Vacuum systems Pump down example

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A clean and dry vacuum system (qpy = 0) with V = 2000 I (as in example 1) is to be pumped down to a pressure of Pend - 10 tAbar. Since this pressure is smaller than the ultimate pressure of the rotary piston pump (Sg max ... 

Once you have successfully removed the bulk of water from the walls of the vacuum system, do not allow it to return. One easy and effective demonstration of the effect of water on vacuum is to pump a vacuum system down to some established level after it has been vented with atmosphere. Then, vent the system, filling it with dry nitrogen or argon back to atmospheric pressure. Now, repump the system back to the same vacuum as before. It should take about one-tenth the time. This example demonstrates why the ability to bake out a vacuum system improves the pumping speed by speeding up the removal (outgassing) of water vapor from the system s walls. It also demonstrates that once a vacuum system has been successfully pumped down, you do not want to re-expose it to the atmosphere. If you need to expose sections of your vacuum system to the atmosphere (for example, traps or mechanical pumps), section off these parts with valves and stopcocks so that the rest of the system can remain in a dry vacuum state. 

Ion Pump. A pump now much used in ultrahigh vacuum systems is the ion pump, of which the Varian Vac-Ion pump is an example. In this pump a gas discharge such as is commonly observed in air and other gases at about 10 Torr (e.g., neon signs, mercury and sodium lamps) is generated by a strong electric field and maintained all the way down... 

These flow resistance considerations are important to vacuum systems when through these, gas flows are restricted in connection with pump-down processes or venting processes or when measuring leaks, for example. 

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