Edwards ELD500 Series

D135-10-880C - Description
Page 10
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 outside, can enter through
any leaks due to the pressure differential for detection by the ELD500.
The test sample is evacuated - START button (Figure 16 item 10) - by the backing pump or the
external pump. In the case of larger test samples an additional external partial 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, p2 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 p1 will continue to drop. When the pressure drops below p1 <
0.2 mbar, the ELD500 will switch to the FINE mode, that is, 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:
1. 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.).
2. The advantages offered by the counter flow principle can still be utilized
In the FINE mode the full sensitivity of the ELD500 is reached.
Because of the higher base pressure of the diaphragm pump the switching from GROSS to
FINE mode of the ELD500 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 opens step by step. When valve V4b is
open completely, pressure < 0.1 mbar, V4a will open also to get the maximum pumping
speed. In PRECISION mode the ELD500 DRY opens the valve V4b only, with the disadvantage
of low pumping speed but with the highest sensitivity.
When the leak detection process is stopped – STOP-button – all valves except valve V2a are
closed.
Valve V3 is opened during venting of the inlet or test sample.