MKS HPS 945 Series, Operation And Maintenance Manual

Page: 23 / 38

17
Digital Pirani vacuum Sensor System
Adjustment
potentiometers
Zero
Adjust
Atmosphere
Adjust
Calibrating for Zero and Atmosphere
Vacuum and atmosphere adjustment potentiometers are located on the front
panel as shown below. The two adjustments can be made independently, but
because of some interaction between them, we recommend making the
adjustments in the order below.
For best results, let the Series 945 Pirani System remain at the
calibration pressure for at least 15 minutes before adjusting
the potentiometers.
1. To adjust for atmosphere, an accurate, high
pressure reference sensor is required. With the
reference sensor at a known pressure (e.g., 50 to
90 Torr), adjust the
Atm potentiometer on the
Series 945 Controller until the display indicates the
correct pressure.
2. To adjust for vacuum, the Pirani Sensor needs
to be on a vacuum system pumped down to less
than 1.0 x 10
-5 Torr. At this pressure, the Series
945 Controller display should read L0 -4 Torr
( L0 -1 for microns, L0 -4 for mbar, or L0 -2
for Pa). If the display shows TL -4 Torr, turn the
Vac potentiometer clockwise until L0 -4 Torr is
displayed. If the reading is 1 -4 Torr or higher,
turn the
Vac potentiometer counterclockwise until
L0 -4 Torr is displayed.
Adjusting the Set Point
To adjust a set point to open or close the relay contact at a particular
pressure, press the view button for the desired set point on the front panel.
Keeping this button depressed, use a small screwdriver to adjust the
corresponding potentiometer until the reading coincides with the desired set
point pressure.
When an LED is on, the measured pressure is below the set point value, the
normally open relay contact is closed, and the normally closed contact is
open. When an LED is off, the measured pressure is above the set point
value, the normally open relay contact is open, and the normally closed
contact is closed.
To prevent the relays from turning on at any pressure, adjust the trim pots
counter clockwise until nc is displayed.
Venting to Atmosphere
The Pirani Sensor senses pressure by measuring heat loss from a heated
fine wire. Sudden venting at its port can greatly stress the Sensor physically.