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Vacuum System
5
Vacuum seals
Vacuum seals
Several types of Viton elastomer O-ring seals are used to prevent air leaks into the
vacuum manifold. All these O-rings, and the surfaces to which they must seal,
must be kept clean and protected from nicks and scratches. A single hair, piece of
lint, or scratch can produce a serious vacuum leak. Two of the O-rings are lightly
lubricated with Apiezon-L vacuum grease: the side plate O-ring and the vent valve
O-ring.
Face seals
A face seal is an O-ring that fits in a shallow groove. The sealing surface is usually
a flat plate. The manifold side plate and end plate O-rings fit into grooves around
the large openings in the vacuum manifold. The side plate swings into place
against the side plate O-ring, and must be held in place when the MSD is turned on
for pumpdown in order to assure a good seal.
The front and rear end plates are screwed onto the manifold, and should not need
to be removed. The GC/MSD interface fastens to the manifold with three screws.
The calibration valve assembly is fastened onto the front end plate by two screws.
The vent valve knob threads into the front end plate. Small O-rings in grooves in
the front end plate provide vacuum seals.
The diffusion pump baffle adapter has a groove for its O-ring. The baffle adapter is
clamped to the manifold with 4 claw grips.
KF (NW) seals
Most of the seals for the high vacuum pumps, foreline gauge, and foreline pump
are KF seals. KF seals have an O-ring supported by a centering ring. The centering
ring can be either on the inside or the outside of the O-ring. The clamp presses two
flanges against the O-ring, making a seal. KF clamps must not be overtightened.
Compression seals
A compression fitting consists of a threaded fitting on the vacuum manifold and a
threaded collar with a ferrule and O-ring. A cylindrical part fits inside the collar.
Tightening the collar presses the ferrule, compressing the O-ring around the part.
The triode gauge tube and calibration vial use compression seals.
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