Page 167 - Instrumentation Reference Book 3E

Non-absolute gauges 151

S

B = 0.031
Figure 10.10 Cylindrical form of Penning gauge

I
6=003T B=rero

Figure 10.13 The hot-cathode ionization gauge

Operation The gauge head is furnished with
I stable electrical supplies as shown in Figure
Figure 'I 0.11 Electron paths in the Penning gauge. 10.14. The filament is heated to produce electrons
which are attracted to the grid where a fraction of
them is immediately collected. The remainder
oscillate several times through the grid wires
before collection; forming ions by collision with
the gas molecules. The electron current i is meas-
ured by MI and is usually between 0.1 mA and
5.0mA. Ions formed between the grid and ion-
collector constitute an ion current iL shown by
MZ. Electrons are prevented from reaching the
ion-collector by the application of a negative bias
I I of 20 V. Ions formed between the filament and the
P Pressure grid are attracted by the filament where their
Figure 10.12 Hysteresis in the Penning gauge
impact etches its surface and shortens its life. This
is particularly the case when the gauge is operated
at pressures above 1 Pa, and if active gases such
10.3.2.3 The hot-cathode ionization gauge as oxygen are present.
(Buckley 1916)
Construction The gauge head shown in Figure Outgassing Since the gauge is highly sensitive.
10.13 is a special triode valve, usually with a hard the gas molecules covering the electrodes and
glass envelope. Stainless steel may also be used, envelope must be removed by heating them to
or a nude form of gauge in which the gauge the highest safe temperature. For the filament,
electrodes are inserted directly into the vacuum the required temperature can be obtained by
vessel. The filament F, of heavy-gauge tungsten, increasing the filament current, while the grid
operates at about 2000K, and may be readily and ion-collector may be heated by electron bom-
damaged by accidental inrushes of air. A filament bardment, using the filament as the source. Com-
of iridium coated with thorium oxide operating at mercial gauge control units make provision for
a lower temperature is almost indestructible. this treatment. The envelope is heated in an oven,
Around the filament is a molybdenum or tung- or by means of a hot-air gun.
sten grid G also heavily constructed, and outside
the grid is a cylindrical ion-collector C of nickel.
Since the ion current received by this electrode Pumping During operation, the gauge removes
is very small, special care is taken with its gas molecules from the vacuum apparatus, and
insulation. thus behaves as a pump. Two processes are