Check the oil level and condition of the oil.
Level should be past mid point on sight glass / sight window of pump.
Oil should appear amber in color and not appear overly dark.  Odor should be neutral and not smell “burnt”.

Close the valve on the inlet of the pump. This may be considered the system valve or main valve.  This will in effect “dead head” the pump and allow pump to achieve it’s ultimate vacuum pressure.  The best vacuum the pump can obtain.

If intake of pump not vented to atmospheric pressure vent pump inlet to atmospheric pressure.

Turn on the pump. Pump should make a gurgling sound and then settle down as pressure runs through high pressure ranges from 760mm - .5mm.  This time frame varies depending on amount of space to be evacuated between pump and system valve.

Once the pump has stabilized (quieted down), open the main valve slowly. The pump should not labor to much as pressure rises and it works to reduce the “system” towards “ultimate pressure”.  If pump sees a large system it may be symptomatic of pump to make again the gurgling noise, generate oil mist caused by high gas flows, or actually emit oil droplets from the exhaust port of the pump.  If you slowly open the valve on larger systems the pump will behave more predictably.  As your system pumps down, you can slowly open the main valve to increase pumping speed and attack the low pressure, “higher vacuum” range using the pumps complete capability.
 

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 regular basis.

Ok ready to start up the system.  Turn on roughing pump (vane pump and rough system down to hopefully like 50 micron.

Classically diffusion 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 diffusion pump.

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.