Technical Information
Ionization Gauge Instruction Manual - 274026 - Rev. B
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Pressure Indication
Ionization gauge controllers measure the positive ion current in amperes
from the ion gauge, but the readout is in pressure units. These pressure units
are direct reading only for the gas of which is specified - usually nitrogen or
air. Other gases may give much different readings than does nitrogen.
Gauge Tube Sensitivity
To present conversions from gas types, the gauge sensitivity,
K is defined as:
where i
+
is the positive ion current to the ion collector, i_ is the electron
emission current from the filament to the grid, and P is the pressure. The
glass envelope gauge tube and the I
2
R degassable nude gauge have
K=10/Torr for nitrogen (or air), and the electron bombardment degassable
nude gauge has K=25/Torr for nitrogen (or air).
Gauge sensitivities for various gases are tabulated in reference material
either directly in Torr
-1
or listed, such as:
Gas Type Conversions
There are two ways to read the pressure of a gas other than the gas for which
a gauge is specified: 1) perform a mathematical conversion on the direct
pressure readout (usually nitrogen/air); 2) use an emission current other than
the value for which the ion gauge controller is set.
1) To correct for an ion gauge controller which is set up for a different
sensitivity (K
N2, cont
) than the ion gauge ( K
N2, tube
), the following correction
to the pressure readout will yield the nitrogen equivalent pressure:
