946 Controller Instruction Manual
pg. 112
a constant,
P
is the pressure, and
n
is a constant (in the range of 1.00 to 1.15). This equation is valid for
pressures ranging from 10
-3
to 10
-8
Torr, depending on the series resistor used. At pressures around 10
-6
Torr, the sensitivities of 1 to 10A/Torr are not unusual.
The initiation of electron impact events within a Cold Cathode sensor depends upon certain chance
events such as field emission or cosmic ray production of the first free electron. Electron-molecule
collisions thereafter produce additional electron/ion pairs during the electrons’ transit between the
electrodes. The discharge rapidly builds to a stable value. Start of the discharge normally requires a very
short time at 10
-6
Torr or above, a few minutes at 10
-8
Torr, and longer times at lower pressures.
Anode
Cathode
Ion
+
e-
e-
e-
Magnetic field
into the page
Figure 11-2 Electron Orbits and Ion Production in an Inverted Magnetron
MKS Cold Cathode sensors use an inverted magnetron with separate feed-throughs for the anode high
voltage and the cathode current. This configuration has a cylindrical cathode, a central wire anode, and
an external cylindrical magnet which provides an axial field. The cathode is isolated from the grounded
metal housing. The inverted magnetron geometry produces more stable signal output, and also works
well to low pressure without risk of extinguishing the discharge. The range of the MKS Instrument’s cold
cathode sensors is extended to 10
-2
Torr by using a large series resistor to decrease sputtering within the
sensor. Additionally, the voltage across the tube is pressure dependent in the range between 10
-4
to 10
-2
Torr and this is used in determining the pressure reading.
11.1.2 Maintenance of Series 431/422 Cold Cathode Sensor
11.1.2.1 Disassemble the Series 431/422 Sensor
The Series 431/422 sensor consists of three subassemblies – the backshell; the internal; and the body.
Only the internal and body subassemblies are exposed to vacuum.
To disassemble the sensor, remove the backshell subassembly as follows (Steps 1 through 4 are not
necessary when replacing internal parts):
1. Remove the two
4-40 x ¼" Phillips head SEMS screws
2
and slide the
backshell
9
off
the sensor.