If your system hase a
secondary diffusion pump often used to reach high vacuum pressures less than
.0001mm Hg you should also read below.
Diffusion pumps may be
activated only when system pressure is reduced below .5mm hg vacuum or 500
micron, ideally south of 50 micron. Use of a rotary vane pump to “rough”
the system down is typical for this function. Sometimes these pumps are
known as roughing pumps for this very reason, sometimes forepumps.
Make sure water or fan
is turned on for diffusion pump cooling. A diffusion pump needs a cooling
mechanism to function. Water flow through tubing on the pump is the
standard but also some “air cooled” fan based units are also available.
Check level and
condition of diffusion pump oil. These oils are typically silicone based
and are very expensive, thank you. If they are exposed to high pressures,
they will be degraded and may need replacement. Diff Oil typically is
clear. Because diff oils are “boiled” in functional application therefore
they typically have pretty high temperature boiling points and very low
vaporization points. Many diff pumps do not have a sight window so checking
the oil condition becomes more involved but it should be accomplished on a
Ok ready to start up the
system. Turn on roughing pump (vane pump and rough system down to hopefully
like 50 micron.
pump system design has a large diameter valve on the top of the diffusion
pump before the system. This is often referred to as a throttling valve.
They come in various designs. This should be closed before the diffusion
pump is actuated on.
Turn on diffusion pump.
There is typically no noise here because of the process that a diffusion
pump employs to do its’ work. Basically in theory you are turning on a
stove burner when you fire up the diffusion pump. The heating coil or
mantle starts to heat up and boil the diffusion pump oil in the base of the
diffusion pump. Heat up time varies with pump but allow at least 10 minutes
to get the diffusion pump rolling.
Hopefully by now the
system pressure is down to vacuum pressure that allows you to engage the
Slowly open throttling
valve. The roughing pump may make a little noise as the diffusion pump is
compressing and passing on it’s high pressure range to the roughing pump.
Take it slow and easy it will get there. Once you have opened fully the
throttling valve close the rough pump connection -> system valve while
maintaining the rough pump -> outlet of diffusion pump valve in open
position. At this point the roughing pump is pulling on the outlet of the
diffusion pump only.
Shut down is essentially
the reverse program making sure diff pump is offline first. Close
throttling valve and turn off diffusion pump. Close diffusion pump outlet
-> rough pump valve after diffusion pump temperature is reduced below
boiling point of diffusion pump fluid. Open roughing pump -> system pump
valve so roughing pump is now only working against the system. Use vacuum
breaker valve to initiate a leak into system and immediately turn off
roughing pump. Let system come up to atmospheric pressure using adjustment
as desired of vacuum breaker valve of system. When pressure equalized.
Close system -> rough pump valve. System is now ready to process again.