Vacuum System 6
5977B Series MSD Troubleshooting and Maintenance Manual 171
A cooling fan is located between the diffusion pump and the front cover of the
MSD. The fan draws air through the cover and blows it over the pump. Without
this cooling, the pump fluid vapor would not condense correctly, and would
diffuse into the analyzer chamber.
The foreline pump is connected by the foreline hose to the outlet of the
diffusion pump. It removes the gas molecules that reach the outlet.
The diffusion pump operation is controlled by the AC board. The AC board
automatically turns on the diffusion pump heater as soon as the foreline pump
lowers the pressure in the analyzer chamber below approximately 300 mTorr
(0.3 Torr). Until the foreline pressure drops below 300 mTorr, the diffusion
pump heater will not turn on. If the pressure does not drop below 300 mTorr
within 7 minutes of turning the MSD on, the foreline pump will shut off.
During operation, if the foreline pressure rises above 400 mTorr, the diffusion
pump heater will turn off. The AC board allows the analyzer electronics to
turn on when the diffusion pump is hot.
The diffusion pump typically maintains an indicated pressure below
1.0 × 10
-4
Torr for GC helium carrier gas flows up to 2 mL/min. High vacuum
(manifold) pressure can only be measured if your MSD is equipped with the
optional gauge controller.
The small size of the diffusion pump allows it to heat up and cool down
quickly. This simplifies pumpdown and venting. From initial power-on, the
system can pump down to operating pressure in approximately 15 minutes.
If the power fails, the diffusion pump fluid stops boiling before the analyzer
chamber pressure begins to rise significantly. This helps prevent back
diffusion of pump fluid into the analyzer chamber. Your data system has
pumpdown and venting programs to guide you through these procedures.
Follow the instruction carefully.
Two thermal switches monitor diffusion pump operational readiness. See
Table 11.
Tab le 11 Diffusion pump thermal switches
Thermal switch Too cold Too hot
Normal state Normally open Normally closed
Changes at 170 °C rising; 140 °C
falling
365 °C rising
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