937B Controller Instruction Manual
pg. 42
appropriate. However, if the protection set point is triggered, the auto control will be
disabled, and the HC sensor can then only be turned ON manually.
•
SAFE: The filament for a Hot Cathode sensor will be automatically turned OFF by the
controlling sensor, but the HC can be turned ON manually.
•
OFF: The Hot Cathode must be turned ON/OFF manually. Even if a control channel is
selected, the HC will not be turned ON or OFF.
6.6 Power Control of a Pressure Sensor
If flammable or explosive gas is used or present (such as during a regeneration of a LN
2
cooled cold
trap), the Convection Pirani, Convectron or Pirani sensor must be turned OFF to avoid potential
explosion.
Only the Pirani (PR), the Convectional Pirani (CP), the Cold cathode (CC) and the Hot Cathode (HC) can
be turned off from the front panel.
6.6.1
Power (including degas) Control of a Sensor using Front Panel
Control Button
To turn ON or OFF a sensor (PR/CP/CC/HC) using the front panel button:
1. Use the UP or DOWN arrow keys to select the desired sensor (PR/CP/CC/HC) to be turned ON
or OFF. The active channel (sensor) is indicted by the illuminated green LED to the left of the
sensor’s displayed pressure.
2. For either a cold or Hot Cathode sensor, make sure it is not controlled automatically by another
sensor. If it is in the AUTO control mode, disable the control set point before using the front panel
key to switch the sensor.
3. The Sensor ON/OFF key switches the corresponding sensor ON and OFF.
When a Hot Cathode is turned ON, the filament power may turn OFF automatically if
the pressure is higher than the protection set point.
When Cold Cathode sensors are turned ON at very low pressures, the sensor may take time to
start as the discharge current does not build up immediately.
Prolonged operation of cold cathode sensors at higher pressures will degrade their performance
due to sputtering inside the cell reducing the operating service time before cleaning. Additionally,
operation at pressures above 5x10
-1
Torr can result in the sensor falsely indicating a much lower
pressure. This phenomenon is called rollback and is due to high concentrations of charge
particles making gas conductive at high pressure. With the protect set point feature, the possibility
of operating in ‘rollback’ is minimized.
Operation of Hot Cathode sensors at high pressure may lead to filament burnout. For this reason,
the Protection Set Point is always enabled for HC sensor, so they are automatically turned OFF
once pressure exceeds the protection set point.
4. The Degas ON/OFF key switches the degassing of a hot cathode sensor ON and OFF.
To turn ON/OFF degas for a hot cathode gauge; the procedure is almost identical to the sensor
power control, except that the DEGAS ON/OFF key is used. When a HC is in degas, a small
DG
indicator will appear on the display line of the HC. Below the
DG
will be a timer indicating the
remaining time for degas.