NAS 1638 is a standard for classification of particle contamination in oils and fluids.

It is an old standard, originally employed in 1964 to classify the cleanliness levels in hydraulic components and systems within the US aerospace industry.

NAS stands for National Aerospace Standard.

 NAS1638 has been the most commonly used cleanliness standard in the oil-, offshore and subsea industry of Norway. In recent years, it has been more or less replaced by the SAE AS4059 standard, even though NAS 1638 is still being used to classify cleanliness levels in hydraulic components and systems for the petroleum sector in Norway.

A cleanliness code is given based on the number of particles within 5 different particle size classes pr. 100 ml  :

• 05-15  μm
• 15-25  μm
• 25-50 μm
• 50-100 μm
•     >100 μm

The cleanliness codes vary between 00, being the cleanest, and 12, being the dirtiest code.

The highest code in the 5 different particle size classes indicates the cleanliness class.

The cleanliness standard NAS1638 was originally developed in 1964.