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As the system progressively becomes more contaminated, the gain also
increases (becomes closer to 10).
To correct this condition, the Tune function must be used (as described in
Section 2.9). If the Tune function has been performed, and either G2 or G3
value is greater than 8, the mass spectrometer assembly should be removed,
cleaned and re-calibrated as described in Sections 4.7 and 4.8.
Internal Rotary Vane Pump Oil Contamination
Repeated exposure to high levels of helium, particularly when testing objects
with large leaks, may cause the mechanical pump’s oil to become
contaminated. Helium, along with other gases, become dissolved in the oil
and trapped in the internal mechanical pump. This type of contamination is
found when:
• A high background of helium is measured that cannot be
pumped away.
• An unstable leak rate caused by excess helium that is released
from the internal mechanical pump at regular intervals
A high background level of helium will prevent the MS-40 from testing in its
most sensitive ranges. An unstable leak rate will complicate testing and will
make the tuning and calibration functions unreliable.
The gas ballast may be opened periodically to “de-gas” the pump’s oil. This
should be done only if the background helium rate is excessive or if the leak
rate becomes unstable as a result of helium loading. The pump gas ballast is
connected to a “purge” valve in the vacuum system, which allows the ballast
to be opened to atmosphere through a sintered filter.
Purging the internal pump with dry nitrogen will help prevent contamination.
Nitrogen is connected through a 1/4” bulkhead marked N2 on the rear panel.
The nitrogen line must be connected internally by removing the sintered filter
(item 13, figure 5-2) and replacing it with a 1/4” NPT to 1/4” poly-flo tubing
elbow adapter (P/N 1620-726-00) and connecting a length of poly-flo tubing
between the adapter and the inside of the bulkhead fitting.
The pump may be purged either to atmosphere through the sintered filter, or
with dry nitrogen. Dry nitrogen is recommended in areas with high ambient
helium. The dry nitrogen supply should be limited to 0.5 to 1.5 PSI so that
only a gentle stream of nitrogen enters the ballast port.
1109-186-00 Rev. C 4-10
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