Stanford Research Systems RGA100, Руководство по эксплуатации и программированию

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2 Appendix B
SRS Residual Gas Analyzer
Sample Valve
Aperature
Process
Hybrid Turbo Pump
Diaphragm Pump
RGA
Hi-C Valve
Figure 1: Schematic of a mid-vacuum pressure reduction system
Apertures can be readily designed for process pressures in the range from 10
-3
mbar to 10
mbar. If the process always operates within a small range, the aperture can be optimized
to deliver gas to the RGA at about 10
-6
to 10
-5
mbar. By operating the RGA at its
optimum pressure the data acquisition time is kept to a minimum and the full dynamic
range in partial pressure is available. For many applications, the process is operated at
one pressure and the aperture can be optimized. If the process pressure varies over a
range of 2 decades or more, the aperture size must be compromised to tolerate the
pressure range. For example, consider a process pressure that varies from 10
-1
to 10
mbar. The aperture would be designed to drop the pressure from 10 mbar to 10
-5
mbar.
When the process pressure was at 10
-1
mbar, the pressure at the RGA would be 10
-7
mbar.
The minimum detectable partial pressure (MDPP) of the RGA does not depend on the
operating pressure; for a Faraday cup detector it is about 10
-10
mbar. Therefore the
dynamic range of the measurement varies from 5 decades at high process pressure to only
3 decades at the low pressures. For applications where the full dynamic range is not
needed, operating the RGA at low pressure may be acceptable. If the full dynamic range
is required over a variety of process pressures, a variable reduction is required. Suitable
variable leak valves are available, but are significantly more expensive than a fixed
aperture. Another method of increasing the dynamic range and data acquisition rate is to
use an RGA with an electron multiplier. The electron multiplier provides gains up to 10
6
and lowers the MDPP to under 10
-13
mbar. This lower MDPP allows the RGA to provide
large dynamic range even at low operating pressures.
A high operating pressure (or throughput of the aperture) at the RGA also improves the
signal to background ratio. In this context, signal is the gas that is drawn through the
aperture and background is outgassing from the system plus backstreaming through the
turbo pump. The ultimate vacuum of many turbo pump packages is about 10
-9
mbar.
The outgassing background will be mostly hydrogen, water, and nitrogen. The
backstreaming background will be air. If measurements are being made near these