Models 947, 948 and 960 Leak Detectors Operations and Service Manual
G-8
G.7 Mass Spectrometer Leak Detector—Simplified Description
The 947/948/960 consist 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
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° (see Figure G-5). Only helium ions reach the collector.
An extremely stable electrometer provides an electron current to the collector, which
neutralizes the current produced by the collection of helium ions. The
feedback
current is
presented 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 system 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 electronics 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 (see Figure G-5). This prevents overloading the vacuum pumping
system.
Figure G-5
Magnetic Separation Principle
Summary of Contents for 948
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