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ISO 11171 Standard

The release of SRM2806b by the National Institute of Standards and Technology (NIST) in 2015 marked a significant shift in the calibration standards for optical particle counters. While this new calibration fluid brings increased accuracy, it has also introduced challenges for laboratories and industries reliant on consistent particle count data. Here's a concise overview of the transition, its implications, and the proposed path forward.

Background: The Evolution of Calibration Standards

In 1999, ISO replaced the ISO4402 calibration standard with ISO11171, introducing ISO Medium Test Dust (ISO MTD) to replace the previously used Air Cleaner Fine Test Dust (ACFTD). This shift brought a new level of precision to optical particle counters, as particles were now measured based on circular equivalent diameter rather than longest chord length. However, it also caused higher contamination levels to be recorded, creating disruption across industries.

To address this, the ISO 4406 cleanliness code reporting standard was revised to introduce new particle size thresholds—4µm(c), 6µm(c), and 14µm(c)—minimizing discrepancies between old and new data.

The Introduction of SRM2806b

For over a decade, the industry enjoyed stability with two calibration fluids—SRM2806 and SRM2806a—both yielding consistent particle count values. However, the release of SRM2806b in 2015 disrupted this balance. Certified particle counts for SRM2806b are considerably higher than its predecessors due to:

  1. Higher Dust Concentration: SRM2806b contains a higher concentration of test dust.
  2. Improved Certification Accuracy: Advances in measuring technology and analysis methods enhanced the certification process.
These improvements led to a significant shift in particle size thresholds and, consequently, higher particle counts in sample analyses.

Challenges with SRM2806b Adoption

The adoption of SRM2806b has created issues reminiscent of those experienced in 1998 when ISO MTD replaced ACFTD. Key challenges include:

  • Increased Count Values: Particle counts calibrated with SRM2806b are significantly higher, causing cleanliness codes to rise.
  • Inconsistent Historical Comparisons: Data calibrated with SRM2806b cannot be directly compared to historical results, leading to confusion in trend analyses.
  • Industry-Wide Impact: The higher counts impact filter efficiency evaluations, contamination assessments, and adherence to technical specifications, all of which must be revised.

Investigating the Sources of Count Differences

To understand the discrepancies between SRM2806a and SRM2806b, secondary calibration fluids traceable to each standard were analyzed. The findings revealed:

  • A 25% higher concentration of test dust in SRM2806b compared to SRM2806a.
  • The improved certification process contributed significantly to the observed count differences.
When real-world samples were analyzed using the two standards, particle counts based on SRM2806b calibration were consistently higher, with some cleanliness codes increasing by two units.

Proposed ISO Revisions to Address the Challenges

To mitigate the impact of SRM2806b, the ISO committee responsible for ISO11171 has proposed a minor revision to the standard. Key elements of the proposed solution include:

  1. Dual Reporting Options: Results can be reported as >4µm(c), >6µm(c), and >14µm(c) for SRM2806a traceability, or >4µm(b), >6µm(b), and >14µm(b) for SRM2806b traceability. This distinction helps clarify the standard used for calibration.
  2. Equivalent Micron Sizes: Laboratories can use equivalent particle size thresholds for SRM2806b (e.g., 4.45µm(b) for 4µm(c)) to maintain consistency with historical data.
  3. Broader Calibration Applicability: While the revision focuses on ISO 4406 cleanliness codes, it also allows for calibration of larger particle sizes (e.g., >21µm) used in AS4059 standards, ensuring broad applicability.

Implications for Industry

The changes introduced by SRM2806b have far-reaching consequences, including:

  • Higher contamination levels being reported, potentially misrepresenting the condition of fluid systems.
  • The need for revised technical specifications to accommodate new cleanliness codes.
  • Challenges in comparing historical and current data, requiring careful interpretation.
While the proposed ISO revisions provide short-term solutions, the industry must collectively determine how to address these issues in the long term. The adoption of SRM2806b underscores the importance of accuracy in particle counting while highlighting the complexities involved in maintaining consistency across evolving standards.

Conclusion: A Path Forward

The introduction of SRM2806b is a double-edged sword: it enhances calibration accuracy but disrupts historical data consistency. The proposed revisions to ISO11171 offer a practical interim solution by allowing dual reporting options and equivalent size thresholds. However, the industry must adapt to the realities of SRM2806b by revising specifications and establishing new benchmarks for contamination analysis.

In the meantime, laboratories and end-users should remain vigilant in interpreting cleanliness codes, understanding that >4µm(b), >6µm(b), and >14µm(b) will inherently reflect higher contamination levels than their micron(c) counterparts. With thoughtful implementation of ISO11171 revisions and proactive industry collaboration, the challenges posed by SRM2806b can be effectively managed, paving the way for a more accurate and robust particle counting framework.

Posted By Alistair Geach on January 8, 2017