ISO/TS 4807:2022

Reference materials for particle size measurement - Specification of requirements

This document is intended to support users of reference materials (RMs) for particle size analysis to identify suitable RMs (certified or not) for their needs. In line with the focus on users, questions on sample preparation that go beyond preparation of the sample as received by the user will not be covered by this document. This document describes the fundamental requirements that RMs (certified or not) for the determination of particle size shall fulfil in order to be fit for a given purpose. The document is limited to a description of the fundamental principles – the discussion whether a certain numerical value is fit for purpose is beyond the scope of this document. The scope of this document is limited to RMs (certified or not) in the form of particles. This document does not deal with any other form of RMs, like calibration grids.

Matériaux de référence pour la mesure de taille de particules — Spécification des exigences

General Information

• Status: Published
• Publication Date: 28-Jun-2022
• ICS: 19.120 - Particle size analysis. Sieving
• Technical Committee: ISO/TC 24/SC 4 - Particle characterization
• Drafting Committee: ISO/TC 24/SC 4/WG 11 - Sample preparation and reference materials
• Current Stage: 9093 - International Standard confirmed
• Start Date: 04-Sep-2025
• Completion Date: 13-Dec-2025

Overview

ISO/TS 4807:2022 - "Reference materials for particle size measurement - Specification of requirements" provides user-focused guidance to help select and use reference materials (RMs) for particle size analysis. The technical specification describes the fundamental requirements that particle-form RMs (certified or non‑certified) must meet to be fit for purpose. It is limited to particulate RMs and does not cover other RM forms (e.g., calibration grids) or sample preparation beyond the sample as received by the user.

Key topics and requirements

• Measurand definitions: Clarifies how particle size measurands should be specified, including operationally defined measurands and the need to document measurement procedures (e.g., equivalent diameters and conditions under which they represent actual particle size).
• Metrological traceability: Describes the expectation that measurement results can be linked to references through a documented chain of calibrations, with associated uncertainties.
• Types of RMs: Differentiates certified vs non‑certified RMs, primary/secondary/tertiary CRMs, and material morphology (spherical/non‑spherical, monomodal/monodisperse). (Example definition: a monodisperse material is characterized in the document by a specific percentile ratio criterion.)
• Material properties: Addresses porous vs dense particles, homogeneity and stability requirements relevant to particle size RMs.
• Practical handling: Provides guidance on storage, handling and use to preserve RM integrity.
• Use‑case requirements: Specifies RM attributes for:
  • Instrument verification / design qualification
  • Installation qualification
  • Calibration (including certified value traceability and uncertainty)
  • Operational qualification / proficiency testing
  • Statistical quality control and performance qualification
• Documentation expectations: For CRMs, certificates must state the assigned value, uncertainty and metrological traceability.

Applications and intended users

ISO/TS 4807 is practical for:

• Analytical laboratories performing particle size characterization (laser diffraction, dynamic light scattering, sieving, image analysis)
• RM producers and certification bodies developing particle RMs or CRMs
• Instrument manufacturers specifying verification and calibration materials
• Quality assurance / regulatory teams seeking traceable measurement systems
• Proficiency testing organizers and method developers setting acceptance criteria

This specification helps users choose RMs that support calibration, method validation, instrument qualification and ongoing quality control for reliable particle size measurement.

Related standards

• ISO Guide 30 and ISO/IEC Guide 99 (terminology and metrological concepts)
• ISO/TS 14411-1 (referenced for monodisperse material criteria)
• ISO 17034 (reference material producers / CRMs)

Keywords: ISO/TS 4807:2022, reference materials, particle size measurement, RMs, CRMs, metrological traceability, particle characterization, calibration, instrument verification.

Standards Content (Sample)

TECHNICAL ISO/TS
SPECIFICATION 4807
First edition
2022-06
Reference materials for particle size
measurement — Specification of
requirements
Matériaux de référence pour la mesure de taille de particules —
Spécification des exigences
Reference number
© ISO 2022
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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3  Terms and definitions . 1
4 Abbreviated terms . 4
5 Basic principles . 5
5.1 Measurand definitions in particle size analysis . 5
5.1.1 General . 5
5.1.2 Operationally defined measurands in particle characterisation: Equivalent
diameters . 5
5.1.3 Required detail of procedure description for operationally defined
measurands. 7
5.1.4 Conditions for equivalent diameters to coincide with the actual particle
diameter . 7
5.2 Metrological traceability of size measurement results . . 8
5.2.1 General . 8
5.2.2 Establishing versus verifying traceability in particle characterisation . 11
5.3 Types of RMs . 11
5.3.1 Certified and non-certified RMs/certified and non-certified values . 11
5.3.2 Primary/secondary/tertiary CRMs .12
5.3.3 Spherical/non-spherical and monodisperse/polydisperse RMs .13
5.4 Porous/dense RMs . . 14
6 Practical handling .14
7  Requirements for specific uses .14
7.1 General . 14
7.2 Instrument verification/design qualification . 14
7.3 Installation qualification . 14
7.3.1 General . 14
7.3.2 Type of material . 14
7.3.3 Kind of quantity of the assigned value . 15
7.3.4 Degree of homogeneity . 15
7.4 Calibration . 16
7.4.1 General . 16
7.4.2 Type of material . 16
7.4.3 Traceability of the certified values . 16
7.4.4 Kind of quantity of the certified property . 16
7.4.5 Uncertainty of the certified value . 17
7.5 Operational qualification/demonstration of proficiency . 17
7.5.1 General . 17
7.5.2 Type of material . 17
7.5.3 Kind of quantity of the certified property . 18
7.5.4 Uncertainty of certified values . 20
7.6 Statistical quality control/performance qualification . 20
7.6.1 General .20
7.6.2 Type of material .20
7.6.3 Kind of quantity .20
7.6.4 Degree of homogeneity . 21
7.7 Summary . 21
Bibliography .24
iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.
ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 24, Particle characterization including
sieving, Subcommittee SC 4, Particle characterization.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
TECHNICAL SPECIFICATION ISO/TS 4807:2022(E)
Reference materials for particle size measurement —
Specification of requirements
1 Scope
This document is intended to support users of reference materials (RMs) for particle size analysis
to identify suitable RMs (certified or not) for their needs. In line with the focus on users, questions
on sample preparation that go beyond preparation of the sample as received by the user will not be
covered by this document.
This document describes the fundamental requirements that RMs (certified or not) for the
determination of particle size shall fulfil in order to be fit for a given purpose. The document is limited
to a description of the fundamental principles – the discussion whether a certain numerical value is fit
for purpose is beyond the scope of this document.
The scope of this document is limited to RMs (certified or not) in the form of particles. This document
does not deal with any other form of RMs, like calibration grids.
2 Normative references
There are no normative references in this document.
3  Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
3.1
kind of quantity
aspect common to mutually comparable quantities
Note 1 to entry: The division of ‘quantity’ according to ‘kind of quantity’ is to some extent arbitrary.
EXAMPLE The quantities diameter, circumference, and wavelength are generally considered to be quantities
of the same kind, namely of the kind of quantity called length.
Note 2 to entry: Quantities of the same kind within a given system of quantities have the same quantity dimension.
However, quantities of the same dimension are not necessarily of the same kind.
[SOURCE: ISO/IEC Guide 99:2007, 1.2, modified — Note 3 to entry and EXAMPLES 2 and 3 have been
deleted.]
3.2
measurand
quantity intended to be measured
Note 1 to entry: The specification of a measurand requires knowledge of the kind of quantity (3.1), description of
the state of the phenomenon, body, or substance carrying the quantity, including any relevant component, and
the chemical entities involved.
[SOURCE: ISO/IEC Guide 99:2007, 2.3, modified — Notes 2 and 3 to entry and all the EXAMPLES have
been deleted.]
3.3
operationally defined measurand
method-defined measurand
measurand (3.2) that is defined by reference to a documented and widely accepted measurement
procedure to which only results obtained by the same procedure can be compared
Note 1 to entry: A term for measurands that are independent of a procedure does not exist. The term “non-
operationally defined measurand” is used in this document.
[SOURCE: ISO 17034:2016, 3.7, modified — the second term has been added and Note 1 to entry has
been replaced.]
3.4
metrological traceability
property of a measurement result whereby the result can be related to a reference through a
documented unbroken chain of calibrations (3.12), each contributing to the measurement uncertainty
Note 1 to entry: For this definition, a ‘reference’ can be a definition of a measurement unit through its practical
realization, or a measurement procedure including the measurement unit for a non-ordinal quantity, or a
measurement standard.
[SOURCE: ISO/IEC Guide 99:2007, 2.41, modified — Notes 2 to 8 to entries have been deleted.]
3.5
monomodal material
material consisting of particles where the particle size density distribution has only one maximum
Note 1 to entry: A monomodal material is not monodisperse if the width of the distribution is larger than the
limits described for monodisperse mateials (3.6).
3.6
monodisperse material
material consisting of particles with narrow particle size distribution
Note 1 to entry: For this document, a material is considered monodisperse if the width of the distribution of the
number-based diameter expressed as x /x is 1,12 or less (where x is 10 % percentile of the cumulative particle
90 10 10
size distribution and x is 90 % percentile of the cumulative particle size distribution), which corresponds
to a relative standard deviation of the distribution of 4,4 %. The limit 1,12 is taken from the requirements for
monodisperse pickets from ISO/TS 14411-1. Such narrow size distributions are typically found in polymer latex
materials.
3.7
spherical particle
particle with an aspect ratio of 0,95 or above in all three dimensions
Note 1 to entry: particles with small outgrows or that are not smooth can nevertheless fulfil this definition of
sphericity.
3.8
reference material
RM
material, sufficiently homogeneous and stable with respect to one or more specified properties, which
has been established to be fit for its intended use in a measurement process
Note 1 to entry: RM is a generic term comprising both certified and non-certified RMs. There is no term explicitly
referring to RMs without any assigned certified value (3.10). In this document, the term “reference material/
RM” is used for the superordinate, i.e. certified and non-certified RMs, whereas “non-certified RM” is used to
explicitly refer to materials without certified values.
Note 2 to entry: Properties can be quantitative or qualitative, e.g. identity of substances or species.
Note 3 to entry: Uses may include the calibration (3.12) of a measurement system, assessment of a measurement
procedure, assigning values to other materials, and quality control.
[SOURCE: ISO Guide 30:2015, 2.1.1, modified — Note 1 to entry has been expanded and Note 4 to entry
has been deleted.]
3.9
certified reference material
CRM
reference material (3.8) characterized by a metrologically valid procedure for one or more specified
properties, accompanied by an RM certificate that provides the value of the specified property, its
associated uncertainty, and a statement of metrological traceability (3.4)
Note 1 to entry: The concept of value includes a nominal property or a qualitative attribute such as identity or
sequence. Uncertainties for such attributes may be expressed as probabilities or levels of confidence.
[SOURCE: ISO Guide 30:2015, 2.1.2, modified — Notes 2 to 4 to entry have been deleted.]
3.10
certified value
value, assigned to a property of a reference material (3.8) that is accompanied by an uncertainty
statement and a statement of metrological traceability (3.4), identified as such in the RM certificate
[SOURCE: ISO Guide 30:2015, 2.2.3]
3.11
indicative value
information value
informative value
value of a quantity or property, of a reference material (3.8), which is provided for information only
Note 1 to entry: An indicative value cannot be used as a reference in a metrological traceability (3.4) chain
[SOURCE: ISO Guide 30:2015, 2.2.4]
3.12
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 99:2007, 2.39]
3.13
design qualification
DQ
process for verification that the proposed specification for the facility, equipment, or system meets the
expectation for the intended use
[SOURCE: ISO 11139:2018, 3.220.1]
3.14
installation qualification
IQ
process of establishing by objective evidence that all key aspects of the process equipment and ancillary
system installation comply with the approved specification
[SOURCE: ISO 11139:2018, 3.220.2]
3.15
performance qualification
PQ
process of establishing by objective evidence that the process, under anticipated conditions, consistently
produces a product which meets all predetermined requirements
[SOURCE: ISO 11139:2018, 3.220.4]
3.16
operational qualification
OQ
process of obtaining and documenting evidence that installed equipment operates within predetermined
limits when used in accordance with its operational procedures
[SOURCE: ISO 11139:2018, 3.220.3]
3.17
proficiency test
evaluation of participant performance against pre-established criteria by means of interlaboratory
comparisons
[SOURCE: ISO/IEC 17043:2010, 3.7, modified — Notes to entry 1 and 2 have been removed.]
3.18
statistical quality control
part of quality control in which statistical methods are used (such as estimation and tests of parameters
and sampling inspection)
EXAMPLE The use of quality control charts.
[SOURCE: ISO 12491:1997, 3.2, modified — the EXAMPLE has been added.]
4 Abbreviated terms
CRM Certified reference material
DLS Dynamic light scattering
DMA Differential mobility analysis
DQ Sesign qualification
ESZ Electric sensing zone
IQ Installation qualification
OQ Operational qualification
PQ Performance qualification
RM Reference material
SAXS Small angle X-ray scatteringPQ
SI International system of units
SQC Statistical quality control
5 Basic principles
5.1  Measurand definitions in particle size analysis
5.1.1 General
In general, two kinds of measurands can be distinguished.
— Non-operationally defined measurands are measurands where a physical unit can be directly related
to a property of a particle and where no further information is required in order to interpret the
value of this quantity. Examples for non-operationally defined measurands are a mass of a particle
or a distance between two points.
— Operationally defined measurands are measurands that are the result of a specific set of operations.
The quantity values of operationally defined measurands are only meaningful in connection with
this set of operations. Deviation from the specified set of operations does not only result in a wrong
result, but actually means that a different quantity is measured.
EXAMPLE 1 The impact toughness of a material as determined by for example, ISO 148-1. This is the energy
required to break a sample of specified dimensions (1 cm × 1 cm × 5 cm) that has a notch of specified width
depths with a hammer of specified dimensions. Deviation from the specifications of ISO 148-1 means that a
different procedure was applied and that the values obtained are not comparable to the impact toughness of
ISO 148-1. Note that the results of impact toughness measurements are expressed in joule, an SI unit. This shows
that operationally defined measurands can be expressed in SI units.
Meaningful comparisons of numerical values can only be made for quantities of the same kind. This
is immediately obvious for some non-operationally defined measurands: a comparison of a mass and
a length is meaningless. As indicated in Note 2 to entry of 3.1, expression in the same unit is required
but not sufficient in order to make results comparable. This is especially important for operationally
defined measurands.
EXAMPLE 2 In the example of impact toughness above, the energy required to break a sample of different
dimensions (e.g. 2 cm × 1 cm × 5 cm) is still expressed in J but it is impossible to say if a material with an impact
toughness of 85 J measured on a 2 cm × 1 cm × 5 cm sample is tougher than a material with an impact toughness
of 70 J as measured according to ISO 148-1.
This means that one should not expect that different operationally defined measurands yield the same
value. Samples may exist that give the same result for two unrelated methods, but this may be due
to coincidence. Conflicting values do not mean that one of the values is wrong, but simply reflect the
different response for the sample measured.
As will be explained below, the same principle applies to results from different methods for particle
size determinations: although the results can all be expressed (and traceable to) as metres, they are in
fact different kinds of quantities and not comparable unless very specific conditions are met.
5.1.2  Operationally defined measurands in particle characterisation: Equivalent diameters
None of the methods used for particle sizing actually measures a particle diameter. Doing so requires
applying a caliper to a particle or every individual particle of the sample. This is clearly impractical and
all particle sizing methods actually measure particle properties different from particle diameters and
relate these properties to the particle size. Examples of measured material properties for some particle
characterisation methods are given in Table 1.
Table 1 — Selected measurement principles in particle characterisation, their measured
properties and information on how this property is expressed
Method Measured property Result are expressed as distribution of
Diameters of spheres with the same sedimen-
Sedimentation analysis Speed of sedimentation
tation velocity (equivalent Stokes’ diameter)
Diameters of spheres with the same diffusion
Dynamic light scattering,
Speed of diffusion coefficient (equivalent hydrodynamic
particle tracking analysis
diameter)
Differential mobility Electrical mobility of charged aerosol Diameters of spheres with the same electrical
analysis particles mobility
Drop in resistance when a particle passes
Electrical sensing zone Diameters of spheres with the same volume
through an aperture
Diameters of circles with the same circum-
Length (diameter, circumference) or area of a ference or area, also direct measurement
Image analysis
projection or reflection of the particle of maximum and minimum Feret diameter
possible
Light scattering particle Intensity of the light scattered by individual
counters particles
Diameters of spheres of the same light scat-
tering/extinction
Light extinction particle Extinction of light caused by individual
counters particles
Ultrasonic attenuation Frequency-dependent attenuation of Diameters of (usually spherical) particles
spectroscopy ultrasound which give the same attenuation spectrum
Single particle inductively
Diameters of spheres of the same mass of the
coupled plasma mass Mass of the selected element(s) per particle
selected element/compound
spectrometry
Angular distribution of elastically scattered Diameters of (usually spherical) particles
Small angle X-ray scattering
X-rays with the same angular distribution of X-rays
Diameters of spheres with the same angular
Laser diffraction Angular distribution of scattered light
distribution of light
Mass fractions passing sieves of specified
Sieving analysis Mass of material that passes a sieve
aperture size
NOTE Results can also differ in the way they are weighted (intensity, number, area etc.).
These different properties are subsequently expressed as lengths, namely as diameters of spheres
that show the same response, for example, having the same speed of sedimentation. These diameters
are called “equivalent diameters”. Equivalent diameters are operationally defined measurands: they
depend on the property measured (projected area, sedimentation velocity, etc.) and the definition to
which shape the property should be equivalent (e.g. equivalent sphere, cube, tetrahedron).
As none of the methods used for particle sizing actually measure the particle diameter, all results of
particle sizing methods are operationally defined. This also means that one should not expect that
different methods yield the same value unless the particles measured fulfil very specific requirements
(see 5.1.4). This non-comparability is clear when one looks at the properties actually measured, but is
hidden by the expression of these properties in the dimension of length. It is not surprising that the
speed of diffusion differs from the projected area but the fact that both are expressed as lengths of
equivalent spheres falsely suggests otherwise.
Conceptually there is no difference between the determination of the equivalent diameter of a single
particle and the determination of the distribution of equivalent diameters in an ensemble method:
in each case, a property is measured and related to spherical particles that behave the same way for
the chosen property. While relating the measured property to spherical particles is more complex for
ensemble methods, it is conceptually not different from relating the property of a single particle to the
same property of a sphere.
EXAMPLE In laser diffraction, the diffraction pattern of a sample is measured. Applying a chosen theory that
models the diffraction pattern of spherical particles, the particle size distribution of an ensemble of spherical
particles is calculated that show the same diffraction pattern.
5.1.3  Required detail of procedure description for operationally defined measurands
As discussed above, the results of operationally defined measurands are only meaningful within
the clearly specified measurement procedure. In general, a measurement procedure for particle
characterisation consists of the following steps, each of which can influence the measurement result:
— sample preparation/dispersion: dispersing medium (e.g. air, liquid), kind and amount of energy
used (air pressure, ultrasound, stirring), addition of dispersion facilitating agents, geometry of the
sample cell, etc.;
— measurement of a property of the dispersed particles: property measured (e.g. sedimentation
velocity, diffraction pattern, diffusion coefficient), instrument parameters (e.g. geometry, laser
wavelength);
— evaluation, i.e. relating the measured property to the particle size (distribution) of equivalent
particles assumption of shape of the particles (spheres, cylinders, spheroids, etc.): the model, data
evaluation algorithm;
In an extreme case, the description may be so specific that results are valid only for a specific instrument
using a specific evaluation algorithm.
In other cases, the measured property is independent of many of these instrument-related parameters
and the same result can in principle be obtained by a variety of instrument configurations.
The level of detail required for a clear definition of the kind of quantity depends on the type of material
and can range from extreme detailed to rather simple.
It is the responsibility of producers of all RMs, in particular of CRMs, to clearly define the detail of the
measurement procedure to which the assigned values refer.
5.1.4 Conditions for equivalent diameters to coincide with the actual particle diameter
While results of particle sizing methods are operationally defined and only meaningful in the context
of the measurement method, there are some samples for which the equivalent diameter approaches
the geometric diameter of a sphere within the measurement uncertainty. The equivalent diameter can
coincide with the actual particle diameter under the following conditions.
a) The material consists of spherical particles. Spherical particles are the only particles that can be
characterised by a single length, the diameter.
b) The material is a monodisperse material. The response of different sizing techniques is weighted
differently depending on the property measured and how it is measured (e.g. for dynamic light
scattering (DLS): scattering intensity scales with the sixth power of the diameter). As size
polydispersity decreases, this weighting becomes less significant. For ideally monodisperse
particles, there is no influence due to weighting.
c) There are no other factors that influence the particle diameter as measured. A plethora of factors
can influence the apparent or real particle diameter even for spherical, monodisperse materials. For
example, particles can shrink in air or in vacuum, which means the actual diameter in air or in an
electron microscope can differ from the actual diameter in suspension. Molecules of the dispersing
liquid can adhere to the particle in dispersion, thus indicating a larger diameter in suspension
than in air. Molecules of dissolved salts can adhere to the particle when turned into an aerosol,
thus increasing the particle diameter of aerosolized particles compared to the same particles in
suspension.
The relative influence of most of these effects decreases as the particle diameter increases: an
adsorbing liquid layer of 1 nm to 2 nm is relevant for a particle diameter of 10 nm but irrelevant for
particles with diameters of 10 μm.
When these conditions are met the equivalent diameter often coincides with the actual diameter and
different equivalent diameters based on different measurement principles will often have the same
value. In these cases, it is therefore possible to use values determined by one method as reference
values for another, unrelated method.
5.2 Metrological traceability of size measurement results
5.2.1 General
As 3.3 highlights, metrological traceability is the property of a measurement result, i.e. the numerical
value that is assigned to a measurand of a certain kind of quantity. Traceability describes by which
calibrations (or comparisons) a measurement result is related to the stated reference.
EXAMPLE The goal is the determination of the length of a structure in a micrograph. The calibration of the
image magnification relates the number of pixels to the stated distance of lines on the calibration grid. The stated
distance of the lines of the calibration grid is related by measurement to the SI length unit (metre). This two-step
calibration makes the measured length traceable to the metre.
In the example above, the traceability of the measurement result is achieved by two sequential
calibration steps. The term “traceability chain” is used to describe such linear, sequential schemes that
establish traceability. However, many measurements have several, unrelated input quantities. The value
used for each of these input quantities shall be traceable to a stated reference to ensure that the final
result is traceable to the given reference. Such multiple references result in a “traceability network”
rather than a linear chain. It is irrelevant for the reference to which a measurement result is traceable
whether this traceability was achieved in one or multiple steps. Three examples of traceability networks
in particle sizing are shown in Figures 1, 2 and 3.
Key
A calibration of the magnification, pixels/gridline
B calibration of the calibration grid, gridlines/metre
C diameter, traceable to the SI via calibration of magnification and grid
NOTE SI Logo from BIPM.org under Creative Commons Licence CC BY-ND 4.0.
Figure 1 — Traceability chain/network of image analysis
Key
X time
Y voltage
A measured pulse height of the sample
B calibration of the pulse height with a CRM of diameter of d
CRM
C measurement of d by image analysis via calibration of the magnification and calibration of the grid
CRM
D relation of the measured pulse height of the sample to the pulse height of the CRM
E equivalent diameter of the sample, traceable to the SI via calibration of the pulse height of the ESZ and the
calibration of the magnification and grid for the determination of d
CRM
NOTE SI Logo from BIPM.org under Creative Commons Licence CC BY-ND 4.0.
Figure 2 — Traceability chain/network of electrical sensing zone
Key
X time
Y intensity
A calibration of the clock
B calibration of the temperature sensor
C viscosity determined via calibrated temperature sensor
D equivalent diameter, traceable to the SI via calibration of the clock and temoerature sensor
T temperature
NOTE 1 No calibration in length is required to obtain results traceable to the metre in the case of dynamic
light scattering.
NOTE 2 SI Logo from BIPM.org under Creative Commons Licence CC BY-ND 4.0.
Figure 3 — Traceability chain/network of dynamic light scattering
An important feature of the calibrations involved in establishing metrological traceability is that each
calibration (or comparison) contributes to the measurement uncertainty, even if this contribution may
be small or insignificant. In contrast, factors that do not contribute to the measurement uncertainty are
also not part of the traceability chain/network.
EXAMPLE 1 In the example above, the calibration of a number of pixels/distance between gridlines can only
be performed within a certain uncertainty. The distance between gridlines has an uncertainty that is derived
from the accuracy with which this distance was determined. Both of these uncertainties are part of the combined
measurement uncertainty of the image analysis, hence both are part of the traceability network.
EXAMPLE 2 A laboratory performs a DLS experiment and uses a CRM to demonstrate proper functioning of
the instrument. It compares the certified values with the measured values, but does not use the certified values
to adjust any parameters of the evaluation algorithm. Therefore, the CRM used is not part of the traceability
chain and the uncertainty of the certified value does not contribute to the overall measurement uncertainty. The
difference between verification and calibration is further explained in 5.2.2.
It is possible to convert input factors from part of the traceability network to the definition of the
property and vice versa by defining an input factor, which normally should be measured, as a certain
number. This number then becomes part of the definition of the measurands and, as it is defined without
uncertainty, also does not contribute to the measurement uncertainty.
EXAMPLE 3 The measured particle size in sedimentation analysis depends, among other, on the density of the
particle. If the value for this density is derived from measurements, these measurements need to be traceable
and the uncertainty of those results contributes to the overall measurement uncertainty. On the other hand, one
can also assume a certain value for the density, for example, 2,2 g/cm for amorphous silicon dioxide. In this case,
the definition of the measurand is “as obtained by sedimentation analysis, using a density value of 2,2 g/cm for
silicon dioxide”.
The definition of an operationally defined measurand itself is unrelated to the traceability of the
measurement result. The fact that a measurand is operationally defined does not mean that the quantity
values assigned to this measurand cannot be metrologically traceable. Traceability of the quantity
values is ensured by proper calibration of the relevant input factors. If this is achieved, then the results
are comparable with the results of another laboratory for the same operationally defined property.
EXAMPLE 4 Proper calibration of magnification and image processing ensures SI-traceability of the area-
equivalent circular diameter from an image analysis. As such, this value can be compared to the results from
other laboratories, as long as these are also the area-equivalent circular diameters of image analysis. Despite its
traceability to the metre, the result is not comparable to a result from laser diffraction (even if this result is also
traceable to the SI), as the two refer to different kinds of quantity.
5.2.2 Establishing versus verifying traceability in particle characterisation
There is an important difference between establishing and verifying traceability: as stated above,
establishing traceability requires calibration of input parameters and these calibrations all contribute
to the measurement uncertainty. Although results of properly calibrated instruments should deliver
traceable results, it is good practice to check the correctness of the calibration to verify the traceability
of results by measuring a CRM. Agreement of the measurement result with the certified values confirms:
a) correctness of the calibration; an incorrect calibration (e.g. wrong line width for a grid) will lead to
deviating results;
b) proper calibration of all other relevant input parameters; insufficient calibration of (even unknown)
relevant input parameters will lead to deviating results.
However, as no adjustment of instrument parameters is performed after this verification, this CRM
used to verify traceability is important, but not a part of the traceability chain.
EXAMPLE An electrical sensing zone (ESZ) instrument is calibrated with CRM A by adjusting the
nominal aperture size based on the certified diameter of this CRM. Subsequently, CRM B is used to verify that
the calibration was appropriate and that all other input factors are properly calibrated. CRM A is part of the
traceability chain, as its values are used to adjust the measurement result, but CRM B is not.
Many particle characterisation instruments are calibrated by the manufacturer, but the users cannot
directly calibrate their instruments or verify correct calibration of the relevant input parameters. An
example is the distance and angle of the detector from the sample in laser diffraction, where calibration
by the user requires dismantling of the instruments. In this case users need to assume proper
calibration of the input parameters by the manufacturer (an assumption that can be supported by
certification of the manufacturer to relevant ISO standards, reputation, experience of the manufacturer,
results of instruments from the manufacturer in proficiency tests etc.). CRMs (often particle size CRMs)
are subsequently used to verify the assumption of proper calibration by the manufacturer. This means
that measurements of these CRMs are important to verify traceability, but do not actually establish
traceability.
5.3 Types of RMs
5.3.1  Certified and non-certified RMs/certified and non-certified values
Within the framework of ISO 17034, the term “reference material (RM)” is the superordinate of “certified
reference material (CRM)”. There is currently no generally accepted term for non-certified materials.
Each RM, certified or not, shall be sufficiently homogeneous and stable for one or more clearly stated
measurand to serve as a reference in a measurement. Homogeneity and stability for one measurand
does not usually imply homogeneity and stability for other measurands. This means a material may be
an RM for one measurand, but not an RM for a different measurand.
Although the term CRM suggests that being certified or not is a status of the complete material, it is
more accurate to refer to certified values and non-certified values. Certified values shall come with
stated uncertainties and stated traceability, whereas neither uncertainty nor traceability is required
for non-certified values. ISO Guide 30 uses the term “indicative value,” as well as the terms “information
value” and “informative value,” to mean “non-certified value”.
CRMs can be used for calibration and assigning values to other materials for that specific certified
property value. Non-certified RMs can be used for statistical quality control, such as demonstrating a
measurement system is under statistical control, performs as expected and provides reliable resu
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