Description and Working Principle
39
iina
74
e 0
4
.f
m
te
chn
ica
l ha
nd
boo
k
(1
40
8)
4.3
Working Modes
4.3.1
Vacuum Mode
As mentioned (See Chapter
) the sample has to be evacuated
to allow Helium which is sprayed on the outside to enter through any leaks due to the
pressure difference.
When pressing the
Valves V1a and V1b open and the sample is
pumped down by the scroll pump. The booster turbo pump starts. At the same time
valve V2 is closed to avoid an unacceptable pressure increase in the turbo pump and
the mass spectrometer. With valve V2 being closed the turbomolecular pump is
operated without being backed up by the scroll pump. Since the mass spectrometer
is already under vacuum no further gas is pumped. Thus the pressure p
2
remains
constant or increases only slowly.
The following diagrams show the gas flow during evacuation and during the modes
GROSS, FINE and ULTRA.
The condition for the evacuation process described is maintained until the inlet
pressure p
1
has dropped below 15 mbar. Now valve V2 opens. Possibly present
helium may now flow upstream against the pumping direction of the turbo molecular
pump into the mass spectrometer where it is detected. This mode is called GROSS,
the detection limit is 1×10
-7
mbar l/s. At 1 mbar the booster turbo pump becomes
effective.
Since the scroll pump continues to evacuate the test sample the inlet pressure p
1
will
continue to drop. Below 2 mbar the UL5000 will switch to FINE mode, i.e. V1a and
V4a will open and valve V1b will close. The gas stream enters the turbo pump at an
intermediate level. The sensitivity of the system now is higher, the detection limit is
1×10
-10
mbar l/s.
Fig. 4-7: left: Evacuation (no measurement), right: GROSS Mode
