ISO/TR 22681:2019

TECHNICAL ISO/TR
REPORT 22681
First edition
2019-09
Hydraulic fluid power — Impact and
use of ISO 11171:2016 µm(b) and
µm(c) particle size designations on
particle count and filter test data
Reference number
ISO/TR 22681:2019(E)
©
ISO 2019

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ISO/TR 22681:2019(E)

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ISO/TR 22681:2019(E)

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Origins of the particle size shift . 1
5 Implications of the particle size shift . 3
6 Use of revised ISO 11171 . 6
Bibliography . 8
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ISO/TR 22681:2019(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
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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).
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.org/iso/foreword .html.
This document was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee
SC 6, Contamination control.
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.
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ISO/TR 22681:2019(E)

Introduction
A minor revision to ISO 11171:2010 was approved during 2016. The revision was necessitated by a
particle size shift resulting from the replacement of the particle counter calibration material, National
Institute of Standards and Technology (NIST) SRM 2806a, with SRM 2806b. Prior to this revision,
particle count data based upon ISO 11171 was reported in units of µm(c). Following the introduction
of SRM 2806b, users of particle count data in the absence of this revision could not discern whether
particle sizes being reported were based upon SRM 2806a or SRM 2806b. Hypothetically, a particle size
reported as 20 µm(c) could actually be as large as 20 µm or as small as 18 µm depending upon whether
SRM 2806a or SRM 2806b were used. This approximately 10 % shift in particle size can become
significant in terms of the actual numbers of particles counted.
To minimize confusion and provide for clear communication, ISO 11171:2010, 6.8 and 7.1 were revised
to provide a means for reporting particle size that clearly identifies the basis for reported particle
size and provides the industry with tools to relate past SRM 2806a and new SRM 2806b data without
extensive revisions to existing standards, specifications, and other literature. It provides a historically
consistent, traceable definition of µm(c), while allowing an option to report sizes in terms of a defined
µm(b) as needed. This document summarizes the underlying reasons for the minor revision and its
practical impact on the industry.
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TECHNICAL REPORT ISO/TR 22681:2019(E)
Hydraulic fluid power — Impact and use of ISO 11171:2016
µm(b) and µm(c) particle size designations on particle
count and filter test data
1 Scope
This document explains the use of the two acceptable methods of reporting particle size, µm(c)
and µm(b), that are defined in ISO 11171:2016. It also explains the reasons for the existence of two
alternative size reporting methods and its implications with respect to particle count and filter Beta
Ratio data.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https: //www .iso .org/obp
— IEC Electropedia: available at http: //www .electropedia .org/
4 Origins of the particle size shift
ISO 11171 provides direct traceability to the standard international (SI) definition of a metre through
NIST SRM 2806. ISO 11171 has been widely used to calibrate automatic particle counters (APCs) for
hydraulic, lube oil, diesel fuel, and other non-aqueous liquid applications since 1999. NIST SRM 2806 is
the primary calibration material used in this document.
During 2010, the original supply of SRM 2806, also sold as SRM 2806a, was exhausted. For ease of
communication, the term “SRM 2806a” is used henceforth in this document to refer to both designations,
SRM 2806 and SRM 2806a, of the original calibration material.
During 2014, its replacement, SRM 2806b, was released. The SRM 2806b production and certification
process was overseen by an international group of experts from ISO/TC 131/SC 6. The specifications for
SRM 2806b were better defined than those for SRM 2806a and it was produced by a different supplier.
Advances in sample preparation and metrology were used to produce and certify SRM 2806b. A critical
difference between SRM 2806a and SRM 2806b is that particle sizing was done manually from SEM
micrographs with SRM 2806a, while automated image analysis was used with SRM 2806b.
This allowed an order of magnitude of more particles to be analysed and in a manner not subject to
human bias. The end result was a certified calibration material, SRM 2806b, with better precision in
the size distribution and a reduction in uncertainty compared to its predecessor, SRM 2806a.
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ISO/TR 22681:2019(E)

An international round robin of ISO 11171 found that particle size, as defined using SRM 2806b
compared to SRM 2806a, had shifted. The relationship between SRM 2806a and SRM 2806b sizes was
determined from data submitted by 15 participating labs and is given by the following formula:
d = 0,898 d
c b
where
d is the particle size in µm(c) obtained using an APC calibrated with SRM 2806 and SRM 2806a;
c
d is the particle size in µm(b) obtained using an APC calibrated with SRM 2806b.
b
As a result of the minor revision to ISO 11171, particle size reported in µm(c) is traceable to SRM 2806a
using the conversion equation, or by direct measurement using an APC calibrated with SRM 2806a
or SRM 2806a-traceable secondary samples in the case of data generated prior to 2016. Particle sizes
traceable to SRM 2806b are reported in units of µm(b). Users of particle count data have the option to
report particle size in units of either µm(c) or µm(b), or both, as appropriate. This particle size shift is
within the published uncertainty of SRM 2806a, but it is great enough to affect particle count and filter
test data. The magnitude of the particle size shift is illustrated graphically in Figure 1.
In Figure 1, particle size in µm(c) as determined using ISO 11171 and SRM 2806a is plotted on the
X-axis. The corresponding particle size as determined using other means, i.e. using SRM 2806b or AC
Fine Test Dust, is plotted on the Y-axis. The bold line shows the relationship between µm(c) and µm(b).
The relationship passes through the origin with a slope of 1,114.
For reference, the thin line shows the relationship between µm(c) and µm sizes determined using the
1)
obsolete ISO 4402 calibration method and AC Fine Test Dust. This relationship does not pass through
the origin and the slope is steeper. Figure 1 demonstrates that the particle size shift with SRM 2806b
is significant, yet less than what was observed when the industry transitioned from ISO 4402 to
ISO 11171.
1) Withdrawn standard.
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ISO/TR 22681:2019(E)

Key
Y particle size µm
X particle size, µm (c)
1 µm(b) ISO 11171
2 AC fine test dust, ISO 4402
Figure 1 — Relationship between particle sizes defined using SRM 2806a and those obtained
using SRM 2806b, and using AC Fine Test Dust
5 Implications of the particle size shift
Prior to the revision in ISO 11171, there was no convenient method for users of particle count data to
determine whether or not their data was affected by the particle size shift. Users could not tell whether
particle count results were based upon SRM 2806a or SRM 2806b calibrations. As a result of the revision,
in most cases particle count and size data reported in units of µm(c) can be used in the same manner
that was don
...