Inficon Transpector CPM User Manual

6–9
Transpector CPM Operating Manual
The highest oxide, WO
3
is yellow in color and has a melting point of 1473°C. No
boiling point or sublimation temperature is reported, but presumably it is far more
volatile than the pure metal itself.
At normal filament operating temperature, tungsten will readily react with oxygen
or water vapor to form oxides.
 If the oxygen (or water vapor) is very high, WO
3
is formed. This would be
observed if an attempt had been made to turn on the filament in air at 760 Torr.
 At lower oxygen (or water vapor) partial pressures, lower tungsten oxides are
formed, giving rise to the blue color observed on the electron repeller. Since the
tungsten oxides are far more volatile than the pure tungsten, when they form
on the filament surface, they evaporate away, depositing on the nearest
relatively cool surface.
Figure 6-1 depicts a graph correlating tungsten filament mean time between failure
(MTBF) in seconds with oxygen or water vapor partial pressure in Torr. The graph
predicts a filament lifetime of approximately 200 seconds at 1 Torr, the maximum
pressure at which Transpector can maintain electron emission.
Figure 6-1 Tungsten Filament MTBF
At 1 Torr at the filament, the pressure inside the anode cylinder will be
approximately five times higher (assuming the bulk of the gas load is entering the
CPM manifold through the closed ion source).
At 5 Torr, the collisional mean-free-path for ions will be less than 0.01 mm. The
distance from the ion creation region to the total pressure collector electrode is
about 4 mm, or approximately 400 mean-free-paths away. At this pressure, the
scattering losses may sufficiently reduce the total pressure ion current so that it
remains less than the over-pressure trip current. Under these conditions, the
Tungsten Filament MTBF
y = 144.92x
-0.9306
R
2
= 0.9895
1.E-01
1.E+00
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03
O
2
or H
2
O Partial Pressure at the Filament (Torr)
MTBF (seconds)
1 year
1 month
1 day
1 hour
1 minute

Inficon Transpector CPM Specifications

General

Brand	Inficon
Model	Transpector CPM
Category	Industrial Equipment
Language	English