In 2015, the United States National Institute of Standards and Technology (NIST) released the latest batch of calibration fluid for optical particle counters. The fluid, identified as SRM2806b, has certified values that are considerably higher than those of the previous batches of calibration fluid.

According to Bill Quesnel of CINRG Systems Inc., Burlington, Ontario, Canada, When optical particle counters are calibrated with fluids traceable to SRM2806b, they show alarming shifts in threshold settings for the various particle sizes. When samples are subsequently tested, count data and cleanliness codes are significantly higher compared to historical data.

In a presentation at OilDoc in Rosenheim, Germany, in January, Quesnel reported on an investigation by CINRG that examined the data from two calibration fluids to estimate the certification error between the two standards and to determine comparative particle counts. As a result of similar tests, he said, the committee responsible for ISO 11171 proposed a revision to the standard to manage the problem.

The ISO 11171 calibration standard for optical particle counters was issued in 1999 to account for the introduction of ISO Medium Test Dust to replace Air Cleaner Fine Test Dust used previously. There was much confusion surrounding the changes introduced in 1999 and a degree of resistance from oil labs to adopt the new standards, Quesnel related. But there is no doubt the changes eventually had a beneficial effect on the quality and reproducibility of count data produced by oil labs.

Since 1999, the industry has enjoyed a little more than 15 years of consistency with the calibration fluids based on the new standard. This consistency was interrupted in mid-2014 when NIST release the third batch of calibration fluid, SRM2806b, which had certified counts that were considerably higher than those of the previous two batches, Quesnel said.

The higher counts were due to improvements in metrology that produced more accurate particle counts. Although test dust content increased in SRM2806b, the new nominal content was not published by NIST. This missing information made it difficult to assess whether the relative contributions were the result of higher dust concentration or a more accurate procedure.

To quantify the relative contributions of these two variables, CINRG used the previous SRM2806a and the newer SRM2806b standards to calibrate an optical particle counting system and analyzed the data to compare the two fluids. The company concluded that it is reasonable to assume particle counts will increase by at least 40 percent or more after an optical particle counter is calibrated with NIST SRM2806b.

Quesnel explained that this increase is a major concern because the higher counts will have an impact on hydraulic, lubricant and fuel applications globally. Samples will appear to be dirtier and filters will appear to be less efficient, while it will be impossible to compare new … data with historical data, he said. This will have far-reaching consequences because a vast number of technical specifications will need to be revised to accommodate the changing numbers.

The committee responsible for ISO 11171 investigated the performance of the new and old calibration fluids, and drafted a revision to ISO 11171 that will minimize the impact of the new calibration fluid. Quesnel reported that the final draft has passed the ballot and has been published as ISO 11171:2016.

The revision allows recalculation of particle sizes measured with the new calibration fluid to an equivalent value for the old calibration fluid. This will yield results that are consistent with a calibration that would historically be traceable to SRM2806a.

Laboratories can purchase a calibration fluid traceable to SRM2806b and then have two options with regard to how they calibrate their particle counters, explained Quesnel. And the laboratory can continue to report particle counts as it has done in the past with relatively little change in count data or ISO cleanliness codes due the use of the new calibration fluid.

Only 4-, 6- and 14-μm particle sizes were examined because they are the only counts used to determine ISO 4406 cleanliness codes. But particle counters also measure particles greater than 21, 38 and 70 μm because these counts are needed to report sample cleanliness to AS4059. The 21-μm settings can be derived in the same way as the smaller sizes. Settings for particles larger than 30 μm are typically determined using different fluids and, therefore, are unaffected by the new calibration fluid.