![]() ![]() ![]() From this basic distribution, a series of 14 classes was created covering very clean to very dirty levels, where the interval between each class is double the contamination level. This distribution was based on particle contamination inside delivered aircraft hydraulic components in the 1960s. The concept of the code can be seen in Table 1 and it is based upon a fixed particle size distribution of the contaminant over a size range of 5 to 100 µm. NAS 1638 Contamination Classes NAS 1638 was the forerunner of other contamination coding. If you obtain the latest issue of NAS 1638 you will find the following statements: “Inactive for new designs after May 30, 2001, see AS4059C” and “6.1.3 This standard should not be used with Automatic Particle Counting.” This is a result of recent changes to the ISO contamination standards for automatic particle counter (APC) calibration 3, which necessitated the review of NAS 1638 and resulted in its withdrawal for newly designed systems. It led to the development of other coding systems, the most notable being the International Organization for Standardization (ISO) 4406 2. ![]()
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