ISO 20998-2:2022

INTERNATIONAL ISO
STANDARD 20998-2
Second edition
2022-08
Measurement and characterization of
particles by acoustic methods —
Part 2:
Linear theory
Caractérisation des particules par des méthodes acoustiques —
Partie 2: Théorie linéaire
Reference number
© ISO 2022
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviations .2
5 Mechanism of attenuation (dilute case) . 4
5.1 Introduction . 4
5.2 Excess attenuation coefficient . 4
5.3 Specific attenuation mechanisms . 5
5.3.1 Scattering . 5
5.3.2 Thermal losses . 5
5.3.3 Viscoinertial losses . . 5
5.3.4 Non-monotonic relaxation mechanisms . 5
5.4 Linear models . 5
5.4.1 Review . 5
5.4.2 Physical parameters . 6
6 Determination of particle size . 7
6.1 Introduction . 7
6.2 Inversion approaches used to determine PSD . 8
6.2.1 Optimization of a PSD function . 8
6.2.2 Regularization . 8
6.3 Limits of application . 9
7 Instrument qualification .9
7.1 Calibration . 9
7.2 Precision . . . 9
7.2.1 Reference samples . 9
7.2.2 Repeatability . 10
7.2.3 Reproducibility . 10
7.3 Accuracy . 10
7.3.1 Qualification procedure . 10
7.3.2 Reference samples . 10
7.3.3 Instrument preparation . 10
7.3.4 Accuracy test . 10
7.3.5 Qualification acceptance criteria. 10
8 Reporting of results .11
Annex A (informative) Viscoinertial loss model .12
Annex B (informative) ECAH theory and limitations .13
Annex C (informative) Example of a semi-empirical model .17
Annex D (informative) Iterative fitting .20
Annex E (informative) Physical parameter values for selected materials .22
Annex F (informative) Practical example of PSD measurement .23
Bibliography .32
iii
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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents 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).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 24, Particle characterization including
sieving, Subcommittee SC 4, Particle characterization.
This second edition cancels and replaces the first edition (ISO 20998-2:2013), which has been
technically revised.
The main changes are as follows:
— References to relaxation mechanisms that affect attenuation
— Additional explanatory notes for Table 1
— Clarification of notation used in Formula (9)
— Minor editorial changes
A list of all parts in the ISO 20998 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
It is well known that ultrasonic spectroscopy can be used to measure particle size distribution (PSD) in
[1],[2],[3],[4]
colloids, dispersions, and emulsions . The basic concept is to measure the frequency-dependent
attenuation or velocity of the ultrasound as it passes through the sample. The attenuation spectrum
is affected by scattering or absorption of ultrasound by particles in the sample, and it is a function
[5],[6],[7]
of the size distribution and concentration of particles . Once this relationship is established by
empirical observation or by theoretical calculations, one can estimate the PSD from the ultrasonic
data. Ultrasonic techniques are useful for dynamic online measurements in concentrated slurries and
emulsions.
Traditionally, such measurements have been made off-line in a quality control lab, 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 be
[8]
observed directly in real time . This data can be used in process control schemes to improve both the
manufacturing process and the product performance.
While it is possible to determine the particle size distribution from either the attenuation spectrum or
phase velocity spectrum, the use of attenuation data alone is recommended. The relative variation in
phase velocity due to changing particle size is small compared to the mean velocity, so it is often difficult
to determine the phase velocity with a high degree of accuracy, particularly at ambient temperature.
Likewise, the combined use of attenuation and velocity spectra to determine the particle size is not
recommended. The presence of measurement errors (i.e. “noise”) in the magnitude and phase spectra
can increase the ill-posed nature of the problem and reduce the stability of the inversion.
v
INTERNATIONAL STANDARD ISO 20998-2:2022(E)
Measurement and characterization of particles by acoustic
methods —
Part 2:
Linear theory
1 Scope
This document specifies requirements for ultrasonic attenuation spectroscopy methods for determining
the size distributions of a particulate phase dispersed in a liquid at dilute concentrations, where the
ultrasonic attenuation spectrum is a linear function of the particle volume fraction. In this regime
particle-particle interactions are negligible. Colloids, dilute dispersions, and emulsions are within the
scope of this document. The typical particle size for such analysis ranges from 10 nm to 3 mm, alth
...