Granville-Phillips Series 330, Инструкция по эксплуатации

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Series 330 Vacuum Gauge Controller
Instruction Manual - 330004 - Rev. 02
3.2 Theory of Operation
3.2.1 Ionization Gauge Theory of Operation
The functional parts of a typical ionization gauge are the filament (cathode), grid (anode) and ion
collector, shown schematically in Figure 3-5. These electrodes are maintained by the gauge
controller at +30, +180, and 0 Vdc, relative to ground, respectively.
The filament is heated to such a temperature that electrons are emitted, and accelerated toward the
grid by the potential difference between the grid and filament. Most of the electrons eventually
collide with the grid, but many first traverse the region inside the grid one or more times.
When an energetic electron collides with a gas molecule, an electron may be dislodged from the
molecule, leaving it with a positive charge. Most ions are then accelerated to the collector. The rate
at which electron collisions with molecules occur is proportional to the density of gas molecules,
and hence the ion current is proportional to the gas density (or pressure, at constant temperature).
The amount of ion current for a given emission current and pressure depends on the ion gauge
design. This gives rise to the definition of ion gauge
sensitivity , frequently denoted by S :
Bayard-Alpert type gauges typically have sensitivities of 10/Torr when used with nitrogen or air.
Refer to Section 4.3.5 on page 36 for more information on gas sensitivities for other gases.
The ion gauge controller varies the heating current to the filament to maintain a constant electron
emission, and measures the ion current to the collector. The pressure is then calculated from these
data.
Ion gauge degas is accomplished by resistance heating (I
2
R). During I
2
R degas, a large current is
passed through the grid structure, raising its temperature and driving off contaminants.
NOTE: Some ionization gauge designs do not allow I
2
R degas.
Figure 3-5 Ionization Gauge Schematic
S =
EMISSION CURRENT X PRESSURE
ION CURRENT