Total Pressure Gauges

Vacuum Gauges where the
Pressure Readings depend of
the Type of Gas
Thermal Conductivity Gauge
(Pirani)
This measurement principle utilizes the
thermal conductivity of gases for the
purpose of pressure measurements in
the range from 10 -4 mbar/Torr to
atmospheric pressure. Today, only the
principle of the controlled Pirani gauge
is used by Oerlikon Leybold Vacuum in
order to attain a quick response. The
filament within the gauge head forms
one arm of a Wheatstone bridge. The
heating voltage which is applied to the
bridge is controlled in such a way, that
the filament resistance and thus the
temperature of the filament remains
constant regardless of the quantity of
heat given off by the filament. Since
the heat transfer from the filament to
the gas increases with increasing pressures,
the voltage across the bridge is
a measure of the pressure.
Improvements with regard to temperature
compensation have resulted in
stable pressure readings also in the
face of large temperature changes, in
particular when measuring low pressures.
Cold Cathode Ionization
Vacuum Gauge (Penning)
Here the pressure is measured through
a gas discharge within a gauge head
whereby the gas discharge is ignited
by applying a high tension. The resulting
ion current is output as a signal
which is proportional to the prevailing
pressure. The gas discharge is maintained
also at low pressures with the
aid of a magnet.
New concepts for the design of such
sensors permit safe and reliable operation
of these so-called Penning sensors
in the pressure range from 10 -2
to 1 x 10 -9 mbar/Torr.
Hot Cathode Ionization
Vacuum Gauge
These sensors commonly use three
electrodes. A hot cathode emits electrons
which impinge on an anode. The
gas, the pressure of which is to be
measured, is thus ionized. The resulting
positive ion current is detected
through the third electrode - the socalled
ion detector - and this current is
used as the signal which is proportional
to the pressure.
The hot cathode sensors which are
mostly used today, are based on the
Bayard-Alpert principle. 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 -1 mbar/Torr down to
10 -10 mbar/Torr. For the measurement
of pressures below 10 -10 mbar/Torr
so-called extractor ionization sensors
after Redhead are employed. In extractor
ionization gauges the created ions
are focused onto a very thin and short
ion detector. Due to the geometrical
arrangement of this system, interfering
influences such as X-ray effects and
ion desorption can be almost completely
eliminated. The extractor ionization
gauge permits pressure measurements
in the range from
10 -4 to 10 -12 mbar/Torr.
Selection of the right Vacuum
Gauge
When selecting a suitable instrument
for pressure measurements, the pressure
range is not the only critera. The
operating conditions for the instrument
play an important part. If, for example,
there is the risk of excessive contamination,
vibrations, or if air inrushes are
to be expected etc., the instrument
must be rugged enough. Thus for industrial
applications diaphragm gauges,
controlled thermal conductivity
gauges as well as cold cathode
ionization gauges after Penning are
strongly recommended. Precision
instruments are very often quite sensitive
to rough operating conditions.
These should therefore only be used
while observing the corresponding
applications information.
Oerlikon Leybold Vacuum Full Line Catalog
07 C16