July/August 2019

HYDRAULICS 28 HYDRAULICS & PNEUMATICS July/August 2019 www.hpmag.co.uk Refractive Index to the fluids used by the hydraulics industry and both gave variable results. In 1997, ISO were informed that ACFTD would no longer be available and had to develop an alternative. The alternative chosen was SAE 5-80, a test dust of identical material to ACFTD, but with significantly less numbers of particles below 5µm, which lessened coincidence errors in the APC. This was renamed ISO Medium Test Dust (ISOMTD) and formed part of a family of test dusts defined in ISO 12103 [2]. The particle size distribution (PSD) of calibration samples (SRM 2806) was certified by the National Institute of Science & Technology (NIST) in the USA using a Scanning Electron Microscope (SEM) with Image Analysis software, and traceability was obtained. The PSD of the dust was obtained by sizing in the particles in the sample on the basis of the diameter of a sphere whose area was the same as the particle, hence the term ‘equivalent spherical diameter’. Unfortunately, this new measurement gave slightly different counts to the previous ACFTD calibration and, to lessen the effects on industry, ISO selected the measurement size that would give notionally the same particle count had the APC had been calibrated using ACFTD to ISO 4402. This is seen in Table 1. As the equivalent sizes were often decimal sizes, the nearest integer size was selected to simplify communication, as seen in Table 1. To distinguish between the ‘old’ and ‘new’ calibration data, the new size was labelled µm(c). This new method of calibration has been published as ISO 11171 [3] and gives a more accurate and traceable calibration. This will enhance both the repeatability of measurement with the same instrument and the reproducibility of measurement between different instruments and laboratories. The standard also included procedures for determining the performance and accuracy of the APC. The latest situation The number of people using the NIST SRM 2806 calibration samples has increased subtantially the last 20 years and there is a limit to the numbers of suspensions that can be produced in any one batch. Thus, re-certification is necessary when a new batch is produced. As the first two batches (SRM 2806 and 2806a) were produced from the same material, recertification was not necessary, and the original certification applied. The next batch (SRM 2806b) was certified using an improved SEM with automated analysis to improve both the speed of analysis and its accuracy. A 10% increase in particle size was observed and this caused a shift in the calibration curve, for example, 10 µm(c) particles became 11 µm(b) particles. This 10% shift in particle size has a significant impact on observed particle counts, and reported filter removal ratings. To compensate for this a size correction factor of 0.898 has to be applied to correct a 2806(b) calibration to a 2806(a) and µm(c), i.e. µm(c) sizes =µm(b) x 0.898. Unfortunately, the latest version of ISO 11171: 2016 – which is still current – was somewhat ambiguous and it gives two options to obtain the calibration curve which will report different results on the same sample. This is in the process of being revised. Note that it is inevitable that there will be differences in the sizing of these particles during certification because the angular shaped particles will adopt different orientations on the analysis membrane and thus present a different area to the detector. This is despite the extraordinary lengths that NIST go to in order to control variability. ISO TC131/SC6 recently took a decision to ‘standardise’ on µm(c) as the measurement standard in ISO 11171 and all subsequent editions will feature this. SC6 have also instructed NIST to develop a ‘Consensus Standard’ which will simply give the numbers of particles per mL at the µm(c) size in the calibration suspension, thus greatly simplifying the issue. What to do about your current calibration. The actions necessary will depend on the calibration method used. If the calibration uses µm(c) then there are no actions necessary as this is the standardised unit now and in the future; both historical data can be compared and current specifications can be used. If µm(b) calibration is used, it: will give increased counts compared to µm(c) data may lead to failure to meet µm(c) specifications may result in extra time and money being expended to determine if the increased counts are due to real issues will give discontinuities with historical data may cause investment in testing to validate revised specifications for entire size range necessitate a repeat of the above factors when both subsequent batches of SRM 2806(x) and ISO 11171 revisions are released So, it is recommended that users of SRM 2806(b) calibration data convert the µm(b) sizes to µm(c) using the conversion factor in ISO 11171:2016, i.e. 0.898, to give the equivalent decimal µm(c) sizes. The calibration threshold values can be plotted against the decimal µm(c) values to obtain the µm(c) calibration curve for that calibration and the integer µm(c) sizes obtained by interpolation. Alternatively, a curve fitting routine can be used for more accurate interpolation. The ISO committee recommend the constrained cubic spline interpolation method developed by C.J.C. Kruger [4]. This curve fitting routine can be used to obtain the revised µm(c) calibration data and also to obtain the particle count data on the basis of µm(c) calibration to see if the SRM 2806(b) data exceed earlier specifications. Future calibrations As stated above, NIST are working on the so-called ‘Consensus Standard’ so that future calibration samples will be certified to the µm(c) base and calibration. Thus, it will be a simple matter of setting the APC to record the certified numbers at the sizes required that are given on the NIST certification sheet. This will greatly simplify the process of APC calibration, overcome possible confusion caused by a recertification process, improve the supply of calibration samples and avoid running out of samples. It will also greatly reduce the amount of time spent by members of the ISO Working Group that has to be involved with the validation of any new batch of suspension material. References [1] ISO 4402: ‘Hydraulic fluid power – Calibration of automatic count instruments for particles suspended in liquids – Method using classified AC Fine Test Dust’, International Standards Organisation, Geneva, Switzerland, 1991. [2] ISO 12103-1: ‘Road vehicles – Part 1: Arizona test dust’, International Standards Org., Geneva, Switzerland, 2016. [3] ISO 11171: ‘Hydraulic fluid power – Calibration of automatic particle counters for liquids’, International Standards Org., Geneva, Switzerland, 2016 [4] C.J.C Kruger: ‘Constrained Cubic Spline Interpolation for Chemical Engineering Applications’, Korf Technology Ltd, Ontario, Canada, 2003, www.korf.co.uk/spline.pdf. Table 1. Changes to calibrated particle size