Stanford Research Systems RGA100, Руководство по эксплуатации и программированию

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Appendix B
SRS Residual Gas Analyzer
1
Appendix B
Using SRS RGA’’s to Sample High
Pressure Gasses
Introduction
The types of analysis performed by an RGA are useful in many applications other than
vacuum systems. But, the RGA is intrinsically a vacuum instrument that operates best
under 10
-5
mbar. The instruments response becomes non-linear above 10
-5
mbar. To
sample gases at higher pressures, a pressure reduction system is needed. These systems
are basically a restriction and a vacuum pump package. Common restrictions are
pinholes and capillaries, which can provide pressure reductions of more than 6 decades of
pressure. The vacuum pump package consists of a turbomolecular pump and a backing
pump. In addition to achieving the desired pressure reduction, the design of a system
should provide for a fast response and high signal to background ratio.
At pressures common to vacuum processes, a simple aperture based pressure reduction
system is suitable. At atmospheric and higher pressures, a two stage reduction based on a
capillary and aperture is used. These two systems will be used to illustrate the design of
pressure reduction systems for RGA’’s.
Vacuum Process Sampling (10 to 10-5 mbar)
Figure 1 shows a schematic of a basic pressure reduction system. The system has two
paths to the RGA: a high conductance path and an aperture path. The high conductance
path (through Valve Hi-C) is provided so that the RGA can monitor the ultimate vacuum
of systems before a process begins. The Hi-C path is also used when leak testing the
vacuum system with the RGA Software’’s leak test mode. The aperture path provides the
pressure reduction for when the vacuum process is operating at pressures up to 10 mbar.
The design of the aperture path is straightforward. First, the RGA operating pressure is
chosen, for example 10
-6
mbar. The pressure, P, and the pumping speed, S, determine
the throughput of the aperture, Q, by the equation Q = P S. To keep the system cost low,
a small turbo pump is chosen. For a 70 liter s
-1
pump, the required throughput is 7
u
10
-5
mbar liter s
-1
. The throughput of the aperture is related to its conductivity, C, and the
pressure drop,
'
P, by the equation Q = C
'
P. The process pressure determines the
pressure drop thereby determining the required conductivity. Formulas for conductivity
of various geometries,
e . g . capillaries, and pinholes, are available in many references
(some are listed at the end of this note).