction in count in a channel with the largest number decreases in proportion to the
dilution. This should always be done when analysing very narrow size distributions, as this is where the effect
of coincidence is most noticeable.
6.4 Dead time
In instruments using digital pulse processing routines, to analyse the signal it is scanned at high frequency.
Information on pulse parameters, such as maximum pulse height, maximum pulse width, mid-pulse height,
mid-pulse width and pulse area, is stored for subsequent analysis. In this case, analog-to-digital conversion of
the pulse with storage of the size value for the pulse is not performed in real time and dead time losses are
avoided.
In instruments in which pulse-height analysis routines are used in real time to process the data, it is possible
that the analyser may not count particles for a given time after receiving a pulse, since it takes a finite time to
process each pulse. Dead time is not a function of pulse height. Therefore, the loss will be proportional to the
counts in each channel and will not affect the size distribution.
To minimize the effect of dead time, the analyser should be used with the lower threshold set to exclude
thermal and electronic noise, as indicated at A in Figure 2. Additionally, the concentration of particles should
be maintained below 5 % coincidence levels.
Key
X channels
Y counts
NOTE Counts at channels below A are noise counts. True particle counts are at the higher channels.
Figure 2 — Typical results
7 Repeatability of counts
In a correctly performed analysis, the number of counts in a size interval is a random variable which follows a
Poisson distribution. In this, the variance is equal to the expected (mean) value. This indicates that the
standard deviation of a number of counts, n, with mean, N, approximates to N . Both the variance and the
standard deviation can be used in statistical tests on the correctness of instrument operation or sample
preparation. The statistical chi-squared test can be used to test whether obtained data follow a Poisson
distribution or not. In this, the apparent and the theoretical variance for a given number of measurements and
a given probability are related. An example is given in Annex C. This statistical test can be performed on
single size intervals, groups of size intervals, or on the total particle count.
8 Operational procedures
8.1 Instrument location
The instrument should be sited in a clean environment that is free from electrical interference and vibration. If
organic solvents are to be used, the area should be well ventilated.
8.2 Linearity of the aperture/amplifier system
The linearity of the aperture/amplifier system can be checked using four materials consisting of near mono-
sized particles with a certified modal diameter. In a suitable electrolyte solution, the instrument is calibrated
with particles at about 0,3 D (see 8.10.2). Three further sizes of particles are then added to the suspension,
one of size of about 0,1 D, one of size of about 0,2, D and one of size about 0,5 D. The suspension is re-
analysed and the size corresponding to these extra peaks must correspond to the quoted size of the particles
to within 5 %.
6 © ISO 2007 – All rights reserved

8.3 Linearity of the counting system
The linearity of the counting system can be tested by obtaining three counts at an arbitrary concentration. The
concentration is then reduced and three further counts obtained. Coincidence-corrected counts shall be used.
The ratio of the mean of the counts should be the same as the dilution. If the agreement is not within 5 %, the
test should be repeated comparing the two lowest dilutions. Subsequent analyses should be carried out at the
dilution giving the best results.
8.4 Choice of electrolyte solution
8.4.1 General
An electrolyte solution should be selected in which the sample is stable. The electrolyte solution should not
dissolve, flocculate, react or, once a good dispersion is achieved, not change the state of dispersion of the
sample in the measurement time, typically up to five minutes. Particles insoluble in water can be analyzed in a
variety of aqueous electrolyte solutions. Particles soluble in water can often be analyzed in methanol or in Iso-
propanol. See Annex D for recommended electrolyte solutions. When using small apertures (20 µm, 30 µm
and 50 µm) or large apertures (400 µm, 560 µm, 1 000 µm and 2 000 µm), special care shall be taken due to
their particular characteristics.
8.4.2 Special considerations for small apertures (D u 50 µm)
Where possible, the electrolyte solution should consist of a 4 % sodium chloride solution or one of equivalent
conductivity. It should be membrane filtered twice at 0,2 µm.
8.4.3 Special considerations for large apertures (D W 400 µm)
To prevent turbulence that can cause noise signals due to fast flow through the aperture, the viscosity of the
electrolyte solution may be increased by the addition of glucose or glycerol; 10 % glycerol is recommended for
560 µm and 400 µm apertures, and 30 % glycerol for the 2 000 µm and 1 000 µm apertures.
8.5 Preparation of electrolyte solution
An electrolyte solution should be well filtered with a membrane filter for which the pore size is less than the
diameter of the smallest particle measured, as it is essential that its background count should be as low as
practicable. It should be noted that quoted values for filters are not absolute. Usually a mean pore size is
given. The width of the distribution of pores around this mean varies depending on filter type and
manufacturer. This will affect the choice of filter size used. All glassware and apparatus used should be pre-
rinsed with filtered electrolyte solution or other suitable liquids. Background counts should not exceed the
values given in Table 2 or yield a total equivalent volume in excess of 0,1 % of the total volume of particles
subsequently measured in the analysis volume.
Table 2 — Counts for background for typical aperture diameters
b
Aperture diameter Analysis
Background counts
a
D
volume
V
m
µm ml
1 000 2 2
560 2 10
400 2 25
280 2 75
200 2 200
140 2 600
100 0,5 400
70 0,5 1 200
50 0,05 300
30 0,05 1 500
20 0,05 5 000
a
For other analysis volumes, use pro rata values.

b
Suggested maximum counts.
8.6 Recommended sampling, sample splitting, sample preparation and dispersion
8.6.1 General
See ISO 14488 for guidance on the sampling and sample-splitting procedure. Select a dispersant and a
dispersion method from the recommendations in ISO 14887 or Annex D. The expertise of the laboratory
performing the analysis with respect to the sample under test may also be utilized.
8.6.2 Method 1: Using a paste
The sample should be subdivided to about 0,2 cm . If the sample is in the form of a powder, it should be
worked and kneaded gently with a flexible spatula with a few drops of suitable dispersant to break down
agglomerates. Transfer a mass of about 20 mg to 50 mg of the paste to a round-bottomed beaker and thin it
with dispersant, followed by a few drops of electrolyte solution. Almost fill the beaker with electrolyte solution
and place it in an ultrasonic bath with suitable power and frequency for 1 min, stirring occasionally. A stop
watch is recommended for a reproducible dispersion technique. One suitable design of beaker of 400 ml
capacity with a baffle is shown in Figure 3. If the sample is not required to be fully dispersed, it may be added
to the electrolyte solution and dispersant while stirring.
NOTE The use of high energy ultrasonic baths and probes, blenders and mixers can cause both agglomeration and
fracture of particles.
8.6.3 Method 2: Alternative method applicable to low-density particles of less than 50 µm
Subdivide the sample into portions of about 1 g. Mix a portion with the dispersant and add it to the electrolyte
solution. Then place the beaker (see Figure 3) containing the suspension in an ultrasonic bath for about 45 s.
After stirring this stock suspension well, withdraw 5 ml using a pipette and add to approximately 400 ml of
electrolyte solution. Place in the ultrasonic bath for a further 15 s. When using this method, it is important that
at least two samples are withdrawn from the stock suspension and analysed to ensure repeatability of the
aliquot sampling and the analysis.
8 © ISO 2007 – All rights reserved

Key
1 aperture tube
2 stirrer
3 baffle
Figure 3 — Example of a beaker with baffle and stirrer
8.6.4 Suspensions and emulsions
Suspensions and emulsions should be diluted by addition of smaller volumes of diluent to the emulsion, not by
addition of the emulsion to a larger volume of diluent. Dilution should be performed stepwise with mixing
performed at each step. To avoid “dilution shock”, oil-in-water emulsions may be initially diluted with distilled
or de-ionized water.
8.6.5 Verification of the dispersion
A small sample of the dispersion may be placed on a microscope slide and used to verify the degree of
dispersion and to estimate the size range of the particles using an optical microscope.
8.7 Choice of aperture(s) and analysis volume(s)
From the microscope examination (8.6.5), estimate the diameter of the largest particles present. Choose an
aperture for the size analysis such that the diameter of the largest particles to be analysed is less than
approximately 60 % of the diameter of the aperture, selected to reduce the possibility of blocking the aperture.
For particles that are spherical or nearly spherical, an aperture such that the diameter of the largest particles
is less than 80 % of the diameter of the aperture may be chosen. If there is a considerable proportion of
sample below the lower size limit of that aperture (1,5 % of its diameter), a second and possibly a third smaller
aperture will be needed (see Annex B). An alternative method to determine the amount of particles not
accounted for with a specific aperture is to perform a mass balance (see Annex E).
Select a suitable analysis volume with reference to Table 1 or select a suitable time of accumulation. It may be
necessary to analyse a number of analysis volumes or to accumulate for a long time to obtain an acceptable
precision, e.g. 50 000 particles will yield a precision (relative standard deviation) of 0,4 %. Counting fewer
particles will reduce the precision, but this may be necessary when using the larger apertures (see Clause 7
and Annex C).
8.8 Clearing an aperture blockage
Apertures below 100 µm in diameter may become blocked with extraneous particles, particularly if care is not
exercised in the clean handling, careful filtration and thorough rinsing of beakers and associated equipment. A
blockage or a partial blockage can be seen by means of the viewing optics provided with the analyser. A
blockage may also be indicated by measuring the flow time through the aperture or by measuring the
electrical resistance of the aperture. A blockage will cause a longer flow time and a higher resistance. A
blockage can also be revealed by an examination of the particle pulse train, which is recorded with some
instruments. A blockage will cause a clearly visible shift in the pulse train. In some instruments, there are
means to automatically detect and remove blockages. Blockages can also be removed by one of the following
techniques.
a) Back flushing: Reversing the flow through the aperture may be sufficient to clear a blockage.
b) Boiling: It is possible to use the heating effect of the current to boil the blockage out. This is done by using
a high aperture current.
c) Brushing: It is often possible to brush the particles off the aperture by using a small high-quality soft-hair
brush with the hairs cut short. Care should be taken not to damage the aperture.
d) Air pressure.
e) Ultrasonic cleaning: With the aperture tube filled with electrolyte solution, the end is dipped into a low-
power ultrasonic bath for about 1 s. Repeat this operation as necessary. This method is very effective but
extreme care should be taken as it is possible to damage the aperture.
CAUTION — This method should not be used for apertures of 50 µm or less.
8.9 Stability of dispersion
With the most suitable aperture fitted, and the suspension prepared, dry the outside of the beaker and place it
on the sample stand of the instrument. Adjust the stirrer for maximum effect without creating a vortex which
will entrain bubbles. Then check the stability of the dispersion during the analysis time. Make a full size
distribution analysis as soon as possible after dispersion; then stir the dispersion for 5 min to 10 min and then
reanalyse. Cumulative counts are recorded at size levels close to 30 % and 5 % of the aperture diameter
(denoted as x and x respectively). Changes in the counts greater than those expected from statistics will
max min
indicate that the dispersion is not stable (see Clause 7 and Annex C). Additional verification of stability can
also be performed in instruments that record raw pulse data. Inhomogeneity across the pulse train during the
time of analysis may indicate a change in the stability of dispersion. Table 3 details some possible causes.
Table 3 — Examples of suspected phenomena in dispersion
Change in count at
Suggests
x x
max min
No change no change stable dispersion
Increase increase crystallization, precipitation
Decrease decrease dissolution
Decrease increase size reduction, deflocculation
Increase decrease flocculation, agglomeration
Decrease no change settling of large particles

10 © ISO 2007 – All rights reserved

8.10 Calibration
8.10.1 General
Electrical sensing zone instruments are calibrated using polymer latex microspheres of known size and
narrow size distribution.
Another method, which is an absolute method, is the mass integration method. Here the weighed mass is
[11], [12]
compared to the mass found as determined by the instrument . This calibration method is directly
traceable and there is no assumption made about the shape, porosity or electric conductivity of the particles.
The mass integration method is described in Annex E.
Special care shall be taken for porous particles. Such particles may have an interconnected pore system
which, at least partly, is being filled with electrolyte solution during the sample preparation procedure. This
electrolyte solution will, to a certain extent, not contribute to the impedance change in the sensing zone when
the particle passes through it. Therefore, a porous particle generates a pulse with lower amplitude than a solid
particle of equivalent envelope volume. The difference is not negligible; for some porous materials the size
can be as little as half that of the envelope size. For the calibration for the measurement of porous particles,
see Annex A.
8.10.2 Calibration procedure — Microsphere calibration
Microspheres with narrow size distribution with a single mode, characterized by a variety of other methods,
are available. They should be characterized traceably to the metre, to a Community Bureau of Reference
reference material (BCR), a National Institute of Standards and Technology reference material (NIST) or
similar reference material. The calibration method used depends on the assayed size parameter of the
microspheres and the analyser used (contact the instrument manufacturers for details). One method is to
obtain a histogram (frequency) plot of the number of particles against channels of equal width (on a linear
scale). The size at the centre of the channel with the greatest number of particles corresponds closely to the
modal size of the calibration material if the distribution is symmetrical. If the distribution is not symmetrical, the
fractional channel position is calculated from the counts in channels on either side of the central channel. The
calibration factor is the ratio of the modal size of the calibration material to the size reported on the instrument.
Calibration should be checked on a regular basis to ensure that the change in the calibration constant is less
than 1,0 %, or every time an aperture tube or an electrolyte solution is changed. See Annex F for a method for
calibration of frequently used apertures.
9 Analysis
Most powders have a particle size range that is sufficiently narrow for a satisfactory analysis to be carried out
using one aperture. Where the size range of a powder is too wide for a single aperture, two or more apertures
should be used. If over 1,5 % by volume of the particles fall in the smallest size interval, it is advisable to use
the multiple aperture method (see Annex B). For certain sample materials, a mass balance may also be
performed (see Annex E).
When the particles are dispersed satisfactorily, following the foregoing procedures, the analysis can begin.
Select the analysis volume, or the number of repeat measurements of the analysis volume, or the time for
accumulation, in such a way that a particle size distribution with sufficient precision is obtained (see Clause 7
and Annex C). Counting fewer particles will reduce the precision, but this may be necessary when using the
larger aperture tubes. It is advisable that at least three, and preferably five, replicates be measured. To ensure
that the sample subdivision has been carried out well, the whole procedure should be repeated with at least
one other, but preferably more, sample(s) from the stock suspension or from the dry powder subdivision.
Report all the measured data on a suitable data sheet.
10 Calculation of results
Modern instruments measure the volume and the number of particles within various size channels directly, so
no data conversion is needed. Some analysers count the number of particles above, or between, variable
equivalent-volume particle diameters, and therefore conversion of data to volume percentage may be
required. In the event of requiring number data to be converted to, and presented as, volume data, it is usual
[13]
for the method of Simpson’s rule to be used. Since the volume of each particle is measured, the numbers
of particles within a size interval (size channel) can be multiplied by the arithmetic mean volume of the
channel in order to present the total particle volume within the channel. In this way, the total volume of all
particles within all size channels can be calculated, and the percentage by volume size distribution calculated.
For the calculation to be reasonably accurate, the size interval should be narrow, i.e. a large number of
channels should be counted. For a more accurate method and the calculation of moments of the distribution,
see ISO 9276-2. Modern analysers perform the calculation automatically. The volume-percentage distribution
so calculated is identical to the mass (or weight) distribution if all the particles have the same specific gravity
(immersed density).
11 Instrument qualification
11.1 General
The instrument is qualified through the verification of the linearity of the aperture/amplifier system (see 8.2),
verification of the linearity of the counting system (see 8.3) and the verification of the calibration constant (see
Annex F).
11.2 Report
It is essential that the whole qualification procedure and all data be reported in full detail on a suitable data
sheet.
12 © ISO 2007 – All rights reserved

Annex A
(informative)
Calibration for the measurement of porous and conductive particles
A.1 General
Particles of conductive materials (e.g. metal particles) can be accurately measured by the electrical sensing
zone method, provided that the voltage applied across the aperture does not prohibit the formation of a
[4]
surface layer, the so-called Helmholz layer. This voltage may typically be 10 V to 15 V .
Porous particles have a pore structure which may be filled with electrolyte solution during the sample
preparation procedure. This electrolyte solution will, at least to a certain extent, not contribute to the
impedance change in the sensing zone of the aperture when the particle passes through it. Therefore, a
porous particle generates a pulse with lower amplitude than a solid particle of equivalent envelope volume. If
the calibration of the instrument is done with solid particles and no correction is carried out for the effect of the
porosity, the measured size will be too small (see Figure A.1). A technique to solve this problem is to fill the
pores either with an organic substance that is solid at room temperature or with a solvent that is immiscible
[14], [15]
with water . However, this technique cannot be used with porous particles made of natural polymers
because they change size in organic solvents. Porous particles can be accurately measured after calibration
of the measuring instrument using the mean size of a narrowly sized fraction of the material under
investigation as described in A.3.

Key
1 porous particle
2 reported size with correction for the porosity (envelope size)
3 reported size without correction for the porosity
Figure A.1 — Diagram illustrating the response of a porous particle
A.2 Particles of conductive materials
To obtain acceptable results for conductive particles, such as metal particles, a distribution is obtained under
normal conditions. The analysis is then repeated using half the current and twice the gain. The distributions
should be the same. If they are not, the procedure should be repeated using an even lower current. Some
metal particles are highly conductive (e.g. copper, silver and platinum). These particles do not easily form
surface layers but they can be correctly sized if a very low voltage is applied and the barrier increased by
adding a 0,5 % solution of Cetrimide.
A.3 Porous particles
A.3.1 General
In order to compensate for the effects of the porosity, the scaling of pulse height is done with the material
under test. A narrow size-range fraction of that material is prepared by sieving or a similar separation method.
The mean diameter is then determined by computer-aided microscopy, so-called image analysis, or by
measurements on photographs. Finally the material is analysed by the electrical sensing zone (ESZ)
[9], [10]
instrument and the known mean diameter is used for calibration of the instrument .
A.3.2 Sample preparation
In the case of sieving, the sieves should have openings as close as possible to the modal diameter of the
sample (e.g. 5 µm to 10 µm on each side). Wet sieving with electro-formed sieves is preferred. The sieving
fluid may be chosen according to the expertise of the laboratory to fit the actual material as well as possible.
The size of porous materials may vary with the ion strength of the suspension medium. Therefore, the fraction
shall be transferred to the electrolyte solution in which the size is to be expressed. The sample is then allowed
to stand overnight or treated in an ultrasonic bath in order to substitute the liquid inside the particle for
electrolyte solution.
A.3.3 Microscopy and ESZ measurements
An aliquot of the suspension of the sieved fraction is transferred to a microscope slide and covered with a
cover glass. Then photographs are taken, or the particle size is measured directly with an image analyser. It is
[9]
important that the sample is not allowed to dry. At least 400 particles should be measured . The number of
particles to be measured can also be calculated using the procedure described in ISO 13322-1. The modal
diameter of the number distribution should be the preferred size parameter for calibration, but the median
diameter either in number or in volume distribution may also be used. Finally, the particle size is measured
with the ESZ instrument which has been calibrated with microspheres.
A.4 Calculations for microscopy
A.4.1 General
The mean diameters of the microspheres and the sieved fraction are used to calculate a response factor
which can be used for future calibrations.
A.4.2 Symbols
d certified mean diameter of the microspheres used for primary calibration
m
d mean diameter of the sieved fraction as determined using microscopy
micr
d mean diameter of the sieved fraction as determined using the ESZ instrument
ESZ
14 © ISO 2007 – All rights reserved

d reference diameter of the microspheres
ref
f response factor
resp
A.4.3 Calculations
The mean diameters are used to calculate a response factor
d
micr
f = (A.1)
resp
d
ESZ
The microspheres may then be given a reference diameter
df=⋅d (A.2)
ref resp m
This reference diameter may then be used for future calibrations.
Annex B
(informative)
Technique using two (or more) apertures
B.1 General
When it is necessary to use more than one aperture to obtain a complete analysis, it is necessary to divide the
suspension into two or more fractions of a suitable particle size that can be measured by each aperture. This
is usually accomplished by wet sieving.
B.2 Separation
After analysis with the larger aperture, the beaker and the remaining suspension are weighed and the
suspension passed through an electroformed o
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