Vacuum System 6
5977B Series MSD Troubleshooting and Maintenance Manual 179
Micro-Ion Vacuum Gauge
The G3397B Micro-Ion vacuum gauge is standard on CI MSDs and optional on
EI MSDs. It consists of the sensing element (an ionization-type gauge) and the
necessary electronics to support it. Both parts are mounted in a single
package.
The ionization gauge creates a current when energized electrons collide with
gas molecules. The electronics provide the voltages required, measure the
current produced, and produce an output signal that is used by the MSD
software.
The Micro-Ion vacuum gauge mounts on the end of the analyzer chamber and
is open to it. This allows you to monitor chamber pressure in daily operation
and in troubleshooting.
The gauge is calibrated for nitrogen (N
2
). The carrier gas is usually helium,
which does not ionize as readily as nitrogen. Therefore, the indicated pressure
for helium is approximately six times lower than the absolute pressure. For
example, a reading of 2.0 × 10
-5
Torr versus an absolute pressure of 1.2 × 10
-4
Torr.
In a CI MSD, the indicated pressure reflects the contribution of both the
carrier gas and the reagent gas. The distinction between indicated and
absolute pressure is not important for normal operation of the MSD.
Of greater concern are changes in pressure from hour to hour or day to day.
These changes can indicate air leaks or other problems with the vacuum
system. All the pressures listed in this manual are indicated pressures for
helium carrier gas. The gauge controller setpoints are also indicated
pressures.
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