One of the challenges to achieving high- or ultra-high vacuum is overall system cleanliness. System contamination can come from the pumping system in the form of back streamed pump oil vapors, the ambient atmosphere (for example, if your next door neighbor is a steel mill), and humans. Humans are the worst culprit and every effort must be made to keep humans out of contact with vacuum systems.
Some factoids on humans:
- One fingerprint contains 1e19 molecules. That is nearly the same number of molecules in a cubic centimeter of air!
- We shed about 150 hairs a day.
- Our eyebrows are full of dust mites and the population grows over time.
- Whenever we speak, we spit–and spittle is both ballistic and sticky.
- Our clothing emits enormous amounts of particulate matter.
In order to protect a vacuum system from contamination a high level of process discipline, coupled with specific cleanroom oriented protective gear, is required.
Fingerprints
Our hands support a wide variety of chemicals, including (Ref. 1):
• Inorganic ions (Na+, Cl-)
• Proteins, amino acids
• Lipids
• Water
This is in addition to whatever we had for lunch, like a ham sandwich. All of these substances are very sticky and not easily removed with soap or solvents. The graph below details the impact of one fingerprint on base pressure in a typical system.
Figure 1: A fingerprint with detail of its secretion of oily, waxy material (Ref 3)
Figure 2: Impact on base pressure of one fingerprint after 10 hours of pumping to high vacuum (Ref 2)
As was mentioned in previous posts, we always recommend that vacuum technologists have a log book for their vacuum systems and that they record the best base pressure and rate-of-rise for the system after it has been cleaned in some repeatable way. No heroic measures–cleaned the way it is always cleaned. With that data in hand the impact of contamination can quickly be seen as a limit, or shift, of base pressure.
Obviously, the interior of a vacuum chamber and its components must be clean, pristine, and NEVER TOUCHED BY HUMAN HANDS. This requires that all hands be gloved and all tools to be used inside a vacuum system be properly cleaned for vacuum service. Refer to our earlier posts on vacuum component cleaning.
Dressing for Vacuum Success
From the list above, in addition to being a source for ions, proteins, amino acids and lipids, we also shed lots of fibrous material and maybe a few insects. Accordingly, vacuum systems need to be protected from humans through proper cleanroom garb including masks (spittle protection), bear covers, hair covers, shoe covers, and bunny suits in the extreme.
The achievement of high and ultra-high vacuum is a challenge with the universe fighting success from many angles. The complexity of the hardware involved and the interaction of materials, pumps, and processes require a high level of vacuum cleanliness. The human factor may be more easily understood and controlled with proper vacuum hygiene and some basic vacuum materials cleaning processes.
For more on vacuum readiness visit the Technical Resource section of our web site at https://www.lesker.com/newweb/techinfo.cfm
References:
1. Chemistry for Liberal Studies - Forensic Academy / Dr. Stephanie R. Dillon, The Chemical Components of Fingerprints, Florida State University, available on-line at https://www.chem.fsu.edu/chemlab/chm1020c/Lecture%2011/01.php
2. Contamination in Vacuum Systems: Sources and Remedies, Phil Danielson, available through Normandale Community College, A shorter version appeared in R&D Magazine, May 2001.
3. Microscopic examination of fingerprint residues: Opportunities for fundamental studies, Moret et al., http://dx.doi.org/10.1016/j.forsciint.2015.05.027
Category: Vacuum Systems
Sub-Category: Vacuum Pump Repair & Services
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