Determination of particle size distribution -- Single particle light interaction methods

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

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Status
Published
Publication Date
17-May-2018
Current Stage
6060 - International Standard published
Start Date
27-Apr-2018
Completion Date
18-May-2018
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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 21501-4:2018(E)
ISO 2018
---------------------- Page: 1 ----------------------
ISO 21501-4:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© 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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Phone: +41 22 749 01 11
Fax: +41 22 749 09 47
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2018 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 21501-4:2018(E)
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

© ISO 2018 – All rights reserved iii
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ISO 21501-4:2018(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 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
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ISO 21501-4:2018(E)
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.
© ISO 2018 – All rights reserved v
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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
© ISO 2018 – All rights reserved 1
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ISO 21501-4:2018(E)
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
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ISO 21501-4:2018(E)

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.
© ISO 2018 – All rights reserved 3
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ISO 21501-4:2018(E)
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
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ISO 21501-4:2018(E)
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'−
ε = (1)
where
© ISO 2018 – All rights reserved 5
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ISO 21501-4:2018(E)
x is the size setting specified for the LSAPC;

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 .

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
V median voltage
V upper voltage limit
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
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ISO 21501-4:2018(E)
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
V median voltage
V upper voltage limit
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

to the size settings (or threshold sizes) x given as specifications of the LSAPC (see Figure 4). In this

determination, a theoretical response curve based on Mie theory may be used to calculate V from

experimentally observed V .

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

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.

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

can be the threshold channel of the built-in PHA which is intended to be assigned to x . For the sake of simplicity

in description, it is assumed that electric comparators are employed in the LSAPC for the rest of this document,

unless otherwise stated.
© ISO 2018 – All rights reserved 7
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ISO 21501-4:2018(E)
Key
X particle size
Y pulse height voltage
1 response curve
x certified size of the calibration particles
V median voltage corresponding to x
m c
x size setting specified for the LSAPC
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.

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
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ISO 21501-4:2018(E)
Key
X particle size
Y pulse height voltage
1 expected response curve
2 actual response curve
x certified size of the calibration particles
V median voltage corresponding to x
m c
x size setting specified for the LSAPC
x ' actual size setting corresponding to V
i ti
V voltage read out from the electric comparator
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):
η= (2)
© ISO 2018 – All rights reserved 9
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ISO 21501-4:2018(E)
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 con
...

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