ISO 21501-4:2007

INTERNATIONAL ISO
STANDARD 21501-4
First edition
2007-05-15

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 21501-4:2007(E)
©
ISO 2007

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ISO 21501-4:2007(E)
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ISO 21501-4:2007(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope . 1
2 Terms and definitions. 2
3 Requirements . 2
3.1 Size calibration. 2
3.2 Verification of size setting . 2
3.3 Counting efficiency. 2
3.4 Size resolution . 2
3.5 False count rate . 3
3.6 Maximum particle number concentration . 3
3.7 Sampling flow rate. 3
3.8 Sampling time . 3
3.9 Response rate . 3
3.10 Calibration interval . 3
3.11 Test report . 3
4 Test method. 4
4.1 Size calibration. 4
4.2 Verification of size setting . 6
4.3 Counting efficiency. 6
4.4 Size resolution . 6
4.5 False count rate . 7
4.6 Maximum particle number concentration . 7
4.7 Sampling flow rate. 8
4.8 Sampling time . 8
4.9 Response rate . 8
4.10 Calibration . 8
Annex A (informative) Uncertainty of particle size calibration. 9
Annex B (informative) Counting efficiency. 11
Annex C (informative) Size resolution. 12
Annex D (informative) False count rate. 13
Annex E (informative) Response rate. 15
Bibliography . 16

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ISO 21501-4:2007(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 21501-4 was prepared by Technical Committee ISO/TC 24, Sieves, sieving and other sizing methods,
Subcommittee SC 4, Sizing by methods other than sieving.
This first edition of ISO 21501-4, together with ISO 21501-2 and ISO 21501-3, cancels and replaces
ISO 13323-1:2000, which has been technically revised.
ISO 21501 consists of the following parts, under the general title Determination of particle size distribution —
Single particle light interaction methods:
⎯ Part 2: Light scattering liquid-borne particle counter
⎯ Part 3: Light extinction liquid-borne particle counter
⎯ Part 4: Light scattering airborne particle counter for clean spaces
The following part is under preparation:
⎯ Part 1: Light scattering aerosol spectrometer
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ISO 21501-4:2007(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 part of
ISO 21501 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.
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INTERNATIONAL STANDARD ISO 21501-4:2007(E)

Determination of particle size distribution — Single particle
light interaction methods —
Part 4:
Light scattering airborne particle counter for clean spaces
1 Scope
This part of ISO 21501 describes a calibration and verification method for a light scattering airborne particle
counter (LSAPC), which is used to measure the size and particle number concentration of particles
suspended in air. The light scattering method described in this part of ISO 21501 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 part of ISO 21501 are used for the classification of air cleanliness in
cleanrooms and associated controlled environments in accordance with ISO 14644-1, as well as the
measurement of number and size distribution of particles in various environments.
The following are within the scope of this part of ISO 21501:
⎯ size calibration;
⎯ verification of size setting;
⎯ counting efficiency;
⎯ size resolution;
⎯ false count rate;
⎯ maximum particle number concentration;
⎯ sampling flow rate;
⎯ sampling time;
⎯ sampling volume;
⎯ calibration interval;
⎯ test report.
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ISO 21501-4:2007(E)
2 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
2.1
calibration particles
mono-disperse spherical particle with a known mean particle size, e.g. polystyrene latex (PSL) particle, that is
traceable to an international standard of length, and where the standard uncertainty of the mean particle size
is equal to or less than ± 2,5 %
NOTE The refractive index of calibration particles is close to 1,59 at a wavelength of 589 nm (sodium D line).
2.2
counting efficiency
ratio of the measured result of a light scattering airborne particle counter (LSAPC) to that of a reference
instrument using the same sample
2.3
particle counter
instrument that counts the number of particles and measures their size using the light scattering method or the
light extinction method
2.4
pulse height analyser
PHA
instrument that analyses the distribution of pulse heights
2.5
size resolution
measure of the ability of an instrument to distinguish between particles of different sizes
3 Requirements
3.1 Size calibration
The recommended procedure for size calibration is described in 4.1.
3.2 Verification of size setting
The error in the detectable minimum particle size and other sizes specified by the manufacturer of an LSAPC
shall be equal to or less than ± 10 % when the test is carried out by the method described in 4.2.
3.3 Counting efficiency
The counting efficiency shall be (50 ± 20) % for calibration particles with a size close to the minimum
detectable size, and it shall be (100 ± 10) % for calibration particles with a size of 1,5 times to 2 times larger
than the minimum detectable particle size.
3.4 Size resolution
The size resolution shall be equal to or less than 15 % for calibration particles of a size specified by the
manufacturer.
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ISO 21501-4:2007(E)
3.5 False count rate
The false count rate is determined by measuring the particle number concentration in the unit of counts per
cubic meter at the minimum reported size range when sampling clean air.
3.6 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 equal to or less
than 10 %.
NOTE When the particle number concentration is higher than the maximum particle number concentration, the
number of uncounted particles increases because of an enhanced probability of multiple particles existing in the sensing
volume (coincidence error) and/or saturation of the electronic system.
3.7 Sampling flow rate
The standard uncertainty of volumetric flow rate shall be equal to or less than ± 5 %.
If the LSAPC does not have a flow rate control system this subclause does not apply, however the
manufacturer shall specify the allowable limit of its flow rate of the LSAPC.
3.8 Sampling time
The standard uncertainty in the duration of sampling time shall be equal to or less than ± 1 % of the preset
value.
If the LSAPC does not have a sampling time control system, this subclause does not apply.
3.9 Response rate
The response rate of the LSAPC obtained according to the test method given in 4.9 shall be equal to or less
than 0,5 %.
3.10 Calibration interval
It is recommended that the calibration interval of an LSAPC be one year or less.
3.11 Test report
The following minimum information shall be recorded:
a) date of calibration;
b) calibration particle sizes;
c) flow rate;
d) size resolution (with the particle size used);
e) counting efficiency;
f) false count rate;
g) voltage limit or channel of an internal pulse height analyser (PHA).
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ISO 21501-4:2007(E)
4 Test method
4.1 Size calibration
When calibrating an LSAPC with calibration particles of known size, the median voltage (or internal PHA
channel), corresponds to the particle size (see Figure 1). The median voltage (or internal PHA channel)
should be determined by using a particle counter with variable voltage limit (or internal PHA channel) settings.
The median voltage (or internal PHA channel) is the voltage (or internal PHA channel) that equally divides the
total number of pulses counted. When a particle counter with variable voltage limit settings is not available, a
PHA can be used in place of the counter.

Key
X pulse height voltage (or channel)
Y density
1 pulse height distribution with PSL particles
V lower voltage limit
l
V median voltage
m
V upper voltage limit
u
Figure 1 — Pulse height distribution of PSL particle signals
When noise signals appear as if there are many small particles in the sample, the median voltage (or internal
PHA channel) shall be determined by discarding the pulses due to “false particles” [see Figure 2 a)]. The
discarding should only be done when the density at the peak du
...