HP 5973, Hardware Manual

Page: 198 / 297

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5 Vacuum System
Diffusion pump and fan
Diffusion pump and fan
The diffusion pump creates the low pressure (high vacuum) required for correct
analyzer operation. The diffusion pump in the MSD is an air-cooled vapor diffusion
pump with 90 liters/second capacity. It mounts with a KF50 fitting to a baffle
adapter clamped to the bottom of the vacuum manifold.
The diffusion pump has a cylindrical body surrounded by fins to help dissipate
heat. Its inlet is open to the interior of the vacuum manifold, through the adapter
and baffle. A structure called the stack is located at the center of the pump body.
An electric heater is located at the bottom of the stack.
The diffusion pump transports gas by momentum transfer. The heater boils a spe-
cial fluid (a polyphenyl ether) inside the stack. As the vapor pressure increases,
the pump fluid vapor is forced out and downward through nozzles in the stack.
The vapor forced out of these nozzles strikes the gas molecules that are present.
This forces the gas molecules down toward the outlet near the bottom of the
pump. Another nozzle in the stack points directly at the outlet and forces the gas
molecules out. The vapor condenses on the sides of the pump and the liquid
drains down to the bottom. The liquid is boiled again and is reused continuously.
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 turns on
the diffusion pump heater automatically as soon as the foreline pump lowers the
pressure in the vacuum manifold below approximately 300 mTorr (0.3 Torr). If
the foreline pressure rises above 400 mTorr, the ac board shuts off the heater. 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 1.4
×
10
-4
Torr
for GC helium carrier gas flows up to 2 ml/minute. High vacuum (manifold) pres-
sure can only be measured if your MSD is equipped with the optional gauge con-
troller.
See Also Gauge controller , page 208
Table 2. Typical MSD pressure readings for various carrier gas flow rates,
page 198