INFICON CATALOG 2000-2001 - Schoonover, Inc.

Vacuum Gauges
Transmitter Technology
Hot Cathode Ionization Vacuum Gauges
In a hot cathode ionization gauge, the electrons emitted from the surface of
a hot filament are attracted to a highly transparent grid made from very thin
wire and at a positive electrical potential. With the grid so open, there is a
very high probability that the electron will pass right through the grid and
not strike a wire. If the grid is surrounded by a screen at a negative
electrical potential, the electron will be repelled by this screen and be
attracted back to the grid. This process can happen many times before the
electron finally hits the grid and is lost. As a result, very long electron paths
can be achieved in a small volume, increasing the probability of ionizing a
gas molecule. The resulting ions are attracted directly to the collector.
Most hot cathode sensors used are based on this Bayard-Alpert
arrangement of electrodes. With this electrode arrangement it is possible
to make measurements in the pressure range from 10 -10 to 10 -2 mbar (Torr).
Other electrode arrangements permit access to a higher range of pressures
from 10 -10 mbar (Torr) up to 10 -1 mbar (Torr). For measurement of pressures
below 10 -10 mbar (Torr), extractor ionization sensors are employed. In
extractor ionization gauges, ions are focused on a very thin and short ion
detector. Due to the geometry of this system, interfering influences such
as X-ray effects and ion desorption are almost completely eliminated.
The extractor ionization gauge permits pressure measurements in the
range from 10 -12 to 10 -4 mbar (Torr).
INFICON improvements in electrode arrangements provide a wider
measurement range combined with excellent repeatability. The Bayard-
Alpert Pirani Combination Gauge further improves the measurement range
in an easy to use package.
Cold Cathode Ionization Vacuum Gauges (Penning)
The cold cathode ionization gauge dispenses with the hot filament and uses
a combination of electric and magnetic fields. If one applies a potential of
a few kilovolts between a parallel plate capacitor in the presence of a gas
pressure of a few mbar (Torr), a glow discharge will result. In the glow
discharge, energetic free electrons interact with neutral molecules to
produce ions and more free electrons. The ions are attracted to the negative
electrode and when they strike it, secondary electrons are created, which
are attracted to the positive electrode. The glow discharge continues as
long as the number of electrons being created is equal to or greater than
the number being removed at the positive electrode.
Transmitter Technology
Transmitters convert an analog input signal (pressure) measurement value
into an analog normed electronic measurement signal, i.e. 0 Ð 10 V.
This measurement signal is typically available as a linear or logarithmic
output signal. The advantages of transmitters over conventional gauges
with a separate signal processing device are:
♦ Sensor and electronics in one compact package
♦ Large cost savings
♦ Direct connection to customer control units
♦ A simple formula to convert the measurement signal into any desired
pressure unit
♦ Clear error messages can be transmitted because of the structured
measurement signals
♦ Digital interfaces (RS 232 C, Profibus, DeviceNetª) allow direct
measurement readings in digital form
♦ Integrated setpoint for direct process control based on
pressure measurement
♦ Measurement signals can be transmitted over greater distances
The INFICON transmitter vacuum gauge line provides unparalleled
performance for system manufacturers taking advantage of unique
technologies combined with cost saving potential. The transmitter
technology combines the sensor with the control and evaluation electronics
in one compact package. Several output concepts like 0 Ð 10 V analog
output, digital RS 232 C and fieldbus communications are available.
INFICON transmitter technology reduces the costs for vacuum
measurement and system integration to a minimum.
The Penning gauge design employs a ring as the positive electrode
(anode), which is centrally located between two electrically connected,
negatively charged, parallel plates (cathodes). Electrons are attracted to the
plane of the ring. Upon arrival, these electrons pass through the ring, and
continue to oscillate between the two cathode plates. This configuration
lowers the pressure at which a discharge can be sustained by increasing
the electron path length. By introducing a magnetic field this electron path
becomes a spiral, significantly increasing the path length. This further
lowers the pressure at which the discharge can be maintained.
INFICON design concepts permit safe and reliable operation of ÒPenningÓ
sensors in the pressure range from 1á10 -9 mbar to 1á10 -2 mbar (Torr).
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