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Introduction and background
IEST-STD-CC1246D was published in 2002(1) and is the latest revision of the standard that was first published in 1962 as MIL-STD-1246.2 The standard was written in response to a need to define quantitative cleanliness levels for products that included components and fluids. Levels were defined for both particulate and nonvolatile residue (NVR) molecular contaminants.
Based on the technology and the needs at that time, it was determined that particles of sizes less than 1 µm and NVR levels of less than 1 mg were not significant.
This lognormal approximation provides a good representation of particles on or in precision-cleaned or filtered products. Larger particles tend to be more easily removed than smaller particles, resulting in larger numbers of small particles remaining in or on the product.
IEST-STD-CC1246D uses the same equation, but the units were changed to be consistent with the International System of Units/metric (SI) usage. The ft^sup 2^ was replaced with 0.1 m^sup 2^ and all fluid volumes are now per 0.1 liter. The difference between ft^sup 2^ and 0.1 m^sup 2^ was deemed to be insignificant considering the uncertainties in counting particles, especially using manual counting methods.
Figures 1 and 2 show the particulate cleanliness levels as defined in IEST-STD-CC1246D.
Originally, the range of particulate cleanliness levels was from 10 to 1000 and was extended to 1 in revision B.4
NVR levels were originally specified as mg/ft^sup 2^ for surfaces and mg/0.1 liter for liquids. Gases were not included.
Revision C
The U.S. Army Missile Command approached the IEST to revise the standard. IEST Working Group 901 was formed to work on the revision, and MIL-STD-1246C was published in 1994.5
The rapid growth in contamination-control technology and the demand for more stringent cleanliness requirements resulted in significant changes to the standard.
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