Contamination Control
“New Particle Analysis
Standard Reference Material”
by Gary L. Horton, Ph.D.
 

From the article originally published in "Lubrication & Fluid Power - Journal of Machinery Fluids, Tests, and Standards"Summary
A new traceable U.S. National Institute of Standards (NIST) Standard Reference Material (SRM) used for International Standards Organization (ISO) calibration procedures for automatic particle counters has been introduced. The SRM addresses past shortcomings in standards and methods for particle size analysis. Several benefits which provide greater credibility and technical validity to the very important area of contamination control and management are a result of the developments. ISO 9000 certified operations conducting particle analysis, desiring to maintain certification, will need to include these new standards in their procedures.Background

Since the 1960s, automatic particle counters (APC) have been calibrated with hydraulic fluid suspensions of dust collected from Arizona and prepared in size ranges (0-100 um) suitable for measurements of interest to the contamination control community. The AC Division of General Motors conducted these preparations and acted as a supplier until 1992, when they discontinued the preparation of Air Cleaner Fine Test Dust (ACFTD).



Another issue impacting this area related to ongoing concern, since the early 1980s, about accuracy of particle sizing of ACFTD in the <10 um range. For these reasons, the National Fluid Power Association Contamination Control Committee T 2.9 initiated efforts in the early 1990s leading to the introduction of a new particle analysis NIST Standard Reference Material and ISO standard procedures for calibration of automatic particle counters.

“ISO 9000 certified operations desiring to maintain certification
will need to include these new standards in their procedures.”

New Particle Dust Standard
ISO 12103-A3 particle dust (also known as ISO Medium Test Dust or ISO MTD) is the new dust reference material for size calibrations. Like the previously used reference dust (ACFTD), ISO MTD derives from the same Arizona source and contains silica particles in the 0-100 um range. However, ISO MTD is prepared by a different process and has a different size distribution. Most importantly, particle sizes in the new calibration fluid were determined by scanning electron microscopy (SEM), as opposed to optical microscopy for ACFTD.



New Primary Standard Reference Material Fluid
The new ISO standard procedure for calibrations of APCs is ISO 11171, replacing ISO 4402 (Figure 1). Note that particle size dimension designations by ISO 11171 and um(c) to indicate a certified source, NIST SRM 2806. SRM 2806 is a suspension of 2.8 mg/liter of ISO 12103-A3 dust in super clean MIL-H-5606 Hydraulic Fluid. As mentioned, particle size counts in SRM 2806 were performed by NIST using scanning electron microscopy, which is much more accurate and precise than optical microscopy sizing. Additionally, the new sizing procedure establishes a projected area equivalent diameter for the particle vs. the longest chord dimension for the old procedure (Figure 2).Secondary Standard Reference Material
Although NIST SRM 2806 is required for primary calibrations of APCs, ISO 11171 allows use of secondary standards for economical reasons (i.e., cost of NIST SRM 2806) to conduct periodic verifications of working APCs or to calibrate more than one APC in a laboratory (Figure 1). Secondary standards must be prepared following ISO 11171, Annex F, using an APC primary calibrated with SRM 2806.



Measurements: New SRM and ISO calibration procedure
Along with the greatly improved smaller size detection and accuracy of SEM, this approach results in a major difference in measured particle size counts at <10 um, comparing ISO 4402 with ISO 11171 procedures (Figure 3). For the reasons, described above, measured sizes are different between the old and new systems, as shown in Figure 4. This means, that future particle



size measurements, following ISO 11171 with calibrations using SRM 2806 will provide richer data in small particle content with improved quality and reproducibility, overall. Some important feature differences between the old and new calibration fluids are summarized in Figure 5. Traceability to respected standards organizations and certification of data are at the heart of quality programs involving measurements impacting the quality of products and services. The output products of analytical measurements by analysts are dimensional number values. The value to the user of these dimensional number values is directly dependent on their accuracy and precision. It is exceedingly important that users of analytical data have confidence in that data. Major human and material resources are often at risk.New ISO Procedures & ISO 9000/QS 9000 Certification
Quality programs based on ISO 9000 or QS 9000 certification will need to address the issue of converting to new calibration

“Quality programs based on ISO 9000 or QS 9000 certification will need to
address the issue of converting to new calibration procedures.”

• ISO 4402 replaced by ISO 11171 (Calibration of Automatic Particle Counters for Liquids)
• ISO 11943 (New) (On-line Automatic Particle Counting Systems for Liquids – Methods of Calibration and Validation) Note: this method required ISO 11171 calibration
• ISO 4572 replaced by ISO 16889 (Multi-pass Method for Evaluating Filtration Performance of a Filter Element) Note: this method requires ISO 11171 calibration
• ISO 4406 (Update) (Fluids – Method for Coding the Level of Contamination by Solid Particles)

Contamination Codes
As shown in Figure 4, calibrated sizes compared at 5 um(old) and 6 um(c)(new), and compared at 15 um(old)

and 14um(c)(new) are close.



Consequently, contamination code values, by the updated ISO 4406, will not change for the 2 larger size ranges, but, now includes an additional smaller official value at 4 um(c) as shown in Figure 6. It is important to note that none of the procedure and code changes result in any change in actual contamination level of a given sample.



Conclusions
Implementation of calibrations using SRM 2806 and ISO 11171 will lead to overall improved data quality and integrity (Figure 7). Accuracy and precision will improve for measurements for on-site and analytical service labs. Repeatability at individual labs and reproducibility between labs will improve, contributing to improved confidence and acceptance of data and promoting collaboration and interchange on programs. Additional benefits will include establishing performance parameters and limits of individual APCs, such that appropriate



selection anduse of equipment is ensured. Finally, ISO 9000 and QS 9000 certification conformance will be realized by implementation of the new particle analysis calibration reference material and associated ISO procedures...References: