Converting of ISO purity class to NAS class - is that possible?
Year of publication: 2002, updated: 10-2024
Many machine failures are due to contamination of hydraulic oil. To assess the cleanliness of an oil, a particle count is carried out. The number of particles detected for specific particle sizes, such as 5 μm, allows the determination of the corresponding cleanliness classes.
In Europe, the cleanliness class is specified according to ISO standard 4406, while in the USA, classification according to SAE AS 4059 (formerly NAS 1638) is common. The two standards, NAS (National Aerospace Standard) and ISO (International Organization for Standardization), differ significantly in the evaluation of the measured particle counts.
In determining the cleanliness class according to ISO 4406, all particles are counted cumulatively, i.e., they are added together. Particles of sizes >4, >6, and >14 μm are considered.
The cleanliness class according to ISO is indicated in three classes, for example, as "20/15/12," where each number represents one of the three counting classes. In a specification with two numbers, such as "15/12," the smallest ISO size class is not considered because the particles were, for example, visually counted using a microscope.
In determining the NAS class, particles are counted differentially, meaning that each channel is evaluated separately.
The number of particles in the sizes 5-15 μm, 15-25 μm, 25-50 μm, 50-100 μm, and >100 μm is determined. Particles smaller than 5 μm are not considered, unlike in the ISO standard.
In SAE AS 4059 (where particles are counted cumulatively, like in ISO 4406), an order number (000 to 12) is assigned for each of the six counting ranges (size codes).
The highest of the six order numbers determined, taking into account the corresponding size code (A-F), defines the cleanliness class according to SAE AS 4059, e.g., "9B."
SAE AS 4059 also includes a table that links to the NAS class, allowing for a reference to a NAS class.
Due to these different counting methods, an exact correlation between NAS class and ISO cleanliness class is not possible.
At the OELCHECK laboratory, both the ISO cleanliness class and the SAE class are routinely determined and reported on the laboratory report.
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