particle counter calibration

I00-10.1

The New NIST Traceable Particle Counter Calibration Standard— Transitioning from ISO 4402 to ISO 11171

Barry M. Verdegan, Nelson Industries, Inc.

Brian W. Schwandt, Nelson Industries, Inc.

Christopher E. Holm, Nelson Industries, Inc.

Kendall McBroom, Nelson Industries, Inc.

Jean Yves Picard, Nelson Industries, Inc.

3333 N. Mayfair Road • Milwaukee, WI 53222-3219 • +1 414-778-3344

TechnicalPaper Series

Presented at the International Exposition for Power Transmissionand Technical Conference4-6 April 2000

LEGEND

Sample Identification Number: I00-3.2

I = Volume number (only one volume)00 = 2000, year of the conference3 = Conference Session Number2 = Second paper in the Session’s presentation order

All papers presented at the 2000 International Exposition for Power Transmissionand Technical Conference are available in one volume, Proceedings of the 48thNational Conference on Fluid Power.

No part of this publication may be reproduced by any means, in an electronicretrieval system or otherwise, without the prior written permission of the author(s).

Statements and opinions advanced in this paper are that of the author(s) and arehis/her responsibility, not those of the National Fluid Power Association. For permis-sion to publish this paper in full or in part, contact the author(s) directly.

The New NIST Traceable Particle Counter Calibration Standard - Transitioning from ISO 4402 to ISO 11171

IO0-10.l

Barry M. Verdegan*, Brian W. Schwandt*, Christopher E. Holm*, Kendall McBroom* and Jean Yves Picard**

Fleetguard/Nelson *Stoughton, Wl USA

**Quimper, France

ABSTRACT

Liquid automatic particle counters (APCs) are used to monitor contamination levels in hydraulic oil, to establish component and assembly cleanliness level specifications, and to determine filter efficiencies and size ratings. In the past, particle counter calibration for hydraulic applications used ISO 4402 and AC Fine Test Dust. The shortcomings of this method were known, but until recently there was no better method. This changed with the passage of ISO 11171, an NIST (National Institute of Standards and Technology) traceable method of calibration. As a result of this and other improvements in ISO filter test standards, it is anticipated that the quality and reliability of particle count and filter test data will improve, increasing their usefulness to the hydraulics, automotive and aerospace industries. However, to make the switch from ISO 4402 to ISO 11171, laboratory personnel, engineers, sales and marketing must make certain accommodations with regard to the procedures used and the way results are reported. This paper discusses the impact of the change in calibration and offers suggestions on how to facilitate transition between the two calibration methods.

INTRODUCTION

In 1999, ISO adopted the most sweeping revisions to contamination control standards to have occurred since the advent of the multi-pass filter test in the early 1970's. With the passage of ISO 11171 (1), ISO 11943 (2), ISO 16889 (3), and ISO 4406 (4), the industry will, for the first time, have NIST traceability in particle size measurement and the resultant filter efficiency and particle size distribution data. ISO TR 16386 (5) provides additional information regarding the impact of the new standards. ISO 11171 establishes an NIST traceable calibration method for liquid automatic particle counters (APCs) and replaces ISO 4402 (6). ISO 11943 is the new ISO calibration method for on-line APCs. ISO 16889 multi-pass filter test method replaces ISO 4572 (7). ISO 4406: 1999 is the new 3-digit solid contamination code and replaces earlier 2-digit versions

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(8). This paper discusses the major innovations brought about by these standards, their impact on the industry, and ways of making the transition from old to new standards.

ISO 11171 PARTICLE COUNTER CALIBRATION

ISO 11171 provides the underpinnings for the sweeping changes in contamination control standards. ISO 11171 introduces four major advances relative to its predecessor, ISO 4402. They are: (1) the use of NIST certified calibration suspensions, (2) a heavy reliance on statistical procedures to ensure data quality and identify sources of error, (3) the establishment of minimal APC performance specifications, and (4) the determination of APC operating limits. This section discusses the importance of these.

ISO 11171 uses NIST certified calibration suspensions instead of AC Fine Test Dust (ACFTD). There are fundamental problems with the ISO 4402 ACFTD approach. ACFTD, itself, exhibited batch-to-batch variability in particle size distribution. In addition, ACFTD is no longer commercially available. The ISO 4402 published particle size distribution for ACFTD is based on the longest chord dimension of particles as measured by optical microscopy in the late 1960s. However, light extinction APCs measure the area equivalent diameter of particles, not their longest chord dimension. The accuracy of the ISO 4402 size distribution is limited by the procedures used to generate it. When the distribution was generated, accurate measurement of particles smaller than 10 p m using optical techniques was difficult. Hence, the published size distribution is based on a mathematical fit of experimental data. The resultant equation does not accurately describe the size distribution at small sizes (9). In contrast, the NIST distribution is based on actual equivalent diameter data obtained by SEM (10). Test dust variability is minimized through the use of ISO Medium Test Dust (ISO MTD, 11). Statistical techniques were used to evaluate and control all sources of variability and uncertainty (12). Through the use of NIST

calibration materials, traceability, as promulgated by ISO 9000 (13) and QS9000 (14), in contamination control is finally achieved.

Statistics are used in the actual calibration procedure itself, as well as in the preparation of NIST calibration samples. Statistics provide two significant benefits to the industry. For those using particle count and filter test data, statistics ensure that only "good" data is used to generate calibration curves. Thus, the resultant calibration curves and filter performance data are more accurate and repeatable. For laboratory personnel, statistics increase efficiency and reduce wasted time by allowing early identification of analytical problems and by permitting outlying data points to be rejected without repeating the entire procedure.

To take advantage of statistical information, lab personnel must understand how to use and interpret the results. ISO 11171 requires that at least 500 particles in a sample be counted to reduce uncertainty in the results. In addition, it requires that critical samples be analyzed in triplicate and that multiple counts for each sample be obtained. Data from multiple counts helps identify systematic errors, particularly particle settling or aeration. The existence of a trend in data from consecutive counts, e.g., continuous increase or decrease in counts, is evidence of a problem that must be corrected. Analysis of multiple identical samples tests for reproducible analytical technique, contamination, and representative samples. Clause 6.3 (1) establishes data acceptance criteria using DQ, the ratio of the range in data to the mean. If variability is excessive (the range large relative to the mean), the results are unacceptable. Before discarding all of the data, however, an outlier test should be performed. An outlier is an individual data point far removed from the other results which can be rejected (ignored) on statistical grounds. The D O outlier test in 6.3 (1) allows such data points to be identified and discarded without the need to embark on time-consuming efforts to identify the problem and repeat the analysis.

In contrast to ISO 4402, ISO 11171 establishes minimal APC performance specifications for volume measurement reproducibility, resolution and counting accuracy. In order to convert particle counts to particle concentration, volume must be accurately and reproducibly measured. According to Annex A (1), APCs must be able to reproducibly measure volume with a coefficient of variation _<3%. Resolution is a measure of the ability of an APC to distinguish between different sizes. Good resolution is essential to obtain agreement between different sensors or APCs, e.g., upstream and downstream sensors on a multi-pass test stand. Annex D (1) requires that resolution be _<10%. A 10% resolution means that an APC set at 10 #m can distinguish between 9, 10 and 11 #m particles. Counting accuracy requirements are set forth in Annex E (1). It requires that a sample of ISO Ultra Fine Test Dust (ISO UFTD) analyzed by an APC agrees within published

346

limits based on results obtained in an international round robin. ISO UFTD is used in order to ensure that APCs used in the industry count particles in a similar manner. Collectively, the instrument performance specifications help ensure that the APC accurately measure volume, sizes and counts particles.

Particle count results are only meaningful if the APC is used properly within its operating limits. ISO 11171 requires operators to determine the threshold noise level, coincidence error limit, and flow rate limits of the APC. The threshold noise level is the voltage level below which electrical noise becomes significant. At threshold (channel) settings below this, significant numbers of counts are due to electrical noise, rather than actual particles being counted. Operationally, the threshold noise level determines the smallest particle size that can be counted by the APC. Coincidence error is the single greatest source of error in particle counting and results in both sizing and counting errors. It occurs when two or more particles are present in the sensing zone at the same time. As a result, the multiple particles are seen as one larger particle. The coincidence error limit, as determined in Annex B (1), establishes the number concentration above which this source of error becomes significant. To avoid coincidence error, sample concentrations must be below this limit. Samples with higher concentrations must be diluted with clean dilution fluid in order to attain concentration low enough for counting. Annex C (1) describes how to determine the flow rate limits of the APC. As a matter of routine, APCs should always be used with the same working flow rate. However, occasions may occur when a different flow rate is used, such as when analyzing samples with differing viscosity or when the flow control system is not working properly. In order to know whether or not the data is acceptable, operators need to know the flow rate limits of the APC. Particles passing through the sensing zone too quickly do not give the electronics enough time to fully develop the electrical signal and may be undersized. Conversely, particles moving too slowly may be indistinguishable from the background or be mis- sized as particles change orientation in the sensing zone. In either case, significant errors may occur.

IMPACTS OF ISO 11171, 16889 AND 4406:1999

ISO has taken several steps in order to minimize confusion during the transition from old to new standards. New standards numbers have been assigned to the 1999 standards utilizing NIST traceable data to distinguish them from the old procedures and the old standards have been withdrawn. For particle counter calibration, ISO 11171 refers to the new NIST traceable APC calibration procedure while ISO 4402 refers to the old ACFTD method which has been withdrawn. For multi-pass testing, ISO 16889 refers to the new NIST traceable method that uses ISO MTD while ISO 4572 refers to the old ACFTD procedure which has also been withdrawn in favor of the new method. For reporting contamination levels using ISO Codes, ISO 4406:1999 uses a three digit code while ISO 4406:1987 uses a two
digit code. When reporting results, documents should

Type of Data ISO Standard Material

Table 1. Comparison of Data Reporting Formats Particle Size Filtration Ratio Solid Contamination Concentration

4402 11171 4572 16889 4406:1987 4406:1999 ACFTD NIST SRM 2806 ACFTD ISO MTD ACFTD NIST SRM 2806

Report Data as: jJm iJnl(c) either explicitly state the ISO standard used to generate data or use the short hand method shown in Table 1. For example, ISO 11171 particle size data is expressed as X pro(c), while ISO 4402 data is simply X pm (where X refers to the size of the particle). Similarly, ISO 16889 beta ratios are expressed as 13x~c~ = Y, while ISO 4572 data is of the form ~x = Y (where Y refers to the beta ratio of the filter). ISO 4406:1999 code results appear in the form A/B/C while ISO 4406 :1987 code results appear in the form B/C where A, B, and C refer code numbers reflecting the concentrat ions of particles larger the size indicated by the code.

The change to NIST traceable units of measure ISO 11171 has industry-wide ramifications, in terms of reported contaminant sizes and concentrations, ISO code results, and filter beta ratios. With ISO 11171, particle size are redefined. This affects how filters are tested and how particle concentration and removal results are reported. The new standards do not affect the actual performance of a filter nor its effectiveness in protecting components nor actual contamination levels in oil. The key to making a smooth transition between standards is to recognize the need to change our benchmarks and reference points, much as we did when learning to use SI (metric) units of measure.

Particle sizes obtained using the ISO 11171 calibration are not the same as those obtained in the past with ISO 4402. The relationship between the two is shown in Figure 1 (1). For ISO 4402 sizes smaller than 10 p.m, the corresponding ISO 11171 size is larger in magnitude. For ISO 4402 sizes 10 #m and larger, the corresponding ISO 11171 size is smaller. The size conversion shown in Fig. 1 was generated from Annex G of ISO 11171. The data used is an average of results from 3 laboratories and several different types of instruments. It is important to note that the exact size conversion for a specific laboratory may be different, depending on past ISO 4402 calibration history. As was noted earlier, batch-to-batch variability in ACFTD, instrument performance, and the ability of the calibration technician are sources of variability with the old ISO 4402 that will be reflected in the size conversion to the new standard. In precise work in which small differences in particle size are significant, the exact relationship between the old and new calibration for a specific instrument and lab should be determined. For most cases, the size conversion given in ISO 11171 is adequate, as different batches of calibration dust, instrument condition, and analytical techniques all
changed over time.
347

~(c) B/C NB/C Comparing beta ratios obtained using the old (ISO 4572) and new (ISO 16889) methods is more difficult, since not only the APC calibration method, but also the test dust has changed (from ACFTD to ISO MTD). This is illustrated in Fig. 2. In general, beta ratios for the same particle size will be higher with ISO 16889 than for ISO 4572 for particles larger than 10 ~m. In the authors' experience, for particle sizes 10 pm and larger old and new results can be compared with reasonable accuracy using a size conversion table or chart, e.g., Fig. 1. This is because APC calibration is the primary effect on beta ratio for these "large" particles. For smaller sizes, however, filters must be retested using ISO 16889 in order to properly account for contaminant size distribution effects on beta ratio. At sizes smaller than 10 pro, beta ratio at a particular size may increase or decrease depending on filter design. This is because of complex interactions between the effects of the calibration change and test dust.

The ISO solid contamination code is an abbreviated, cursory method for communicating particle size and concentration information. In this light, the transition from ISO 4406:1987 to ISO 4406:1999 is relatively easy. The new 1999 standard reports results for three size classes, corresponding to 4, 6 and 14 u.m(c), while the old 1987 standard reports only two classes, corresponding to 5 and 15 #m. The 6 and 14 p.m(c) classes correspond to approximately the same sizes as the old 5 and 15 p.m classes. In general, the same data reported using the two methods will agree within ±1 ISO code number at these size, well within the limits of the

40

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d ._N 03 e .2

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30

2O

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0 10 20 30 ISO 4402 Particle Size, pm

Fig. 1. Relating ISO 4402 (ACFTD) Sizes to ISO

I I

4O

11171 (NIST) Sizes

code, as shown in Fig. 3. In comparing historical, two digit code data to new, three digit code data, it is important to remember that the code is a cursory communication tool. Differences of ±1 code number are not significant and may well be more an artifact of how near the observed concentration is to a cut-off point between size classes, than to a real difference in contamination levels. Perhaps the greatest source of concern deals with the smallest size class. Prior to ISO 4406: 1999, there was no adopted ISO or national standard for the third digit. Nonetheless, some companies adopted 2 #m (ACFTD) as their own internal, third digit. This was a convenient size that was readily measurable by most APCs. The 2 #m ACFTD size corresponds to 4.6 p_m(c). ISO chose not to use this size class for two primary reasons. First, the use of decimal point size classes was avoided, in order to make it convenient for APC operators. Second, while 2 and 5 pm are 3 #m apart with the old ISO 4402, they are really only 1.8 #m(c) with the new NIST traceable ISO 11171.

Since ISO 11171 includes explicit APC performance specifications, it directly impacts the APC instrumentation used by the industry. Not all existing APCs meet the new specifications and those that do not will need to be repaired or replaced. Further, some that do meet the specifications may be unable to count at the 4.0 pm(c), the smallest size class in ISO 4406: 1999. Users of the new standards and users of the data need to be vigilant to ensure that APCs meet the new performance specifications. It can be tempting to ignore the specifications, but severe consequences may result from failure to heed them. For example, the smallest particle size that can be counted is 1.5 times the threshold noise level of the APC. If an APC is set at a lower threshold than this in an attempt to count smaller sizes, particle counts will be artificially higher due to electrical noise. Another example of the effect of failure to meet the performance specification is when APCs do

ooo,

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0 5 10 15 20 Particle Size, ~m

Fig. 2. Comparison of Filter Performance Data

99.9

99 o ~ , ~

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for Filters Tested using ISO 4572 and ISO 16889

348

not meet the resolution (Annex D) or counting accuracy specification (Annex E). If the system is used on a multi- pass test stand, agreement between upstream and downstream sensors can only be obtained under certain conditions and erroneous beta ratios (high or low depending on sensor location) are obtained. Failure to meet APC performance specifications may also lead to poor repeatability in filter testing, rejection of filters during qualification testing, artificially low beta ratios, and noncompliance with ISO or QS standards.

ISO 11171 - HINTS TO EASE THE TRANSITION

Upon first reading, ISO 11171 is intimidating. However, calibration can be expedited with a little forethought and planning. Operators must first decide which parts of the calibration must be performed. Table 2 shows when the various parts of the calibration need to be performed. If it is a routine calibration and there is no reason to suspect that the calibration has changed, only the size calibration procedure described in clause 6 of the standard need be performed. The operator may suspect that the calibration has changed if test results start to shift or if routine quality control samples (e.g., such as samples prepared in annex E or F) deviate outside of control limits. If the instrument has been serviced in some way that may affect its ability to size and count particles or measure sample volume or flow rate, specific parts of the procedure need to be done (refer to Table 2) in addition to the sizing calibration. If the instrument has never been fully calibrated with ISO 1171, it is necessary to perform the entire calibration, including clauses 5 and 6, as well as, annexesA, B, C, Dand E. This can be time consuming and many labs consider this to be beyond their expertise. If this is the case, the manufacturer of the particle counter or other outside expert able to conduct the full calibration can be used, providing the APC is shown to meet all performance specifications and that full documentation of the results

1000000 ~ ~ 1987 1999

100000 - A 913 1711013

,oooo !&Ol ! C 20/15 26/20/15 . - ~.j 1000

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20

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Fig. 3. ISO Code Results Reported using ISO
4406:1987 and ISO 4406:1999

artifact with

Table 2. APC Calibration Schedule

IF:

New APC or f irst ISO 11171 calibration Last calibration >6-12 months Suspect calibration shift Optics repaired or readjusted Sensor repaired or readjusted Volume measurement components repaired or readjusted Operation not involve repair or readjustment of

Then perform the operations described in the indicated

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portion of ISO 11171:

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X X X X

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No action necessary

levels, working flow rate, coincidence error limit, etc.

APC, sensor or volume measurement system is supplied to the user. Although this is often the path of least resistance, labs that take the time to go through the entire procedure find that the extra invested effort pays off shortly and that it is more than just a good way to train APC operators and to improve sample handling and analysis procedures.

In order to reduce expenses, ISO 11171 provides users have the option of using either a primary or secondary calibration method. With the primary method, the sizing calibration is done using NIST SRM2806 samples. A minimum of 3 samples is required for sizing calibration. Since SRM2806 costs about $350 per sample, primary calibration for laboratories with multiple APCs could become prohibitively expensive without the secondary calibration option. With the secondary method, ISO MTD suspensions prepared by a source other than NIST are used. Secondary samples must meet quality criteria described by the procedure and their size distribution must be determined using an APC calibrated using the full primary method. For laboratories having multiple APCs or multi-pass test stands, the primary method is typically used to calibrate a "golden particle counter" and the secondary method is used to calibrate all others with traceability obtained through the golden counter.

To reduce costs, some are tempted to conduct the entire sizing calibration using a single sample of SRM2806. This is in contradiction to the standard and greatly increases the risk of artifacts that could result in erroneous calibrations. In addition to sampling handling and analytical errors, there is a finite probability that the actual size distribution of a particular bottle of SRM2806 differs significantly from the averages given on the NIST certificate. To minimize the impact of these artifacts and to increase the likelihood of their detection, ISO 11171 requires that 3 separate samples be analyzed. Significant differences among the 3 is indicative of an

the potential of skewing the calibration

349

results. Often the artifact is the result of faulty equipment or poor particle counting technique. Therefore, discrepancies in the results or a failure of the results to pass the data quality criteria is the first indication of a problem, not only with the calibration, but also with the equipment or analysis technique. In any event, the problem must be identified and corrected before proceeding. Failure to do so renders the calibration and subsequent particle count data meaningless.

The first time a full ISO 11171 is conducted can be time- consuming, requiring 16-160 hours. Subsequent routine calibrations, in which only the sizing calibration need be performed require much less time. Here are some helpful tips that can be used to speed up the process: • Before starting, review and understand the

procedure. Familiarize yourself with the overall process and do no attempt illegitimate short cuts.

• Ensure that all equipment required for the calibration, including the APC, batch sampler, and flow rate measurement instrumentation, is in good working order and that all fluids, sample bottles, glassware, etc. meet or exceed the standards cleanliness specifications.

• Use only ISO MTD from the same batch used to prepare SRM 2806 and use only UFTD from the same batch used to generate Table 7 (1). (NOTE - These are available from NIST as RM 8631 and RM 8632, respectively.)

• Use the past calibration and performance history of the APC in making decisions about threshold settings and other discretionary aspects of the procedure. Even if the APC is new and only latex calibration curves are available, these provide an indication of the size-voltage relationships, noise

14. Quality System Requirements QS-9000

• Use computer spreadsheets to facilitate data analysis, such as data quality testing, curve fitting, interpolation, and resolution determination

• Use annex F to create many "extra" ISO MTD and UFTD samples for use in trouble-shooting and to verify APC performance and calibration between regularly scheduled calibrations.

• Carefully watch the data and statistical analysis results for trends and indicators of excessive variability, as these are often early warning of larger sample handling or sample analysis problems.

• Analyze a "practice sample" at many different voltage setting before actually starting the sizing calibration and counting accuracy procedures, in order to ensure that the APC is set at approximately the correct settings before starting. This could save time that would otherwise be spent "guessing" at the correct settings.

• Make up enough ISO UFTD samples to complete all phases of the calibration, since identical samples are used in the preliminary instrument check, to determine coincidence error limit, to determine flow rate limits, and to verify counting accuracy.

• Before conducting the entire resolution procedure, first determine the half-count setting of the latex using a single latex sample and adjusting the channels as needed to fulfill the half-count condition described in Annex D (1), then use these results as the starting point for the complete resolution determination.

CONCLUSIONS

In 1999, ISO adopted the most sweeping revisions to contamination control standards to have occurred since the early 1970's. With the passage of ISO 11171 (1), ISO 11943 (2), ISO 16889 (3), and ISO 4406:1999 (4), the industry has, for the first time, NIST traceability in particle size measurement, filter efficiency (beta ratio), and particle size distribution data. As a result, it is anticipated that the quality and reliability of particle count and filter test data will improve, increasing their usefulness to the hydraulics, automotive and aerospace industries. However, this will force the industry to make accommodations with regard to the way results are reported. When comparing results, users will need to ensure that common APC calibration and multi-pass filter test procedures were used. Filter and APC manufacturers need to educate users of their products concerning the impact of these changes. Those utilizing the unofficial 2-5-15 ~m ISO code for cleanliness specifications will need consider whether or not to adopt the new ISO 4406: 1999 4-6-14 p.m(c) solid contamination code. During the transition between standards, test reports and sales literature may need to report particle counts and beta ratios using both old and new methods. Despite the inconvenience, the new standards offer major advantages and improvements over the old standards, including : • First time ever NIST traceability,
• Reduced variability in test results, and
350

Ability to accurately count particles and analyze filter performance at very small particles sizes (1 ~m(c) NIST is about 4 times smaller than 1 ~m ACFTD).

ACKNOWLEDGMENTS

The authors gratefully acknowledge the work of NFPA Technical Committees T2.9 and T3.10 and of ISO TC131/SC6 in the development of the new standards. They would also like to acknowledge the work of Doug Paepke, Larry Liebmann, Jim Neese, Peter Rao and their other Fleetguard/Nelson colleagues in the experimental work reported in this paper.

REFERENCES

1. ISO 11171:1999 Hydraulic fluid power - Calibration of liquid automatic particle counters.

2. ISO 11943:1999 Hydraulic fluid power - On-line liquid automatic particle systems - Methods of calibration and validation.

3. ISO 16889:1999 Hydraulic fluid power - Filters - Multi- pass method for evaluating filtration performance of a filter element.

4. ISO 4406:1999 Hydraulic fluid power - Code for defining the level of contamination of solid particles.

5. ISO TR 16386:1999 Impact of changes in ISO fluid power particle counting - Contamination control and filter test standards.

6. ISO 4402:1991 Hydraulic fluid power - Calibration of automatic-count instruments for particles suspended in liquids - Method using classified AC Fine Test Dust contaminant.

7. ISO 4572:1981 Hydraulic fluid power- Multi-pass method for evaluating the filtration performance of a fine hydraulic fluid power filter element.

8. ISO 4406:1987 Hydraulic fluid power - Code for defining the level of contamination of solid particles.

9. Verdegan, B., Holm, C., Schwandt, B., "Reducing Variability in Particle Count Results for Oil Samples," Proceedings of the 47th National Conference on Fluid Power, April 23-25, 1996. pp. 335-349.

10. NIST, Certificate - Standard Reference Material 2806 - Medium Test Dust (MTD) in Hydraulic Fluid, 1997.

11. ISO 12103-1:1997 Road vehicles - Test dust for filter evaluation - Part 1: Arizona test dust.

12. Fletcher, R., Verkouteren, J., Windsor, E., Small, J., Steel, E., Bright, D., Liggett, W., "Development of a Standard Reference Material for the Fluid Power Industry: ISO Medium Dust in Oil," Proceedings of the 47th National Conference on Fluid Power, April 23-25, 1996. pp. 351- 364.

13. ISO 9O0O: 1987 Quality management and Quality assurance standards - Guidelines for selection and use.

particle counter calibration

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