Ionization Gauge Instruction Manual - 274026 - Rev. B
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Chapter 4
Technical Information
4.1
Operating Principle
The pressure indication of a hot cathode Bayard-Alpert (B-A) gauge is based
on the ionization of gas molecules by a constant flow of electrons. The
negative electrons are emitted at a well-controlled, selectable rate from a
heated cathode and are accelerated toward a positively charged wire grid
(anode). Electrons pass into the space between the grid and the collector
wire. In this space the electrons collide with gas molecules from the
vacuum system, producing positive ions. The positive ions are then
collected by the ion collector wire which is at nearly ground potential, thus
negative with respect to the grid and is located along the axis of the
cylindrical 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 space for pressures below 1 x 10
-3
Torr. The strength of the ion current is then indicated on a electrometer that
is calibrated in units of pressure. Since as the pressure indication is linear,
the hot cathode Bayard-Alpert gauge is generally considered the most
accurate continuous indicator for pressures below 1 x 10
-3
Torr.
More detailed information regarding the theory of operation for B-A gauges
is presented in the Operation Chapter of this Instruction Manual.
The low end of the operating range of a B-A gauge is determined by the
X-ray limit of this type of gauge. The X-ray limit varies with different gauge
designs. X-rays are produced when the electrons emitted by the cathode
impact the grid and support wires. Because of the geometry of the B-A
gauge, only a small fraction of the emitted X-rays are intercepted by the ion
collector. When the X-rays strike the collector wire they cause electrons to
be photoelectrically ejected from the collector. This X-ray current limits the
pressures that can be measured, and is equivalent to a pressure reading in
the 10
-11
Torr range. Earlier gauges that had a cylindrical collector outside
the grid experienced an X-ray limit of about 10
-8
Torr. The X-ray limit refers
to the lowest pressure indication that may be obtained in a gauge when all
the output current is due to X-ray induced photoemission and there is an
absence of gas.
The X-ray limit will be increased as a result of hydrocarbon contamination
of the electrodes, since contaminated surfaces release more secondary
electrons under X-ray bombardment. Such contamination can generally be
removed by thorough degassing of the electrodes
More detailed information regarding the X-ray limit of B-A gauges is
presented in the Operation Chapter of this Instruction Manual.
