Converting of ISO purity class to NAS class - is that possible?

Year of publication: 2002
Updated on: 09-2023


Many machine failures are due to contamination of the hydraulic oil. To assess the purity of an oil, a particle count is performed. With the number of particles determined for certain particle sizes, such as 5 μm, assigned cleanliness classes can be given.


In Europe, the purity class is given according to ISO standard 4406, in the USA the classification according to SAE AS 4059 (formerly: NAS 1638) is common. However, the two standards, NAS (National Aerospace Standard) and ISO (International Organisation for Standardisation), differ considerably in the evaluation of the measured particle counts: 


 When determining the purity class according to ISO, all particles are counted cumulatively, i.e. they are added together. Particles of sizes >4, >6 and >14 μm are taken into account. The purity class is indicated in 3 classes, for example "20/15/12", where each number stands for one of the 3 counting classes. In the case of a specification with two numbers, such as "15/12", the smallest ISO size class is not taken into account, because, for example, the particles were counted optically with a microscope. 


 When determining the NAS class, the particles are counted differentially, i.e., each channel is counted separately. The number of particles of sizes 5-15 μm, 15-25 μm, 25-50 μm, 50-100 μm and >100 μm are determined. I.e., particles smaller than 5 μm are not taken into account, in contrast to the ISO standard. In AS4059, a NAS number (0 to 12) is determined for each of the 5 counting ranges. The highest of the 5 determined NAS numbers results in the NAS class, e.g. "NAS 9". 


Due to these different counting methods, an exact assignment between NAS class and ISO purity class is not possible. In the OELCHECK laboratory, both the ISO purity class and the NAS class are determined as standard when determining the purity class and are also printed on the laboratory report.