1.5 Description of the
Functions of the UL 200
The mass spectrometer (2/7) only operates under high
vacuum conditions, i.e. the pressure here must always
remain below 10
-3
mbar.This vacuum is generated by the
turbomolecular pump (2/6) with the support of the rotary
vane pump (2/16). Except during the evacuation phase,
the valve (2/3) between these two pumps always remains
open so that this is ensured. In this state the UL 200
reverts to the standby mode; after completion of run-up,
for example. All other valves are then closed. The pres-
sure pV between the two pumps is measured with a Pira-
ni gauge (2/4) and this pressure lies in the range bet-
ween 3 to 10
-3
mbar while in the measurement mode.
This pressure must not exceed a value of 3 mbar as
otherwise the turbomolecular pump will not be capable of
maintaining the vacuum in the mass spectrometer.
1.5.1 Vacuum Method
For the purpose of leak detection on a test sample (vacu-
um method), the sample has to be evacuated so that
helium which is sprayed on to the outside, can enter
through any leaks due to the pressure differential for
detection by the leak detector.
The test sample is evacuated - START push-button (5/3)
- by the rotary vane vacuum pump (2/16). In the case of
larger test samples an additional external partial flow
pump (2/15) with a corresponding linking valve (2/12)
may be connected in parallel as required.
Inlet valve (2/14) is opened so that the evacuation may
take place. At the same time valve (2/3) is closed in order
to prevent an unacceptable pressure increase in the
mass spectrometer.
In this context (valve (2/3) closed) the turbomolecular
pump is operated without being supported by the rotary
vane pump. Since generally no gas is pumped out of the
mass spectrometer, p
V
remains constant or increases
only slowly.
The condition for the evacuation process described here
is maintained until the inlet pressure p
E
(2/13) has drop-
ped ≤ 3 mbar. Now the valves (2/5) and (2/3) are opened
and (2/14) is closed. Possibly present helium may now
flow upstream against the pumping direction of the tur-
bomolecular pump (2/6) into the mass spectrometer
where it is detected. This measurement mode is called
GROSS. In this mode, leak rates down to 10
-9
mbar·l·s
-1
can be detected.
Since the rotary vane vacuum pump (2/16) continues to
evacuate the test sample via valves (2/3) and (2/5) the
inlet pressure p
E
will continue to drop. When the pressu-
re drops below p
E
< 0.2 mbar, the UL 200 will switch to
the FINE mode, i.e. valve (2/5) closes and valve (2/8)
opens so that the gas flow enters the turbomolecular
pump (2/6) at the side. This offers two advantages:
a) A part of the high pumping speed of the turbomole-
cular pump remains available for further evacuation of
the test sample.The response time of the leak detec-
tor is reduced (response time is inversely proportional
to pumping speed).
b) The advantages offered by the counterflow principle
can still be utilized.
In the FINE mode the full sensitivity of the UL 200 is rea-
ched.
When the leak detection process is stopped - STOP
push-button (5/2) - all valves except valve (2/3) are clo-
sed.
Valve (2/10) is opened during venting.
1.5.2 Partial Flow Method
In the partial flow mode the test sample is additionally
evacuated by an auxiliary pump. Using the optional par-
tial flow pump set (see Chapter 1.3.2) offers to the user
the following advantages:
- faster response
- entry into the measure mode already at an inlet pres-
sure of the 1000 mbar
- faster venting of large test objects
1.5.3 Sniffer Mode
The UL 200 may simply be converted into a sniffer leak
detector via two rugged sniffer lines (Cat. No. 140 21 or
140 24).
For this the KF flange of the sniffer line is connected to
the inlet flange (3/2) and the sniffer mode is selected
through menu item 2 (see Chapter 2.5.4). After pressing
START, the inlet valve (2/14) opens. The sniffer lines
have been designed in such a way that the UL 200 is
operated in the FINE mode (as described in Chapter
1.5.1).
A calibration according to Chapter 2.7.3 is recommen-
ded.
In the measurement mode the helium present in the
ambient air is now indicated as the leak rate (about
2·10
-6
mbar·l·s
-1
). Smaller leaks may be detected by
pressing the ZERO push-button (Chapter 2.4.4).
7
TH 10.211/8.02 - 12/97
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