Aerosol particle number concentration -- Calibration of condensation particle counters

ISO 27891:2015 describes methods to determine the detection efficiency of condensation particle counters (CPCs) at particle number concentrations ranging between 1 cm-3 and 105 cm-3, together with the associated measurement uncertainty. In general, the detection efficiency will depend on the particle number concentration, the particle size, and the particle composition. The particle sizes covered by the methods described in this International Standard range from approximately 5 nm to 1 000 nm. The methods can therefore be used both to determine a CPC calibration factor to be applied across the range of larger particle sizes where the detection efficiency is relatively constant (the plateau efficiency), and to characterize the drop in CPC detection efficiency at small particle sizes, near the lower detection limit. These parameters are described in more detail in Annex A. The methods are suitable for CPCs whose inlet flows are between approximately 0,1 l/min and 5 l/min. This International Standard describes a method for estimating the uncertainty of a CPC calibration performed according to this International Standard.

Densité de particules d'aérosol -- Étalonnage de compteurs de particules d'aérosol à condensation

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Published
Publication Date
26-Feb-2015
Current Stage
9093 - International Standard confirmed
Start Date
15-Jun-2020
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INTERNATIONAL ISO
STANDARD 27891
First edition
2015-03-01
Aerosol particle number
concentration — Calibration of
condensation particle counters
Densité de particules d’aérosol — Étalonnage de compteurs de
particules d’aérosol à condensation
Reference number
ISO 27891:2015(E)
ISO 2015
---------------------- Page: 1 ----------------------
ISO 27891:2015(E)
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© ISO 2015

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ii © ISO 2015 – All rights reserved
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ISO 27891:2015(E)
Contents Page

Foreword ..........................................................................................................................................................................................................................................v

Introduction ................................................................................................................................................................................................................................vi

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols .......................................................................................................................................................................................................................... 5

5 Calibration using reference instruments — General principles ......................................................................... 8

5.1 General principles ................................................................................................................................................................................ 8

5.2 Objectives for the calibration aerosol ................................................................................................................................. 9

5.3 Setup overview ....................................................................................................................................................................................... 9

5.4 Components and their requirements ...............................................................................................................................10

5.4.1 Primary aerosol source ...........................................................................................................................................10

5.4.2 Charge conditioner .....................................................................................................................................................11

5.4.3 DEMC .......................................................................................................................................................................................11

5.4.4 Make-up or bleed air .................................................................................................................................................11

5.4.5 Mixing device, flow splitter and connection tubing ........................................................................12

5.4.6 Reference instrument: FCAE or CPC ............................................................................................................12

5.4.7 Other tools ..........................................................................................................................................................................14

5.5 Differences between FCAE and CPC as a reference instrument ................................................................14

6 Calibration using an FCAE as reference instrument .....................................................................................................15

6.1 Overview of the setup and calibration procedure .................................................................................................15

6.2 Preparation .............................................................................................................................................................................................18

6.2.1 General preparation ...................................................................................................................................................18

6.2.2 Primary aerosol .............................................................................................................................................................18

6.2.3 Other equipment ...........................................................................................................................................................18

6.2.4 DEMC .......................................................................................................................................................................................18

6.2.5 FCAE.........................................................................................................................................................................................19

6.2.6 Test CPC ................................................................................................................................................................................20

6.2.7 Check of the complete setup ...............................................................................................................................21

6.3 Calibration procedure of detection efficiency ...........................................................................................................23

6.3.1 General...................................................................................................................................................................................23

6.3.2 DEMC diameter adjustment ................................................................................................................................23

6.3.3 Primary aerosol adjustment ...............................................................................................................................23

6.3.4 Splitter bias β measurement ..............................................................................................................................24

6.3.5 Test CPC efficiency measurement ..................................................................................................................24

6.3.6 Measurement of different particle concentrations .........................................................................26

6.3.7 Measurement of different sizes ........................................................................................................................26

6.3.8 Repetition of first measurement point ......................................................................................................26

6.3.9 Preparation of the calibration certificate ................................................................................................26

6.4 Measurement uncertainty ..........................................................................................................................................................26

6.4.1 General...................................................................................................................................................................................26

6.4.2 Particle size ...................................................................... ..................................................................................................27

6.4.3 Detection efficiency ....................................................................................................................................................27

6.4.4 Particle number concentration ........................................................................................................................28

© ISO 2015 – All rights reserved iii
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ISO 27891:2015(E)
Contents Page

7 Calibration using a CPC as reference instrument ............................................................................................................28

7.1 Overview of the setup and calibration procedure .................................................................................................28

7.2 Preparation .............................................................................................................................................................................................31

7.2.1 General preparation ...................................................................................................................................................31

7.2.2 Primary aerosol .............................................................................................................................................................31

7.2.3 Other equipment ...........................................................................................................................................................31

7.2.4 DEMC .......................................................................................................................................................................................31

7.2.5 Reference CPC..................................................................................................................................................................32

7.2.6 Test CPC ................................................................................................................................................................................33

7.2.7 Check of the complete setup ...............................................................................................................................33

7.3 Calibration procedure of detection efficiency ...........................................................................................................35

7.3.1 General...................................................................................................................................................................................35

7.3.2 DEMC diameter adjustment ................................................................................................................................35

7.3.3 Primary aerosol adjustment ...............................................................................................................................36

7.3.4 Splitter bias β measurement ...............................................................................................................................36

7.3.5 Test CPC efficiency measurement ..................................................................................................................37

7.3.6 Measurement of different particle concentrations .........................................................................38

7.3.7 Measurement of different sizes ........................................................................................................................38

7.3.8 Repetition of first measurement point ......................................................................................................38

7.3.9 Preparation of the calibration certificate ................................................................................................38

7.4 Measurement uncertainty ..........................................................................................................................................................38

7.4.1 General...................................................................................................................................................................................38

7.4.2 Particle size ...................................................................... ..................................................................................................39

7.4.3 Detection efficiency ....................................................................................................................................................39

7.4.4 Particle number concentration ........................................................................................................................40

8 Reporting of results ........................................................................................................................................................................................40

Annex A (informative) CPC performance characteristics ............................................................................................................42

Annex B (informative) Effect of particle surface properties on the CPC detection efficiency ................51

Annex C (informative) Example calibration certificates ...............................................................................................................53

Annex D (normative) Calculation of the CPC detection efficiency ......................................................................................62

Annex E (informative) Traceability diagram .............................................................................................................................................73

Annex F (informative) Diluters ................................................................................................................................................................................75

Annex G (normative) Evaluation of the concentration bias correction factor between the

inlets of the reference instrument and test CPC ...............................................................................................................78

Annex H (informative) Extension of calibration range to lower concentrations .................................................83

Annex I (informative) Example of a detection efficiency measurement ......................................................................90

Annex J (normative) Volumetric flow rate calibration ...............................................................................................................106

Annex K (normative) Testing the charge conditioner and the DEMC at maximum particle

number concentration .............................................................................................................................................................................108

Annex L (informative) A recommended data recording method when using a reference FCAE .....109

Annex M (informative) Uncertainty of detection efficiency due to particle size uncertainty ............111

Annex N (informative) Application of calibration results .......................................................................................................113

Bibliography .........................................................................................................................................................................................................................116

iv © ISO 2015 – All rights reserved
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ISO 27891:2015(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 meaning of ISO specific terms and expressions related to conformity

assessment, as well as information about ISO’s adherence to the WTO principles in the Technical Barriers

to Trade (TBT), see the following URL: Foreword — Supplementary information.

The committee responsible for this document is ISO/TC 24, Particle characterization including sieving,

Subcommittee SC 4, Particle characterization.
© ISO 2015 – All rights reserved v
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ISO 27891:2015(E)
Introduction

A condensation particle counter (CPC) is a measuring device for the number concentration of small

aerosol particles. The common principle of all different CPC types is that condensation of supersaturated

vapours is used to grow ultra-fine and nanoparticles to droplets of sizes that can be detected optically.

[44]

The counting of the droplets is performed via optical light scattering. The droplet passes through

a detection area where it is illuminated by a focused light beam and a portion of the scattered light is

detected with a photodetector. The frequency of this event leads, with the known volume of sampled

air, to the particle number concentration. At low concentrations, the CPC counts individual particles and

allows an absolute determination of particle number concentration.

Commercially available CPCs employ different working fluids to generate the vapour, e.g. 1-butanol,

2-propanol, or water. Moreover, different principles are in use to achieve the needed supersaturation

in the sample air. The most common CPC uses laminar flow and diffusional heat transfer. The diffusion

constant of the working fluid determines the needed heating or cooling steps to initiate condensation

and hence, the principle design of a laminar flow CPC. Less common are turbulent mixing CPCs: in these

CPCs, the supersaturation is achieved by turbulently mixing the sample air with a particle free gas flow

saturated with the working fluid. Figure 1 shows a schematic of the probably most common CPC type

with a laminar flow through a heated saturator and a cooled condenser.
Key
1 aerosol inlet 7 droplet
2 working fluid reservoir 8 light source
3 heated saturator 9 illumination optics
4 nanoparticle 10 receiving optics
5 thermoelectric cooling and heating device 11 photodetector
6 condenser 12 aerosol outlet
Figure 1 — Principle of a laminar flow CPC
vi © ISO 2015 – All rights reserved
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ISO 27891:2015(E)

The accuracy of CPC measurements, however, depends on various influences. For example, if the flow

rate had an error, the concentration would have an error. Coincidence error at very high concentration,

inefficient activation of particle growth at very small sizes, and losses of particles during transport from

the inlet to the detection section are other possible sources of errors. For accurate measurement, the

CPC shall be calibrated.

“Calibration” of the CPC is usually done using a Faraday-cup aerosol electrometer (FCAE) as reference

[33][36]

instrument. In many cases, the purpose of the “calibration” is to determine the limit of particle

detection at very small size. The FCAE has been used as the reference since the detection efficiency of

the FCAE was considered to be unity at any size. The detection efficiency of a CPC is determined as the

ratio of the concentration indicated by the CPC under calibration to that by the FCAE, while aerosols of

singly charged, size-classified particles of the same number concentration are supplied simultaneously

to both instruments.

This International Standard sets out two distinct methods of CPC calibration: the characterization of a

CPC by comparison with an FCAE, which is the same as the traditional approach described above; and

by comparison with a reference CPC. An FCAE that has a reputable calibration certificate, covering the

relevant particle number concentrations, sizes, and composition, can be used. In the latter case, the

reference CPC is one that has a reputable calibration certificate, again covering the relevant particle

number concentrations, sizes, and composition. A reputable calibration certificate shall mean either

one that has been produced by a laboratory accredited to ISO/IEC 17025 or an equivalent standard,

where the type and range of calibration is within the laboratory’s accredited scope, or a European

Designated Institute or a National Metrology Institute that offers the relevant calibration service and

whose measurements fulfil the requirements of ISO/IEC 17025.

Two major sources of errors are known in CPC calibration: the presence of multiply charged particles

and the bias of the particle concentrations between the inlet of the CPC under calibration and that of

the reference instrument. Evaluation of these factors and corrections for them shall be included in the

calibration procedure, the methods of which are specified in this International Standard.

This International Standard is aimed at

— users of CPCs (e.g. for environmental or vehicle emissions purposes) who have internal

calibration programmes,

— CPC manufacturers who certify and recertify the performance of their instruments, and

— technical laboratories who offer the calibration of CPCs as a service, which can include National

Metrology Institutes who are setting up national facilities to support number concentration

measurements.
© ISO 2015 – All rights reserved vii
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INTERNATIONAL STANDARD ISO 27891:2015(E)
Aerosol particle number concentration — Calibration of
condensation particle counters
1 Scope

This International Standard describes methods to determine the detection efficiency of condensation

-3 5 -3

particle counters (CPCs) at particle number concentrations ranging between 1 cm and 10 cm , together

with the associated measurement uncertainty. In general, the detection efficiency will depend on the

particle number concentration, the particle size, and the particle composition. The particle sizes covered

by the methods described in this International Standard range from approximately 5 nm to 1 000 nm.

The methods can therefore be used both to determine a CPC calibration factor to be applied across the

range of larger particle sizes where the detection efficiency is relatively constant (the plateau efficiency),

and to characterize the drop in CPC detection efficiency at small particle sizes, near the lower detection

limit. These parameters are described in more detail in Annex A.

The methods are suitable for CPCs whose inlet flows are between approximately 0,1 l/min and 5 l/min.

This International Standard describes a method for estimating the uncertainty of a CPC calibration

performed according to this International Standard.
2 Normative references

The following documents, in whole or in part, are normatively referenced in this document and are

indispensable for its application. For dated references, only the edition cited applies. For undated

references, the latest edition of the referenced document (including any amendments) applies.

ISO 15900, Determination of particle size distribution — Differential electrical mobility analysis for

aerosol particles
3 Terms and definitions
For the purpose of this document, the following terms and definitions apply.
3.1
aerosol
system of solid or liquid particles suspended in gas
3.2
bipolar charger

particle charge conditioner to attain the equilibrium, known size-dependent charge distribution by

exposing aerosol particles to both positive and negative ions within the device

Note 1 to entry: Exposing aerosol particles to an electrically neutral cloud of positive and negative gas charges

with sufficiently high charge concentration and for a sufficiently long period of time leads to an equilibrium with

the net charge of the aerosol nearly zero (also known as charge neutralization).
© ISO 2015 – All rights reserved 1
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ISO 27891:2015(E)
3.3
calibration

operation that, under specified conditions, in a first step, establishes a relation between the quantity

values with measurement uncertainties provided by measurement standards and corresponding

indications with associated measurement uncertainties and, in a second step, uses this information to

establish a relation for obtaining a measurement result from an indication

Note 1 to entry: A calibration may be expressed by a statement, calibration function, calibration diagram,

calibration curve, or calibration table. In some cases, it may consist of an additive or multiplicative correction of

the indication with associated measurement uncertainty.

Note 2 to entry: Calibration should not be confused with adjustment of a measuring system, often mistakenly

called “self-calibration”, nor with verification of calibration.

Note 3 to entry: Often, the first step alone in the above definition is perceived as being calibration.

[SOURCE: ISO/IEC Guide 99]
3.4
calibration aerosol

charge conditioned and size classified primary aerosol with particle number concentration adjusted for

the calibration measurement, as delivered by the flow splitter
3.5
calibration particle material
material of the particles of the calibration aerosol
3.6
charge concentration
concentration of the net electrical charges per unit volume
Note 1 to entry: Charge concentration is the measurand of the FCAE.

Note 2 to entry: FCAE measurement can be displayed as charge concentration, C , (e.g. in fC/cm ), charge number

concentration, C , (e.g. in cm ) or electrical current, I , (e.g. in fA). Using the elementary charge, e, and the

FCAE

volumetric FCAE inlet flow rate, q , these displayed values are related as follows:

FCAE
CC==eI ()qe×
NQ FCAE FCAE
3 -3

EXAMPLE A charge concentration of 1 fC/cm corresponds to a charge number concentration of 6241 cm .

When the volumetric FCAE inlet flow rate is 1 l/min, the resulting electrical current is 16,67 fA.

3.7
charge conditioning

process that establishes a steady state charge distribution on the sampled aerosol

3.8
coefficient of variation
ratio of the standard deviation to the arithmetic mean value
3.9
coincidence error

probability of the presence of more than one particles inside the sensing zone simultaneously

Note 1 to entry: Coincidence error is related to particle number concentration, flow velocity through the sensing

zone and size of sensing zone.
2 © ISO 2015 – All rights reserved
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ISO 27891:2015(E)
3.10
condensation particle counter
CPC
instrument that measures the particle number concentration of an aerosol

Note 1 to entry: The sizes of particles detected are usually smaller than several hundred nanometres and larger

than a few nanometres.

Note 2 to entry: In some cases, a CPC may be called a condensation nucleus counter (CNC).

Note 3 to entry: The CPC used as the reference instrument is called the “reference CPC” throughout this

International Standard.

Note 4 to entry: The CPC under calibration is called the “test CPC” throughout this International Standard.

[SOURCE: ISO 15900:2009, modified]
3.11
detection efficiency

ratio of the concentration reported by an instrument to the actual concentration at the inlet of the instrument

3.12
differential electrical mobility classifier
DEMC

classifier that is able to select aerosol particles according to their electrical mobility and pass them to its exit

Note 1 to entry: A DEMC classifies aerosol particles by balancing the electrical force on each particle with its

aerodynamic drag force in an electrical field. Classified particles are in a narrow range of electrical mobility

determined by the operating conditions and physical dimensions of the DEMC. Classified particles can have

different sizes due to difference in the number of charges that they have.
[SOURCE: ISO 15900:2009, modified]
3.13
differential mobility analyzing system
DMAS

system to measure the size distribution of submicrometre aerosol particles consisting o

...

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