ISO/TR 16144:2002
(Main)Hydraulic fluid power — Calibration of liquid automatic particle counters — Procedures used to certify the standard reference material SRM 2806
Hydraulic fluid power — Calibration of liquid automatic particle counters — Procedures used to certify the standard reference material SRM 2806
ISO/TR 16144:2002 describes the procedures used by the United States National Institute of Standards and Technology (NIST) for the certification of the calibration material SRM 2806, which is used in the primary calibration of liquid automatic particle counters. SRM 2806 is a suspension of ISO MTD in hydraulic fluid with a number size distribution certified using a scanning electron microscope (SEM) and image analysis techniques.
Transmissions hydrauliques — Étalonnage des compteurs automatiques de particules en suspension dans les liquides — Procédures utilisées pour certifier le matériau de référence normalisé SRM 2806
L'ISO/TR 16144 décrit les procédures utilisées par le National Institute of Standards and Technology (NIST) des États-Unis pour la certification du matériau d'étalonnage SRM 2806, qui est utilisé pour l'étalonnage primaire des compteurs automatiques de particules en suspension dans les liquides. Le SRM 2806 est une suspension d'ISO MTD dans un fluide hydraulique avec une distribution granulométrique en nombre certifiée à l'aide d'un microscope électronique à balayage (MEB) et de techniques d'analyse d'image.
Fluidna tehnika - Hidravlika - Kalibriranje naprav za avtomatsko štetje delcev, izločenih v tekočinah - Uporabljeni postopki za certificiranje standardnih referenčnih materialov po SRM 2806
General Information
Buy Standard
Standards Content (Sample)
SLOVENSKI STANDARD
SIST ISO/TR 16144:2003
01-julij-2003
)OXLGQDWHKQLND+LGUDYOLND.DOLEULUDQMHQDSUDY]DDYWRPDWVNRãWHWMHGHOFHY
L]ORþHQLKYWHNRþLQDK8SRUDEOMHQLSRVWRSNL]DFHUWLILFLUDQMHVWDQGDUGQLK
UHIHUHQþQLKPDWHULDORYSR650
Hydraulic fluid power -- Calibration of liquid automatic particle counters -- Procedures
used to certify the standard reference material SRM 2806
Transmissions hydrauliques -- Étalonnage des compteurs automatiques de particules en
suspension dans les liquides -- Procédures utilisées pour certifier le matériau de
référence normalisé SRM 2806
Ta slovenski standard je istoveten z: ISO/TR 16144:2002
ICS:
23.100.60 )LOWULWHVQLODLQ Filters, seals and
RQHVQDåHYDQMHWHNRþLQ contamination of fluids
SIST ISO/TR 16144:2003 en
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
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SIST ISO/TR 16144:2003
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SIST ISO/TR 16144:2003
TECHNICAL ISO/TR
REPORT 16144
First edition
2002-09-15
Hydraulic fluid power — Calibration of
liquid automatic particle counters —
Procedures used to certify the standard
reference material SRM 2806
Transmissions hydrauliques — Étalonnage des compteurs automatiques
de particules en suspension dans les liquides — Procédures utilisées pour
certifier le matériau de référence normalisé SRM 2806
Reference number
ISO/TR 16144:2002(E)
©
ISO 2002
---------------------- Page: 3 ----------------------
SIST ISO/TR 16144:2003
ISO/TR 16144:2002(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not
be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this
file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this
area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2002
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body
in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
ii © ISO 2002 – All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO/TR 16144:2003
ISO/TR 16144:2002(E)
Contents Page
Foreword . iv
Introduction. v
1 Scope. 1
2 Equipment and material. 1
2.1 Test powder . 1
2.2 Test fluid. 1
2.3 Sample preparation loop . 2
2.4 Membrane preparation equipment . 2
2.5 Scanning electron microscope and image analyser .2
3 Equipment validation. 3
3.1 Sample preparation validation. 3
3.2 Microscope calibration validation . 4
3.3 Membrane preparation validation. 4
3.4 Membrane and SRM 2806 stability testing . 6
4 Test procedure . 6
4.1 Calibration suspension preparation SRM 2806 . 6
4.2 Membrane preparation. 7
4.3 Membrane examination and particle counting. 7
5 Data processing . 11
Bibliography. 16
© ISO 2002 – All rights reserved iii
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SIST ISO/TR 16144:2003
ISO/TR 16144:2002(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that which is
normally published as an International Standard (“state of the art”, for example), it may decide by a simple majority
vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature
and does not have to be reviewed until the data it provides are considered to be no longer valid or useful.
Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 16144 was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 6,
Contamination control.
iv © ISO 2002 – All rights reserved
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SIST ISO/TR 16144:2003
ISO/TR 16144:2002(E)
Introduction
Solid particulates are a major contributor to wear in hydraulic systems. The fluid power industry, the aerospace
industry and the military sector utilize optical automatic particle counter (APC) technologies to assess the level of
hydraulic oil contamination by suspended particulate. The amount of contamination is often related to the integrity
of the system and the usage of the fluid. APCs are also employed in various oil filter testing operations by the
[1]1)
manufacturers and the users. The standard method ISO 4402 has been used for nearly 30 years to calibrate
optical particle counters in terms of particle size as a function of particle concentration.
The calibration material used in ISO 4402:1991 is Air Cleaner Fine Test Dust (ACFTD) produced in the past by a
division of General Motors Corporation. This material consists of a polydisperse dust having the largest number of
particles, as indicated in ISO 4402:1991, with the size range of 1 µm to 80 µm diameter (particle concentration
increases with decreasing diameter). There is a low concentration of particles reported to extend out to
approximately 100 µm. Some problems have arisen with the use of ACFTD in such calibration procedures. Firstly,
there has been ongoing concern that the particle size distribution is not accurate in the small particle size regime
[2], [3], [4], [5]
(< 10 µm) of the distribution . Many researchers have noted that there are more sub-10 µm particles in
ACFTD than reported by ISO 4402:1991. Secondly, but not less importantly, the production of ACFTD has been
discontinued by the supplier.
Thus there is a need to investigate, design and devise a new standard method (Hydraulic fluid power — Calibration
[6]
method for liquid automatic particle counters) using a new Standard Reference Material (SRM) . The National
Institute of Standards and Technology (NIST) was requested to develop an SRM for use by the fluid power
industry. Users will benefit from improved precision since there is a central source of only one material and
[7]
increased accuracy resulting from the size characterization . The new SRM, designated as SRM 2806, is
composed of ISO Medium Test Dust (ISO MTD) suspended in MIL-H-5606 hydraulic fluid. The number of particles
per millilitre greater than specified sizes has been determined for this material.
1) Cancelled in 1999 and replaced by ISO 11171:1999.
© ISO 2002 – All rights reserved v
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SIST ISO/TR 16144:2003
TECHNICAL REPORT ISO/TR 16144:2002(E)
Hydraulic fluid power — Calibration of liquid automatic particle
counters — Procedures used to certify the standard reference
material SRM 2806
1 Scope
This Technical Report describes the procedures used by the United States National Institute of Standards and
Technology (NIST) for the certification of the calibration material SRM 2806, which is used in the primary calibration
of liquid automatic particle counters.
SRM 2806 is a suspension of ISO MTD in hydraulic fluid with a number size distribution certified using a scanning
electron microscope (SEM) and image analysis techniques.
2 Equipment and material
2.1 Test powder
2.1.1 Standard reference material SRM 2806
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 and their
[8]
properties are specified in ISO 12103-1 .
The powder used to prepare SRM 2806 is an ISO 12103-A3 grade, also called ISO MTD, with supplier batch
number 4390C.
2.1.2 Reference materials RM 8631 and RM 8632
Reference materials RM 8631 and RM 8632 are composed of ISO MTD and ISO ultra fine test dust lot numbers
4390C (same lot as the SRM 2806) and 4476 J, respectively. These RMs provide materials to make secondary
[9] [10]
standards used in support of ISO 11171 and SRM 2806 . The RM was received in 3,6 kg bottles. This dust
was dried and spin-riffled into 147 aliquots, each of 20 mg. The material was examined for homogeneity using
optical particle counters after suspension in clean oil.
2.2 Test fluid
Test fluid in which ISO MTD is suspended is a hydraulic fluid widely used worldwide for filter testing. This oil is
defined in American national standards as MIL-H 5606 and in French national standards as AIR 3520, and in the
NATO specification H 515.
[11]
Its physical-chemical properties are defined in annex A of ISO 16889:1999 .
To ease particle dispersion, a small quantity (50 µg/g) of an antistatic agent is added to the oil so that its
conductivity is 1 500 pS/m ± 100 pS/m.
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2.3 Sample preparation loop
In view of supplying worldwide demand for several years with the SRM 2806 (supplied in bottles of 400 ml), it was
necessary to prepare and store a great number of bottles for further sales.
Because of the settling velocity of the larger silica grains, a special mixing loop was built with mechanical and
hydraulic components which were used to eliminate grinding the powder in suspension. It was designed according
[12]
to the recommendations of ISO 11943 .
To guarantee bottle sample homogeneity, a supplementary volume of oil was necessary to allow sampling of
control bottles used as described in 3.1.2.
The schematic of the sample preparation loop is given in Figure 1.
Key
1 Fluid reservoir (200 l)
2 Circulating pump
3 Clean-up filter
4 Sampling tap
Figure 1 — Schematic of calibration suspension preparation loop
2.4 Membrane preparation equipment
Particles are filtered on 25 mm diameter polycarbonate membranes, 0,2 µm pore diameter using the equipment
[13]
commonly used for determining hydraulic fluid particulate contamination by gravimetry according to ISO 4405 or
[14]
by microscopic counting according to ISO 4407 .
2.5 Scanning electron microscope and image analyser
The scanning electron microscope used to examine particles is a JEOL 840. The images were produced by
electron backscattering and collected on a MicroVax and analysed using LISPIX, a public domain image
processing software developed at NIST. LISPIX currently runs on any computer.
2 © ISO 2002 – All rights reserved
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3 Equipment validation
3.1 Sample preparation validation
3.1.1 General
Quality assurance for both production and testing was developed by a task force composed of North American
members from two filter manufacturers, a particle counter manufacturer, an independent laboratory and NIST. APC
measurements were made by both the independent laboratory and NIST, with NIST performing the data analysis.
3.1.2 Homogeneity testing/batch screening
An experimental sampling design was developed and implemented at NIST to measure the bottle-to-bottle
homogeneity and, at the same time, to identify possible systematic errors in the instrumental measurements. In the
production process, four bottles (a, b, c, d) were filled at any one time. There were 320 bottles per batch and
bottles were numerically labelled sequentially from 1 (a, b, c, d), 2 (a, b, c, d), ., to 80 (a, b, c, d) as they were
produced. Selected bottles from each batch were tested for homogeneity at both the independent laboratory and
NIST using APCs with extinction sensors calibrated according to ISO 4402:1991. Four bottles (a, b, c, d) were
sampled and analysed from approximately the following four points in the production cycle: 5 %, 30 %, 60 %, and
95 %. Another set of four bottles that were produced directly adjacent to the first four were then analysed. For
example, the first 16 bottles 5 a, 5 b, 5 c, 5 d, 25 (a, b, c, d), 50 (a, b, c, d) and 75 (a, b, c, d) were analysed in that
order. Then bottles 6 (a, b, c, d), 26 (a, b, c, d), 51 (a, b, c, d), and 76 (a, b, c, d) were analysed all by the same
calibrated APC. With three replicates for each bottle, this totalled 96 measurements. Each batch of 320 bottles was
subjected to this procedure or a modified version of this test. A batch of material was deemed homogeneous if the
coefficient of variation for the number of particles larger than 5 µm, 7 µm, 10 µm, 20 µm and 30 µm did not exceed
4 %, 4 %, 4 %, 5 % and 7 % respectively and there were no systematic variations in the batch. The cumulative
particle size distribution was determined for the nominal size range of 1 µm to 80 µm particle diameter and
measurements were compared for the same batch of materials.
3.1.3 Homogeneity
To provide high precision measurement capability for a user community, a standard reference material should be
as homogeneous as possible. Special efforts were made to assure that this material was made with a low bottle-to-
bottle variation within the batch. Within batch variability for the SRM is presented in Table 1 expressed as relative
standard deviation for within batch measurements. Figure 2 shows the batch-to-batch comparison in histogram
form. The histogram is composed of the mean values of the cumulative particle counts for the same volume of fluid
analysed.
Table 1 — Variability found within a batch of material
Greater than size Relative standard deviation
µm %
5 1,1
10 1,3
15 2,0
20 3,8
30 6,7
© ISO 2002 – All rights reserved 3
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Figure 2 — Particle counts in four batches of ISO MTD to verify bottle-to-bottle homogeneity
3.2 Microscope calibration validation
3.2.1 Microscope calibration
SRM 484d, a NIST scanning electron microscope magnification standard, is mounted in the x and y direction
(orthogonal) on the SEM sample stage and used in conjunction with each sample to calibrate the x-y length for
particle sizing. SRM 1960, 1 µm polystyrene spheres, were examined by the same procedures used for the dust
particles, in order to verify the procedure. Elemental analysis is conducted for a subset of dust particles in the filter
sample using energy dispersive X-ray spectroscopy to assure, within the limits of the experiment, that only mineral
dust is analysed and that other contaminating particulate material is not present.
3.2.2 Traceability
[15]
Particle size is traceable to a primary measurement, optical interferometry, through the NIST SRM 484d . This
SRM standard was imaged for each data set collected and for each magnification. From the certified lengths and
the measured number of pixels, a pixel-to-length relation was derived and was used to convert the particle images
represented in pixels to area in square micrometres. Uncertainties were determined for these conversions (reported
as length uncertainty). They are composed of a combination of uncertainty in the pixel determination and the
[10], [15]
reported uncertainty in the SRM .
3.3 Membrane preparation validation
3.3.1 Sampling from the filter
One component of the total measurement uncertainty results from sampling. Analysis of a large number of fields
indicated a non-uniform particle coverage on the filter. Non-uniform particle deposition was observed on many of
4 © ISO 2002 – All rights reserved
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ISO/TR 16144:2002(E)
the filters after particle separation from the hydraulic oil. Presumably, this was the result of vigorous rinsing with the
clean solvent that caused particles to be preferentially relocated toward the centre of the filter. To overcome this
sampling problem, the SEM was made to select randomly fields-of-view of particles on the filter surface. Figure 3
shows a schematic of a filter with the regions from which micrograph field of view images were sampled and their
respective particle counts represented in grey-level format. The darkest fields contain the largest counts and are
generally located in the interior region of the filter, while the low-count fields are found closer to the edge of the
filter. In this case, a non-random sampling of only interior fields would give an elevated particle concentration. Note
that the edge fields of view that overlap the particle-free boundary are included in the sample population. These
edge regions have their area corrected by extracting the particle-free area observed in the micrographs.
NOTE The darker squares correspond to fields with the highest number of particles counted and the lighter squares
indicate low particle coverage.
Figure 3 — Schematic of a filter surface showing the location from which fields were sampled and
micrographs obtained
3.3.2 Image analysis
The image analysis was carried out on the original images; none of the features of the particles was altered or
enhanced. Thresholding was accomplished visually for each image to maximize the particle thresholded area
without introducing background pixels or false particles into the analysis. Once thresholded, the software
determined the number of pixels comprising the particles, i.e. the areas. Each magnification that spans a portion of
the particle size distribution is analysed separately. The particles have brighter edge regions when compared to the
particle interiors as illustrated by Figure 4. The pixel scan across the horizontal line is shown as an insert in the
figure. This is observed in electron micrographs because the scattering beam electrons can more readily escape
from the particle edges and be detected whereas electrons penetrating the central part of the particle have less
[16]
probability of detection . Since the edges are bright, they are almost always above threshold and included in the
particle. The software “fills” all hollow particles, i.e. particles having pixels below threshold in their central region.
Thus, the critical step in determining accurate particle area is to identify the particle boundary or edge. There is a
complication in that the true particle edge can never be known to be better than a pixel width. Edge determinations
[17]
for linear microstructures by electron microscopy for metrology are discussed at length . To minimize the area
uncertainty, particles need to be represented by a large number of pixels. For example, a 1 µm sphere is
represented by approximately 270 pixels at 3 300 × magnification. One would not want to analyse the same particle
at 300 × where the particle would be represented by only 2 or 3 pixels. The lowest number of pixels used in the
design analysis is, in most cases, between 13 and 50 pixels.
© ISO 2002 – All rights reserved 5
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ISO/TR 16144:2002(E)
NOTE The background filter has a grey level of 251 and the edges have a grey level of 60 to 70 while the centre of the
particle is approximately 150 to 180. This image was obtained with NIH Image, a public domain software package, and has an
inverted grey level assignment with respect to Lispix.
Figure 4 — SEM image of ISO medium test dust showing the particle edge brightening
3.3.3 Projected area diameter of stable oriented particles
The polycarbonate filter material used in this analysis has a planar surface. Consequently, microscopic images are
collected for particles laying flat on a planar surface. Particles should settle in their most stable configuration on the
filter and thus should exhibit on average their largest projected area. In the APC application, the material is normally
measured with the particles suspended in random orientation or oriented with respect to the fluid flow. The values for
the particles characterized are in terms of their projected area diameter found from their most stable orientation.
3.4 Membrane and SRM 2806 stability testing
SRM 2806 will be tested at 6-month intervals to ensure that the particle size distribution is not changing with time.
APCs calibrated to standard polystyrene spheres will be used for this stability monitoring. A historical record of the
size distribution will be made from the time a batch of material arrives at NIST until it is sold. Spot microscopy
checks will be performed on selected bottles as necessary if the APC measurements indicate any changes in the
material.
4 Test procedure
4.1 Calibration suspension preparation SRM 2806
The SRM material was produced from ISO 12103-A3 medium test dust suspended in hydraulic oil at known
concentrations. The ISO MTD, lot number 4390C, specified in ISO 12103 was suspended at a concentration of
6 © ISO 2002 – All rights reserved
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SIST ISO/TR 16144:2003
ISO/TR 16144:2002(E)
2,8 mg of dust per litre of MIL-H-5606 hydraulic fluid. The material was produced in 320-bottle batches, each bottle
containing approximately 400 ml of the suspension.
4.2 Membrane preparation
Before microscopy on the individual particles could be performed, particles had to be separated from the hydraulic
fluid by filtration. All the apparatus associated with the procedure was carefully cleaned with demineralized particle
free water (determined by APC measurements) and rinsed with heptane solvent that was prefiltered by 0,2 µm pore
membrane filter. Filtered heptane was used as the clean solvent to remove the hydraulic oil from the filter and to
wash the filtering apparatus. The filtrations were performed in a class 100 cleanroom to minimize possible
contamination by ambient airborne particles. Polycarbonate filters (25 mm diameter with 0,2 µm pore size) were
used to filter the particles from the oil. These filters have high collection efficiency for particles greater than 0,2 µm
[18], [19]
and provide a smooth, planar surface for electron microscopy .
[20]
The procedure, adapted from an existing Society of Automotive Engineers (SAE) Method entailed producing
three filters per sample for analysis:
1) the filter, solvent and apparatus blank were obtained by flushing several hundred millilitres of filtered
solvent through a clean filter while at the same time washing the walls of the filter funnel;
2) filtering a known volume (at standard temperature) of SRM 2806 hydraulic oil suspension through a
second filter followed by funnel wall washing;
3) extensively washing the funnel down with filtered solvent onto a third clean filter.
The first procedure provided assessment of the cleanliness of the blank filter material, of the filtering apparatus, of
the filtered solvent and of the overall sample processing. Then, in the second procedure, particles in individual
bottles of SRM 2806 were resuspended by sonication, mechanical shaking and then resonication. Following
resuspension and mixing, 10 ml or 30 ml volumes were carefully pipetted from the bottle and flushed through the
filter using the prefiltered solvent. The walls of the funnel and pipette were extensively flushed with solvent. Finally,
for the third procedure a new clean filter was installed and the same funnel was washed down again to ensure that
all particles had been removed from the walls. All three filters were examined by electron microscopy and the
measurement results obtained from them are used in the data analysis.
4.3 Membrane examination and particle counting
4.3.1 Sampling microscopic fields of view
Sampling is one of the most important elements of the procedure to properly characterize the particulate material.
Collecting an accurate representative sample is critical. If sampling is incorrectly done, size distribution analysis
can be biased due to a number of problems that are a function of filtering the material. An example of this is large
or small particle segregation and nonuniform particle coverage. The sampling process is constrained in that the
entire filter cannot be sampled due to time and image storing limitations. Thus a subset of the filter area has to be
carefully chosen to provide a representative sample. One way to accomplish this task is random sampling from all
possible regions on the sample-containing filter where there are filtered particles. Practically, this means the
sampling has to be conducted on the filter in the entire circular region defined by the funnel placement including the
regions where the filter is in contact with the funnel edge.
4.3.2 Scanning electron microscopy
The scanning electron microscope (SEM) is used in this work to collect images of the particles that are in turn used
to attain the primary particle size/count metrology necessary for this SRM. The SEM is used because it is well
suited for imaging particles in the range of particle size of concern, 1 µm to 100 µm. Backscatter electron imaging
provides the maximum grey level contrast for subsequent processing of the digital images. The technique is well
[21], [22], [23], [24], 25]
established . The entire 25 mm diameter polycarbonate filter containing the particle sample is
gold-coated using a low temperature plasma source and subsequently mounted onto the sample stage in the SEM.
The SEM is computer controlled for automated sample stage movement and image collection.
© ISO 2002 – Al
...
TECHNICAL ISO/TR
REPORT 16144
First edition
2002-09-15
Hydraulic fluid power — Calibration of
liquid automatic particle counters —
Procedures used to certify the standard
reference material SRM 2806
Transmissions hydrauliques — Étalonnage des compteurs automatiques
de particules en suspension dans les liquides — Procédures utilisées pour
certifier le matériau de référence normalisé SRM 2806
Reference number
ISO/TR 16144:2002(E)
©
ISO 2002
---------------------- Page: 1 ----------------------
ISO/TR 16144:2002(E)
PDF disclaimer
This PDF file may contain embedded typefaces. In accordance with Adobe's licensing policy, this file may be printed or viewed but shall not
be edited unless the typefaces which are embedded are licensed to and installed on the computer performing the editing. In downloading this
file, parties accept therein the responsibility of not infringing Adobe's licensing policy. The ISO Central Secretariat accepts no liability in this
area.
Adobe is a trademark of Adobe Systems Incorporated.
Details of the software products used to create this PDF file can be found in the General Info relative to the file; the PDF-creation parameters
were optimized for printing. Every care has been taken to ensure that the file is suitable for use by ISO member bodies. In the unlikely event
that a problem relating to it is found, please inform the Central Secretariat at the address given below.
© ISO 2002
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from either ISO at the address below or ISO's member body
in the country of the requester.
ISO copyright office
Case postale 56 • CH-1211 Geneva 20
Tel. + 41 22 749 01 11
Fax + 41 22 749 09 47
E-mail copyright@iso.ch
Web www.iso.ch
Printed in Switzerland
ii © ISO 2002 – All rights reserved
---------------------- Page: 2 ----------------------
ISO/TR 16144:2002(E)
Contents Page
Foreword . iv
Introduction. v
1 Scope. 1
2 Equipment and material. 1
2.1 Test powder . 1
2.2 Test fluid. 1
2.3 Sample preparation loop . 2
2.4 Membrane preparation equipment . 2
2.5 Scanning electron microscope and image analyser .2
3 Equipment validation. 3
3.1 Sample preparation validation. 3
3.2 Microscope calibration validation . 4
3.3 Membrane preparation validation. 4
3.4 Membrane and SRM 2806 stability testing . 6
4 Test procedure . 6
4.1 Calibration suspension preparation SRM 2806 . 6
4.2 Membrane preparation. 7
4.3 Membrane examination and particle counting. 7
5 Data processing . 11
Bibliography. 16
© ISO 2002 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO/TR 16144:2002(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO
member bodies). The work of preparing International Standards is normally carried out through ISO technical
committees. Each member body interested in a subject for which a technical committee has been established has
the right to be represented on that committee. International organizations, governmental and non-governmental, in
liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical
Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 3.
The main task of technical committees is to prepare International Standards. Draft International Standards adopted
by the technical committees are circulated to the member bodies for voting. Publication as an International
Standard requires approval by at least 75 % of the member bodies casting a vote.
In exceptional circumstances, when a technical committee has collected data of a different kind from that which is
normally published as an International Standard (“state of the art”, for example), it may decide by a simple majority
vote of its participating members to publish a Technical Report. A Technical Report is entirely informative in nature
and does not have to be reviewed until the data it provides are considered to be no longer valid or useful.
Attention is drawn to the possibility that some of the elements of this Technical Report may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO/TR 16144 was prepared by Technical Committee ISO/TC 131, Fluid power systems, Subcommittee SC 6,
Contamination control.
iv © ISO 2002 – All rights reserved
---------------------- Page: 4 ----------------------
ISO/TR 16144:2002(E)
Introduction
Solid particulates are a major contributor to wear in hydraulic systems. The fluid power industry, the aerospace
industry and the military sector utilize optical automatic particle counter (APC) technologies to assess the level of
hydraulic oil contamination by suspended particulate. The amount of contamination is often related to the integrity
of the system and the usage of the fluid. APCs are also employed in various oil filter testing operations by the
[1]1)
manufacturers and the users. The standard method ISO 4402 has been used for nearly 30 years to calibrate
optical particle counters in terms of particle size as a function of particle concentration.
The calibration material used in ISO 4402:1991 is Air Cleaner Fine Test Dust (ACFTD) produced in the past by a
division of General Motors Corporation. This material consists of a polydisperse dust having the largest number of
particles, as indicated in ISO 4402:1991, with the size range of 1 µm to 80 µm diameter (particle concentration
increases with decreasing diameter). There is a low concentration of particles reported to extend out to
approximately 100 µm. Some problems have arisen with the use of ACFTD in such calibration procedures. Firstly,
there has been ongoing concern that the particle size distribution is not accurate in the small particle size regime
[2], [3], [4], [5]
(< 10 µm) of the distribution . Many researchers have noted that there are more sub-10 µm particles in
ACFTD than reported by ISO 4402:1991. Secondly, but not less importantly, the production of ACFTD has been
discontinued by the supplier.
Thus there is a need to investigate, design and devise a new standard method (Hydraulic fluid power — Calibration
[6]
method for liquid automatic particle counters) using a new Standard Reference Material (SRM) . The National
Institute of Standards and Technology (NIST) was requested to develop an SRM for use by the fluid power
industry. Users will benefit from improved precision since there is a central source of only one material and
[7]
increased accuracy resulting from the size characterization . The new SRM, designated as SRM 2806, is
composed of ISO Medium Test Dust (ISO MTD) suspended in MIL-H-5606 hydraulic fluid. The number of particles
per millilitre greater than specified sizes has been determined for this material.
1) Cancelled in 1999 and replaced by ISO 11171:1999.
© ISO 2002 – All rights reserved v
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TECHNICAL REPORT ISO/TR 16144:2002(E)
Hydraulic fluid power — Calibration of liquid automatic particle
counters — Procedures used to certify the standard reference
material SRM 2806
1 Scope
This Technical Report describes the procedures used by the United States National Institute of Standards and
Technology (NIST) for the certification of the calibration material SRM 2806, which is used in the primary calibration
of liquid automatic particle counters.
SRM 2806 is a suspension of ISO MTD in hydraulic fluid with a number size distribution certified using a scanning
electron microscope (SEM) and image analysis techniques.
2 Equipment and material
2.1 Test powder
2.1.1 Standard reference material SRM 2806
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 and their
[8]
properties are specified in ISO 12103-1 .
The powder used to prepare SRM 2806 is an ISO 12103-A3 grade, also called ISO MTD, with supplier batch
number 4390C.
2.1.2 Reference materials RM 8631 and RM 8632
Reference materials RM 8631 and RM 8632 are composed of ISO MTD and ISO ultra fine test dust lot numbers
4390C (same lot as the SRM 2806) and 4476 J, respectively. These RMs provide materials to make secondary
[9] [10]
standards used in support of ISO 11171 and SRM 2806 . The RM was received in 3,6 kg bottles. This dust
was dried and spin-riffled into 147 aliquots, each of 20 mg. The material was examined for homogeneity using
optical particle counters after suspension in clean oil.
2.2 Test fluid
Test fluid in which ISO MTD is suspended is a hydraulic fluid widely used worldwide for filter testing. This oil is
defined in American national standards as MIL-H 5606 and in French national standards as AIR 3520, and in the
NATO specification H 515.
[11]
Its physical-chemical properties are defined in annex A of ISO 16889:1999 .
To ease particle dispersion, a small quantity (50 µg/g) of an antistatic agent is added to the oil so that its
conductivity is 1 500 pS/m ± 100 pS/m.
© ISO 2002 – All rights reserved 1
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ISO/TR 16144:2002(E)
2.3 Sample preparation loop
In view of supplying worldwide demand for several years with the SRM 2806 (supplied in bottles of 400 ml), it was
necessary to prepare and store a great number of bottles for further sales.
Because of the settling velocity of the larger silica grains, a special mixing loop was built with mechanical and
hydraulic components which were used to eliminate grinding the powder in suspension. It was designed according
[12]
to the recommendations of ISO 11943 .
To guarantee bottle sample homogeneity, a supplementary volume of oil was necessary to allow sampling of
control bottles used as described in 3.1.2.
The schematic of the sample preparation loop is given in Figure 1.
Key
1 Fluid reservoir (200 l)
2 Circulating pump
3 Clean-up filter
4 Sampling tap
Figure 1 — Schematic of calibration suspension preparation loop
2.4 Membrane preparation equipment
Particles are filtered on 25 mm diameter polycarbonate membranes, 0,2 µm pore diameter using the equipment
[13]
commonly used for determining hydraulic fluid particulate contamination by gravimetry according to ISO 4405 or
[14]
by microscopic counting according to ISO 4407 .
2.5 Scanning electron microscope and image analyser
The scanning electron microscope used to examine particles is a JEOL 840. The images were produced by
electron backscattering and collected on a MicroVax and analysed using LISPIX, a public domain image
processing software developed at NIST. LISPIX currently runs on any computer.
2 © ISO 2002 – All rights reserved
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ISO/TR 16144:2002(E)
3 Equipment validation
3.1 Sample preparation validation
3.1.1 General
Quality assurance for both production and testing was developed by a task force composed of North American
members from two filter manufacturers, a particle counter manufacturer, an independent laboratory and NIST. APC
measurements were made by both the independent laboratory and NIST, with NIST performing the data analysis.
3.1.2 Homogeneity testing/batch screening
An experimental sampling design was developed and implemented at NIST to measure the bottle-to-bottle
homogeneity and, at the same time, to identify possible systematic errors in the instrumental measurements. In the
production process, four bottles (a, b, c, d) were filled at any one time. There were 320 bottles per batch and
bottles were numerically labelled sequentially from 1 (a, b, c, d), 2 (a, b, c, d), ., to 80 (a, b, c, d) as they were
produced. Selected bottles from each batch were tested for homogeneity at both the independent laboratory and
NIST using APCs with extinction sensors calibrated according to ISO 4402:1991. Four bottles (a, b, c, d) were
sampled and analysed from approximately the following four points in the production cycle: 5 %, 30 %, 60 %, and
95 %. Another set of four bottles that were produced directly adjacent to the first four were then analysed. For
example, the first 16 bottles 5 a, 5 b, 5 c, 5 d, 25 (a, b, c, d), 50 (a, b, c, d) and 75 (a, b, c, d) were analysed in that
order. Then bottles 6 (a, b, c, d), 26 (a, b, c, d), 51 (a, b, c, d), and 76 (a, b, c, d) were analysed all by the same
calibrated APC. With three replicates for each bottle, this totalled 96 measurements. Each batch of 320 bottles was
subjected to this procedure or a modified version of this test. A batch of material was deemed homogeneous if the
coefficient of variation for the number of particles larger than 5 µm, 7 µm, 10 µm, 20 µm and 30 µm did not exceed
4 %, 4 %, 4 %, 5 % and 7 % respectively and there were no systematic variations in the batch. The cumulative
particle size distribution was determined for the nominal size range of 1 µm to 80 µm particle diameter and
measurements were compared for the same batch of materials.
3.1.3 Homogeneity
To provide high precision measurement capability for a user community, a standard reference material should be
as homogeneous as possible. Special efforts were made to assure that this material was made with a low bottle-to-
bottle variation within the batch. Within batch variability for the SRM is presented in Table 1 expressed as relative
standard deviation for within batch measurements. Figure 2 shows the batch-to-batch comparison in histogram
form. The histogram is composed of the mean values of the cumulative particle counts for the same volume of fluid
analysed.
Table 1 — Variability found within a batch of material
Greater than size Relative standard deviation
µm %
5 1,1
10 1,3
15 2,0
20 3,8
30 6,7
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ISO/TR 16144:2002(E)
Figure 2 — Particle counts in four batches of ISO MTD to verify bottle-to-bottle homogeneity
3.2 Microscope calibration validation
3.2.1 Microscope calibration
SRM 484d, a NIST scanning electron microscope magnification standard, is mounted in the x and y direction
(orthogonal) on the SEM sample stage and used in conjunction with each sample to calibrate the x-y length for
particle sizing. SRM 1960, 1 µm polystyrene spheres, were examined by the same procedures used for the dust
particles, in order to verify the procedure. Elemental analysis is conducted for a subset of dust particles in the filter
sample using energy dispersive X-ray spectroscopy to assure, within the limits of the experiment, that only mineral
dust is analysed and that other contaminating particulate material is not present.
3.2.2 Traceability
[15]
Particle size is traceable to a primary measurement, optical interferometry, through the NIST SRM 484d . This
SRM standard was imaged for each data set collected and for each magnification. From the certified lengths and
the measured number of pixels, a pixel-to-length relation was derived and was used to convert the particle images
represented in pixels to area in square micrometres. Uncertainties were determined for these conversions (reported
as length uncertainty). They are composed of a combination of uncertainty in the pixel determination and the
[10], [15]
reported uncertainty in the SRM .
3.3 Membrane preparation validation
3.3.1 Sampling from the filter
One component of the total measurement uncertainty results from sampling. Analysis of a large number of fields
indicated a non-uniform particle coverage on the filter. Non-uniform particle deposition was observed on many of
4 © ISO 2002 – All rights reserved
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ISO/TR 16144:2002(E)
the filters after particle separation from the hydraulic oil. Presumably, this was the result of vigorous rinsing with the
clean solvent that caused particles to be preferentially relocated toward the centre of the filter. To overcome this
sampling problem, the SEM was made to select randomly fields-of-view of particles on the filter surface. Figure 3
shows a schematic of a filter with the regions from which micrograph field of view images were sampled and their
respective particle counts represented in grey-level format. The darkest fields contain the largest counts and are
generally located in the interior region of the filter, while the low-count fields are found closer to the edge of the
filter. In this case, a non-random sampling of only interior fields would give an elevated particle concentration. Note
that the edge fields of view that overlap the particle-free boundary are included in the sample population. These
edge regions have their area corrected by extracting the particle-free area observed in the micrographs.
NOTE The darker squares correspond to fields with the highest number of particles counted and the lighter squares
indicate low particle coverage.
Figure 3 — Schematic of a filter surface showing the location from which fields were sampled and
micrographs obtained
3.3.2 Image analysis
The image analysis was carried out on the original images; none of the features of the particles was altered or
enhanced. Thresholding was accomplished visually for each image to maximize the particle thresholded area
without introducing background pixels or false particles into the analysis. Once thresholded, the software
determined the number of pixels comprising the particles, i.e. the areas. Each magnification that spans a portion of
the particle size distribution is analysed separately. The particles have brighter edge regions when compared to the
particle interiors as illustrated by Figure 4. The pixel scan across the horizontal line is shown as an insert in the
figure. This is observed in electron micrographs because the scattering beam electrons can more readily escape
from the particle edges and be detected whereas electrons penetrating the central part of the particle have less
[16]
probability of detection . Since the edges are bright, they are almost always above threshold and included in the
particle. The software “fills” all hollow particles, i.e. particles having pixels below threshold in their central region.
Thus, the critical step in determining accurate particle area is to identify the particle boundary or edge. There is a
complication in that the true particle edge can never be known to be better than a pixel width. Edge determinations
[17]
for linear microstructures by electron microscopy for metrology are discussed at length . To minimize the area
uncertainty, particles need to be represented by a large number of pixels. For example, a 1 µm sphere is
represented by approximately 270 pixels at 3 300 × magnification. One would not want to analyse the same particle
at 300 × where the particle would be represented by only 2 or 3 pixels. The lowest number of pixels used in the
design analysis is, in most cases, between 13 and 50 pixels.
© ISO 2002 – All rights reserved 5
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ISO/TR 16144:2002(E)
NOTE The background filter has a grey level of 251 and the edges have a grey level of 60 to 70 while the centre of the
particle is approximately 150 to 180. This image was obtained with NIH Image, a public domain software package, and has an
inverted grey level assignment with respect to Lispix.
Figure 4 — SEM image of ISO medium test dust showing the particle edge brightening
3.3.3 Projected area diameter of stable oriented particles
The polycarbonate filter material used in this analysis has a planar surface. Consequently, microscopic images are
collected for particles laying flat on a planar surface. Particles should settle in their most stable configuration on the
filter and thus should exhibit on average their largest projected area. In the APC application, the material is normally
measured with the particles suspended in random orientation or oriented with respect to the fluid flow. The values for
the particles characterized are in terms of their projected area diameter found from their most stable orientation.
3.4 Membrane and SRM 2806 stability testing
SRM 2806 will be tested at 6-month intervals to ensure that the particle size distribution is not changing with time.
APCs calibrated to standard polystyrene spheres will be used for this stability monitoring. A historical record of the
size distribution will be made from the time a batch of material arrives at NIST until it is sold. Spot microscopy
checks will be performed on selected bottles as necessary if the APC measurements indicate any changes in the
material.
4 Test procedure
4.1 Calibration suspension preparation SRM 2806
The SRM material was produced from ISO 12103-A3 medium test dust suspended in hydraulic oil at known
concentrations. The ISO MTD, lot number 4390C, specified in ISO 12103 was suspended at a concentration of
6 © ISO 2002 – All rights reserved
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ISO/TR 16144:2002(E)
2,8 mg of dust per litre of MIL-H-5606 hydraulic fluid. The material was produced in 320-bottle batches, each bottle
containing approximately 400 ml of the suspension.
4.2 Membrane preparation
Before microscopy on the individual particles could be performed, particles had to be separated from the hydraulic
fluid by filtration. All the apparatus associated with the procedure was carefully cleaned with demineralized particle
free water (determined by APC measurements) and rinsed with heptane solvent that was prefiltered by 0,2 µm pore
membrane filter. Filtered heptane was used as the clean solvent to remove the hydraulic oil from the filter and to
wash the filtering apparatus. The filtrations were performed in a class 100 cleanroom to minimize possible
contamination by ambient airborne particles. Polycarbonate filters (25 mm diameter with 0,2 µm pore size) were
used to filter the particles from the oil. These filters have high collection efficiency for particles greater than 0,2 µm
[18], [19]
and provide a smooth, planar surface for electron microscopy .
[20]
The procedure, adapted from an existing Society of Automotive Engineers (SAE) Method entailed producing
three filters per sample for analysis:
1) the filter, solvent and apparatus blank were obtained by flushing several hundred millilitres of filtered
solvent through a clean filter while at the same time washing the walls of the filter funnel;
2) filtering a known volume (at standard temperature) of SRM 2806 hydraulic oil suspension through a
second filter followed by funnel wall washing;
3) extensively washing the funnel down with filtered solvent onto a third clean filter.
The first procedure provided assessment of the cleanliness of the blank filter material, of the filtering apparatus, of
the filtered solvent and of the overall sample processing. Then, in the second procedure, particles in individual
bottles of SRM 2806 were resuspended by sonication, mechanical shaking and then resonication. Following
resuspension and mixing, 10 ml or 30 ml volumes were carefully pipetted from the bottle and flushed through the
filter using the prefiltered solvent. The walls of the funnel and pipette were extensively flushed with solvent. Finally,
for the third procedure a new clean filter was installed and the same funnel was washed down again to ensure that
all particles had been removed from the walls. All three filters were examined by electron microscopy and the
measurement results obtained from them are used in the data analysis.
4.3 Membrane examination and particle counting
4.3.1 Sampling microscopic fields of view
Sampling is one of the most important elements of the procedure to properly characterize the particulate material.
Collecting an accurate representative sample is critical. If sampling is incorrectly done, size distribution analysis
can be biased due to a number of problems that are a function of filtering the material. An example of this is large
or small particle segregation and nonuniform particle coverage. The sampling process is constrained in that the
entire filter cannot be sampled due to time and image storing limitations. Thus a subset of the filter area has to be
carefully chosen to provide a representative sample. One way to accomplish this task is random sampling from all
possible regions on the sample-containing filter where there are filtered particles. Practically, this means the
sampling has to be conducted on the filter in the entire circular region defined by the funnel placement including the
regions where the filter is in contact with the funnel edge.
4.3.2 Scanning electron microscopy
The scanning electron microscope (SEM) is used in this work to collect images of the particles that are in turn used
to attain the primary particle size/count metrology necessary for this SRM. The SEM is used because it is well
suited for imaging particles in the range of particle size of concern, 1 µm to 100 µm. Backscatter electron imaging
provides the maximum grey level contrast for subsequent processing of the digital images. The technique is well
[21], [22], [23], [24], 25]
established . The entire 25 mm diameter polycarbonate filter containing the particle sample is
gold-coated using a low temperature plasma source and subsequently mounted onto the sample stage in the SEM.
The SEM is computer controlled for automated sample stage movement and image collection.
© ISO 2002 – All rights reserved 7
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ISO/TR 16144:2002(E)
The software selects a true set of random fields-of-view to be collected and computes the most efficient route to
scan across the sample surface. The stage then steps through the sequence, the sample is brought to optical
focus, and backscatter electron 12-bit images are collected and stored for each field. This is illustrated in Figure 5
where a random field selection from a circular filter surface is simulated and one of the associated electron images
is “grabbed” by the interfaced computer. The gain of the SEM was adjusted for each filter and each magnification to
assure the particles have as high a possible contrast over the filter substrate with a dynamic range of 256 without
saturation. Gain settings were adjusted to include all features of the filter and particles. The grey level value for the
filter (background peak in the histogram) was a nominal 20 to 30 with particle grey levels ranging from greater than
40 to 50 out of 256. All images are archived on two CD-ROMs so that a permanent record of the data is available.
Magnifications of 100 ×, 300 ×, 500 ×, 1 500 × and 3 300 × are used to interrogate and span the particle size range
2 2
of interest. These magnifications correspond to fields of view with areas of approximately 0,8 mm , 0,09 mm ,
2 2 2
0,03 mm , 0,004 mm an
...
RAPPORT ISO/TR
TECHNIQUE 16144
Première édition
2002-09-15
Transmissions hydrauliques — Étalonnage
des compteurs automatiques de particules
en suspension dans les liquides —
Procédures utilisées pour certifier le
matériau de référence normalisé SRM 2806
Hydraulic fluid power — Calibration of liquid automatic particle counters —
Procedures used to certify the standard reference material SRM 2806
Numéro de référence
ISO/TR 16144:2002(F)
©
ISO 2002
---------------------- Page: 1 ----------------------
ISO/TR 16144:2002(F)
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© ISO 2002
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forme que ce soit et par aucun procédé, électronique ou mécanique, y compris la photocopie et les microfilms, sans l'accord écrit de l'ISO à
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Imprimé en Suisse
ii © ISO 2002 – Tous droits réservés
---------------------- Page: 2 ----------------------
ISO/TR 16144:2002(F)
Sommaire Page
Avant-propos . iv
Introduction. v
1 Domaine d'application . 1
2 Équipement et matériau. 1
2.1 Poudre d'essai. 1
2.2 Fluide d'essai. 1
2.3 Boucle de préparation des échantillons. 2
2.4 Appareillage pour la préparation des membranes . 2
2.5 Microscope électronique à balayage et analyseur d'image. 2
3 Validation de l'équipement. 3
3.1 Validation de la préparation des échantillons. 3
3.2 Validation de l'étalonnage du microscope . 4
3.3 Validation de la préparation de la membrane . 5
3.4 Membrane et essai de stabilité du SRM 2806. 7
4 Protocole d'essai. 7
4.1 Préparation de la suspension d'étalonnage SRM 2806. 7
4.2 Préparation de la membrane. 7
4.3 Examen de la membrane et comptage des particules . 8
5 Traitement des données. 12
Bibliographie. 19
© ISO 2002 – Tous droits réservés iii
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ISO/TR 16144:2002(F)
Avant-propos
L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes nationaux de
normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est en général confiée aux
comités techniques de l'ISO. Chaque comité membre intéressé par une étude a le droit de faire partie du comité
technique créé à cet effet. Les organisations internationales, gouvernementales et non gouvernementales, en
liaison avec l'ISO participent également aux travaux. L'ISO collabore étroitement avec la Commission
électrotechnique internationale (CEI) en ce qui concerne la normalisation électrotechnique.
Les Normes internationales sont rédigées conformément aux règles données dans les Directives ISO/CEI,
Partie 3.
La tâche principale des comités techniques est d'élaborer les Normes internationales. Les projets de Normes
internationales adoptés par les comités techniques sont soumis aux comités membres pour vote. Leur publication
comme Normes internationales requiert l'approbation de 75 % au moins des comités membres votants.
Exceptionnellement, lorsqu'un comité technique a réuni des données de nature différente de celles qui sont
normalement publiées comme Normes internationales (ceci pouvant comprendre des informations sur l'état de la
technique par exemple), il peut décider, à la majorité simple de ses membres, de publier un Rapport technique.
Les Rapports techniques sont de nature purement informative et ne doivent pas nécessairement être révisés avant
que les données fournies ne soient plus jugées valables ou utiles.
L'attention est appelée sur le fait que certains des éléments du présent Rapport technique peuvent faire l'objet de
droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour responsable de ne pas
avoir identifié de tels droits de propriété et averti de leur existence.
L'ISO/TR 16144 a été élaboré par le comité technique ISO/TC 131, Transmissions hydrauliques et pneumatiques,
sous-comité SC 6, Contrôle de la contamination.
iv © ISO 2002 – Tous droits réservés
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ISO/TR 16144:2002(F)
Introduction
Dans les systèmes hydrauliques, la contribution des particules solides à l'usure est importante. L'industrie des
transmissions hydrauliques, l'industrie aérospatiale et le secteur militaire utilisent les technologies des compteurs
optiques automatiques de particules (CAP) pour évaluer le degré de contamination de l'huile hydraulique par des
particules en suspension. L'importance de la contamination est souvent liée à l'intégrité du système et à l'utilisation
du fluide. Les CAP sont également employés dans divers essais de filtres à huile par leurs fabricants et leurs
[1]1)
utilisateurs. La méthode normalisée ISO 4402:1991 a été utilisée pendant près de 30 ans pour étalonner des
compteurs optiques de particules en termes de granulométrie en fonction de la concentration des particules.
Le matériau d'étalonnage utilisé dans l'ISO 4402:1991 est de la fine poudre d'essai pour épurateur d'air (ACFTD)
que produisait un département de l'entreprise General Motors. Ce matériau est constitué d'une poudre
polydispersée ayant le plus grand nombre de particules, comme indiqué dans l'ISO 4402:1991, dans la plage de
dimensions allant de 1 µm à 80 µm de diamètre (la concentration des particules augmente avec la diminution du
diamètre). Au-delà d'environ 100 µm, on observe une faible concentration de particules. L'emploi de l'ACFTD a
parfois posé des problèmes d'étalonnage à ces dimensions. Le premier sujet de préoccupation récurrent a été le
manque d'exactitude de la distribution granulométrique dans le domaine de faible granulométrie (< 10 µm) de la
[2], [3], [4], [5]
distribution . De nombreux chercheurs ont noté qu'il y a dans l'ACFTD plus de particules inférieures à
10 µm qu'indiqué dans l'ISO 4402:1991. Second facteur, non moins important, le fournisseur a cessé de produire
l'ACFTD.
Il a été jugé indispensable de produire une nouvelle méthode normalisée (Transmissions hydrauliques —
Étalonnage des compteurs automatiques de particules en suspension dans les liquides) utilisant un nouveau
[6]
matériau de référence normalisé (SRM) . Il a été demandé au National Institute of Standards and Technology
(NIST) d'élaborer un SRM à utiliser par le secteur des transmissions hydrauliques. Les utilisateurs y gagneront en
précision, le matériau unique ayant une provenance unique, et l'exactitude étant accrue du fait de la caractérisation
[7]
des dimensions . Le nouveau SRM nommé le SRM 2806, est composé d'un matériau de référence normalisé
(ISO MTD) en suspension dans un fluide hydraulique MIL-H 5606. Le nombre de particules par millilitre supérieur
aux dimensions spécifiées a été déterminé pour ce matériau.
1) Annulée en 1999 et remplacée par l'ISO 11171:1999.
© ISO 2002 – Tous droits réservés v
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RAPPORT TECHNIQUE ISO/TR 16144:2002(F)
Transmissions hydrauliques — Étalonnage des compteurs
automatiques de particules en suspension dans les liquides —
Procédures utilisées pour certifier le matériau de référence
normalisé SRM 2806
1 Domaine d'application
Le présent Rapport technique décrit les procédures utilisées par le National Institute of Standards and Technology
(NIST) des États-Unis pour la certification du matériau d'étalonnage SRM 2806, qui est utilisé pour l'étalonnage
primaire des compteurs automatiques de particules en suspension dans les liquides.
Le SRM 2806 est une suspension d'ISO MTD dans un fluide hydraulique avec une distribution granulométrique en
nombre certifiée à l'aide d'un microscope électronique à balayage (MEB) et de techniques d'analyse d'image.
2 Équipement et matériau
2.1 Poudre d'essai
2.1.1 Matériau de référence normalisé SRM 2806
Le matériau particulaire utilisé est une poudre de silice obtenue par broyage au jet de sable du désert d'Arizona,
soumise ensuite à une classification par air pour donner une distribution granulométrique cohérente. Il existe
[8]
plusieurs qualités de poudre selon leur granulométrie, et leurs propriétés sont spécifiées dans l'ISO 12103-1 .
La poudre utilisée pour préparer le SRM 2806 est une poudre ISO 12103-A3, également appelée ISO MTD;
numéro de lot du fournisseur 4390C.
2.1.2 Matériaux de référence RM 8631 et RM 8632
Les matériaux de référence RM 8631 et RM 8632 sont composés d'ISO MTD et de poudre d'essai ISO ultra-fine,
numéro de lot 4390C (même lot que le SRM 2806) et 4476 J, respectivement. Ces matériaux de référence (MR)
[9]
servent de matériaux pour la préparation des étalons secondaires utilisés à l'appui de l'ISO 11171 et du
[10]
SRM 2806 . Le MR a été reçu en bouteilles de 3,6 kg. La poudre a été séchée et fractionnée en 147 aliquotes de
20 g chacun. L'homogénéité du matériau a été examinée à l'aide de compteurs optiques de particules après mise
en suspension dans une huile propre.
2.2 Fluide d'essai
Le fluide d'essai dans lequel l'ISO MTD est mis en suspension est un fluide hydraulique largement utilisé dans le
monde entier pour les essais de filtres. Cette huile est définie dans les normes nationales américaine comme la
MIL-H 5606 et française AIR 3520, ainsi que dans la spécification de l'OTAN H 515.
[11]
Ses propriétés physico-chimiques sont définies dans l'annexe A de l'ISO 16889:1999 .
Pour faciliter la dispersion des particules, une faible quantité (50 µg/g) d'agent antistatique est ajoutée à l'huile de
façon que sa conductivité résiduelle soit de 1 500 pS/m ± 100 pS/m.
© ISO 2002 – Tous droits réservés 1
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ISO/TR 16144:2002(F)
2.3 Boucle de préparation des échantillons
Compte tenu de la demande mondiale de SRM 2806 (livré en flacons de 400 ml) pendant de nombreuses années,
il a été nécessaire de préparer et de stocker un grand nombre de flacons pour les ventes ultérieures.
En raison de la vitesse de sédimentation des gros grains de silice, une boucle de mélange spéciale a été réalisée,
avec des composantes mécaniques et hydrauliques utilisées afin d'éliminer les particules en suspension broyées.
[12]
Sa conception suit les recommandations de l'ISO 11943 .
Pour garantir l'homogénéité de l'échantillon d'un flacon à l'autre, un volume supplémentaire d'huile a été
nécessaire pour l'échantillonnage des flacons de contrôle utilisés comme décrit en 3.1.2.
Le schéma de la boucle de préparation des échantillons est représenté à la Figure 1.
2.4 Appareillage pour la préparation des membranes
Les particules sont filtrées sur des membranes de polycarbonate de 25 mm de diamètre, de diamètre de pore
0,2 µm, au moyen de l'appareillage communément utilisé pour déterminer la contamination particulaire des fluides
[13] [14]
hydrauliques par la méthode gravimétrique de l'ISO 4405 ou par comptage au microscope selon l'ISO 4407
2.5 Microscope électronique à balayage et analyseur d'image
Le microscope électronique à balayage (MEB) utilisé pour examiner les particules est un modèle JEOL 840. Les
images ont été produites par rétrodiffusion électronique, recueillies sur un MicroVax et analysées avec LISPIX,
logiciel de traitement de données du domaine public développé au NIST. LISPIX fonctionne actuellement sur
n'importe quel ordinateur.
Légende
1 Réservoir de fluide (200 l)
2 Pompe de circulation
3 Filtre de dépollution
4 Prise d'échantillon
Figure 1 — Schéma de la boucle de préparation de la suspension d'étalonnage
2 © ISO 2002 – Tous droits réservés
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ISO/TR 16144:2002(F)
3 Validation de l'équipement
3.1 Validation de la préparation des échantillons
3.1.1 Généralités
L'assurance de la qualité tant en production qu'en essai a été établie par un groupe de travail composé de
membres américains de deux fabricants de filtres, un fabricant de compteurs de particules, un laboratoire
indépendant et le NIST. Les mesurages au CAP ont été effectués par le laboratoire indépendant et par le NIST, ce
dernier étant chargé de l'analyse des données.
3.1.2 Essai d'homogénéité/criblage des lots
Un modèle d'échantillonnage expérimental a été conçu et mis en œuvre au NIST pour mesurer l'homogénéité de
flacon à flacon et, dans le même temps, identifier les erreurs de mesure systématiques possibles. Dans le
processus de production, quatre flacons (a, b, c, d) ont été remplis ensemble à un moment donné. Au total, 320
flacons par lot ont été étiquetés séquentiellement avec des numéros de 1 (a, b, c, d), 2 (a, b, c, d), ., 80 (a, b, c, d)
au fur et à mesure de leur production. Un choix de flacons prélevés dans chaque lot a été soumis à l'essai
d'homogénéité dans le laboratoire indépendant et au NIST, au moyen de plusieurs CAP munis de capteurs à
absorption de lumière, étalonnés selon l'ISO 4402:1991. Quatre flacons (a, b, c, d) ont été prélevés et analysés à
partir des quatre points approximatifs suivants dans le cycle de production: 5 %, 30 %, 60 % et 95 %. Une autre
série de quatre flacons contigus aux quatre premiers a été analysée. Par exemple, les 16 premiers flacons 5 a, 5 b,
5 c, 5 d, 25 (a, b, c, d), 50 (a, b, c, d) et 75 (a, b, c, d) ont été analysés dans cet ordre. Ensuite les flacons 6 (a, b,
c, d), 26 (a, b, c, d), 51 (a, b, c, d) et 76 (a, b, c, d) sont analysés par le même CAP étalonné. Avec trois
sous-échantillons pour chaque flacon, il a été procédé au total à 96 mesurages. Chaque lot de 320 flacons a été
soumis à ce mode opératoire ou à une version modifiée de cet essai. Un lot de matériaux a été jugé homogène si
le coefficient de variation pour le nombre de particules supérieur à 5 µm, 7 µm, 10 µm, 20 µm et 30 µm ne
dépassait pas 4 %, 4 %, 4 %, 5 % et 7 %, respectivement, et si aucune variation systématique n'était présente
dans le lot. La distribution granulométrique cumulée a été déterminée dans la plage de dimensions nominales
(diamètres) de particule de 1 µm à 80 µm et les mesures ont été comparées pour le même lot de matériaux.
3.1.3 Homogénéité
Afin que l'ensemble des utilisateurs dispose d'une capacité de mesurage de haute précision, il convient qu'un
matériau de référence normalisé soit aussi homogène que possible. Des efforts particuliers ont été déployés pour
assurer que ce matériau soit produit avec une faible variation d'un flacon à l'autre dans le lot. La variabilité du SRM
au sein du lot est présentée dans le Tableau 1, exprimée sous la forme de l'écart-type relatif pour les mesurages
au sein du lot. La Figure 2 représente la comparaison entre lots sous la forme d'un histogramme. Cet histogramme
est constitué des valeurs moyennes des comptages cumulés des particules pour le même volume de fluide
analysé.
Tableau 1 — Variabilité observée au sein d'un lot de matériaux
Supérieur à la dimension Écart-type relatif
µm %
5 1,1
10 1,3
15 2,0
20 3,8
30 6,7
© ISO 2002 – Tous droits réservés 3
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ISO/TR 16144:2002(F)
Figure 2 — Comptage des particules dans quatre lots d'ISO MTD pour vérifier l'homogénéité entre flacons
3.2 Validation de l'étalonnage du microscope
3.2.1 Étalonnage du microscope
Le SRM 484d, étalon pour grossissement au microscope électronique à balayage (MEB) du NIST, est monté dans
les directions x et y (orthogonale) sur la platine porte-échantillon du MEB et utilisé conjointement avec chaque
échantillon afin d'étalonner la longueur x-y pour la mesure dimensionnelle des particules. Le SRM 1960, constitué
de billes de polystyrène de 1 µm, a été examiné selon les mêmes modes opératoires que ceux utilisés pour les
particules de poudre, afin de vérifier le mode opératoire. L'analyse élémentaire est conduite sur un sous-ensemble
de particules de poudre de l'échantillon de filtre par spectroscopie de rayons X à dispersion d'énergie, pour
garantir, dans les limites de l'expérience, que seule la poudre minérale est analysée et qu'aucun autre matériau
particulaire contaminant n'est présent.
3.2.2 Traçabilité
La granulométrie est traçable jusqu'à un mesurage primaire, l'interférométrie optique, avec le SRM 484d du
[15]
NIST . L'image de cet étalon SRM a été saisie pour chaque série de données recueillies et pour chaque
grossissement. À partir des longueurs certifiées et du nombre de pixels mesurés, une relation pixel-longueur a été
calculée pour convertir les images des particules, représentées en pixels, en surfaces en micromètres carrés. Les
incertitudes ont été déterminées pour ces conversions (et consignées sous forme d'incertitudes de longueur). Elles
sont constituées d'une combinaison de l'incertitude de la détermination des pixels et de l'incertitude déclarée dans
[10], [15]
le SRM .
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ISO/TR 16144:2002(F)
3.3 Validation de la préparation de la membrane
3.3.1 Échantillonnage à partir du filtre
Un facteur de l'incertitude totale de mesure découle de l'échantillonnage. L'analyse d'un grand nombre de champs
a indiqué la présence d'une répartition de particules non uniforme sur le filtre. Un dépôt non uniforme de particules
est observé sur bon nombre de filtres après séparation des particules de l'huile hydraulique. Il est supposé qu'il
s'agit du résultat d'un rinçage vigoureux avec un solvant propre qui a pour effet de rassembler les particules au
centre du filtre. Pour remédier à ce problème d'échantillonnage, le MEB a été réglé pour choisir au hasard des
champs d'observation des particules sur la surface du filtre. La Figure 3 représente un schéma du filtre, avec les
régions sur lesquelles les images micrographiques du champ d'observation ont été échantillonnées et le comptage
correspondant de particules représentées sous forme de carrés en niveaux de gris. Les champs les plus sombres
contiennent le nombre de particules le plus élevé et sont en général situés vers l'intérieur du filtre alors que les
champs à faible nombre de particules se situent plus près du bord du filtre. Dans ce cas, un échantillonnage non
aléatoire des seuls champs intérieurs donnerait une concentration élevée de particules. À noter que les champs
d'observation situés sur les bords, qui chevauchent les limites exemptes de particules, sont inclus dans la
population de l'échantillonnage. La correction de la surface de ces zones limites est opérée en soustrayant la zone
exempte de particules observée dans les micrographes.
NOTE Les carrés les plus sombres correspondent aux champs où le nombre de particules comptées est le plus élevé et
les carrés gris clair indiquent une faible répartition de particules.
Figure 3 — Schéma d'une surface de filtre représentant l'emplacement où chaque champ
a été échantillonné et où des micrographies ont été obtenus
3.3.2 Analyse d'image
L'analyse d'image a été effectuée sur les images d'origine. Aucune caractéristique des particules n'a été modifiée
ou renforcée. Le seuillage a été réalisé à l'œil nu pour chaque image afin de maximiser la surface grisée de
particules sans introduire de pixels de fond ou de fausses particules dans l'analyse. Une fois le seuillage réglé, le
logiciel a déterminé le nombre de pixels occupé par les particules, c'est-à-dire les surfaces. Chaque grossissement
qui recouvre un segment de la distribution granulométrique est analysé séparément. Les particules présentent des
zones périphériques plus claires que les zones intérieures, comme illustré à la Figure 4. Le balayage horizontal
des pixels est représenté en encadré dans la figure. Ce phénomène est observé dans les micrographies
électroniques en raison du fait que les électrons du faisceau de diffusion peuvent s'échapper plus facilement des
contours des particules et être détectés alors que la probabilité de détecter les électrons qui pénètrent dans la
© ISO 2002 – Tous droits réservés 5
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ISO/TR 16144:2002(F)
[16]
partie centrale de la particule est moindre . Étant donné que les contours sont lumineux, ils sont presque toujours
au-dessus du seuil et inclus dans la particule. Le logiciel «remplit» toutes les particules creuses, c'est-à-dire les
particules dont la région centrale a un nombre de pixels inférieur au seuil. Ainsi, l'étape critique dans la détermination
de la surface exacte des particules est d'identifier leurs limites ou leurs contours. Une source de complication vient du
fait que le contour véritable d'une particule ne peut jamais être connu avec une précision supérieure à celle de la
largeur du pixel. La détermination des contours pour les microstructures linéaires par microscopie électronique à des
[17]
fins métrologiques est amplement étudiée . Pour réduire au minimum l'incertitude liée à la surface, les particules
doivent être représentées par un nombre élevé de pixels. Par exemple une sphère de 1 µm est représentée par
environ 270 pixels à un grossissement de × 3 300. Il ne serait guère possible d'analyser la même particule avec un
grossissement de × 300, la particule étant alors représentée par seulement 2 ou 3 pixels. Le nombre le plus faible de
pixels utilisés pour l'analyse se situe dans la plupart des cas entre 13 et 50 pixels.
NOTE Le niveau de gris est de 251 pour le filtre de fond, de 60 à 70 pour les bords et de 150 à 180 environ pour le centre
de la particule. Cette image a été obtenue avec NIH Image, logiciel du domaine public, et a une attribution d'échelle de gris
inversée par rapport à Lispix.
Figure 4 — Image MEB d'une poudre d'essai moyenne ISO représentant
l'éclaircissement du contour de la particule
3.3.3 Diamètre de la surface projetée de particules stables et orientées
Le matériau du filtre en polycarbonate utilisé dans cette analyse a une surface plane. En conséquence, les images
microscopiques sont celles de particules posées à plat sur une surface plane. Il convient que les particules se
déposent dans leur position la plus stable sur le filtre et présentent ainsi en moyenne leur surface projetée la plus
grande. Dans l'application CAP, le matériau est normalement mesuré avec les particules en suspension dans une
orientation aléatoire ou orientées par rapport à l'écoulement du fluide. Les valeurs du diamètre équivalent de la
surface projetée des particules sont observées dans leur orientation la plus stable.
6 © ISO 2002 – Tous droits réservés
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ISO/TR 16144:2002(F)
3.4 Membrane et essai de stabilité du SRM 2806
Le SRM 2806 sera contrôlé à 6 mois d'intervalle pour s'assurer que sa distribution granulométrique ne varie pas
avec le temps. Des CAP étalonnés avec des billes de polystyrène normalisées seront utilisés pour cette
surveillance de la stabilité. Un historique de la granulométrie sera dressé dès réception du matériau au NIST,
jusqu'à ce qu'il soit vendu. Des vérifications ponctuelles au microscope seront réalisées sur des flacons choisis,
selon la nécessité, lorsque les mesures du CAP indiquent des changements quelconques dans le matériau.
4 Protocole d'essai
4.1 Préparation de la suspension d'étalonnage SRM 2806
Le matériau SRM a été produit à partir de poudre d'essai moyenne ISO 12103-A3 en suspension dans de l'huile
hydraulique à des concentrations connues. L'ISO MTD numéro de lot 4390C spécifiée dans l'ISO 12103 a été mise
en suspension à une concentration de 2,8 mg de poudre par litre de fluide hydraulique MIL-H-5606. Le matériau a
été produit par lots de 320 flacons, chaque flacon renfermant environ 400 ml de la suspension.
4.2 Préparation de la membrane
Avant qu'il ne soit possible de réaliser la microscopie des particules individuelles, il a fallu séparer les particules du
fluide hydraulique par filtration. Tous les appareillages associés à ce mode opératoire ont été soigneusement
nettoyés avec de l'eau déminéralisée exempte de particules (déterminée par des mesures au CAP) et rincés à
l'aide d'un solvant heptane préfiltré sur une membrane de pores de 0,2 µm. L'heptane filtré a été utilisé comme
solvant de nettoyage pour éliminer le fluide hydraulique du filtre et pour laver l'appareillage de filtration. Les
filtrations ont été réalisées dans une salle propre de la classe 100 afin de réduire au minimum toute possibilité de
contamination par des particules en suspension dans l'air ambiant. Des filtres de polycarbonate (diamètre de
25 mm avec une dimension des pores de 0,2 µm) ont été utilisés pour filtrer les particules en suspension dans
l'huile. Ces filtres ont une efficacité élevée pour les particules supérieures à 0,2 µm et présentent une surface plane
[18], [19]
régulière pour la microscopie électronique .
Ce mode opératoire, qui est une adaptation d'une méthode existante de la Society of Automotive Engineers
[20]
(SAE) nécessite la production de trois filtres par échantillon pour analyse:
1) le blanc du filtre, du solvant et de l'appareillage a été obtenu par rinçage avec plusieurs centaines de
millilitres de solvant filtré au travers d'un filtre propre tout en lavant dans le même temps les parois de
l'entonnoir de filtre;
2) filtration d'un volume connu (à une température normalisée) de la suspension d'huile hydraulique
SRM 2806 au travers d'un second filtre suivi d'un nettoyage des parois de l'entonnoir;
3) nettoyage complet de l'entonnoir avec un solvant filtré sur un troisième filtre propre.
Le premier mode opératoire a donné une évaluation de la propreté du matériau de filtre à blanc, de l'appareillage
de filtration, du solvant filtré et de l'ensemble des opérations de traitement de l'échantillon. Ensuite, avec le mode
opératoire suivant, les particules ont été remises en suspension dans des flacons individuels de SRM 2806 par
sonification, agitation mécanique puis resonification. Après remise en suspension et mélange, des volumes de
10 ml ou 30 ml ont été prélevés soigneusement du flacon à l'aide d'une pipette et filtrés dans le solvant préfiltré.
Les parois de l'entonnoir et la pipette ont été rincées abondamment au solvant. Enfin, pour le troisième mode
opératoire, un nouveau filtre propre a été mis en place et le même entonnoir a été à nouveau lavé pour s'assurer
de l'élimination
...
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