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Side note: Most of the pictures here are linked to external material, including: videos, articles, slideshows, and product pages. Click to explore; though they aren't necessary, and can be tangential or beyond the scope of this introductory guide.
Happy readingLearning!
Starting at the bottom, Setting the scene
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Ok, so you know how I said molecules bounce off walls? Not actually true. ! 😬 I lied to keep it simple, but I think you're ready for the truth: It turns out that any time a gas molecule hits a surface it actually sticks to that surface, just like how your breath fogs the mirror; this called adsorption (not to be confused with absorption).
And also just like fog on a mirror vanishes over time, those stuck molecules will eventually jump back off of the surface (I.e. evaporate, outgas, desorb), flying of in a random direction just as fast as it was when it hit the surface.
How long a molecule stays stuck before desorbing from a surface that depends on the type of molecule, the temperature, and the what the surface is made of. Evaporation of a molecule can happen anytime between almost instantly and basically never. And we'll never be a able to predict the moment exactly, but we often usually have a general idea for how long.
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When a
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molecule is flying around a vacuum chamber, it's contributing to the total pressure, but a molecule is trapped on the surface doesn't contribute to pressure because it's not a gas, it's trapped on the surface! So if a type molecule tends to jump off surfaces rarely, the pressure can stay low. And also if a certain type molecule tends to jump off
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quickly (from a given surface at a given temperature
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) it will hop around enough to quickly find its way into a pump, again the pressure drops .
The trouble for
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lowering pressure comes when a given type molecule jumps around enough to raise the pressure, but not enough to easily find it's way into the pump.
Contamination: molecules sticking to walls intermittently
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molecules jumping around intermittently keeping the pressure too high
Main culprits
Water - from moisture in the air
attaches to chamber surfaces
Hydrocarbons (oils, plastics)
oops: leave a piece of plastic inside the vacuum chamber, then pump it down:
the plastic will outgas and coat the chamber walls with a fine layer of hydrocarbons
you know what's really oily? You.
how to defeat contamination
Prevent it!
use gloves, change them frequently
clean anything going inside the vacuum chamber
opening the chamber? pump nitrogen into it first, keep the nitrogen flowing while the chamber is open - prevents water from getting in
nitrogen gas has no water vapor, unlike air.
Bake it
making the chamber hot will get help those sticky those sticky molecules (water and oil) move along fasterto jump around more
we wrap the chamber in electrical heating tape to heat it from the outside in. it takes days.
detailed discussion the see pages on baking
water: 150°C
hydrocarbons: 250°C
Leaks: always happening, hopefully tiny
all vacuum chambers leak, but the leak doesn't have to come from outside the chamber
sources of leaks
really big leaks can be heard, usually from something like a mechanical connection seal that wasn't tightened
and then something, like smaller leaks come from various places, often for example a hair , sitting on a gasket
gas permeability
seal or ????
tiny tiny leaks: individual gas wriggle their way through materials (gas permeability), especially rubber, making a very slow leak.
leaks can come from within the chamber
gas trapped in a pocket slowly leaks into the rest of the chamber - a virtual leak, eventually it runs out, but can take weeks.
for detailed discussion on leaks, how to find them, and what to do about it for detailed discussion see the Leak Checking pages
Time to talk numbers (maybe skip this portion)
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