Model 979 Series Helium Mass Spectrometer Leak Detector
C-8
DRAFT 9/15/03
C.7 Mass Spectrometer Leak Detector—Simplified Description
Each Model 979 consists basically of an analytical sensing tube called a
spectrometer tube
,
electronics to operate the tube, and a vacuum system to maintain a very high vacuum
within this tube (usually less than 0.1 millitorr or about one ten-millionth of ordinary atmo-
spheric pressure). In addition, a rough vacuum pump and a system of valves is provided to
enable test cycles to be carried out (see Figure C-4).
Figure C-4
Mass Spectrometer Leak Detector
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-5). 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 provides suitable voltages to operate the spectrometer tube and controls and
instrumentation for the vacuum system.
Electronics
Leak Indicator
and
Vacuum
Roughing
979
Spectrometer
Tube
Test
Piece
System
Pump
Summary of Contents for 979 Series
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