Varian 990 dCLD II, Operation Manual

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Model 990 dCLD II Component Leak Detector
C-8
C.7 Mass Spectrometer Leak Detector—Simplified Description
Each Model 990 dCLD II consists basically of an analytical sensing tube called a spectrom-
eter tube , electronics to operate the tube, and a vacuum system to maintain a very high vac-
uum within this tube (usually less than 0.1 millitorr or about one ten-millionth of ordinary
atmospheric pressure).
In the spectrometer tube, gas molecules are ionized (given a positive electrical charge) by
bombarding them with electrons from a hot thoriated iridium filament. The ions, thus
formed, are accelerated into a magnetic field where the mass 4 (helium) ions are deflected
90 degrees (see Figure C-4). Only helium ions reach the collector.
An extremely stable electrometer provides an electron current to the collector, which neu-
tralizes the current produced by the collection of helium ions. The
feedback current is pre-
sented on the leak rate bar graph. Since this current is directly proportional to the number of
helium ions striking the collector per unit time, the panel leak rate bar graph directly reflects
the concentration of helium in the vacuum system at any time. Any helium entering the sys-
tem causes an increased concentration of helium within the spectrometer tube, which is
reflected as an increase on the leak rate bar graph. In addition to the electrometer, the elec-
tronics also provide suitable voltages to operate the spectrometer tube and controls and
instrumentation for the vacuum system.
Test pieces are generally rough pumped (or, if pressurized, the chamber in which they are to
be tested is rough pumped) by a mechanical vacuum pump before they are connected to
the spectrometer tube. This prevents overloading the vacuum pumping system.
Figure C-4 Magnetic Separation Principle