937B Controller Instruction Manual
pg. 99
Figure 11-5 Bayard Alpert Gauge Structure and Electron Processes
The collection ion current
i
is related to the pressure
P
and emission current
i
by the equation
kPi
i
where,
k
is the sensitivity factor which depends strongly upon the electron accelerating
structure and operating condition. Typical sensitivity factor for nitrogen is around 7.5 to 15 Torr
-1
.
To reduce the outgassing within the hot cathode to a negligible level and minimize the effect of ESD
(electron stimulated desorption) on high vacuum measurement, high temperature degassing techniques
are used to drive off any adsorbed gas molecules from the surface of the anode grid. Electrode heating
during the degas process is accomplished either by electron bombardment (EB degas) or by passing a
high current through the grid (I
2
R degas). EB degas technique is accomplished in the 937B Controller by
increasing the emission current from the filament.
11.4.1 Cleaning the Hot Cathode Sensor
Pump oils and other fluids that condense and/or decompose on surfaces such as the grid and collector,
contaminate the sensor and can cause calibration shift. Degassing of the electrode structure can remove
surface contamination on the grid and collector. Severe contamination of the grid structure may require a
replacement of the sensor.
Although the feed-through insulators are shielded, in some applications conducting films or other forms of
electrically conductive pathways may be formed on insulator surfaces. When this happens, it creates a
leakage path on the insulator that can produce false low pressure reading. In these cases, the sensor
may have to be replaced.
Unlike with Cold Cathode Sensors, it is not advisable to the clean the Hot Cathode
sensor. Attempts to clean the sensor it may either deform or break the gauge structure.
11.4.2 Testing the Hot Cathode Sensor
This test will only identify a non-functional sensor. It will not detect damage from
contamination, misuse or rough handling that can affect the calibration of a Hot Cathode
gauge.
The most common cause of sensor failure is filament failure. To check for this, test the sensor using an
ohmmeter with less than 5 mA of current. The resistance readings of a normal hot cathode sensor are
show in Table 11-1. The resistance between the two pins of each filament is important.
Pin Numbers
Resistance (Ohm) for a
good sensor
Resistance (Ohm) for a
bad sensor
Between F1 pins
0 - 5
Open (>100 Ohm)
Between F2 pins
0 - 5
Open (>100 Ohm)
Any pin to ground/shell
>10
6
Ohm
<10
6
Ohm
Table 11-1 Resistance Readings of a Normal HC Sensor