Contamination Control Terminology

Contamination control terminology often uses common words in a scientific context. This practice can cause contamination control requirements to be misinterpreted if the terminology is not formally defined. The following definitions are included to establish the meaning of terminology used for the MSSL contamination control requirements:


Clean Area

An enclosed, environmentally conditioned area where airborne contaminants are controlled. Clean areas are classified by a number such as 100, 10,000, 100,000, etc., in accordance with FED-STD-209, which describes the maximum number of particles, 0.5 microns in size and larger, permitted per cubic foot of air under certain performance and operating requirements.


Material that degrades the intended function of an instrument or flight hardware. Contamination is usually separated into two types, particles and non-volatile residue (NVR).

Contamination Control

Organized action to control contamination levels.


Particle with a length-to-width ratio exceeding 10:1 and a minimum length of 100 µm.


Small quantity of solid or liquid material with definable shape or mass.

Particle Size

Maximum linear dimension or diameter of a particle.

Sensitive Surface

Flight hardware surface requiring a specific cleanliness level to meet minimum performance requirements.

Gross Cleaning

A cleaning operation performed to achieve a level of product cleanliness as part of good workmanship and good housekeeping practice, for example the removal of oils, grease, oxide films, etc. Gross cleaning does not usually require verification beyond visual appearance as observed without optical aids other than normal corrected vision. This step precedes precision cleaning.

Precision Cleaning

Cleaning of hardware surfaces according to approved engineering methods and procedures to meet specific criteria.

Solvent Flushing

Pressurized stream of filtered solvent directed against a surface to dislodge and rinse away contaminants.

Solvent Washes

Quantitative method of verifying MIL-STD-1246C levels by measuring molecular contamination in a solvent washed over a surface.

Surface Cleanliness Level

An established level of maximum allowable particulate and/or nonvolatile residue (NVR) contamination ranging from visibly clean to specific MIL-STD-1246 levels.

Swab Sample

Qualitative method of identifying contaminants by analyzing residue on a solvent soaked swab that was wiped over a surface.

Nonvolatile Residue (NVR)

Soluble material remaining after evaporation of a volatile solvent, or determined by special purpose analytical instruments, usually in milligrams per unit area.

Tape Lifts

Qualitative method of verifying MIL-STD-1246C particle cleanliness levels by measuring particle contamination on a tape sample that has contacted a surface.

Vapor Degrease

Item to be cleaned is exposed to heated solvent vapors that condense on the part and wash away contaminants. (NOTE: Halogenated solvents used to vapor degrease plastics are often outgassed or leached out later. Therefore, plastics vapor degreased with halogenated solvents must be baked out.)

Visibly Clean

The achievement of a visibly clean surface when viewed without optical aids (except corrected vision) as measured by a specific method. This requirement will be accompanied by a description of the method of verification (e.g., when viewed from an approximate distance using oblique white light of an approximate intensity or under normal shop lighting, etc.).


Cleanliness inspection which specifies and incident light of 50 to 75 foot-candles. The surface to be inspected shall be observed by the unaided (except for corrected vision) eye at a distance of 2 to 5 feet.


Cleanliness inspection which specifies an incident light of 50-75 foot-candles. The surface to be inspected shall be observed by the unaided eye (except for corrected vision at a distance of 12-24 inches).


Cleanliness inspection which specifies an incident light level of 100-125 foot-candles. The surface to be inspected shall be observed by the unaided eye (except for corrected vision) at a distance of 6 to 18 inches.

Visibly Clean Plus Ultraviolet Visibly clean

(as defined above) and inspected with the aid of an ultraviolet light (black light) of 3200 to 3800 Angstrom wavelength (320 to 380 nanometers). Note: Any evidence of fluorescence shall be cause for recleaning. If recleaning does not reduce the fluorescence, an investigation shall be made to determine whether the fluorescing material is contamination or the basic material. This level requires precision cleaning methods, but no particle count.


©MSSL Created by Alex Rousseau last modified: February 2, 2004 15:30