ISO 20998-1:2006

INTERNATIONAL ISO
STANDARD 20998-1
First edition
2006-08-01

Measurement and characterization of
particles by acoustic methods —
Part 1:
Concepts and procedures in ultrasonic
attenuation spectroscopy
Mesurage et caractérisation des particules par des méthodes
acoustiques —
Partie 1: Concepts et modes opératoires en spectroscopie d'atténuation
ultrasonique



Reference number
ISO 20998-1:2006(E)
©
ISO 2006

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ISO 20998-1:2006(E)
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ISO 20998-1:2006(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Terms and definitions. 1
3 Sampling and reference materials . 3
3.1 Sampling considerations . 3
3.2 Reference materials. 4
4 Methods . 4
4.1 Principles. 4
4.2 Apparatus . 5
4.3 Preparation . 6
4.4 Measurement. 8
4.5 Interpretation of measurement data . 9
5 Reporting of results. 10
5.1 Reports for laboratory testing . 10
5.2 Results for in-process and control applications . 10
Annex A (informative) Techniques . 11
Annex B (informative) Application examples . 17
Annex C (informative) Inversion of attenuation spectrum. 18
Bibliography . 20

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ISO 20998-1:2006(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 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 20998-1 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other sizing methods,
Subcommittee SC 4, Sizing by methods other than sieving.
ISO 20998 consists of the following parts, under the general title Measurement and characterization of
particles by acoustic methods:
⎯ Part 1: Concepts and procedures in ultrasonic attenuation spectroscopy
The following parts are under preparation:
⎯ Part 2: Guidelines for linear theory
⎯ Part 3: Guidelines for non-linear theory
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ISO 20998-1:2006(E)
Introduction
It is well known that ultrasonic spectroscopy can be used to measure particle size distribution (PSD) in colloids,
[6][7][8][9]
dispersions, and emulsions (see ). The basic concept is to measure the frequency-dependent
attenuation or velocity of the ultrasound as it passes through the sample. This attenuation includes
contributions due to scattering or absorption by particles in the sample, and the size distribution and
[10][11][12]
concentration of dispersed material determines the attenuation spectrum (see ). Once this
connection is established by empirical observation or by theoretical calculations, one can in principle estimate
the PSD from the ultrasonic data. Ultrasonic techniques are useful for dynamic on-line measurements in
concentrated slurries and emulsions. Traditionally, such measurements have been made off-line in a quality
control laboratory, and constraints imposed by the instrumentation have required the use of diluted samples.
By making in-process ultrasonic measurements at full concentration, one does not risk altering the dispersion
state of the sample. In addition, dynamic processes (such as flocculation, dispersion, and comminution) can
[13]
be observed directly in real time (see ). This data can be used in process control schemes to improve both
the manufacturing process and the product performance.
ISO 20998 consists of three parts:
⎯ Part 1 introduces the terminology, concepts and procedures for measuring ultrasonic attenuation spectra;
⎯ Part 2 provides guidelines for determining particle size information from the measured spectra for cases
where the spectrum is a linear function of the particle volume fraction;
⎯ Part 3 addresses the determination of particle size for cases where the spectrum is not a linear function of
volume fraction.

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INTERNATIONAL STANDARD ISO 20998-1:2006(E)

Measurement and characterization of particles by acoustic
methods —
Part 1:
Concepts and procedures in ultrasonic attenuation
spectroscopy
1 Scope
This part of ISO 20998 describes ultrasonic methods for determining the size distributions of one or more
material phases dispersed in a liquid. Colloids, dispersions, slurries and emulsions are within the scope of this
part of ISO 20998. The typical particle size for such analysis ranges from 10 nm to 3 mm, although particles
outside this range have also been successfully measured. Measurements can be made for concentrations of
the dispersed phase ranging from 0,1 % by volume up to 50 % or more by volume, depending on the density
contrast between the continuous and the dispersed phases. These methods can be used to monitor dynamic
changes in the size distribution, including agglomeration or flocculation in concentrated systems.
2 Terms and definitions
For the purposes of this document, the following terms apply:
2.1
absorption
direct reduction of incident ultrasonic energy by means other than scattering
2.2
attenuation
extinction
total reduction of incident ultrasonic energy, including both scattering and absorption.
NOTE The recommended measurement unit is the decibel (dB), which is defined as 10 times the common (base 10)
logarithm of the ratio of incident intensity to transmitted intensity, or equivalently 20 times the common logarithm of the
ratio of incident amplitude to transmitted amplitude. The neper (Np) is a permitted alternative measurement unit based on
the natural logarithm, rather than the common logarithm. The conversion factor is 1 Np = 8,686 dB.
2.3
attenuation coefficient
extinction coefficient
attenuation (extinction) per unit length of ultrasonic propagation through a material, measured in units of
dB/cm or Np/cm.
NOTE Attenuation coefficients are sometimes scaled by frequency, or frequency-squared, to identify the dominant
attenuation mechanism. For clarity, in this part of ISO 20998, only the attenuation per unit length (in dB/cm) is considered.
2.4
attenuation spectrum
attenuation coefficient measured as a function of frequency
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ISO 20998-1:2006(E)
2.5
bandwidth
range of frequencies contained in an ultrasonic signal, typically measured as the frequency difference
between the -3 dB points on a spectrum analyser
2.6
broadband
characterized as having a bandwidth that is equal to at least half of the centre frequency
2.7
digitization
act of generating a digital (quantized) representation of a continuous signal
NOTE The number of bits determines the resolution (fidelity), and the sampling rate determines the bandwidth
(Nyquist criterion).
2.8
excess attenuation
incremental attenuation caused by the presence of particles in the continuous phase
2.9
Fourier transform
mathematical transform that converts a time-varying signal into its frequency components, which is often
implemented in computers as a Fast Fourier Transform (FFT) algorithm
2.10
interference
wave phenomenon of cancellation or enhancement observed when two or more waves overlap
2.11
intrinsic response
frequency-dependent response of the ultrasonic spectrometer itself
NOTE This is not to be confused with the intrinsic absorption of the sample component materials.
2.12
path length
distance traversed by the ultrasonic wave between the emitting transducer and the receiver
2.13
pulse
wave of sufficiently short duration to contain broadband Fourier components
2.14
reflection
return of an ultrasonic wave at an interface or surface
2.15
scattering
removal of ultrasonic energy from the incident wave by redirection
2.16
spectrum
frequency components of a signal, typically arranged as magnitude versus frequency
2.17
tone burst
short duration of a few cycles of a sinusoidal wave
NOTE Typically, a tone burst consists of 5 to 10 cycles of a sinusoidal wave.
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ISO 20998-1:2006(E)
2.18
transducer
device for generating ultrasound from an electrical signal or vice versa
NOTE Piezoelectric devices are commonly used for this purpose.
2.19
transmission
passage of ultrasound through a sample
2.20
transmission spectrum
transmission value measured as a function of frequency
2.21
transmission value
amplitude of an ultrasonic signal (or a component thereof) that has been transmitted through a sample,
measured in volts or arbitrary units
2.22
ultrasound
high frequency (over 20 kHz) sound waves which propagate through fluids and solids
NOTE The range employed in particle characterization is typically 100 kHz to 100 MHz.
2.23
wave
fluctuation, e.g. pressure, shear or thermal, which propagates through a physical medium
2.24
waveform
shape of the wave when seen on an oscilloscope or digitized display
2.25
wavelength
length of a wave, determined by the distance between corresponding points on successive waves
3 Sampling and reference materials
3.1 Sampling considerations
3.1.1 Dry powders
It is necessary to disperse a dry powder in a liquid before measuring the ultrasonic attenuation spectrum.
A representative sample of the powder shall be used in the preparation of the liquid dispersion. It is
recommended that sampling procedures be carried out in accordance with ISO 14488. Dispersion of the
powder should be carried out in accordance with ISO 14887.
3.1.2 Suspensions and slurries
The apparent particle size in flocculated or poorly-dispersed systems changes as a function of the applied
shear stress. Therefore, unless floc size or quality of a suspension is to be measured, it is recommended that
suspension and slurries be mixed thoroughly before a sample is withdrawn for ultrasonic analysis. The
stability of the suspension impacts the results.
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ISO 20998-1:2006(E)
3.1.3 Emulsions
Many phenomena affect the homogeneity of emulsions, including creaming, droplet coalescence and phase
separation. These changes affect the observed ultrasonic attenuation spectrum. If the initial droplet size
distribution is to be measured for unstable emulsions, it is recommended that the sample be prepared
immediately before the measurement.
3.2 Reference materials
3.2.1 Reference liquid
The use of a reference liquid is required in order to verify correct operation of the ultrasonic spectrometer itself.
[14]
De-gassed clean water at ambient temperature has a relatively low attenuation coefficient (see ). Water is
therefore recommended as a reference liquid for determining the intrinsic response of the spectrometer.
A procedure for de-gassing water is given in IEC 62127-1.
3.2.2 Reference sample
The use of a reference sample is recommended to verify the correct estimation of particle size distribution
from the observed attenuation spectrum, but no reference material has yet been identified for general use.
The user should identify a well-characterized and stable material as a standard sample for monitoring
variability in the size distribution results.
4 Methods
4.1 Principles
As ultrasound passes through a suspension, slurry, colloid or emulsion, it is scattered and absorbed by the
discrete phase, with the result that the intensity of the transmitted sound is diminished. The attenuation
coefficient is a function of ultrasonic frequency and depends on the composition and physical state of the
particulate system. The measurement of the attenuation spectrum can be used to estimate the particle size
distribution and concentration. The necessary apparatus is described in 4.2.
The total measured attenuation is due to the intrinsic absorption of the continuous liquid phase, the intrinsic
[6][7]
absorption of the dispersed phase(s), thermal losses, viscous losses and scattering losses (see ).
The relative importance of these loss mechanisms depends on the material system. A theoretical or empirical
model of these mechanisms can be used to convert the observed data into an estimate of the particle size or
particle size distribution. There is no single general procedure for determining particle size information from
the attenuation spectrum. Different models and procedures are used depending on the application and nature
of the sample, as described in ISO 20998-2.
The attenuation spectrum can be measured as long as the signal-to-noise ratio is adequate. However, an a
priori theoretical model may not exist due to lack of knowledge about the dispersed or continuous phases. In
cases where there is no suitable theoretical or empirical model available to describe the interaction of
ultrasound with the system of interest, the attenuation spectrum can still be used to infer relative changes in
[13]
particle size (see ).
A variety of techniques (see Annex A) have been used to measure ultrasonic spectra. Some of these methods
have been implemented in laboratory instruments and some have been used in industrial applications.
Ultrasonic spectroscopy has been used to measure particle size in a variety of material systems. Example
applications are listed in Annex B.
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ISO 20998-1:2006(E)
4.2 Apparatus
4.2.1 General
As a minimum, the spectrometer consists of an excitation source, one or more ultrasonic transducers, a
sample cell (or flow cell, in the case of in-process instruments), a preamplifier and a means for acquiring the
signal. Each of these components shall be tailored to fit the particular needs of the implementation.
4.2.2 Excitation source
The excitation source produces the electrical signal that is converted by the transducer into ultrasonic waves.
This circuit determines the frequency content of the resulting ultrasonic signal. This source may produce a
continuous wave at a single frequency, a frequency sweep, a set of tone bursts (which may be at various
frequencies), step pulses or broadband pulses. The frequency response (bandwidth) and electrical impedance
of the source should be matched to those of the transducer. The output signal level can range from a few volts
up to a few hundred volts, depending on the application and transducer type.
4.2.3 Transducers
Ultrasonic transducers convert the electrical signal from the excitation source into ultrasonic waves. The active
element within the transducer is typically made of a piezoelectric material (such as barium titanate) or a
piezoelectric polymer film (such as PVDF). When excited by an electrical signal, the piezoelectric element
constricts and relaxes, sending a longitudinal compression wave through the facing material and into the
dispersion. An acoustic delay line or buffer plate may be attached to the front of the transducer to protect it.
The construction of the transducer affects the frequency response. If the backing material heavily damps the
vibration of the element, the natural resonance will be de-tuned, giving a broadband response.
In the through-transmission method (see Annex A) a second transducer is used to detect the transmitted
ultrasonic waves and convert them into electrical signals. Due to the reciprocity theorem, the receiving
characteristics of a transducer are the same as the emission characteristics. In a pulse-echo method, the
same transducer is used to send and receive the ultrasonic pulses. In both arrangements, alignment of the
transducers is important, and typically becomes critical at frequencies of the order of 10 MHz and higher.
Alignment of the transducers is achieved by pivoting them relative to each other, so that the bandwidth and
signal strength of the ultrasonic signal are at a maximum. This action in effect aligns the directional patterns of
emission and reception, which can be skewed with respect to the mechanical axis of the transducers. If the
alignment is not correct, destructive interference at the edge of the receiver distorts the transmitted signal.
4.2.4 Sample cell
The sample cell is used to contain the dispersion and maintain the transducers in alignment with each other.
This element is optional, as some instruments are designed as a probe that is inserted directly into a process
vessel; in such cases, the probe body holds the transducers.
If a sample cell is used, the dispersion sample shall be circulated or stirred in order to prevent sedimentation.
An exception to this requirement can be made in the case of stable suspensions where particles do not settle.
For in-process applications, if a flow cell design is used in which the process fluid (i.e. dispersion) flows
through the cell, it is important to maintain an open bore all the way from inlet to outlet, in order to prevent
plugging.
4.2.5 Preamplifier
The preamplifier is an optional element that boosts the relatively weak signal detected by the receiving
transducer. Typically, this element will add 20 dB to 60 dB of gain to the signal. The bandwidth and input
impedance of the preamplifier shall meet the needs of the transducer.
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ISO 20998-1:2006(E)
4.2.6 Receiver (signal acquisition)
This required element measures the strength of the detected transmission signal. The design of the receiver
depends on the ultrasonic technique. For continuous wave signals, the receiver can be a tuned circuit that
measures the signal strength at that frequency. The output is connected to a low-speed digitizer to capture the
transmission signal. Swept-frequency and tone-burst techniques (see Annex A) also use a tuned receiver,
where the centre frequency is adjusted to track that of the ultrasonic signal. Pulsed systems use a high-speed
digitizer to capture the entire waveform of the received pulse. The digitization rate is typically chosen to be
several times the highest frequency component to be measured (10 MHz to 1 GHz, in common practice). The
waveform is then transformed into frequency components using a Fast Fourier Transform or other discrete
Fourier transform algorithm. The magnitude of the resulting complex array is the energy spectrum of the
transmitted pulse.
4.3 Preparation
4.3.1 Sample requirements
In order to achieve the closest possible correspondence between ultrasonic theory and experimental
observation, it is recommended that the sample be uniformly dispersed and free of air bubbles, which scatter
and attenuate sound. It is recommended that the sample be of relatively low viscosity (under 0,03 Pa·s) in
order to allow trapped air to escape. Enough sample shall be prepared to fill the sample cell completely. When
the sample is placed in the cell, care shall be taken to ensure that no bubbles are left clinging to the
transducer face.
For certain applications (such as monitoring the size of flocs), the sample need not be completely dispersed.
In some cases, it is possible to obtain useful data from systems that do contain bubbles, provided an
adequate ultrasonic signal is received. Trace amounts of very small, well-dispersed bubbles contribute to the
overall attenuation. It is reported that in some cases the frequency range can be chosen to distinguish the
[6][15]
ultrasonic resonance of air bubbles from attenuation due to scattering by particles (see ). Single, large
bubbles can disrupt the ultrasonic signal, but pulsed measurement systems, for example, can detect the loss
of data and compensate by taking more data.
4.3.2 Sample preparation
4.3.2.1 Laboratory applications (off-line applications)
4.3.2.1.1 Suspensions
To avoid multiple scattering and other non-linear concentration effects, an initial analysis sample with a solids
concentration of no more than 5 % by volume should be prepared. If this is not sufficient to give an adequate
attenuation signal, it may be necessary to increase the concentration. The concentration at which non-linearity
becomes an issue is determined by the density contrast between the solids and the suspending liquid, and by
the ratio of the wavelength to the diameter of the particles. In general, low density solids may be prepared at a
higher concentration. For dense or large particles, it is recommended that the sample be agitated or
recirculated to maintain the particles in suspension. It should be noted that poorly dispersed or flocculated
material will be seen as having a larger particle size, but this ability to monitor floc size has practical
applications. The actual dispersion method depends upon the sample material. In the case of powders,
dispersion should be carried out in accordance with ISO 14887.
Subsequent samples at higher or lower concentrations may be prepared as needed.
Slurries are suspensions with a high concentration (over 10 % by volume) of particles. Multiple scattering and
particle-particle interactions lead to non-linear effects in these samples, so the direct application of linear
theories may be inappropriate in this case. In such cases, useful data can still be obtained and interpreted on
the basis of empirical observation. The preparation of these slurries should minimize the amount of trapped air.
It is recommended that the slurry be agitated or recirculated to maintain the particles in suspension.
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ISO 20998-1:2006(E)
4.3.2.1.2 Emulsions
In the case of emulsions with droplets approximately 10 µm in diameter or larger, linear scattering theory
agrees with attenuation spectroscopy measurements even at high concentrations (50 % or more by volume)
[7][13]
of the dispersed phase (see ). For emulsions with droplets smaller than 1 µm, this agreement can break
down at lower concentrations (about 20 % by volume). The preparation of the sample ought to minimize the
amount of trapped air.
4.3.2.2 In-process applications and on-line applications
By definition, in-process applications of ultrasonic spectroscopy constantly examine the process stream, so
there is no sample to be prepared. However, it must be ensured that a representative sample is in the
measurement zone.
4.3.3 Temperature
Temperature affects the material properties of the suspension and consequently the transmission of the
[16]
ultrasonic waves (see ). It is therefore recommended that off-line measurements be conducted while
controlling the temperature of the cell. The temperature of the dispersion shall be determined and recorded
before and after analysis. If the difference is more than 2 °C, then the measurement shall be repeated. In-
process applications that reach a steady-state temperature are exempt from this requirement.
4.3.4 Transducer alignment
Transducers shall be mechanically aligned in order to optimize the transmission spectrum in both the time and
frequency domains. In the case of an adjustable path length, the transducer alignment shall be optimized to
produce the best transmission spectrum at the ends and midpoint of the transducer travel.
4.3.5 System check
Before starting a new round of measurements, laboratory spectroscopic systems shall be checked by
measuring the transmission spectrum at a predetermined path length for a suitable test sample. De-gassed
water may be used as a test sample. The measurement at each frequency (over the useful bandwidth of the
instrument) should agree to within 5 % of previous measurements on the same test sample.
If a particle size distribution is to be estimated on the basis of the ultrasonic attenuation spectrum, the system
check should include a PSD measurement of the standard sample identified in 3.2.2. The median size of the
particles should agree to within 5 % of previous measurements on the same standard.
Spectroscopic systems for in-process applications shall be checked before installation in the process.
4.3.6 Background measurement
Methods that use a fixed path length (i.e. fixed transducers) require measurement of the intrinsic frequency
response of the spectrometer in order to determine the attenuation. In such cases, a suitable calibration fluid
with low ultrasonic attenuation (such as water) is introduced into the cell and the transmission spectrum is
obtained. That spectrum is used as a background signal in subsequent calculations. The background
measurement may use the transmission spectrum generated during the system check.
In-process applications shall rely upon factory calibration of the spectrometer, unless it is possible to introduce
calibration fluid and measure the intrinsic response during preventive maintenance cycles.
Methods in which the acoustic path length is varied may not requir
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