Veeco MS40 Operation And Maintenance Manual page 78

Note that it is possible that the symptoms listed above may also be due to
causes other than contamination. The following problems should be
considered and eliminated prior to proceeding with contamination-related
service:
Sensitivity Check
A sensitivity check provides an indication on how the leak detector is
responding to repeated exposure to gases drawn into its vacuum system.
Matter contained in these gases tend to contaminate the mass spectrometer
and reduce its sensitivity. The unit's computer senses this reduction and
automatically compensates for this loss by increasing the helium signal
amplifier gain (the Amplifier Gain function divided into G2: gain during
reverse mode testing or G3: gain during direct mode testing). Ultimately, if
the unit were to be left unserviced, through continuing computer adjustment in
response to increasing contamination, the "reserve" gain would eventually be
exhausted. A cleaning and re-calibration of the mass spectrometer
components is necessary to prevent this condition.
A sensitivity check is made by pressing the SELECT momentary switches on
the Service Panel until the AMP GAIN function is reached. As stated above,
five Gain measurements are available: G1, G2, G3, G4 and G5. G1, G4 and
G5 are user-settable, so a high value of this gain is not necessarily an
indication of contamination. The SELECT switches are also used to toggle
between the five values. The scale of amplifier gain is from 0.01 to 10.
Typical gains for a clean system are:
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condition indicates that the accumulated contaminants within the
unit have absorbed a high amount of test helium. When this
occurs, place the unit in its "standby" mode for a minimum of
three minutes. The unit will be able to pump away some of the
background helium and to take new measurements. Re-testing
of the test object is recommended at this point. If the situation is
still not remedied, changing the oil in the internal rough pump as
well as a system-wide cleansing of the internal components of
the unit may be needed.
• leak in the vacuum system
• incorrectly tuned spectrometer
• malfunctioning turbo-molecular pump
G2: 1 - 2
G3: 2 – 4
4-9