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4.5 Contamination of the Vacuum System
All leak detectors are subjected to contamination of the vacuum system by
repeated exposure to gases and other matter drawn from test objects during
ordinary testing. This matter will settle throughout the vacuum system.
Special consideration should always be given for contamination of the mass
spectrometer and the oil and filter of the rotary vane mechanical pump. As a
result of contamination, system performance will drop and sensitivity will
decrease. Cleaning and re-calibration of the vacuum system is necessary to
restore the unit to optimum performance.
Indications of Contamination
The vacuum system is considered contaminated if any of the following
conditions exist:
1. The amplifier gain (either G2 or G3) indicator displays a gain of
8 or higher. This indicates that the sensitivity of the mass
spectrometer has decreased and the automatic gain
compensation of the unit is reaching its limit. Verify tuning by
performing the ‘Tune” function (as described in Section (2.9).
If after the Tune function has been performed, the gain value is
not lower than previously reported, the mass spectrometer
should be removed from the high-vacuum system and cleaned
(the procedure is described in Section 4.7).
2. The high-vacuum pressure display is erratic. This can be
observed by selecting the High Vacuum function on the Service
Panel. This requires that the high-vacuum section be opened
and cleaned (this procedure is described in Section 4.7).
3. The plugged test port cannot be pumped to less than 50
millitorr. This condition indicates that the oil in the internal
rough pump (or external rough pump, when equipped) may be
contaminated and needs to be replaced (this procedure is
described in Section 4.4).
4. The foreline pressure cannot be pumped to less than 50 millitorr.
This condition indicates that the oil in the internal rough pump
may be contaminated and needs to be replaced (this procedure is
described in Section 4.3).
5. With the test port plug in place, the leak rate indicator does not
reach a “zero” reading even after prolonged pumping. This
1109-186-00 Rev. C 4-8
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