Agilent Technologies 5975T LTM GC/MSD, Troubleshooting And Maintenance Manual

Page: 198 / 270

198 Troubleshooting and Maintenance Guide
12
Vacuum System
Micro-Ion Vacuum Gauge
The G4363A Micro-Ion vacuum gauge is optional. It consists of
the sensing element (an ionization-type gauge) and the
necessary electronics to support it.
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 readout installed on top of the
instrument.
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.
The distinction between indicated and absolute pressure is not
important for normal operation of the instrument. 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.
WA R N I N G
If you are using H
2
as a carrier gas, do not turn on the Micro-Ion
vacuum gauge if there is any possibility that H
2
has accumulated
in the analyzer chamber. Read “Hydrogen Safety” on page 19
before operating the instrument with hydrogen carrier gas. Pump
down for 25 minutes before you turn on the vacuum gauge.