7.9 Penning (Cold Cathode) Ionization Gauge 11
Gauges are generally calibrated for air (78%
N2, 21% O2, 1% Ar). Some typical sensitivities
are given in Table 7.1. The true pressure
is given by the gauge reading divided by the
gauge sensitivity.
Table 7.1: Gauge Factors for Ionization
Tube
Air 1.00 O2 0.85 N2 1.10
He 0.14 CO2 1.20
For example if the chamber contains Argon
and shows a pressure of 6 × 10−6 Torr, the
true pressure is (6/1.2) × 10−6 = 5 × 10−6
Torr.
The actual gauges can be purchased in glass
tubes or ”nude”. Nude ionization tubes are
mounted on a flange but stick directly into
the chamber. This eliminates any factors relating
to conductance of tubes, and is important
at UHV pressures.