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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 vacuum manifold pressure begins
to rise significantly. This helps prevent backstreaming of diffusion pump fluid into
the vacuum manifold. Your data system has pumpdown and venting programs to
guide you through these procedures. Follow their instructions carefully.
See Also
To pump down the MSD, page 58
To vent the MSD, page 52
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, but would diffuse
into the vacuum manifold.
Diffusion pump operational readiness is monitored by two thermal switches.
Table 7
Diffusion pump thermal switches
7KHUPDO 6ZLWFK
Too Cold
1RUPDO VWDWH
Normally open
&KDQJHV DW
170°C rising; 140°C falling
)XQFWLRQ
Keeps analyzer turned off until
the pump is hot enough to for
adequate vacuum. Prevents
analyzer damage.
0HVVDJH
The high vacuum pump is
not ready
:KDW LW PHDQV
Normal during pumpdown.
:KDW WR GR
Wait for pump to heat up.
Vacuum System
5
Diffusion pump and fan
Too Hot
Normally closed
365°C rising
Shuts off diffusion pump and
analyzer if the pump overheats.
Prevents damage to the pump and
analyzer.
Difficulty with the high vacuum pump
Always indicates a problem.
Check for fan failure. Make sure
pump is cool, and power-cycle MSD
to reset.
199
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