Stanford Research Systems RGA100, Operating Manual And Programming Reference

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8 Appendix A
SRS Residual Gas Analyzer
00:00:00 00:00:37 00:01:15 00:01:53 00:02:31 00:03:09 00:03:46 00:04:24 00:05:02 00:05:40 00:06:18
Time (hh:mm:ss)
-10
1.0x10
-9
1.0x10
-8
1.0x10
-7
1.0x10
-6
1.0x10
-5
1.0x10
-4
1.0x10
mBar
Airlock Sequence
Hydrogen
Water
Nitrogen
Oxygen
Oil
Floor
Figure 4: Airlock Sequence
To make these measurements, the electron multiplier detector has been used with a gain
of 100, which allows all six channels to be recorded every three seconds. The standard
Faraday cup detector is able to detect these partial pressures, but not at this rate. The
““floor”” channel is set to mass 21. There is rarely anything present at this mass, which
allows it to be used as an indicator of the minimum detectable partial pressure.
The sequence starts with the main vacuum chamber at its base pressure of 2
u
10
-8
mbar.
The chamber is pumped by a turbomolecular pump that is backed by a rotary vane pump.
A small load lock is attached to the chamber, which can be rough pumped by the same
mechanical pump and purged with nitrogen.
At 0:30, the isolation valve between the turbo pump exit and mechanical pump is shut so
that the mechanical pump can be used to rough pump the load lock. During this time, the
load lock is repeatedly filled with nitrogen and pumped down. Of interest in the data is
the rise in the hydrogen partial pressure during this step of the sequence. The partial
pressure of hydrogen increases by a factor of 100 while the partial pressure of the other
gasses barely increase. This difference is caused by the low compression ratio that turbo
pumps have for light gasses. The heavier gasses are being compressed into the dead
volume between the turbo pump exit and isolation valve. But, the turbo pump has
insufficient compression ratio to store hydrogen in this manner, causing the partial
pressure of hydrogen to rise.
At 2:30, the load lock has been roughed and the turbo pump foreline isolation valve is
opened. The pressures of H
2
immediately drops back to the base pressure values.