Operation
Ionization Gauge Instruction Manual - 274026 - Rev. B
17
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3.2
Filament Emission
There are two types of materials commonly used for filaments: tungsten or
thoria-coated iridium. The most common style is coated iridium because
they operate at a lower temperature than tungsten, therefore less reactive.
Coated iridium filaments are also more burnout resistant when exposed to
atmospheric pressure while power is on. Tungsten filaments will burn out
immediately if exposed to pressures of 1 x 10
-2
Torr or higher while they're
on. However, tungsten filaments are the best type to use when the chemistry
(such as halogen compounds) of the vacuum process causes premature
failure of coated iridium filaments.
The amount of emission current that a B-A gauge requires for proper
operation depends on many factors such as: the size or style of the gauge,
the process in which the gauge is used, the pressure range of operation, and
the desired accuracy of the indicated pressure. Emission currents are
typically in the range of 25 µA to 10 mA.
Generally, low emission current is used in the high pressure end of the
range of a B-A gauge, which helps avoid the ion turn-around phenomenon
and glow discharge. High emission current is used at ultra high vacuum to
obtain ion currents large enough for convenient measurement. Typical
values are 100 µA at 10
-3
Torr and 10 mA at 10
-9
Torr. The trade-offs on
emission current are that high emission current gives better readout stability
and sensitivity, but more pumping if the gauge is clean, and more
outgassing if the gauge is contaminated. Likewise, low emission currents
minimize gauge outgassing and minimize pumping. However, low
emission currents yield low ion currents which are sensitive to electronic
noise and may be too low for a given electrometer to measure.
3.3
Pumping Capacity
The ionization gauge exhibits a certain pumping capacity. This is due to
both chemical and electrical effects. Chemical pumping is due, in general,
to the affinity of gases for very clean surfaces. As the surface becomes
saturated, the pumping action is diminished and reaches a steady state
value. Readings for a typical glass- tabulated gauge will be 20 to 50 percent
lower during the period of chemical pumping. The duration of the pumping
may be of the order of four hours at I x 10
-9
Torr. Electronic, or ionic
pumping saturates after pumping approximately three months at I x 10
-9
Torr. The most common remedy for pumping effects is to provide a passage
of large conductance between the gauge and the vacuum system.
A third mechanism of pumping in gauges involves chemical reactions with
the hot cathode. A number of gases such as oxygen, nitrogen, water vapor,
and hydrogen have been shown to react with the carbon present in
tungsten. The ratio of these reactions are dependent on the cathode
temperature, and are low enough that serious errors in measurement can be
avoided when high-conductance connections are used. In small systems,
however, the change in gas composition may be significant.
