Cleanliness classes for hydraulic oil and fuels

In order to present particle counts in oil and fuel samples and, depending on the industry, also in water-based hydraulic fluids, in a simplified and uniform manner, they are often reported in cleanliness classes according to the chosen reporting standard.

Below we would like to introduce you to the most common and important international standards:

ISO 4406:2017

-History and scope: In 1999, ISO 4406:1999 replaced the previous ISO 4406:1987.

The latest revision is called ISO 4406:2017.

ISO 4406 is primarily used in the hydraulic industry, but companies in the filtration, fuel and automotive industries also apply this standard.

-Particle sizes: The cleanliness classes according to ISO 4406 are determined by the cumulative particle numbers according to the particle sizes >4µm(c), >6µm(c) and >14µm(c).

To comply with the ISO standardization process, particle sizes have been redefined. Particle sizes are now expressed as µm(c), where (c) indicates that the particle counter is calibrated according to ISO 11171. This allows the old and new measurements to be easily identified and differentiated. 1 µm according to ISO 4402 is approximately equal to 4 µm(c) according to ISO 11171.

-Calibration: The ISO 4406 reporting standard is based on the ISO 11171 calibration standard. The calibration medium used is called ISO MTD (ISO Medium Test Dust) and was introduced after the end of production of ACFTD (Air Cleaner Fine Test Dust), which was previously used to calibrate particle counters according to ISO 4402.

SAE AS4059

-History and scope: SAE AS4059 was developed to succeed the NAS 1638 standard and to eliminate the existing disadvantages. For example, SAE AS4059 has the traceability of the calibration standard used (see below), where NAS 1638 did not have traceability.

SAE AS4059 is primarily used by the military, aerospace, and offshore oil and gas industries.

-Particle sizes: Depending on the application, 5 or 6 cleanliness classes are reported.

For 6 classes they are shown in: > 4 µm(c), 6 µm(c), 14 µm(c), 21 µm(c), 38 µm(c) and 70 µm(c).

For 5 classes they are shown in: >6 µm(c), 14 µm(c), 21 µm(c), 38 µm(c) and 70 µm(c).

This is the last revision which removed the AF (6 channels) and BF (5 channels) from the standard reporting result. The result is reported according to the largest classification code of the particle size channels with the letters “cpc” in front (cumulative particle count). For example, a result is shown as SAE AS4059F cpc 4.

-Calibration: Similar to the ISO 4406 standard, SAE AS4059 is based on the ISO 11171 calibration standard.

GOST 17216:2001

-History and scope: The Russian military standard GOST 17216:2001 plays a central role in expressing the purity of oils and fuel in all military and civil applications. This applies to Russia, all CIS countries and some Asian countries supplying the Russian market.

-Particle sizes: Depending on the interpretation, cleanliness classes are determined for different particle sizes between 0.5 µm and 200 µm.

-Calibration: GOST 17216:2001 is based on the calibration standard ISO 4402.

NAS 1638

-History and Scope: NAS 1638 is the predecessor to SAE AS4059. Because it is the oldest cleanliness standard, it is familiar to most users. Originally developed in the 1960s as a guideline for the cleanliness of hydraulic fluids in the aerospace industry, it was quickly adopted by other industries.

-Particle sizes: The cleanliness classes according to NAS 1638 are determined by the differential numbers of the particle sizes between 5 to 15 µm, 15 to 25 µm, 25 to 50 µm, 50 to 100 µm and >100 µm.

-Calibration: To express results according to NAS 1638, automatic particle counters were usually calibrated according to ISO 4402. Since no particle counters were developed in 1964 when NAS 1638 was published, the first calibration standards were retrofitted to NAS 1638. Production of the ISO 4402 calibration material ACFTD ceased in 1999 and calibrations according to ISO 4402 have not been effective since then.

However, PAMAS still has a large stock of this ACFTD calibration material and can provide calibrations to ISO 4402 upon request if customer applications or legacy projects require this standard.

ISO 4406:1987

-History and scope: The predecessor of ISO 4406:1999 is only relevant today as a reference value or for historical reasons.

-Particle sizes: The cleanliness classes according to ISO 4406:1987 are determined by the cumulative numbers of particle sizes >5 µm and >15 µm (sometimes also >2, >5 and >15 µm).

-Calibration: ISO 4406:1987 is based on the ISO 4402 calibration standard.

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