There are many sources of contamination that effect components to be used in vacuum. For metal parts they range from cutting fluids and coolants used in the machining process to handling by humans and improper storage. Most of these can be mitigated by basic, multi-step, cleaning procedures and proper protection from the environment, including humans.
Humans are one of the worst sources of contamination for vacuum systems and components. We lose about 150 hairs a day. Our eyebrows are full of dust mites. We shed something like 600,000 skin cells and hour. When we speak we spit. A single human fingerprint contains nearly 1019 molecules. That is nearly the same number of molecules in a cubic centimeter at atmosphere of air! Imagine the contamination from a hand print. So the first order of business when allowing a human to clean a part is to protect the part from the human. This means gloves – properly installed without the outside of the gloves touching any human parts. This also means masks – so that when the human speaks/spits the ballistic output does not land on the vacuum components.
Most contaminants from the environment and from intrinsically dirty humans can be removed with some basic steps that must be followed, in order, to get a part clean. This multi-step process works for most metal components:
• Remove hydrocarbons with an organic solvent
• Remove contaminants dissolvable in water with soap and water
• Remove soap with water
• Remove water with alcohol
• Remove alcohol with hot, dry air (max temp of about 120oF – for safety)
• After the part has cooled, secure in appropriate containment
The organic solvents can be acetone, ethanol, methanol and others that are miscible in water (and hopefully not hazardous to humans or the environment). The soap and water step could include something simple like Dawn dishwashing liquid (it works for oily birds) and tap water or something fancier like Liquinox and distilled or deionized water. The water to remove the “soap and water” should be distilled or deionized water, if available. The alcohol should be electronic grade or +99% pure isopropanol. A commercial heat gun (not a hair drier) that can deliver an air stream at +120oF should be sufficient – just be safe – and the parts should not be so hot they cannot be easily handled. Let the part cool on a clean, lint-free cloth. Once cooled, if the part is not going back into vacuum immediately, wrap it in a fresh, dry, lint free cloth and put that wrapped component into a zip lock bag for storage.
The cleaned part will still have adsorbed water on its surfaces but, for high vacuum, much of that will be pumped away after 4 – 10 hours at pressures in the range of 10-7 Torr.
Figure 1: Partial pressure analysis of a vacuum clean component in high vacuum showing peak at AMU 2, 18, 28 and 44, representing hydrogen, water, nitrogen and carbon dioxide.
Verification of cleanliness is another issue. The pragmatic vacuum technologist, like our veteran technical resource, Mike Sadusky, will say if the system pumps down to its best achievable base pressure then the chamber and its components are clean. Those who are more skeptical of their vendors or their co-workers may require something more rigorous, such as a scan of the partial pressure of gases being emitted from a part. This analysis of the residual gases desorbing from the surface of a component, interpreted in atomic mass units, is a highly quantitative assurance of cleanliness. For a clean system, even in ultra-high vacuum, there will always be a signature which includes hydrogen, water, nitrogen and carbon dioxide–but nothing else (see figure 1 for a scan of a part in high vacuum).
Category: Vacuum Systems
Related Topics: Maintenance, Safety