To display particle counts in oil and fuel samples and, depending on the industry, also in water-based hydraulic fluids, in a simplified and uniform way, they are often reported in cleanliness classes according to the chosen reporting standard. In the following we’d like to introduce you to the most common and important international standards.
1. Current cleanliness classes
History and scope: In 1999 ISO 4406:1999 replaced the previous standard ISO 4406:1987. The latest revision is called ISO 4406:2017. ISO 4406 is predominantly used in the hydraulic industry however companies within the filtration, fuel and automotive industries also adopt this standard.
Particle sizes: The cleanliness classes as per ISO 4406 are determined by the cumulative counts in the size bands >4µm(c), >6µm(c) and >14µm(c). To comply with the ISO-standardization process, the particle sizes have been redefined. Particle sizes are now displayed as µm(c). The additional (c) indicates that the particle counter has been calibrated according to ISO 11171. This enables the old and new measurements to be easily identified and differentiated. 1µm as per ISO 4402 equals approximately to 4µm(c) as per ISO 11171.
Calibration: The ISO 4406 reporting standard is based on the calibration standard ISO 11171. The calibration media used is called ISO MTD (ISO Medium Test Dust) and it was introduced after the end of production of ACFTD (Air Cleaner Fine Test Dust) which had been previously used to calibrate particle counters according to ISO 4402.
History and scope: SAE AS4059 was developped to succeed the NAS 1638 standard and to eliminate the existing disadvantages NAS 1638 has, most importantly, the traceability of the used calibration standard. NAS 1638 has no traceability. SAE AS4059 is mainly used by the military and companies from the aviation/aerospace and the offshore oil & gas-industry.
Particle sizes: Depending on the application, 5 or 6 cleanliness classes are reported.
For 6 size channels, they are displayed in: > 4 µm(c), 6 µm(c), 14 µm(c), 21 µm(c), 38 µm(c) and 70 µm(c).
For 5 size channels, they are displayed in: >6 µm(c), 14 µm(c), 21 µm(c), 38 µm(c) und 70 µm(c).
This is the latest revision that has removed the Banding A-F (6 channels) and B-F (5 channels) from the reporting standard result. The result is reported to the largest classification code of the size channels with the letters cpc preceding it. The letters cpc stands for cumulative particle count. For example, a result would be displayed as – SAE AS4059F cpc 4
Calibration: As with the ISO 4406 standard, SAE AS4059 is based on the ISO 11171 calibration standard.
History and scope: The Russian military standard GOST 17216:2001 plays a central role in expressing the cleanliness of oils and fuel in all military and civil applications. This doesn’t just apply for Russia but also for all CIS countries and some Asian countries which supply to the Russian market.
Particle sizes: Depending on the interpretation, cleanliness classes for various particle sizes between 0.5µm and 200µm are determined.
Calibration: GOST 17216:2001 is based on the calibration standard ISO 4402.
2. Historical cleanliness classes
History and scope: NAS 1638 is the predecessor of SAE AS4059. Being the oldest cleanliness standard, it is known to most users. Originally developed in the 1960’s as a guideline for the cleanliness of hydraulic fluids in the aviation industry, it quickly became adopted by other industries.
Particle sizes: The cleanliness classes according to NAS 1638 get determined by the differential counts in the size bands of:
5 to 15 µm, 15 to 25 µm, 25 to 50 µm, 50 to 100 µm and >100 µm.
Calibration: To express results as per NAS 1638, automatic particle counters were usually calibrated according to ISO 4402. Since there weren’t any particle counters developed in 1964 when NAS 1638 was published, the first calibration standards have been retrospectively tailored to NAS 1638. The production of the ISO 4402 calibration material ACFTD stopped in 1999 and calibrations according to ISO 4402 are no longer effective since then. PAMAS, however, have a large stock of this ACFTD calibration material and are able to provide calibrations according to ISO 4402 upon request if the customer applications or legacy projects require this standard.
History and scope: The predecessor of ISO 4406:1999 today only is relevant as a reference value or for historical reasons.
Particle Sizes: The cleanliness classes according to ISO 4406:1987 are determined by the cumulative counts in the size bands >5 µm und >15 µm (sometimes also >2, >5 and >15 µm).
Calibration: ISO 4406:1987 is based on the calibration standard ISO 4402.
This chart gives an overview of the most common cleanliness classes and the calibration standards on which they are based: