937B Controller Instruction Manual
pg. 98
7. Using your thumbs, push the larger end of the spring into the sensor body (9) until it is contained
within the tube's inside diameter.
8. Using the smooth-jaw, needle-nose pliers, work the compression spring (3) down into the sensor
body (9) until it is fully seated in the formed groove.
9. Inspect the ground shield (8) and the grid washer (5) to verify they are centered with respect to
the anode (10).
10. If adjustment is needed, gently reposition the grid washer/cathode assembly, taking care not to
scratch the grid washer (5).
It is recommended that the resistance between the ion current feed-through pin (13) and the grid washer
(5) be measured to verify that the leaf spring (1) is in contact with the cathode (6). The measurement
should indicate a short circuit between them. There should be an open circuit between the ion current
feed-through pin (13) and both the high voltage feed-through pin (2) and the sensor body (9).
Once this procedure is complete, the I-Mag Sensor is ready for installation. If it is not to be installed
immediately, cover the flange with clean, vacuum grade aluminum foil and cap with a flange protector.
11.3.4 Preparing the Sensor for Bakeout
To prepare the sensor for bakeout at up to 400°C (when the sensor has a CF flange and is sealed with a
metal seal), remove the sensor cable and magnet assembly as described in "Disassemble the I-Mag
sensor".
11.4 Maintenance of Low Power Nude and Mini BA Hot Cathode
Sensors
Hot Cathode Theory
Hot Cathode Ionization sensors use the electrons emitted from a hot filament (thermionic electrons) to
create ions in a surrounding gas. The ion numbers are in proportion to the ambient gas pressure.
Electrons are accelerated through the structure by a potential difference between the hot, emitting
filament and a positively charged surrounding grid (anode). The energy acquired by the electrons as they
are accelerated by the electric field is sufficient to ionize resident ambient gas molecules. The positively
charged ions created by this collision ionization are attracted to the negatively biased ion collector where
they are neutralized by an electron current. The gas molecules are singly ionized and there is a one
–to-
one correspondence between the number of ions neutralized and the magnitude of the neutralizing
electron current. Hence the electron current is often called the “ion current” and this is proportional to the
pressure in the sensor
. The “ion current” is measured by the electrometer and converted to a pressure
indication on the display.
The Bayard Alpert (BA) geometry is one of the most popular types of hot cathodes sensors. The main
advantage of the BA configuration is its reduced susceptibility to X-ray induced errors. This is achieved
through the adoption of a small diameter ion collector that minimizes the area exposed to the soft X-ray
emitted from the grid. X-ray emission from the grid is an undesirable side effect of electron impact upon
the grid surface. Some of these emitted X-rays strike the ion collector, releasing electrons by the
photoelectric effect. This photoelectric current is not related to the pressure but is nevertheless added to
the measurement of current determined by the electrometer. The photoelectric current can fully mask the
ion current at low pressure (around 1x10
-10
Torr) which limits the pressure measurement capabilities.
Ion Collector
Electron
Collector
(Grid)
Electron Source
(Filament)
A
+180V
+30V