# Vacuum Pumps

## Introduction

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.

## Conductances

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

$\text{Resistance, equivalent series} = \text{Resistance}_1 + \text{Resistance}_2 + ... + \text{Resistance}_n$

$\frac{1}{\text{Conductance, equivalent series}} = \frac{1}{\text{Conductance}_1} + \frac{1}{\text{Conductance}_2} + ... + \frac{1}{\text{Conductance}_n}$

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

$\frac{1}{\text{S}_{net}} = \frac{1}{S} + \frac{1}{C}$

### Parallel Flowpaths

$\frac{1}{\text{Resistance, equivalent parallel}} = \frac{1}{\text{Resistance}_1} + \frac{1}{\text{Resistance}_2} + ... + \frac{1}{\text{Resistance}_n}$

$\text{Conductance, equivalent parallel} = \text{Conductance}_1 + \text{Conductance}_2 + ... + \text{Conductance}_n$

## Practical Issues

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!