ISO 21501-4:2018/Amd 1:2023

INTERNATIONAL ISO
STANDARD 21501-4
Second edition
2018-05
AMENDMENT 1
2023-02
Determination of particle size
distribution — Single particle light
interaction methods —
Part 4:
Light scattering airborne particle
counter for clean spaces
AMENDMENT 1
Détermination de la distribution granulométrique — Méthodes
d'interaction lumineuse de particules uniques —
Partie 4: Compteur de particules en suspension dans l'air en lumière
dispersée pour espaces propres
AMENDEMENT 1
Reference number
ISO 21501-4:2018/Amd.1:2023(E)
© ISO 2023

---------------------- Page: 1 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023
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
  © ISO 2023 – All rights reserved

---------------------- Page: 2 ----------------------
ISO 21501-4:2018/Amd.1:2023(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.
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.
A list of all parts in the ISO 21501 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.
iii
© ISO 2023 – All rights reserved

---------------------- Page: 3 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
Determination of particle size distribution — Single
particle light interaction methods —
Part 4:
Light scattering airborne particle counter for clean spaces
AMENDMENT 1

3.2
Add the following text at the end of the definition: “or ratio of the particle number measured by an
LSAPC to that introduced to the LSAPC for a given sampling time”, so that the entry reads:
3.2
counting efficiency
ratio of the number concentration measured by a light scattering airborne particle counter (LSAPC)
(3.4) to that measured by a reference instrument for the same test aerosol, or ratio of the particle
number measured by an LSAPC to that introduced to the LSAPC for a given sampling time

6.2
Add the following paragraph at the end of the subclause:
It can be appropriate to evaluate counting efficiency for some applications at sizes larger than
twice the minimum detectable size. It is recognized that the counting efficiency range of 0,90 to
1,10 [(100 ± 10) %] specified above does not remain relevant at all larger sizes due to particle
losses within the LSAPC; depending on the application requirements, a tolerance of ±10 to ±30 % is
recommended at a nominal particle diameter of 5 µm.

7.2
Add the following subclause heading above the first paragraph:
7.2.1  Parallel comparison method

Add the following subclause at the end of subclause 7.2.1:
7.2.2  Generator method
Clause A.2 describes the generator method for evaluating the counting efficiency of LSAPC.
Generator method uses monodisperse particles whose sizes are defined as the volume equivalent
diameter. The method uses an inkjet aerosol generator (IAG) as a monodisperse particle number
standard. In this method, the counting efficiency, η, is evaluated according to Formulae (3) and (4).
N
1
η= (3)
N
0
1
© ISO 2023 – All rights reserved

---------------------- Page: 4 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
Nt=⋅L (4)
00
where
N is the number of particles measured by an LSAPC under test;
1
N is the number of particles introduced to the LSAPC;
0
t
is the sampling time set to the LSAPC;
L is the particle generation rate of the IAG.
0

The counting efficiencies of Formula (2) is equivalent to Formula (3) when N is evaluated by
0
NV=⋅C
00
where V is the volume of test aerosol sampled by the LSAPC.

Renumber subsequent Formulae (3) to (7) as (5) to (8).

Annex A
Replace Annex A with the following:
2
  © ISO 2023 – All rights reserved

---------------------- Page: 5 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
Annex A
(informative)

Counting efficiency
A.1 Introduction
This annex introduces the parallel comparison method and the generator method. The parallel
comparison method is the general method, and the generator method is the alternative method.
Table A.1 summarizes the characteristics of these two methods.
Table A.1 — Characteristics of the parallel comparison method and the generator method
Parallel comparison method Generator method
Liquid or solid particles nebulised from solutions/particle Monodisperse solid or liquid particles are generated
suspensions or dispersed from dry powder; PSL spheres can from aqueous solutions; PSL spheres cannot be used
be used as test particles. as test particles.
Particle size range: typically from 100 nm PSL optical diameter. Particle size range: typically from 0,5 μm.
Since the method can select the particle size by using classi- Not an appropriate method to evaluate the lower
fication devices such as DEMC or AAC, the cut-off region of cut-off diameter of the counting efficiency curve.
the counting efficiency curve can be evaluated.
SI-traceability of the PSL geometric diameter can be established. SI-traceability of the particle volume equivalent
diameter can be established.
The number of particles delivered to a DUT-LSAPC must The number of particles delivered to a DUT- LSAPC
be measured with a reference instrument (e.g. a reference is accurately and precisely known.
LSAPC or CPC).
A.2 Parallel comparison method
A.2.1 Principle
Figures A.1 and A.2 show the test system for counting efficiency. The particle generator generates an
aerosol that consists of dry monodisperse PSL particles (100 nm to 10 μm) suspended in clean air.
PSL particles in the range of 100 nm to 5 μm can be generated by nebulizing aqueous suspensions. After
nebulization of a PSL suspension, the aerosol typically contains residue particles which can bias the
measurement of the counting efficiency. Measurement errors should be minimized by:
— separating the PSL particles from surfactants, for example, in several mixing/settling separation
steps in ultrapure water before preparing the suspension for the aerosol generator;
— using a PSL suspension in the aerosol generator with very low concentration of impurities in the
liquid phase, for example, traces of salt in ultrapure water, to a) achieve a low enough background of
residue particles and b) avoid growth of PSL particles due to coating of impurities after evaporation
of the suspension liquid droplet;
— optimising the concentration of PSL particles in the suspension to avoid measurement bias due to
doublet PSL particles (two PSL particles were contained in a droplet);
— drying the aerosol to remove all suspension liquid from the surface of the PSL particles and to avoid
condensation of suspension liquid vapour on the PSL particles.
3
© ISO 2023 – All rights reserved

---------------------- Page: 6 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
After drying the aerosol, size classifying the PSL particles with a DEMC (compare ISO 15900 and
ISO 27891; commercial DEMCs can be used for particles up to about 1 μm) or an aerodynamic aerosol
[11]
classifier (AAC), applicable up to 5 μm, can be applied if the background of residue particles needs
to be further reduced. This can especially be necessary if the requirements in Clause 7 (see Figure 3)
cannot be fulfilled.
Since PSL aerosol generated from a suspension is electrostatically charged and since DEMC-classified
PSL particles are unipolarly charged, a bipolar diffusion charge conditioner (as known as aerosol
neutralizer) further increases the accuracy of the measurement of the counting efficiency by minimizing
particle losses in both the particle counter to be inspected and the reference particle counter.
After generation and conditioning, the PSL aerosol is fed to the particle counter to be inspected and
the reference particle counter via a device (e.g. a distributing box, see Figures A.1 and A.2) which shall
be designed in such a way that the particle number concentration at the inlet of both particle counters
is as close as possible. The uncertainty associated with the inhomogeneity in the particle number
[1]
concentration should be evaluated according to the procedure given in Clause E.2 .
The counting efficiency is obtained by calculating the ratio of the particle number concentration
measured by the particle counter under test and the particle number concentration measured by the
reference particle counter. The particle number concentration of the sample should be less than 25 %
of the maximum particle number concentration of both the reference particle counter and the particle
counter under test.
Figure A.1 — Example of counting efficiency test system
4
  © ISO 2023 – All rights reserved

---------------------- Page: 7 ----------------------
ISO 21501-4:2018/Amd.1:2023(E)
Key
1 filtered dilution air
2 DEMC or AAC
3 wet or dry PSL dispersion
4 turbulent airjets
5 reference LSAPC
6 LSAPC under test
Figure A.2 — Example of counting efficiency test system
As mentioned before, the method described above is most useful for PSL particles smaller than
approximately 5 μm. If calibration with larger particles (e.g. 10 μm) is required, dry PSL particles
generated with a dry powder dispenser are better suited. The counting efficiency of the LSAPC under
test can decrease considerably for particles with a diameter larger than 1 μm. The monodisperse, dry
PSL powder needs to be free of surfactants to avoid errors during the calibration. Homogenization of
large particles (larger than about 0,5 μm) can require mixing by turbulent airjets as shown in Figure A.2.
Moreover, distributing the aerosol between the reference particle counter and the particle counter to
be inspected in Figure A.1 requires special attention for larger particles since particle losses due to
inertial impaction and gravitational settling become important. To minimize errors, it is recommended
to:
— use a distribution tube in Figure A.2 instead of a distribution box in Figure A.1;
— use isokinetic and isoaxial probes to extract the calibration aerosol for both particle counters;
— use vertical tubing to connect the distribution tube with the particle counters;
— u
...