HAP 0921

www.bfpa.co.uk Contact the BFPA at Tel: +44 (0)1608 647900 • Email: enquiries@bfpa.co.uk • Web: www.bfpa.co.uk level and filter performance specifications, even though nothing had changed. Measurement uncertainty Reported particle concentrations were observed to be 2 to 8 times greater with SRM 2806b calibrations compared to SRM 2806a. The introduction of alternative particle sizes reporting conventions resulted in confusion, as some laboratories reported in µm(c) to be consistent with existing specifications, while others chose to report in µm(b) sizes obtained directly from SRM 2806b. Further, it was apparent that, unless something changed, a size shift could result each time a new batch of SRM 2806 was certified simply due to measurement uncertainty. To address the challenges, ISO/TC 131/SC 6, in partnership with NIST, initiated an ambitious programme to revise ISO 11171 and the SRM 2806 certification protocol. As a result, ISO 11171:2020 establishes µm(c) as the sole particle size reporting convention and eliminates the µm(b) reporting option. SRM 2806 will continue to be used for primary calibration at particle sizes below 30 µm(c) while polystyrene latex will be used for primary calibration at larger sizes. FP022354:16 December 2020 The definition of µm(c) remains consistent with past versions of ISO 11171. To avoid future shifts in particle sizes associated with new batches of SRM 2806, NIST has started certifying SRM 2806, beginning with SRM 2806d, as a consensus standard using round robin data provided by ISO/TC 131/SC 6. A normative dilution procedure for calibration samples has been added to ISO 11171:2020 to minimise coincidence error and enable calibration at smaller particle sizes. The constrained cubic spline method is now specified for use in generating particle counter calibration curves. As a result of these improvements, an inter-laboratory study found that the uncertainty in particle concentration data using ISO 11171:2020 was approximately one third of that observed with the previous version. Further, there is no significant particle size shift nor shift in particle concentration data using ISO 11171:2020 with the newest batch of primary calibration suspensions, SRM 2806d. In the inter-laboratory study, some light scattering particle counters were calibrated to particle sizes as small as 1,5 µm(c). No notable impacts To facilitate implementation of ISO 11171:2020, ISO/TC 131/SC 6 has created ‘ISO/TR 4813 Hydraulic fluid “To facilitate implementation of ISO 11171:2020, ISO/TC 131/SC 6 has created ‘ISO/TR 4813 Hydraulic fluid power – Background, impact and use of ISO 11171:2020 on particle count and filter test data’, which discusses its background and implications. Significantly, there are no notable impacts upon those currently reporting and using the µm(c) particle size convention.” power – Background, impact and use of ISO 11171:2020 on particle count and filter test data’, which discusses its background and implications. Significantly, there are no notable impacts upon those currently reporting and using the µm(c) particle size convention. On the other hand, those using the µm(b) convention should take steps to convert to µm(c). In many cases, this may be as simple as returning to older (before 2012-2016) fluid cleanliness and filter performance specifications that used µm(c). In other cases, more recently developed µm(b) fluid cleanliness and filter performance specifications should be converted to µm(c). ISO/TR 4808 ‘Hydraulic fluid power – Interpolation method for particle count and filter test data’ provides a recommended procedure and guidance for converting µm(b) data and specifications to µm(c) and for the interpolation of data at intermediate particle sizes to ease the transition. With the publication of ISO 11171:2020 and supporting Technical Reports, ISO/TC 131/SC 6 has returned the industry to a single particle size reporting convention, reduced uncertainty in the data, and expanded the range of particle sizes that can be counted while minimising any inconvenience associated with the transition. This enables industry to implement more robust contamination control programmes and improve the reliability of equipment in hydraulic, diesel, aerospace, and similar applications. BFPA's technical and standards development committees play a key role in framing worldwide best practice in the fluid power industry through the development of National, European and International standards. If you would like to make a difference and get involved in this important work please email Yvonne Pearman, BFPA Standards Projects Manager at yvonne@bfpa.co.uk For more technical articles on fluid power please visit the BFPA website www.bfpa.co.uk/technical