Vacuum Pumps

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Lecture date: Friday, 2014.09.19 (lecture recording)

Leybold has published an excellent guide on vacuum technology fundamentals, including full treatment of throughput, conductance, speed, and the other basic vacuum system concepts.


Conductance is the equivalent to conductivity in electrical engineering—it is the ease with which a gas flows through a hole or tube, and mathematically it's the inverse of the resistance to gas flow.

Because it's the inverse of a resistance, the following mathemtical rules apply.

Series Flowpaths[edit]

[math]\text{Resistance, equivalent series} = \text{Resistance}_1 + \text{Resistance}_2 + ... + \text{Resistance}_n[/math]

[math]\frac{1}{\text{Conductance, equivalent series}} = \frac{1}{\text{Conductance}_1} + \frac{1}{\text{Conductance}_2} + ... + \frac{1}{\text{Conductance}_n}[/math]

Pumping speed has the same units as a conductance, so they can also be combined in series:

[math]\frac{1}{\text{S}_{net}} = \frac{1}{S} + \frac{1}{C}[/math]

Parallel Flowpaths[edit]

[math]\frac{1}{\text{Resistance, equivalent parallel}} = \frac{1}{\text{Resistance}_1} + \frac{1}{\text{Resistance}_2} + ... + \frac{1}{\text{Resistance}_n}[/math]

[math]\text{Conductance, equivalent parallel} = \text{Conductance}_1 + \text{Conductance}_2 + ... + \text{Conductance}_n[/math]

Practical Issues[edit]

You'll never pull a perfect vacuum with a vacuum pump.

Real leaks: your seals will physically leak! Metal flanges with soft metal gaskets are used to reduce this as much as possible.

(can't use polymer o-rings—they'll leak!)

(watch for grease—it'll vaporize and go where you don't want it!)

Virtual leaks: species (especially water) will absorb to the walls and then desorb as the pressure drops!

Usually dealt with by pre-cooking the water out of the chamber with heat tape.

Hook up a residual gas analyzer (a simple form of mass spectrometer) and you see air components. Can't find the leak! What do you do? Squirt helium wherever you think there might be a leak, and look for the gas signature on the gas analyzer!