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13.APPENDIX
13.1.Principles of Operation
When gas molecules collide with particles having energy above a certain level, they emit
electrons and are turned into ions. This phenomenon is called gas ionization.
To ionize gas, the filament is heated and the generated thermoelectrons are accelerated.
Since the frequency of thermoelectrons and gas molecules colliding with each other is in
proportion to gas density, the density of the gas molecules (gas pressure) can be known
from the number of ions produced if ions are formed at a fixed ratio by collision.
The following relationship holds between the number of thermoelectrons emitted from
the filament (emission current: Ie), number of ions produced (ion current: Ii) and density
of gas molecules (pressure: P).
Ii = S
⋅
Ie
⋅
P
.................................
(1)
Ii : Ion current [A]
S : Sensitivity [Pa
-1
]
Ie : Emission current [A]
P : Pressure [Pa]
In the equation above, sensitivity S depends on various conditions, such as the structure
and size of the sensor head, voltage applied to each electrode, type of gas, and others.
This gauge indicates Ii in the above equation on the meter after electrically amplifying
it.
Here, it will be understood that a change in sensitivity S will make a difference in the
indicated value of the vacuum gauge even if emission current Ie or pressure P is the
same.
Typical factors that change sensitivity S include:
(1)
Type of sensor head
(2)
Type of gas
(3)
Deterioration of sensor head
The sensitivity in nitrogen gas atmosphere of the sensor head that can be used with
this vacuum gauge is as follows.
GI-M2
GI-D7
GI-N8
Measurement
sensor head
sensitivity factor
Srj(N2)
0.06 Pa
－１
0.0827 Pa
－１
0.153 Pa
－１
Summary of Contents for GI-M2
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