Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Appendix A
SRS Residual Gas Analyzer
11
00:00:00 00:00:48 00:01:36 00:02:24 00:03:12 00:04:01 00:04:49 00:05:37 00:06:25 00:07:14 00:08:02
Time (hh:mm:ss)
0
-10
6.0x10
-9
1.2x10
-9
1.8x10
-9
2.4x10
-9
3.0x10
-9
3.6x10
-9
4.2x10
-9
4.8x10
-9
5.4x10
-9
6.0x10
Torr
Chamber Leak Test
LOW 1.9E-10
Helium
Figure 6: Helium Leak Test Data
The partial pressure of the test gas is directly related to the leak rate into the chamber.
Assuming that the vacuum pump is not operating near its compression limit, the
throughput of the test gas is equal to the product of the partial pressure and the effective
speed of the pump at the RGA ionizer (Q = S P). In Figure 5, the partial pressure was
measured in Torr. The effective speed of the turbo pump for helium was approximately
50 liter s
-1
. The largest peak in this data at 4
u
10
-9
Torr represents a leak of 1.5
u
10
-7
scc sec
-1
. (0.76 scc sec
-1
= 1 Torr liter s
-1
). From this figure we can estimate a minimum
detectable leak of 1
u
10
-8
scc sec
-1
, which is measured with a FC detector. To measure
smaller leaks, a CEM detector can be used, or the turbo pump can be throttled to decrease
the pumping speed.
Supply Gas Valve Seats
Leaks across valves that supply gasses to a vacuum system cannot be detected with
conventional helium leak testers (unless the valve supplies helium). To test a suspect
valve would require removing it and attaching it to the leak tester. Because the RGA can
monitor any gas, this is unnecessary; the valves on gas supply lines can be tested
in situ .
The procedure is simple: monitor the composition of the vacuum system with a high
pressure and then a low pressure behind the valve seat in question. If the partial pressure
of the gas in question changes, the valve seat is leaking.
Supply Gas Manifolds
Leak testing supply gas lines can be a very trying experience. The difficulty is mostly
because compression type fittings do not have a leak test port. Another difficulty is that
supply manifolds commonly have a large number of connections in close proximity.
Because the leak in a compression fitting is inside the fitting, transporting the test gas to