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Troubleshooting
Leakage
If leakage is the suspected cause of poor system perfor-
mance, first check the following items:
❑
Inlet and foreline connections
❑
Drain and fill plugs
❑
Other compression fittings, such as high-vacuum
gauges in the system
❑
Threaded connections, such as a foreline gauge
Before proceeding with a program of step-by-step trou-
bleshooting, check the performance and accuracy of the
vacuum gauges used on the system.
High Throughput Diffusion Pumps
51
Outgassing
High-vacuum systems, even without external leakage,
can have high gas loads due to outgassing from internal
surfaces or processes. The pressure in the system is a
result of gas load divided by pumping speed (p = Q/S). If
the gas load Q exceeds the maximum throughput capa-
bility of the diffusion pump, the diffusion pump will not
function and the pumping action will essentially be due
to the mechanical backing pump.
To estimate the gas load, isolate the system from all
pumps after evacuation and measure the rate of pressure
increase.
The gas load can be estimated from the following
relationship:
Q
where V is the isolated volume,
rise, and
t is the time period of measurement.
V P
=
----------------- -
t
P is the pressure
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