"Can you explain the NAS value in hydraulic and engine oils? Does it vary with a change in viscosity?"
The National Aerospace Standard (NAS) 1638 was developed in the 1960s to help control the contamination levels of hydraulic fluid within hydraulic components. NAS 1638 became a standard not only for the aerospace industry but also throughout many other industries as well. The latest version of NAS 1638 is SAE 4059.
Nevertheless, in many areas of industry, NAS 1638 has largely been outpaced by the ISO 4406 standard in recent years. Both NAS 1638 and ISO 4406 focus on the methods for calculating particle counts or solid contamination levels within a fluid.
NAS 1638 represents the counts of particles in five size groups:
ISO 4406:1999 represents the counts of particles in three size groups:
From these five groups of particle sizes and a basic particle size distribution, NAS 1638 reports the results in 14 distinct classes of particle counts numbered 00, 0 and 1 through 12, with 00 being the cleanest and 12 being the dirtiest. Each proceeding class contains twice the number of particles as the previous class per each of the five particle size groups (see the table below).
NAS 1638 Contamination Classification System
Although the use of NAS 1638 has been decreasing through the years, it is still used in some areas of the world. The most recent update to the standard includes the following: "Inactive for new designs after May 30, 2001, see AS4059C," and "This standard should not be used with automatic particle counting."
AS4059C was put in place to overcome some insufficiencies with NAS 1638 related to the proper direction of the method application.
ISO 4406 utilizes a new calibration method for automatic particle counters (APCs). This method reports particle counts in three code numbers with each number correlating to the count of particles larger than 4, 6 and 14 microns.
Since NAS 1638 (as well as ISO 4406 and AS4059) is designed solely to measure particle counts in terms of size distribution, the effects of viscosity have no role in the reported values.