INFICON UL1000 Helium leak detector - Manual (Page 45)

Description and Working Principle
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4.3 Working Modes
4.3.1 Vacuum Mode
As mentioned (Please refer to Chapter 4.2.1 Vacuum System ) 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 START Button Valves V1a and V1b open and the sample is
pumped down by the roughing pump (UL1000) or scroll pump (UL1000 Fab). 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.
If the pressure p
2
even though increases (e.g because of a very long pumping down
process), then the evacuation will be broken (V1a and V1b closed) at p
2
> 10 mbar
and V2 will open shortly to restore an appropriate foreline (p
2
< 1 mbar).
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
-6
mbar l/s.
Fig. 4-6: left: Evacuation (no measurement), right: GROSS Mode
V4a
V4b
p
2
V2
MS
V6
V7
p
1
V1b
V1a
V5
TL
V8
V5
V4a
V4b
p
2
V2
MS
V6
V7
p
1
V1b
V1a
V5
TL
V8
V5