Varian 959D, Operation Manual

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959D Leak Detector
1-11
1.6.2 Automatic Valve Operation for Large Volume Leak Checks
The 959D valve control system allows large volumes to be pumped without damaging the
leak detector. If the leak detector is not advanced to Test or the test port pressure is above
250 mTorr, the system automatically switches between Start mode and Hold mode every
three minutes. When in Hold mode, V1 is closed, V2 is opened, and the TriScroll roughing
pump clears the foreline of any back pressure that may have built up in the time the leak
detector was pumping on the external volume.
The unit remains in Hold mode for 30 seconds. After this time, the Test valve V2 automati-
cally closes and the Roughing valve V1 opens to continue pumping on the external volume.
This switching continues indefinitely if the leak detector is left in Start mode or until the leak
detector is switched into Test mode. If the test port pressure rises to between 125 mTorr and
300 mTorr, overpressure protection automatically sets the leak detector into Hold mode.
The normal operating states of the valves are shown in Table 1-2.
1.6.3 Main Electronics Assembly and Circuit Boards
The main electronics assembly and circuit boards provide control for the 959D Leak Detec-
tor. The main electronics assembly provides the following secondary controls:
❑
Adjustment of spectrometer tube voltages for leak rate tuning
❑
Volume control and threshold adjustment
❑
Calibration
❑
Filament selection, activation, and emission adjustment
❑
Residual helium signal background check used during prolonged leak checking
Table 1-2 Valve Operating States
Valve
Operating Mode
Roughing
(V1)
Test
(V2)
Vent
(V3)
VENT Closed Open Open
START Open Closed Closed
HOLD Closed Open Closed
TEST Open Open Closed