ISO/TR 6049:2026

Technical
Report
ISO/TR 6049
First edition
Hydraulic fluid power — Procedures
2026-09
used to certify the standard
reference material SRM 2806d used
in ISO 11171
Transmissions hydrauliques — Procédures utilisées pour certifier
le matériau de référence normalisé SRM 2806d utilisé dans l'ISO
Reference number
© ISO 2026
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Equipment and material . 2
4.1 Test powder .2
4.1.1 Reference material for SRM 2806d .2
4.1.2 Reference materials RM 8361 and RM 8632 .2
4.2 Test fluid .2
4.3 Instrumentation .2
4.4 Statistical data analysis .3
5 Material production and description of acceptance testing . 3
5.1 Production of SRM 2806d .3
5.2 Automatic optical particle counter data .4
5.3 Scanning electron microscope .4
5.4 Accelerated ageing .5
5.5 Statistical analysis .5
5.5.1 General .5
5.5.2 Distributional check: histogram .6
5.5.3 Distributional check: normal probability plots .6
5.5.4 Homogeneity and stability plots .7
5.5.5 Homogeneity and stability statistics .7
5.5.6 Homogeneity and Stability Summary Graphics .7
5.6 Summary of acceptance phase .9
6 NIST Certification. 10
6.1 Interlaboratory study .10
[16]
6.2 The interlaboratory study for SRM 2806d [ISO 4813] .10
6.3 Calibration for the ILS . 12
6.4 Example of consensus values determined for SRM 2806d . 12
7 Determination of cumulative particle concentration for SRM 2806d by microscopy . 17
7.1 General .17
7.2 Uncertified results for SRM 2806d microscopy analysis .17
7.3 Comparison of microscopy and ILS results for SRM 2806d .19
7.4 Historic microscopy results . 22
8 Summary .27
Annex A (informative) Procedure used to create SRM 2806d .29
Bibliography .32

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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 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.

iv
Introduction
NIST Standard Reference Material (SRM) 2806d is intended for use as a reference material for calibrating
liquid particle sizing instruments, especially automatic optical particle counters. The fluid power industry
estimates that about 80 % of all hydraulic failures are due to particulate contamination in the hydraulic
fluid. Equipment failure is costly in terms of repairs, lost productivity, safety, and waste generation. Particle
counters are routinely used to monitor fluid cleanliness in operating systems and to assess cleanliness of
equipment coming off production lines as well as of individual components. SRM 2806d is a particle counter
[1]
calibrant for liquid suspensions and a required reference material for ISO 11171 . Industries impacted by
SRM 2806d include hydraulics industry, aerospace, automotive, ship, petroleum and lubricant, gas, power
generation, filter manufacturers and the military.
SRM 2806d is a National Institute of Standards and Technology (NIST) certified standard reference material.
“NIST certified value is a value for which NIST has the highest confidence in that all known or suspected
sources of bias and imprecision have been considered. For NIST to issue an SRM, the reference material at a
minimum must have certain properties:
a) the material must be homogenous;
b) the material must be stable over a suitable length of time;
c) some property of the material to be certified by NIST, a measurand, must be determined with utmost
care, usually by employing two or more measurement techniques;
d) the certified value must be traceable to a primary standard, i.e. the International System of Units (the
SI).
[2]
The measurand values and associated uncertainty in the certified value must be given. Enough information
must be given to provide users with confidence that the certified value is fit for the purpose specified in the
documents supplied to the user.
SRM 2806d – Medium Test Dust in Hydraulic Fluid is composed of MIL-PRF-5606 hydraulic fluid (Exxon-
[19]1) 1)
Mobil) with an added trace amount of SAE 5-80 Medium Test Dust (Particle Technology Inc., Arden
[3]
Hills, MN) . The polydisperse medium test dust came from the same batch of dry dust, 4390C, that was
used to make past SRM 2806, SRM 2806a, SRM 2806b, RM 8631, RM 8631a and RM8631b. The SRM has
been manufactured in approximately 200 l batches with a specified dust level of approximately 6 mg/l, and
split into 400 to 800 individual bottles, each bottle sequentially labelled to denote its place in the production
sequence.
The measurand for method-dependent values is operationally defined by the procedure used to obtain
the value. The values associated with SRM 2806d was partially derived by a method dependent process -
consensus in that an interlaboratory study (ILS) was conducted using automatic particle counters to achieve
this goal. However, SRM 2806d does have SI traceability through SRM 2806b which was certified by SEM
microscopy traceable to the meter through the NIST Line Scan Interferometer.
“A consensus value is derived from suitable number of valid measurement results and represents the best
estimate of the value based on agreement between multiple measurement results obtained from the ILS. The
ILS could be administered by NIST or another organization that has experience in directing the study and
the results and procedures are given with enough detail so that the ILS can be reproduced. There must be an
associated uncertainty in the derived value(s) calculated from accepted statistical means and the value(s)
[2]
and uncertainty needs to be fit for the purpose of the CRM .”
For SRM 2806d, the most relevant characteristics of the materials are the following:
— projected area particle diameter that is traceable to a recognized reference and cumulative particle size
concentration that is obtained by an accepted method;
— material is homogenous – low variation of cumulative particle size concentration from bottle to bottle;
1) This information represents trademarks for the respective companies and is given for the convenience of users of this
document and does not constitute an endorsement by ISO of the product named.

v
— material is stable over the normal use time. This means the hydraulic fluid does not degrade nor does the
particle population change due to say agglomeration or particle formation;
— material is free of any extraneous contaminant;
— production and testing are documented well enough to provide assurance and confidence in the material
and to provide a basis of quality in case something changes within the material.
There are two main steps to certifying a reference material that NIST can provide to its customer base. The
first step is very important because it determines the quality of the material that will be certified by NIST. In
the production and acceptance phase, NIST tests the material to verify that it is stable and homogenous and
that there are no anomalous properties or materials present in the material. Whether or not the material
is acceptable for certification is the critical question to be answered in the first phase. After the material
passes the acceptance criteria, some measurand associated with the material that NIST can both determine
with high confidence and can assign an associated uncertainty in that value is measured. If NIST cannot
have confidence in its determination of the measurand, the material cannot be certified, and cannot become
a NIST SRM, but can become an uncertified reference material. In the case of SRM 2806, the material has
been certified in the past by microscopy (SEM), but more recently by consensus determination as part of an
interlaboratory study. SRM 2806d has both certified and uncertified values associated with it.
Historically, SRM 2806 was certified by determining the cumulative particle size distribution of the medium
test dust particles using scanning electron microscopy to obtain electron micrograph images followed by
image analysis to determine particle area used to calculate equivalent circular diameter of the particles
[4]
traceable to the SI. The historic values of the cumulative particle concentration determined by microscopy
are given in this document for SRM 2806a, SRM 2806b and SRM 2806d. For the first time, in 2019, a NIST
issue of SRM 2806, SRM 2806d, was certified as a consensus standard based on analysis of measurements
from an interlaboratory study (ILS) that included 13 laboratories and 22 automatic particle counter (APC)
measurements with contributions from 5 countries – China, France, Germany, United Kingdom and the
United States. This is the 4th generation of this SRM, but the first based on consensus measurements. The
measurements were made traceable to the meter through a secondary standard that was compared to SRM
2806b, the SRM traceable to the meter. Ideally, it would be highly desirable to obtain traceability to a higher,
more fundamental standard for each consensus determination.
In summary, the steps taken to assure the quality to certify SRM 2806d are the following:
— production and acceptance testing of candidate material;
— homogeneity testing using a statistical experimental design to select bottles and automatic particle
counting by both manufacturer and NIST;
— automatic particle count data subjected to a battery of statistical tests;
— scanning electron microscopy (SEM) measurements on selected samples to verify hydraulic fluid
cleanliness and lack of contamination;
— accelerated ageing process at high and low temperatures to confirm stability;
— certification process of accepted candidate material;
— consensus through interlaboratory study with NIST doing the final data analysis;
— verification using SEM micrograph image collection and image analysis to derive projected area particle
diameter and cumulative particle concentration to compare with the ILS results;
— annual verification by automatic particle counter to confirm that the SRM continues to perform consistent
with the certified values.
The document covers main components of the NIST certification process for SRM 2806 and is divided into
the following main sections:
— production and NIST acceptance testing of the candidate material.
— certification process which has been accomplished by either of two methods:

vi
1) Interlaboratory Study (ILS).
2) Microscopy and image analysis.

vii
Technical Report ISO/TR 6049:2026(en)
Hydraulic fluid power — Procedures used to certify the
standard reference material SRM 2806d used in ISO 11171
1 Scope
This document describes the criteria and procedures used by the United States National Institute of
Standards and Technology (NIST) to certify the calibration material SRM 2806d, which is used in the
primary calibration of liquid automatic particle counters (APCs).
SRM 2806d is a suspension of SAE 5 - 80 μm test dust in hydraulic fluid with a cumulative particle
concentration determined through a consensus measurement processes, interlaboratory study (ILS),
utilizing APCs. The original projected area equivalent diameters for SRM 2806 were certified using a
scanning electron microscope (SEM) and image analysis techniques traceable to the meter through the NIST
line scan interferometer.
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
National Institute of Standards and Technology Standard Reference Material
reference material certified and issued by the National Institute of Standards and Technology (NIST)
that provides a measurand with its associated uncertainty used for calibration or comparison with other
materials.
Note 1 to entry: The common and recognized abbreviation for this is NIST SRM.
3.2
interlaboratory study
ILS
collaborative study of a material that provides enough technically valid measurement results obtained
from appropriate portions of homogenous stable material to enable generation of a consensus value (and
associated uncertainty) that is fit for purpose of the material
Note 1 to entry: In this document, the material is an automatic particle counter calibration fluid.
[SOURCE: Reference [2], 2,4, modified — “NIST” deleted before “material”, “obtained from appropriate
portions of homogenous stable material” added, “(and associated uncertainty) that is fit for purpose of the
material” replaced “whose associated uncertainty is fit for purpose”. Note 1 to entry added.]
3.3
automatic particle counter
APC
instrument that automatically
a) senses individual particles suspended in a controlled volume of fluid using optical light extinction or
light scattering principles,
b) provides electrical response that correlates to the size of particles,
c) sorts or compiles responses (particles) into size ranges, and
d) counts particles in each size range.
3.4
scanning electron microscopy
SEM
microscopy using a scanning electron beam to produce micrograph images
3.5
image analysis
process of performing measurements on the morphology of digital micrograph representations of objects
3.6
statistical analysis
data analysis employing accepted mathematical methods to describe the properties of the data, such as
patterns and trends, and characterize the data based on overall or average behaviour, as well as provide a
description of variation
3.7
equivalent projected area particle diameter
equivalent diameter of a circular particle in two dimensions that has the same area as the particle measured
4 Equipment and material
4.1 Test powder
4.1.1 Reference material for SRM 2806d
The particulate material used is a silica powder made from Arizona desert sand by jet milling and then air
classifying to a consistent particle size distribution. Several grades with different size ranges are available
[3]
and their properties are specified in ISO 12103-1 .
The powder used to prepare SRM 2806d is SAE 5-80 μm test dust with supplier batch number 4390C in the
approximate size range as an ISO 12103-A3 grade, also called ISO MTD.
4.1.2 Reference materials RM 8361 and RM 8632
NIST reference materials RM 8631 and RM 8632 are composed of SAE 5-80 and ISO ultra-fine test dust,
lot numbers 4390C (same lot as the SRM 2806d) and 13458D ultra fine test dust, respectively. These RMs
[5][6][8]
provide materials to make secondary standards used in support of ISO 11171 and SRM 2806d .
4.2 Test fluid
Test fluid in which ISO MTD is suspended is a hydraulic fluid widely used worldwide for filter testing.
This fluid is defined in American National Standards as Exxon Mobil MIL-PRF-5606 hydraulic fluid. Fluid
o 3 3
density measured at 22 C (at NIST) was 0,869 624 g/cm with an uncertainty of 0,0008 g/cm . The index of
refraction was found to be 1,470 5.
4.3 Instrumentation
The ILS employed a diverse set of automatic particle counters normally used by the user group of the
standard. The current ILS contained four different automatic particle counters models that are widely used
in the community.
The NIST laboratory analysis was conducted using two techniques:
a) automatic optical particle counting (APC) using an HRLD 600 sensor (for the initial acceptance) and a
HRLD 150JA (for final ILS) both with an auto sampler operating in the volumetric mode, and
1)
b) microscopy using a commercial available scanning electron microscope, TESCAN MIRA-3 automated
[8][9]
using SEMantics to image the particles and NIST Lispix software to image analyze to determine
[10]
particle diameter and particle concentration .
4.4 Statistical data analysis
Statistical data analysis was conducted at NIST using a graphical Exploratory Data Analysis approach coded
[11][12]
in DATAPLOT. Overall description of the data and statistical analysis associated with SEM particle
[4]
analysis process is available .
5 Material production and description of acceptance testing
5.1 Production of SRM 2806d
Creation and test procedures that were used to assess the quality and suitability of the new material to
become a NIST SRM. The candidate SRM 2806d material was made and tested according to the Specifications
for Manufacturing and Acceptance of New SRM 2806 Material document given in the Annex A.
Hydraulic fluid suspension was made using a test stand (with four delivery nozzles A, B, C, and D) that
circulates the mineral dust - hydraulic fluid suspension to mix the material thoroughly and produce ideally
a large homogeneous suspension. Sequentially 100 bottles were filled from 4 nozzles for a production run of
400 bottles labelled 1A to 100A, 1B to 100B, …, 1D to 100D. Bottles analysed by manufacturer and by NIST
were chosen by a statistical design. Bottles were taken from the global collection 400 bottles as specified
in Annex A, designed to sample the material over the complete production sequence, but randomized for
possible systematic problems related to filling nozzle or circulation flow rate.
From Annex A:
The manufacturer analysed one bottle out of each ten bottles produced for within batch homogeneity as
described in ISO 11171:2022, F.3, F.4 and F.5, except that only a total of 10 % bottle samples were analysed
(10 % total number). NIST received and tested eight bottles for within batch for homogeneity using an
extinction particle counter calibrated to the existing SRM 2806b. The eight bottles consist of two bottles
taken from each quartile of the production cycle. The data is compared to analogous data from the
manufacturer.
The schematic of the test stand used to prepare the bottles of SRM 2806d is shown in Figure 1, taken from
the withdrawn ISO/TR 16144.
Key
1 fluid reservoir (200 l)
2 circulating pump
3 clean-up filter
4 sampling tap
Figure 1 — Schematic of the preparation loop used to mix and fill bottles of SRM 2806d. Sampling
tap (4) has four nozzles
5.2 Automatic optical particle counter data
Liquid borne automatic particle counters are used to qualify the candidate material and ultimately provide
the data to certify this reference material through a multi-lab measurement process, the above-mentioned
ILS.
The procedure for analysis of a single bottle followed the recommended approach described in ISO 11171.
The sensor was calibrated using NIST SRM 2806b (the current valid SRM) to determine the particle number
distribution of candidate SRM. This single calibration was used throughout the measurements.
5.3 Scanning electron microscope
Selected bottles of candidate SRM 2806d were subjected to the resuspension procedure described in
ISO 11171. Approximately 10 mL to 50 mL of hydraulic fluid were poured and then filtered from each sample
using 0,4 μm diameter pore size polycarbonate filters. The mass of the aliquot of filtered hydraulic fluid
was determined by weighing the fluid bottle sample before and after filtration. The filtered samples were
washed three to four times each with a clean, filtered (0,2 μm) reagent grade heptane solvent to remove the
oil residue. The filters were evacuated over 48 h to remove any volatiles. Filters were gold coated with ca.
10 nm thick gold delivered by an Ar plasma coater and were mounted on a metal stage and analysed using a
[8][9]
TESCAN MIRA-3 SEM . The goal of this analysis was to verify the cleanliness of the hydraulic oil samples,
to verify that the hydraulic fluid has not degraded and to verify that there is no contaminate materials in the
suspension.
SEM images are taken of filtered particles from selected bottles of candidate SRM 2806d. A typical electron
[4]
backscatter image is shown in Figure 2. The procedure is detailed elsewhere .

Figure 2 — SEM backscattered electron micrograph of filtered candidate SRM 2806d magnified such
that full scale is 0,5 mm
5.4 Accelerated ageing
Multiple bottles of candidate SRM 2806d material were placed in a 75 °C oven, and bottles were placed in a
refrigerator at approximately 4 °C. The materials were monitored for 90 days to determine if the candidate
SRM 2806d hydraulic fluid degrades. All bottles were unopened, and all contained medium test dust. Clean,
particle free hydraulic fluid was subjected to similar tests.
During the thermal tests, the bottles were examined visually while at their respective temperatures either
value (75 °C and 4 °C) – not cooled/warmed to lab temperatures. After 90 days, the bottles were allowed to
return to room temperature ~22 °C. Immediately after shaking and sonicating each sample, about 50 mL of
oil was poured from each sample, was filtered, and prepared as described.
The contents of all four bottles were analysed with the automatic particle counter calibrated to the same
scale used to qualify the material. Both the SEM images and the particle counter results were compared to
samples of the candidate SRM 2806d that has remained at room temperature.
5.5 Statistical analysis
5.5.1 General
Automatic particle count data described in Clause 5 was subjected to a rigorous set of Exploratory Data
Analysis (EDA). A five-step statistical methodology also supplied to manufacturer was designed to quantify

the quality of the candidate reference material, to aid the manufacturer and to provide firm evaluation
[13]
criteria for NIST acceptance and further work on a specific batch of material. The five steps are:
a) distributional check: histogram;
b) distributional check: normal probability plots;
c) homogeneity and stability plots;
d) homogeneity and stability stats;
e) homogeneity and stability summary graphics.
EDA graphical representation for data analysis, publicly available, was most informative because the
eye was a good gauge of anomalies in the data. It was especially beneficial, as shown in the past, for the
manufacturer to use the analysis package in conjunction with their on-line and bottle data results to
facilitate improvements in their mixing process. Five steps are summarized here.
5.5.2 Distributional check: histogram
This was a check for distributional shape and outliers. Under rather general circumstances, a well-behaved
manufacturing process has a histogram that is bell-shaped and outlier-free. For a given particle diameter,
the histogram as seen in Figure 3 was of the optical particle counter response (cumulative number particles/
ml > the given diameter).
Key
X number of particles > diameter
Y number of counts
Figure 3 — Histogram representation of particle
5.5.3 Distributional check: normal probability plots
Provides a check for distributional normality. If the data was normally distributed, then certain subsequent
statistical tests were valid; non-normality was sometimes a flag for some process abnormality. For a given
diameter, the normal probability plot illustrated in Figure 4 has the ordered response (cumulative number
of particles/mL > the given diameter) on the vertical axis, and theoretical normal-spaced values on the
horizontal axis. Linearity in the plot indicates normality; non-linearity indicates non-normality.
Key
X ordered N(0,1) statistical medians
Y ordered number of particles > diameter
Figure 4 — Example of normal probability plots

5.5.4 Homogeneity and stability plots
Check for the homogeneity and stability of the material manufacturing process--a process in statistical
control will be both homogeneous and stable. Homogeneity here meant that the particle counts are near
equivalent (as per ANOVA or Student’s t-test) between the first half of the manufacturing process and the
second half. Stability meant that the slope of the data over the entire manufacturing range was near-flat. In
Figure 5, vertical axis was the response (cumulative particle concentration); the horizontal axis was coded
time (or ordered bottle) --low numbers indicate early in the manufacturing process and high
numbers indicated later in the process. The plot was augmented with the mean value and with a band in
which approximately 95 % of the values fall (if the process was in statistical control).
Key
X time coded in terms of bottle run number
Y number particles > diameter
band is k = 2 bounds
Figure 5 — Example of homogeneity and stability plots
5.5.5 Homogeneity and stability statistics
Quantitative analogue for Step 3--providing relevant formal test statistics to assess the homogeneity and
stability of the material manufacturing process. The cell shown in Figure 6 provided a list of test statistics
which reflect on the location and variability of the diameter's response over the entire manufacturing range.
The last three lines of the cell provide test statistic results for homogeneity and for stability
Figure 6 — Example of homogeneity and stability statistics
5.5.6 Homogeneity and Stability Summary Graphics
Graphical summary of all the information presented in Step 4. This fifth step has nine plots--one plot for
each of the statistics imbedded in Step 4 illustrated in Figure 7. As seen in Figure 8, horizontal axis for each
plot was the diameter (with typically eight or sixteen values). The vertical axis was the computed statistic
(from Step 4) for each diameter value. The most important plots were plots 8 and 9--presenting the results
of the homogeneity and stability statistics of Step 4 (see Figure 8); the tests stats were presented in green if
the process "passed", and in red if the process "failed".

Key
X diameter (μm)
Y t slope
band indicates acceptable domain
Figure 7 — Example of summary graphics for plot 8
Five-step methodology presented in Figure 8 was applied to both early on-line data at the material
developmental stage by the manufacturer and later in bulk bottled samples at the final NIST SRM material
acceptance stage. The combination of 1) focusing on key statistical properties of a process in statistical
control, along with 2) the graphical nature of the approach enabled the material manufacturer (and
the reference material agency) to immediately gain insight into problems with the data set and thus the
prospective sample.
Again, the five-step procedure was portable and free to the public from https:// www .nist .gov/ itl/ sed/
[11][12]
dataplot . The five-step method was described in a published Reference [13].
Example of the nine summary plots presented in step 5 are shown in Figure 8.

Key
X particle diameter (μm)
Y mean cumulative particle concentration
Y 2 times standard deviation of the data
Y relative 2 times standard deviation of the data (%)
Y relative standard deviation of the data (%)
Y relative range (%)
Y autocorrelation (%)
Y slope
Y tslope
Y anova CFD (%)
NOTE Each frame presents the summary of a statistical test. (Horizontal axis is particle diameter, vertical axis
is denoted above the individual plot.) Green colour associated with data points indicate values are within
specification, red is outside. Complete description in published manuscript [13].
Figure 8 — Graphical summary of the quality as described above in statistical testing section
5.6 Summary of acceptance phase
For the material to be accepted by NIST and proceed to the next phase of certification it had the following
attributes:
— the material meets the specification document requirements;
— the material has acceptable agreement (as defined in the specification document) between manufacturer
and NIST particle measurement values;

— the material passes the five-step statistical test;
— the material does not degrade during the accelerated ageing process;
— the material contains no contaminate particles.
6 NIST Certification
6.1 Interlaboratory study
Values that were NIST certified and made traceable to the SI, the projected area diameters matched to their
associated cumulative particle concentrations were determined by an interlaboratory study (ILS). The
consensus values, values agreed upon based on the ILS measurements results, with the feature that the
calibrant used by all participants in the study was a current certified SRM or secondary material traceable to
the previous edition of SRM 2806d that has SI traceability through SRM 2806b certified by SEM microscopy
traceable to the meter resulted in the best estimate of the cumulative particle concentration.
The ILS was administered by an organization or coordinator that had experience in directing the study
and the results and procedures were given with enough detail so that the ILS could be reproduced. The
credibility of the results was dependent upon an experienced standards organization that can validate the
[2]
results and assign uncertainties fit for purpose of the CRM .
[16]
6.2 The interlaboratory study for SRM 2806d [ISO 4813]
Voluntary participants agreed to perform the measurement protocol without deviating from the detailed
instructions provided by the coordinator. They had the necessary equipment and expertise to collaborate.
The automatic particle counters were current with certain minimum capabilities defined by the ILS. The
particle counters were required to meet performance requirements based on ISO 11171. Participating
laboratories had to qualify by performing tests similar to the instrument performance criteria and
procedures found in ISO 11171 that provide methodology to verify the quality and calibration of the APC.
The results were evaluated by the ILS coordinator before the potential participant joined the ILS.
Each participant had detailed instructions on how to perform the calibrations, instrument verification and
[16]
candidate SRM 2806d measurements (for example, see ISO/TR 4813 . Method of and format for the results
were provided. Only data in the required format were accepted.
[14][15]
ILS experimental design, characteristics of requirements :
a) clearly identified coordinator of the ILS was available for any questions or concerns during the technical
procedures;
b) the design identified the calibrant for the ILS, a secondary calibrant traceable to SRM 2806b, this
included polystyrene or glass microspheres for diameters > 30 μm(c);
c) qualified participants based on qualification and the APC technology (ideally cover all the technologies
currently used in the community);
d) required ISO 11171 with modifications;
e) coordinator provided detailed procedure and spread sheets for participants to analyse candidate SRM
2806d;
f) coordinator analysed participant-submitted data as per ISO 5725-2 or other acceptable method;
g) coordinator transmitted unaltered raw measurement data to NIST for further analysis.
Each participant reported the minimum set of required diameter values and any additional values possible
to support the relevant document standards. An example set is (2, 3, 4, 6, 10, 14, 21, 30, 38 and 50) μm(c),
units defined below. Most APCs had 16 to 18 channels so data for each available channel could have been
provided (desirable). The calibration for the particles > 30 μm(c) was accomplished using polystyrene latex
microspheres as described in ISO 11171.

The data was reported to the coordinator of the ILS in a defined format provided. Materials and
documentation were provided to each member of the ILS. For example, each participant was provided the
following:
— for calibration, three samples of current SRM 2806 or other prescribed calibration suspension;
— three samples of SRM 2806d candidate material to measure;
— ISO 11171 and ILS protocol (provided by coordinator) if different than 11171;
— ILS reporting forms or required data format to submit results to coordinator;
— person or address to send the formatted measurement results; and
— a deadline for submitting results.
An example of the capabilities of the labs involved in the 2019 ISO 11171 ILS is given in Table 1. In this study,
all labs remained anonymous with identifying numbers assigned by an ISO coordinator. The laboratories’
APC characteristics are presented. One of the most important goals of the user community is continuity of
subsequent batches of SRM 2806x traceable back to the original SRM 2806 issued in 1998 [ISO/TR 4808]
[15] [15]
. The values for particle size, have been assigned the notation of micrometer c [μm(c)] [ISO/TR 4808] .
The notation μm(b) was created to identify the equivalent projected area diameters reported in SRM 2806b
certificate. The values labelled μm(b) are traceable to the NIST line scan primary standard.
μm(c) = 0,898 × μm(b)
Table 1 — Data summary for the 13-lab ISO 11171 interlaboratory study performed in 2019
a b c d e f g h i j k l m
1 2 3 4 5 6 7 8 9 10 11 12 13
Meas. Orig. Lab Number Instrument Instrum. Instrum. Coincidence C Instrument Sample Sample Number of
V,vol
dataset dataset ID diameters manufact type noise error resolution volume flowrate voltage
ID ID measured level part./ml % % ml thresholds
reported mV  ml/min
1 1a 1 7 1 E 105 27 000 2,7 13,4 10 25 16
2 2a 2 7 2 E 10 19 000 0,4 5 10 50 12
3 3a 3 7 1 E 150 59 129 0,4 8,5 10 10 16
4 4a 3 7 1 S 80 12 713 0,2 6,8 10 10 16
5 3b 3 7 1 E 150 59 129 0,4 8,5 10 10 16
6 4b 3 7 1 S 80 12 713 0,2 6,8 10 10 16
7 5a 4 15 1 E 200 27 171 1,9 9,5 10 25 16
8 6a 5 7 1 E 120 89 813 0,8 9,6 10 10 18
9 6b 5 7 1 E 120 89 813 0,8 9,6 10 10 18
10 16a 5 15 1 S 40 14 003 0,8 10 10 10 18
11 7a 6 7 3 E 178 30 000 0,4 7,4 10 30 17
12 18a 6 5 3 E 181 30 000 0,4 7,4 10 30 17
13 8a 7 7 2 E 3 40 246 0,6 11,2 10 30 18
14 17a 7 7 2 E 8 15 789 0,6 7 10 20 18
15 9a 8 28 2 E 10 6 000 0,9 4,5 10 25 101
16 9b 8 28 2 E 10 6 000 0,9 4,5 10 25 101
17 10a 9 6 1 E 77 34 251 0,1 6,7 10 25 13
18 11a 10 7 1 E 190 19 418 0,2 9,6 10 25 16
19 12a 11 6 1 E 190 20 743 0,3 6 10 25 24
20 13a 11 7 1 S 150 12 245 0,6 3 25 10 24

TTaabblle 1 e 1 ((ccoonnttiinnueuedd))
a b c d e f g h i j k l m
1 2 3 4 5 6 7 8 9 10 11 12 13
Meas. Orig. Lab Number Instrument Instrum. Instrum. Coincidence C Instrument Sample Sample Number of
V,vol
dataset dataset ID diameters manufact type noise error resolution volume flowrate voltage
ID ID measured
...