4 Operation
33
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
A single-pole, double-throw relay is provided to indicate filament status (the normally open contact
is open when the filament is OFF). Refer to Table 4-1.
Table 4-1 Series 340 Remote Input/Output Connector for the Ion Gauge
4.2 Theory of Operation
4.2.1 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 4-3. 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).
Pin Number
Function
Pin Number
Function
1
Gauge On/Off Remote*
6
Degas On/Off Remote*
2
Ground
7
Ground
3
Not Used
8
Not Used
4
Gauge Status Common
9
Gauge Status N.O. (Normally Open)
5
Gauge Status N.C. (Normally Closed)
*Active low inputs
Figure 4-3
Ionization Gauge Schematic
