Precision cleaning is a practice spanning multiple high-purity industries. Cleanliness is necessary for a system owner to ensure their system performs as designed, without concern for corrosion or fracture from unwanted contamination. Clean is both a perishable condition and a condition requiring further levels of specification, especially in critical applications throughout the aerospace industry. For that reason, the US military drafted MIL STD 1246 in the 1960s to establish cleanliness levels and detail verification methods. However, MIL STD 1246 was replaced in 2002 by the Institute of Environmental Sciences and Technology’s standard IEST-STD-CC1246. At the time of this post, IEST-STD-CC1246, titled “Product Cleanliness Levels – Applications, Requirements, and Determination” is in its “E” revision, which was published in 2013.
IEST-STD-CC1246 Section 2
IEST-STD-CC1246 provides “…methods for specifying and determining product cleanliness levels for contamination-critical products.” With that said, the standard does not contain procedures for the test methods used to measure cleanliness levels. Instead, it only references the test methods. For example, Section 2 of the standard refers to other standards, specifications, and applicable documents such as ASTM F331 and ASTM F311 which are test methods for conducting nonvolatile residue solvent extraction tests and particle count tests, respectively.
IEST-STD-CC1246 Section 5: Particles and Nonvolatile Residue (NVR)
Section 5.2 of IEST-STD-CC1246 defines cleanliness levels and is split into two subsections. The first section is for particulate cleanliness levels and the second is for nonvolatile residue cleanliness levels. Particulate cleanliness levels are classified by a single number which defines a distribution of particle sizes allowed within the class. This distribution is split up into ‘bins’ of particle sizes where only a certain number of particles are allowed within each bin depending on the cleanliness level. Beyond defining the allowable distribution of particle sizes, the number designation also relates to the maximum particle size in which only one particle is allowed in the test sample. Level designations and their size distributions are shown in Table 1 below which is the same as in Section 5.2.1 in the standard.
Table 1. Particle Cleanliness Levels (Bold Column Headers) and Associated Particle Size Distribution
|min (um)||max (um)||25||50||100||200||300||400||500||750||1000|
The first two columns, min and max, are the bin endpoints. The first particle size bin is any particle between 5 and 15 micrometers. The remaining columns, 25 through 1000, are the particle cleanliness levels. All blank fields are bins that do not require counting. Levels 200, 300, and 400 allow more than one particle in their highest nonzero bin because the level designation size falls within the min and max of the bin. The same rule still applies where each designation shouldn’t allow more than one particle above its respective number. For example, with Level 200, although it says 15 particles between 100 and 250 micrometers, 14 of those particles must be between 100 and 200 micrometers and only one may be between 200 and 250 micrometers.
Nonvolatile residue cleanliness levels are shown in Table 2, a re-creation of the same table in Section 5.2.2.
Table 2. Nonvolatile Residue Cleanliness Levels
|Level (µg/cm2 or µg/0.1cm3)||
Maximum allowable NVR limit mass / 0.1 m2 or
mass / 0.1 L
Understanding and Communicating Cleanliness Levels in IEST STD-CC1246
The final part of Section 5 of IEST-STD-CC1246 offers guidance on communicating cleanliness levels for component owners and cleaning service requesters. The particle count level is mentioned first, and NVR level second. In all instances, cleanliness levels defined in the standard are expressed as the maximum amount per either unit area or unit volume, respectively. The standard uses 0.1L of solvent volume to represent 0.1M^2 (1 Ft^2) of surface area. To summarize, a component cleaned to IEST-STD-CC1246 level 50R1 must contain less than or equal to 141, 17, 6, and 1 particle(s) in the first four bins respectively and 1 mg of nonvolatile residue per each 0.1L or 0.1 m2 sampled.
The Role of the Compressed Gas Association’s CGA G-4.1 Specification
Another specification that is involved in industries with contamination-clean components is the Compressed Gas Association’s CGA G-4.1 specification. CGA G-4.1 provides a large amount of commentary on various cleaning techniques and chemistries to suitably clean components for oxygen service. Not only does it mention cleaning techniques, but it also includes contamination limits. Section 9.1 of the 2018 publication of CGA G-4.1 sets the maximum allowable level of NVR to 20 mg/ft2(220 mg). Further on in the section is a recommendation for particulate cleanliness. CGA G4.1 recommends no particles 1000 micrometers or larger and a maximum of two particles between 500 and 1000 micrometers. This recommendation is closest compared to IEST-STD-CC1246 level 1000 but with less total particles between 500 and 1000 micrometers and no maximum particle number for any bins below 500 micrometers.
Comparing IEST-STD-CC1246 and CGA G-4.1 Specifications
Comparing IEST-STD-CC1246 and CGA G-4.1 reveals a large difference between both particle count and NVR levels. NVR testing is not explicitly required by CGA G-4.1 since the requirement is more lenient relative to the cleanliness levels presented in IEST-STD-CC1246. Instead, Section 8.1 of CGA G-4.1 lists NVR inspection techniques with corresponding detection thresholds, shown in Table 3 below. CGA G-4.1 lists the detection thresholds as mg/m2, but they are converted here to mg/0.1m2 to better correlate to Table 2.
Table 3. Minimum Detection Levels for Hydrocarbon Oil Contaminants on Stainless Steel (CGA G-4.1)
Contamination Detection Threshold (mg/0.1m2)
Bright White Light
50 – 170
4 – 150
3 – 60
Water Break Test
3 – 6
Why Astro Pak is Your Trusted Partner in Precision Cleaning
Contamination-critical cleaning is an industry over half a century old and guided by a rich and overlapping history of specifications. From MIL-STD-1246 to the transition into IEST’s stewardship, precision cleaning is an industrial practice led by many specifications. Experience with these documents is paramount to ensuring the right cleanliness is both requested and provided.
Astro Pak brings that experience. Precision cleaning is the original service Astro Pak provided when first incorporated in 1959 and has been processing to all iterations of precision cleaning documentation. Reach out to Astro Pak today for a quote on reliably and precisely cleaned components.