ISO 21501-4:2018

Determination of particle size distribution - Single particle light interaction methods - Part 4: Light scattering airborne particle counter for clean spaces

This document describes a calibration and verification method for a light scattering airborne particle counter (LSAPC), which is used to measure the size distribution and particle number concentration of particles suspended in air. The light scattering method described in this document is based on single particle measurements. The typical size range of particles measured by this method is between 0,1 μm and 10 μm in particle size. Instruments that conform to this document are used for the classification of air cleanliness in cleanrooms and associated controlled environments in accordance with ISO 14644‑1 and ISO 14644‑2, as well as the measurement of number and size distribution of particles in various environments. The following parameters are within the scope of this document: - size setting error; - counting efficiency; - size resolution; - false count; - maximum particle number concentration; - sampling flow rate error; - sampling time error; - response rate; - calibration interval; - reporting results from test and calibration.

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

General Information

• Status: Published
• Publication Date: 17-May-2018
• ICS: 19.120 - Particle size analysis. Sieving
• Technical Committee: ISO/TC 24/SC 4 - Particle characterization
• Drafting Committee: ISO/TC 24/SC 4/WG 9 - Single particle light interaction methods
• Current Stage: 9093 - International Standard confirmed
• Start Date: 06-Sep-2023
• Completion Date: 13-Dec-2025

Relations

• Amended By: ISO 21501-4:2018/Amd 1:2023 - Determination of particle size distribution - Single particle light interaction methods - Part 4: Light scattering airborne particle counter for clean spaces - Amendment 1
• Effective Date: 13-Oct-2020
• Revises: ISO 21501-4:2007 - Determination of particle size distribution - Single particle light interaction methods - Part 4: Light scattering airborne particle counter for clean spaces
• Effective Date: 22-Jan-2011

Overview

ISO 21501-4:2018 specifies calibration and verification methods for light scattering airborne particle counters (LSAPC) used in clean spaces. The standard covers single-particle light interaction methods to measure particle size distribution and particle number concentration of airborne particles typically in the 0.1 µm to 10 µm range. It defines test procedures, performance requirements and reporting for instruments used in air cleanliness classification and monitoring.

Key topics and technical requirements

ISO 21501-4 focuses on performance attributes and test methods for LSAPCs. Key topics include:

• Measurement principle: particle sizing and counting by detection of light scattered when individual particles pass through a light beam (optical sizing based on scattered intensity; Mie theory applies for spherical particles).
• Calibration particles: use of monodisperse, traceable calibration particles (e.g., PSL) with certified mean size and refractive index (~1.59 at 589 nm) and low uncertainty (≤ 2.5%).
• Performance parameters within scope:
  • Size setting error (MPE = 0.10, i.e., 10% of specified size) - verification and calibration requirements.
  • Counting efficiency - required ranges: ~0.30–0.70 near the minimum detectable size and ~0.90–1.10 for particles 1.5–2× the minimum detectable size.
  • Size resolution - maximum allowed resolution ≤ 0.15 (15%).
  • False count - determination of counts in particle-free air and reporting of the 95% upper confidence limit (UCL).
  • Maximum particle number concentration and associated coincidence loss evaluation.
  • Sampling flow rate error, sampling time error, response rate, calibration interval, and reporting of test and calibration results.
• Test procedures and annexes: Clauses and informative annexes detail test methods for size setting, counting efficiency, size resolution, false counts, response rate and uncertainty estimation.

Practical applications and users

ISO 21501-4 is essential for ensuring reliable airborne particle measurements in:

• Cleanroom classification and monitoring (compliance with ISO 14644‑1 and ISO 14644‑2).
• Pharmaceutical manufacturing, semiconductor and electronics fabrication, medical device production, laboratory and aseptic environments.
• Users include cleanroom operators, quality assurance teams, instrument manufacturers, calibration laboratories, and regulatory bodies requiring traceable particle counter performance.

Related standards

• ISO 14644-1 / ISO 14644-2 (cleanroom classification and monitoring)
• Other parts of the ISO 21501 series (particle sizing and counting methods)

Keywords: ISO 21501-4, LSAPC, light scattering airborne particle counter, particle size distribution, cleanrooms, calibration, counting efficiency, size resolution, false count, ISO 14644.

Standards Content (Sample)

INTERNATIONAL ISO
STANDARD 21501-4
Second edition
2018-05
Determination of particle size
distribution — Single particle light
interaction methods —
Part 4:
Light scattering airborne particle
counter for clean spaces
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
Reference number
©
ISO 2018
© ISO 2018
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
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CH-1214 Vernier, Geneva
Phone: [phone]
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Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 2
5 Basic configuration . 3
6 Requirements . 3
6.1 Size setting error . 3
6.2 Counting efficiency . 4
6.3 Size resolution . 4
6.4 False count . 4
6.5 Maximum particle number concentration . 4
6.6 Sampling flow rate error . 4
6.7 Sampling time error . 4
6.8 Response rate . 4
6.9 Calibration interval . 4
6.10 Reporting of test and calibration results . 5
7 Test and calibration procedures . 5
7.1 Size setting . 5
7.1.1 Evaluation of size setting error . 5
7.1.2 Procedure of size setting . 6
7.2 Evaluation of counting efficiency . 9
7.3 Evaluation of size resolution .10
7.4 Evaluation of false count . .11
7.5 Estimation of coincidence loss at the maximum particle number concentration .11
7.6 Evaluation of sampling flow rate error .12
7.7 Evaluation of sampling time error .12
7.8 Evaluation of response rate .12
Annex A (informative) Counting efficiency .14
Annex B (informative) Size resolution .16
Annex C (informative) False count .17
Annex D (informative) Response rate .18
Annex E (informative) Procedure for evaluating the uncertainties of the results of the
performance tests .19
Bibliography .25
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 on 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 the following
URL: 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 21501-4:2007), which has been
technically revised.
The main changes from the previous edition are:
— Clause 4 for “Principle” and Clause 5 for “Basic configuration” have been added;
— “size calibration” and “verification of size setting” have been combined as “size setting error” in the
requirements clause;
— “Test report” (3.11 in the previous edition) has been changed to 6.10 on “Reporting of test and
calibration results”;
— information about uncertainties has been enriched and is now the subject of Annex E.
A list of all parts in the ISO 21501 series can be found on the ISO website.
iv © ISO 2018 – All rights reserved

Introduction
Monitoring particle contamination levels is required in various fields, e.g. in the electronic industry, in
the pharmaceutical industry, in the manufacturing of precision machines and in medical operations.
Particle counters are useful instruments for monitoring particle contamination in air. The purpose of
this document is to provide a calibration procedure and verification method for particle counters, so
as to minimize the inaccuracy in the measurement result by a counter, as well as the differences in the
results measured by different instruments.
INTERNATIONAL STANDARD ISO 21501-4:2018(E)
Determination of particle size distribution — Single
particle light interaction methods —
Part 4:
Light scattering airborne particle counter for clean spaces
1 Scope
This document describes a calibration and verification method for a light scattering airborne particle
counter (LSAPC), which is used to measure the size distribution and particle number concentration of
particles suspended in air. The light scattering method described in this document is based on single
particle measurements. The typical size range of particles measured by this method is between 0,1 μm
and 10 μm in particle size.
Instruments that conform to this document are used for the classification of air cleanliness in
cleanrooms and associated controlled environments in accordance with ISO 14644-1 and ISO 14644-2,
as well as the measurement of number and size distribution of particles in various environments.
The following parameters are within the scope of this document:
— size setting error;
— counting efficiency;
— size resolution;
— false count;
— maximum particle number concentration;
— sampling flow rate error;
— sampling time error;
— response rate;
— calibration interval;
— reporting results from test and calibration.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
3.1
calibration particle
monodisperse spherical particle with a certified mean particle size, e.g. a polystyrene latex (PSL)
particle, where the certified size is traceable to the International System of Units (SI), a relative
standard uncertainty equal to or less than 2,5 %, and a refractive index that is approximately 1,59 at
the wavelength of 589 nm (sodium D line)
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
3.3
false count
apparent count per unit volume when a sample air containing no measurable particles is measured by
the light scattering airborne particle counter (LSAPC) (3.4)
3.4
LSAPC
light scattering airborne particle counter
instrument that measures airborne particle numbers by counting the pulses as the particles pass
through the sensing volume, and also particle size by scattered light intensity
Note 1 to entry: The optical particle size measured by the LSAPC is the light scattering equivalent particle size
and not the geometrical size.
3.5
PHA
pulse height analyser
instrument that analyses the distribution of pulse heights
3.6
size resolution
measure of the ability of an instrument to distinguish between particles of different sizes
3.7
coincidence loss
reduction of particle count caused by multiple particles passing simultaneously through the sensing
volume and/or by the finite processing time of the electronic system
3.8
test aerosol
aerosol to be used for calibration or testing of a light scattering airborne particle counter (LSAPC) (3.4)
that is composed of calibration particles (3.1) suspended in clean air
3.9
MPE
maximum permissible error
limit of error
extreme value of measurement error, with respect to a known reference quantity value, permitted by
specifications or regulations for a given measurement, measuring instrument, or measuring system
Note 1 to entry: This document uses decimal numbers for the requirements to MPEs to avoid confusion that may
arise when relative uncertainties of test results are reported in percent figures.
4 Principle
The measurement principle of the LSAPC is based on detection of light scattered by a particle when the
particle passes through an incident light beam.
2 © ISO 2018 – All rights reserved

The particle size is determined from the intensity of the scattered light, and the number of particles
from the number of light pulses scattered by individual particles.
To be more specific, sample air is drawn from the inlet of the LSAPC at a constant flow rate, and
introduced to the sensing volume of the LSAPC where a light beam is irradiated. When a particle
suspended in the sample air passes through the light beam, it scatters the light, emitting a light pulse.
The light pulse is detected by a photo detector, and converted to an electrical pulse. The electrical
pulse height is proportional to the scattered light intensity, and depends on the optical system design,
the electronic components used, and the light source. The intensity of the scattered light is dependent
on the size, refractive index, and shape of the particle. If the particle is spherical, the scattered light
intensity is described by the Mie theory. In order to establish a relationship between the electrical pulse
height and the particle size, calibration of each LSAPC with use of particles having a well-defined size,
refractive index, and shape is required.
5 Basic configuration
An LSAPC is composed typically of a light source, a sample air suction system, a sensing volume, a
photoelectric conversion device, a pulse height analyser, and a display (see Figure 1). Some LSAPCs do
not contain a sample air suction system and/or a display.
To make the particle size calibration possible, the LSAPC should be constructed so that pulse height
distributions for calibration particles can be measured.
Figure 1 — Example of basic configuration of LSAPC
6 Requirements
6.1 Size setting error
The MPE for size setting in the minimum detectable particle size and other sizes specified by the
manufacturer of an LSAPC is 0,10 (corresponding to 10 % of the specified size).
Size setting shall be conducted before the LSAPC is shipped from the manufacturer and when the size
setting error is found not fulfilled in a periodic calibration.
A recommended procedure for size setting is described in 7.1.2. If other methods are used, their
uncertainty shall be evaluated and described.
6.2 Counting efficiency
The counting efficiency shall be within 0,30 to 0,70 [corresponding to (50 ± 20) %] for calibration
particles with a size close to the minimum detectable particle size, and it shall be within 0,90 to 1,10
[(100 ± 10) %] for calibration particles with a size 1,5 to 2 times larger than the minimum detectable
particle size.
When calibration particles with exactly the same size as the minimum detectable particle size are not
available, particles whose size is within ±5 % of the minimum detectable particle size may be used and
the diameter of the calibration particles shall be reported.
6.3 Size resolution
The size resolution shall be less than or equal to 0,15 (corresponding to 15 % of the specified particle
size), when it is evaluated using calibration particles of a certified average size specified by the
manufacturer.
A recommended procedure is described in 7.3. If other methods are used, their uncertainty shall be
evaluated and described.
6.4 False count
The false count per volume in cubic meters and its 95 % upper confidence limit (UCL) shall be determined
according to 7.4. The 95 % UCL shall be less than or equal to the value specified and reported by the
manufacturer of the LSAPC.
6.5 Maximum particle number concentration
The maximum measurable particle number concentration shall be specified by the manufacturer. The
coincidence loss at the maximum particle number concentration of an LSAPC shall be less than or equal
to 0,1 (10 %).
NOTE The probability of occurrence of coincidence loss increases with increasing particle number
concentration.
6.6 Sampling flow rate error
The MPE of the volumetric sampling flow rate determined according to 7.6 compared to the flow rate
specified by the manufacturer shall be 0,05 (corresponding to 5 %) of the specified flow rate.
6.7 Sampling time error
The MPE in the duration of the sampling time shall be 0,01 (corresponding to 1 %) of the preset value.
If the LSAPC does not have a sampling time control system, this subclause does not apply.
6.8 Response rate
The response rate of the LSAPC obtained according to the test method given in 7.8 shall be equal to or
less than 0,005 (corresponding to 0,5 %).
6.9 Calibration interval
The calibration of the LSAPC should be conducted at an interval equal to or shorter than one year. The
requirements should be met during the calibration interval.
4 © ISO 2018 – All rights reserved

6.10 Reporting of test and calibration results
The following is the minimum information that shall be recorded in a test report:
a) date of test/calibration;
b) test/calibration particles used;
c) results for the parameters:
1) size setting error;
2) counting efficiency;
3) sampling flow rate error;
4) size resolution (with the particle size used);
5) false count;
d) threshold voltage values or channels of the built-in PHA corresponding to the size settings;
e) a statement of the test/calibration method used (e.g. ISO 21501-4);
f) report/certificate identification, test/calibration location, title and identification of test/calibration
provider including signature and date;
g) identification of customer and device under test, including how output was obtained for counting
efficiency (e.g. analogue, display or digital output).
A calibration certificate shall furthermore include:
h) identification and - if possible - statement of metrological traceability of all reference equipment
and calibration particles used;
i) relevant environmental conditions (e.g. temperature, air pressure and humidity) under which the
calibration was performed;
j) a stated uncertainty for each result for the parameters 1 to 4 with reference to the calculation
method (e.g. ISO/IEC Guide 98-3) - Annex E contains procedures for evaluating the uncertainty of
the results of the performance tests recommended in this document for parameters 1 and 2;
k) a stated false count at a 95 % confidence limit (see Annex C).
NOTE Calibration certificates issued by ISO/IEC 17025 accredited laboratories and covering all results for
the parameters 1 to 5 are considered to comply with the requirements above.
7 Test and calibration procedures
7.1 Size setting
7.1.1 Evaluation of size setting error
Calculate the size setting error ε according to Formula (1).
xx'−
ii
ε = (1)
x
i
where
x is the size setting specified for the LSAPC;
i
x ' is the actual size setting corresponding to V (see 7.1.2 for the meaning of V ).
i ti ti
7.1.2 Procedure of size setting
By use of a PHA connected to the output terminal for signal pulses of the LSAPC, or by use of a built-
in PHA if one is contained as a part of the LSAPC, obtain a pulse height distribution for a test aerosol
in which calibration particles are suspended. Let V and V denote the lower and upper voltage limits,
l u
respectively, of the range of pulse heights for the calibration particles (see Figure 2). The median voltage
V of the pulse height distribution in the range from V to V , shall be calculated, and is assigned to the
m l u
certified size of the calibration particles, x .
c
When a built-in PHA is used, the abscissa of the pulse height distribution may be given in channel
number instead of voltage. In this case, the term “voltage” above and in relevant descriptions below
should be interpreted as channel number of the PHA.
Key
X pulse height voltage
Y frequency
1 pulse height distribution
V lower voltage limit
l
V median voltage
m
V upper voltage limit
u
Figure 2 — Pulse height distribution for the test aerosol
If a noise distribution is observed in the pulse height distribution, and if it is separated distinctly from
the main peak corresponding to the calibration particles, the voltages V and V shall be chosen so that
l u
the range (V , V ) encompasses only the main peak [see Figure 3 a)]. If the noise distribution overlaps
l u
with the main peak, V and V shall be chosen so that the range (V , V ) corresponds to the full width
l u l u
at half maximum of the main peak [see Figure 3 b)]. The latter way of determining V and V is allowed
l u
only when the height of the valley between the noise distribution and the main peak is at most half the
main peak height.
6 © ISO 2018 – All rights reserved

a) b)
Key
X pulse height voltage
Y frequency
1 pulse height distribution for calibration particles
2 noise distribution (evaporation residues and/or optical or electrical noises)
V lower voltage limit
l
V median voltage
m
V upper voltage limit
u
Figure 3 — Pulse height distribution for the test aerosol when noise exists
By use of the data pair (x , V ) obtained in this way, or multiple data pairs (x , V ) ( j = 1, 2, .) obtained
c m cj mj
similarly for multiple calibration particles, determine the voltage values V (i = 1, 2, .) that correspond
i
to the size settings (or threshold sizes) x given as specifications of the LSAPC (see Figure 4). In this
i
determination, a theoretical response curve based on Mie theory may be used to calculate V from
i
experimentally observed V .
m
Let V denote the adjustable threshold voltage corresponding to x . For all the size settings x , adjust the
ti i i
value of V to V .
ti i
NOTE 1 The response curve can be calculated according to the Mie theory when the parameter set defining the
optical system of the LSAPC is available. If the parameter set of the optical system is not available, the response
curve in the vicinity of x can still be empirically determined by fitting a simple function, e.g. a quadratic or cubic
i
polynomial, to multiple data pairs (x , V ) obtained for x on either side of x .
cj mj cj i
NOTE 2 The detailed procedure for determining V can vary depending on the model of the LSAPC.
i
NOTE 3 V can be the set voltage of an electric comparator used in the LSAPC, or if a built-in PHA is used, it
ti
can be the threshold channel of the built-in PHA which is intended to be assigned to x . For the sake of simplicity
i
in description, it is assumed that electric comparators are employed in the LSAPC for the rest of this document,
unless otherwise stated.
Key
X particle size
Y pulse height voltage
1 response curve
x certified size of the calibration particles
c
V median voltage corresponding to x
m c
x size setting specified for the LSAPC
i
V voltage corresponding to x
i i
Figure 4 — Size calibration
Read out the value of V set for the electric comparator of the LSAPC. Ideally V corresponds to x ,
ti ti i
but in reality V corresponds to a particle size x ' which may be different from x owing, for example,
ti i i
to a change of the response curve over time. Determine the actual response curve according to the
procedure as described above or to another method which is scientifically documented, and determine
x ' using this curve (see Figure 5). Calculate the size setting error ε according to Formula (1) above.
i
NOTE 4 The expected response curve in Figure 5 is a hypothetical curve on which the threshold voltages of
the electric comparator, V , would correspond exactly to the specified size thresholds x .
ti i
8 © ISO 2018 – All rights reserved

Key
X particle size
Y pulse height voltage
1 expected response curve
2 actual response curve
x certified size of the calibration particles
c
V median voltage corresponding to x
m c
x size setting specified for the LSAPC
i
x ' actual size setting corresponding to V
i ti
V voltage read out from the electric comparator
ti
Figure 5 — Evaluation of size setting error
7.2 Evaluation of counting efficiency
To evaluate the counting efficiency of the LSAPC, use two populations of calibration particles; one that
has a size close to the minimum detectable particle size, and another that has a size 1,5 to 2 times larger
than the minimum detectable particle size.
Tests with other particle sizes may be added, if it is requested by a user of the LSAPC.
Use either a condensation particle counter (CPC) combined with a differential electrical mobility
classifier (DEMC) or a calibrated LSAPC as a reference instrument. The counting efficiency of the
reference instrument shall have a metrological traceability to a national or international standard, or
the International System of Units (SI).
NOTE 1 The condensation particle counter is also referred to as a condensation nucleus counter (CNC).
Measure the number concentrations of test aerosols suspending each of the two kinds of calibration
particles with the LSAPC under test and with the reference instrument (see Annex A). Determine the
counting efficiency according to Formula (2):
C
η= (2)
C
where
η is the counting efficiency;
C is the particle number concentration measured by reference particle counter;
C is the particle number concentration measured by particle counter under test.
For these measurements, the particle number concentration of the test sample should be equal to or
less than 25 % of the maximum particle number concentration of both the LSAPC under test and the
reference instrument.
NOTE 2 When the particle concentration measured by an LSAPC is, as usually is the case, not corrected for the
coincidence loss, the counting efficiency of the LSAPC depends on the particle number concentration stemming
from the coincidence loss. If the maximum particle number concentration is so determined t
...