Ionization Gauge Instruction Manual - 274026 - Rev. B
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Chapter 3
Operation
This chapter explains the basic operating principles of a Bayard-Alpert (B-A)
ionization gauge, including:
• Theory of operation of B-A gauges
• Filament emission of B-A gauges
• Pumping capacity of B-A gauges
• X-ray limit of B-A gauges
• Degas procedure for B-A gauges
• Bakeout procedure for B-A gauges
3.1
Theory of Operation
A Bayard-Alpert vacuum gauge ionizes the gas molecules within the gauge
volume, collects those ions on a thin ion collector wire, and measures the
resulting current to the ion collector to determine the number of molecules
present and indicates a pressure based on that measurement.
A B-A gauge is a hot-filament style ionization gauge. It is called such
because a heated filament (cathode) is used to emit electrons toward a grid
(anode). See Figure 3-1.
The pressure indication of a B-A vacuum gauge is based on the ionization
of the gas molecules by a constant flow of electrons. The negative electrons
are emitted at a well-controlled, selectable rate from a heated filament
(cathode) and are accelerated toward a positively-charged wire grid
(anode). Electrons pass into the space enclosed by the grid. In this space the
electrons collide with the gas molecules that are in the vacuum system, and
produce positive ions. The positive ions are then collected by the ion
collector that is located along the axis of the cylindrical grid. The ion
collector is at nearly ground potential, which is negative with respect to the
grid. At a constant filament-to-grid voltage and electron emission current,
the rate that positive ions are formed is directly proportional to the density
of molecules (pressure) in the gauge for pressures below approximately 1 x
10
-3
Torr. The strength of the ion current is then indicated on an
electrometer that is calibrated in units of pressure.
Because the pressure indication is linear, the hot cathode B-A gauge is
generally considered to be the most accurate continuous indicator for
pressures below 1 x 10
-3
Torr.
