Precision Cleaning and Oxygen Service Cleaning
Precision cleaning is specified as a cleanliness level, usually written to IEST-STD-CC1246 as a particle level plus a nonvolatile residue (NVR) level, and verified by testing the part or its rinse fluid. Oxygen service cleaning adds fire safety requirements from ASTM G93 and program documents such as MSFC-SPEC-164 or KSC-C-123.
Specify a level, not a process
Engineers get better results when the drawing states the cleanliness the surface must meet and lets the cleaning source choose the process that achieves it. A note such as clean to IEST-STD-CC1246 Level 100A, verify by particle count and NVR on the final rinse, package double bagged, is measurable at receiving. A note that says ultrasonic clean in IPA is not, because two shops running the same process can deliver very different surfaces.
The exceptions are when a program has qualified a specific process for compatibility reasons, such as a solvent that must not attack a seal material, or when oxygen system specifications prescribe cleaning steps. In those cases cite the program specification, not a generic process description.
How IEST-STD-CC1246 levels work
IEST-STD-CC1246 grew out of MIL-STD-1246. After five military revisions, the Army commissioned IEST in 1997 to adapt it for industry use, and revision E was issued in 2013. It defines levels for particles and for nonvolatile residue, and the two are written together, for example 300A or 100A/10.
Particle levels. The standard models particle size distribution as a straight line on a log versus log-squared plot, so a level can be defined by one number: the intercept, read as the largest particle size in the distribution. MIL-STD-1246B fixed the slope of that line at -0.926. A part at Level 100 has a distribution whose largest particle is nominally 100 µm, with a defined allowable count in each smaller size bin. Revision D moved the units from per square foot to per 0.1 m² of surface, and fluid volumes to per 0.1 liter.
NVR levels. NVR is the film left after a solvent rinse is evaporated: oils, greases, plasticizers and other molecular contamination. Levels are letters, with fractional designations such as A/10 for tighter requirements. Historically NVR was specified in mg per square foot for surfaces and mg per 0.1 liter for liquids. Use the current table in the standard for the mass each letter represents.
| Item | Basis in CC1246 | How it is checked |
|---|---|---|
| Particle level | Particle count distribution per 0.1 m², slope -0.926 on log vs log² scale | Flush or extract the surface, filter, count by microscope or automatic counter |
| NVR level | Mass of residue per unit area, letter designations | Evaporate a measured solvent rinse and weigh the residue |
| Percent area coverage | Calculated from particle counts using Annex B | Used for optical surfaces where obscuration matters |
Some laboratories now count particles down to 0.3 µm for optical applications, and a presentation at a NASA Goddard contamination workshop describes a cylindrical hemispherical particle shape model that gives obscuration values 19 to 32 percent lower than assuming spherical or cylindrical particles. If your hardware is optical or sensor related, ask how the lab calculates obscuration.
Oxygen service: why cleanliness is a fire problem
In oxygen enriched systems, hydrocarbons and particles are fuel and ignition sources. ASTM G93, now issued as a guide (G93/G93M-25), covers selection of cleaning methods and apparatus for oxygen service and describes cleanliness levels used in practice and how they are achieved and verified. It is explicit that its levels are non-mandatory guidance and that the rationale for cleaning is predominantly the risk of ignition and fire. That means G93 alone is not an acceptance standard. Your drawing or system specification must state the level.
For NASA programs the controlling documents are usually center specifications:
| Document | Scope | Status |
|---|---|---|
| MSFC-SPEC-164 Rev E (2020) | Cleanliness of components for oxygen, oxidizer, fuel and pneumatic systems in space vehicle fluid systems, related GSE and test facilities. Covers cleaning, verification, drying and packaging. Excludes composite surfaces in cryogenic service but may be used for internal cleanliness of metal lined COPVs | Active |
| KSC-C-123 Rev J (2009) | Surface cleanliness of ground support equipment fluid systems, including levels, handling, test methods, packaging and inspection of critical surfaces | Active |
| ASTM G93/G93M-25 | Guide to cleaning methods and cleanliness levels for oxygen enriched environments | Active guide |
Program requirements generally take precedence over these documents, and each user is expected to impose sub-tier documents for processing, solvent selection and verification. Read your contract flow downs before choosing which spec to cite.
Materials, design and the cleaning process
Cleanability is a design property. Blind holes, crevices between pressed parts, thread roots and weld root porosity trap particles and solvent. Design features that help: through holes instead of blind holes where possible, no crevices at press fits, accessible flow paths for flushing, and sub-assembly cleaning before final assembly.
Material compatibility limits the process. Aqueous cleaners can leave residues on porous surfaces, some solvents swell elastomers, and nonmetals used in oxygen service must be compatible with oxygen at the system pressure. The engineering plastics page covers PTFE, PCTFE and other polymers common in oxygen hardware, and the nickel superalloys page covers why nickel alloys are favored in high pressure oxygen components.
Typical flow: pre-clean to remove gross contamination and machining fluids, precision clean, final rinse with a verification sample, dry with filtered gas, then package in a controlled environment. Passivation of stainless parts is usually done before precision cleaning, not after.
Verification and the certificate
Ask what was sampled and how. Particle counts and NVR are usually taken from the final rinse of the actual part or from a representative part in the lot. The certificate should state:
- Specification, revision and required level, for example CC1246E Level 100A
- Verification method, sample fluid, volume and surface area used to normalize results
- Particle counts by size range and NVR mass, with the resulting level achieved
- Cleaning date, lot or batch, and the cleanroom or clean bench class where packaging was done
- Package description and labeling
A cert that says only "cleaned for oxygen service" with no level or data is not verifiable and should be rejected.
Packaging after cleaning
Cleanliness only lasts as long as the package. Common practice is to cap or plug ports with clean closures, bag the part in a clean film compatible with the service fluid, then bag again so the outer bag can be removed at the entrance to a clean area while the inner bag stays intact. Labels should state the cleanliness level, the specification and the date, and should be on the outer bag, not inside with the part. For crating, shock indicators and storage, see the crating and packaging page.
Do not open the inner bag at receiving inspection. Inspect the label and certificate, and leave verification of the clean surface to the point of use or to a lab sample if your procedure requires it. See receiving inspection.
Get parts cleaned and certified
If you need components cleaned to a CC1246 level or to an oxygen service specification, send the part list and the level you need. We source the work from qualified cleaning providers, flow down the specification, verification and packaging requirements, check the certificate data against the PO, and a person replies within one business day. Mutual NDA before files, and controlled data follows our controlled programs process. Related: processes hub, surface finishing, inspection and metrology.
Questions
What does a level like 100A mean?
It combines a particle level and an NVR level from IEST-STD-CC1246. The number is the particle level, defined by a size distribution whose largest particle is nominally that size in micrometers, with allowable counts per 0.1 m² in smaller size bins. The letter is the nonvolatile residue level, the mass of film left after evaporating a solvent rinse. Lower numbers and fractional letters such as A/10 are cleaner.
Is ASTM G93 an acceptance standard?
No. The current edition is a guide. It helps engineers select cleaning methods and specify cleanliness for oxygen enriched service and lists levels used in practice, but it states that its levels are non-mandatory and it does not prescribe specific measurement methods. The required level must come from your drawing, system specification or a program document such as MSFC-SPEC-164.
Which NASA specification applies to oxygen components?
For space vehicle oxygen, oxidizer, fuel and pneumatic components and related GSE and test facilities, MSFC-SPEC-164, revision E, is the usual reference. For ground support equipment fluid systems at Kennedy, KSC-C-123 revision J applies. Program requirements take precedence over both, so check the contract and system specification before selecting one.
Should passivation be done before or after precision cleaning?
Before. Passivation is a chemical process that leaves its own residues and is followed by rinsing, while precision cleaning is the last wet step before verification and packaging. Doing passivation after precision cleaning would undo the verified condition. Sequence the router so machining, deburr, passivation and any marking are complete before the part enters precision cleaning.
How do I keep parts clean through receiving inspection?
Double bag them and inspect only the outer package. The outer bag protects the inner bag during shipping and is removed at the entrance to a clean area, leaving the inner bag sealed until the point of use. Receiving should check labels, certificate data and package integrity. If your procedure requires surface verification, sample at the clean area rather than opening bags on a receiving bench.
Related
Sources
- NASA GSFC: product cleanliness levels presentation (CC1246 history and particle model)
- ANSI Webstore: IEST-STD-CC1246E
- Cleanroom Technology: precision cleaning explained
- Wikipedia: IEST-STD-CC1246
- ASTM G93/G93M-25 cleanliness levels and cleaning methods for oxygen enriched environments
- ASTM G93/G93M-19 scope
- NASA Technical Standards: MSFC-SPEC-164 Rev E
- NASA Technical Standards: KSC-C-123 Rev J
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