MKS HPS 919 Series, Operation And Maintenance Manual

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33
Hot Cathode Ionization High Vacuum Senor System
Setting the Sensor Sensitivity
The ion current obtained at a given pressure and electron emission current
depends upon the sensor design. The sensor sensitivity S is defined by
the equation,
S =
where,
i
i
is ion current in amperes
i
e
is electron emission current in amperes
P is pressure in Torr.
The sensitivity for the Series 919 System's Low Power Nude Sensor is about
9 Torr
-1
for nitrogen or air. Sensitivity values for other gases differ (see Using
the Series 919 System with Other Gases, p. 35).
To set the Series 919 System sensitivity,
1. With Controller power on, depress the Sensitivity push-button (shown
to the left) on the front panel.
2. With a small screwdriver, adjust the Sensitivity potentiometer until
the display indicates the Sensor's sensitivity value as specified by its
manufacturer (e.g. 9.0 x10 0 for the Series 919 Low
Power Nude Sensor).
Sensor Sensitivity for Different Pressure Units
Series 919 Controllers displaying units in Torr are shipped with the sensitivity
set to 9 Torr
-1
. Series 919 Controllers that display pressure in units of Pascal
or mbar must have the sensitivity divided by 1.33 to read the correct
pressure. These Controllers are shipped with a sensitivity of 6.8. The x100
scale factor between Pascal and mbar units is changed in the Controller
electronics.
Example: If a sensor's nominal sensitivity is 12 Torr -1 , divide 12 by 1.33 to
get 9. Set the Controller to a sensitivity of 9.0 x10 0 for Pascal or mbar
units.
Degassing the Sensor
The Series 919 System is designed for I 2 R degas to remove adsorbed gas
from the Sensor. Degas power decreases as cable length increases.
Pressure can still be measured during degas.
i
i
(i
e
x P)
Sensitivity
Front Panel
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