How Do You Remove Air From Something? You Cannot Pull It Out

Nothing pulls air out of a chamber, because air is not attached to anything and cannot be pulled. What happens instead is that a pump provides a space at lower pressure, and molecules moving randomly wander into it and are then trapped and expelled. That distinction matters practically, because it means the rate of removal depends on how often molecules happen to arrive at the opening, which depends on how many remain. As the chamber empties, molecules arrive at the pump more and more rarely, so each successive factor of improvement takes longer than the last, which is why reaching extremely low pressures takes hours or days rather than minutes.

Different pressure ranges need mechanically different machines:

• •Rotary vane pumps, which trap and compress gas mechanically, for rough vacuum
• •Scroll and diaphragm pumps, which do the same without oil
• •Diffusion pumps, which use a jet of hot vapour to drag molecules downwards
• •Turbomolecular pumps, with blades spinning fast enough to knock molecules along
• •Cryogenic pumps, which freeze gases onto a cold surface
• •Ion pumps, which ionise molecules and bury them in a metal surface

No single machine covers the whole range, because the physics at high and low pressure is entirely different. At ordinary pressure, air behaves as a fluid and can be scooped and compressed, which is what a mechanical pump does. Below a certain point molecules stop colliding with each other and travel in straight lines between the walls, so there is no fluid to compress and mechanical scooping becomes useless. The pumps used at that stage work on individual molecules instead, striking them with fast-moving blades or trapping them on cold or reactive surfaces. Those pumps cannot exhaust to atmospheric pressure, so a mechanical pump must work behind them, and a system reaching extreme conditions runs several in series.

Removing air is required for a surprising range of purposes and the reasons differ. Semiconductor manufacturing needs it because the processes deposit material one atomic layer at a time and any stray molecule ruins the result. Electron microscopes need it because electrons are scattered by air within centimetres. Particle accelerators need beams to travel kilometres without colliding with anything. Thermal insulation uses it because a gap with no gas conducts almost no heat. Freeze drying uses it to make ice turn directly to vapour at low temperature, which preserves food and medicines without cooking them. Vacuum forming, packaging, degassing resins and distilling heat-sensitive liquids at low temperature all rely on it industrially.

Beyond a certain point, what fills the chamber is not the original air but material coming out of the walls, and dealing with that dominates the work. Every surface holds a layer of adsorbed water and other molecules that release slowly under vacuum, supplying gas indefinitely, which is why chambers are baked at high temperature for days to drive that material off. Materials themselves release dissolved gas, so plastics and rubbers are excluded and metal and glass are used. Every seal leaks slightly at a rate that eventually dominates. Fingerprints and oil residues are serious contaminants. Reaching extremely low pressures is therefore a matter of cleanliness, materials and patience rather than of a better pump.

Molecules are not pulled out but wander into a trap, so removal slows as the chamber empties and each further improvement takes longer. Mechanical pumps scoop and compress air while it behaves as a fluid, and below that point molecules travel in straight lines and must be struck or trapped individually. Several pumps run in series. The limit comes from gas released by the chamber walls, which is why chambers are baked for days.