Description
GA10218_002_A7 - 9/2015 - © Oerlikon Leybold
13
1.1.4 Vacuum Method
For the purpose of leak detection on a test sample (vacuum method), the sample
has to be evacuated so that Helium or Hydrogen which is sprayed on to the out-
side, can enter through any leaks due to the pressure differential for detection by
the PHOENIX L300i.
The test sample is evacuated - START button (Fig 16./10) - by the backing pump
or the external pump. In the case of larger test samples an additional external par-
tial flow pump with a corresponding linking valve may be connected in parallel as
required.
Inlet valve V1 is opened so that the evacuation can take place. At the same time all
other valves are closed in order to prevent an unacceptable pressure increase in
the mass spectrometer.
In this context (valve V2a closed) the turbo molecular pump is operated without
being supported by the rotary vane pump. Since generally no gas is pumped out of
the mass spectrometer, p
2
remains constant or increases only slowly.
The condition for the evacuation process described here is maintained until the
inlet pressure p1 has dropped <15 mbar. Now the valves V2a and V2b are opened
additionally. Possibly present Helium or Hydrogen may now flow upstream against
the pumping direction of the turbo molecular pump into the mass spectrometer
where it is detected. This measurement mode is called GROSS. In this mode, leak
rates down to 10
-8
mbar l/s can be detected.
Since the rotary vane vacuum pump continues to evacuate the test sample via
valves V2a, V2b and V1 the inlet pressure p
1
will continue to drop. When the pres-
sure drops below p
1
< 0.2 mbar, the PHOENIX L300i will switch to the FINE mode,
i.e. valve V1and V2b closes and valve V4a opens so that the gas flow enters the
turbo molecular pump at the side. This offer two advantages:
In the FINE mode the full sensitivity of the PHOENIX L300i is reached.
Because of the higher base pressure of the diaphragm pump the switching from
GROSS to FINE mode of the PHOENIX L300i DRY is done by the valve V4b. When
the pressure drops below 3,5 mbar the valves V1 and V2b will be close and V4b
Pos. Description
1
MS: Mass Spectrometer, Helium sensor (180° magnetic field mass
spectrometer)
2
Turbo molecular pump (TMP, provides high vacuum conditions in
the MS)
3
Pirani gauge P2 (fore vacuum pressure)
4
Fore pressure pump (provides the fore vacuum pressure for the
TMP and pumps down the parts under test)
5
Inlet Port
6
Pirani gauge P1 (inlet pressure)
V1 … V7: Electromagnetic Valves to control the gas flows
a)
A part of the high pumping speed of the turbo molecular pump
remains available for further evacuation of the test sample. The
response time is inversely proportional to pumping speed.).
b)
The advantages offered by the counter flow principle can still be uti-
lized
