Systems evacuated with oil-based pumps may be victims of backstreaming of pump oil vapors into the fore- line, vacuum chamber, and upstream throughout the system. The back- streaming referred to here is the act of pump oil vapors moving against the flow of molecules traveling from the vacuum chamber to the pump(s). In this case, we refer to these oil vapors as moving upstream, and eventually into the vacuum chamber. At high pressures, where the number of molecules from the chamber outnumber pump oil vapor molecules emanating from the pumping system, flow upstream is greatly mitigated. Oil vapor back- streaming is significantly more pronounced at lower operating pressures.
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