TECHNICAL INFORMATION
PUMP TROUBLESHOOTING
SYMPTOM
POSSIBLE CAUSE
CORRECTION PROCEDURE
Reduce pressure to 10
-5
Torr rec-
ommended 10
-4
Torr minimum.
1) – Slow starting
Starting vacuum pressure too high.
2) – Slow starting (more than 30
minutes).
Air leaks
which limit pressure to
above 10
-6
Torr (mbar) and cause
longer starting time.
Leak check the vacuum system
with a helium leak detector.
3) – Slow pump-down due to long
exposure of viton parts to air.
Viton releases considerable gas
after long exposure to air.
(A bell-jar system which reached
1.5 x 10
-8
Torr (2 x 10
-8
mbar) in 24
hours after 30 minutes air expo-
sure, will only reach 7.5 x 10
-8
Torr
(1 x 10
-7
mbar) in 24 hours after 20
hours air exposure).
With the system under vacuum,
pump for several days, or heat to
100-150°C for up to 15 hours.
4) – Slow pump-down due to ab-
sorption of vapours on pump and
system walls.
Vapours and gases admitted to a
system are absorbed
on the walls
of the system and pump. Subse-
quent reduction in pressure de-
pends on the rate of depletion of
this vapour. Heavy hydrocarbons
are most troublesome because of
their relative low vapour pressure
and they are very difficult to re-
move, even by baking.
Bake the system walls, thereby
accelerating the desorption proc-
ess. Baking mobilizes the vapours
so they can be cracked and
pumped by discharge (see para.
“BAKEOUT OPERATION”).
5) – Slow starting or slow pump-
down.
High voltage feedthrough is leak-
ing.
Replace the feedthrough.
6) – Current higher than expected
at any given pressure.
Ion pump leakage current causing
higher pressure reading.
Highpot the pump.
7) – System fails to achieve de-
sired UHV vacuum pressure.
System not fully baked. Water va-
pour limits base pressure.
Bake the system walls, thereby
accelerating the desorption proc-
ess. Baking mobilizes the vapours
so they can be cracked and
pumped by discharge (see para.
“BAKEOUT OPERATION”).
8) – System fails to achieve de-
sired UHV vacuum pressure.
System not appropriately cleaned
for UHV. Excessive outgassing
from walls limits base pressure.
Clean all components for UHV and
bake the system again.
20
87-900-106-01(B)
