are attracted by the positively charged anode (1/2). But
they do not immediately impinge on the anode (1/2);
instead they oscillate to and fro several times until they
finally arrive at the filament of the anode (1/2).
On this path they ionize the gas atoms by impact.These
ions are extracted from the ion source by a ground-
connected extractor orifice (1/10) and enter the magne-
tic separation system.
In order to avoid deposits of polymerized hydrocarbons
(insulating layers which may impair sensitivity) the anode
is heated while the UL 200 runs up.
Due to the operating temperature of the ion source
(cathode heater), this is no longer required after the
instrument has run up and the anode heater is then swit-
ched off for this reason.
Cathodes (1/1) and (1/3) are made of iridium tape which
is coated with yttrium oxide. Due to this coating, the ope-
rating temperature of the iridium filaments is much lower
compared to tungsten filaments, and these offer excel-
lent resistant to burnout, imbrittlement, oxygen, water
vapour and hydrocarbons.
1.4.5 Separation System
A magnetic sector field (1/8) with 180 ° deflection is used
as the separating system. Due to the inhomogeneous
magnetic field perpendicular to the ion beam, additional
focusing of the ions is attained in this direction (Z-focu-
sing) resulting in a high sensitivity.
Unwanted ions from other gases are kept away in additi-
on by means of the intermediate orifice (1/9).
1.4.6 Ion Collector
The ion collector (1/5) is composed of a tube-shaped
suppressor and the actual collecting plate.
The suppressor (1/7) carries a positive potential which is
slightly less than the anode potential. It is the purpose of
the suppressor to prevent scattered ions which have a
lower nominal energy from arriving at the collector. This
improves resolution at low detection rates.
The impinging ions are collected by the collecting plate
where they are neutralized. The corresponding current
flows into the connected amplifier.
1.4.7 Electrometer Amplifier
The electrometer amplifier amplifies the current which is
generated on the ion collector of the mass spectrometer.
Gain of this amplifier can be switched over in four stages.
The control assembly is responsible for the switching
over and processing of the analogue output signal. The
detection limit in the most sensitive range amounts to
approximately 1·10
-15
A.
1.4.8 Mass Spectrometer Supply
The subassembly generates all the voltages and cur-
rents required for operation of the mass spectrometer:
- anode heating voltage
- cathode heating voltage
- anode voltage
- suppressor voltage
Errors in the mass spectrometer are detected and sig-
nalled to the control assembly. The control assembly
monitors the mass spectrometer supply.
1.4.9 Control
The control assembly is the central assembly of the leak
detector’s electronics. All other subassemblies are con-
trolled and monitored by this assembly. The micropro-
cessor which is located here is thus continuously infor-
med about the status of the entire leak detector and can
respond accordingly. In order to accept commands from
the operator and to output measured values and messa-
ges, the control subassembly is linked to the operating
and display subassembly.
1.4.10 Operation and Remote Control
These two subassemblies are used to communicate with
the operator. They accept commands from the key pad
and output measurement results and messages via the
display components.
6
TH 10.211/8.02 - 12/97
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