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Hydrocarbons (oils, plastics)
oopswhat if: leave a piece of plastic plastic inside the vacuum chamber (oops!), 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.
Dust
it's all around us, slowly settling on surfaces.
Dust always matters. We try to avoid it. Sometimes a lot. Sometimes not so much.
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
- worried about dust? don't let it get in your chamber!
wear special dust-free robes and hats. Do vacuum work in a special dust free room.
Bake it
making the chamber hot will get help those sticky molecules (water and oil) to jump around more
we wrap the chamber in electrical heating tape to heat it from the outside in. it takes days.
detailed discussion see pages on baking
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Leaks: always happening, hopefully tiny
all vacuum chambers leak, but the leak doesn't have to come from outside the chamber
sources of leaks
leaking in from the outside
- really big leaks can be heard, usually from something like a seal that wasn't tightened
- smaller leaks come from various places, often for example a hair sitting on a seal or ????
- tiny tiny leaks: individual gas molecules wriggle their way through materials (gas permeability), especially rubber, making a very slow leak.
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all vacuum chambers leak, but the leak doesn't have to come from outside the chamber...
leaks
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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.
the The above diagram's screw has a hole drilled through the middle to allow the trapped gas to escape quickly. these are called 'vented screws' and are used commonly inside vacuum chambers. But there are other places gas can get trapped besides at the bottom of screw holes.
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How do we find small leaks?
with helium.
For detailed discussion on leaks, how to find them, and what to do about them see the Leak Checking pages
Time to talk numbers (
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sorry)
Units Used with Pressure
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we use torr to describe the pressure of our vacuum systems
we use psi (pounds per square inch) to describe the pressure of our compressed systems
compressed: air, nitrogen, helium, argon, etc...
gas flow rate: liters per second we use liters per second (l/s) to describe the rate of gas flowing through something
Log Scales (making numbers lie)
not Not this log scale:
it's math. sorry.
Log scale allows people to create graphs that show tiny things next to giant things really well by stretching out the distance between tiny numbers, and compressing the distance between huge numbers. the powers of 10 does this really well.
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Exponents and powers of 10
the little number above the 10's, called the exponent, is the number of zeros; a negative exponent.
you can remember 'exponent' because it exposes the number of zeros.
A negative exponent is how many zeros past the decimal place.
linear vs logarithmic
in the linear scale the distance between 0 and 500 is a small portion of the vertical aspect of the graph, but in the log scale that same distance takes up over half the graph; this is the stretching.
common units that use log scales: sound (decibels: dB), earthquakes (Richter magnitude)
UHV: how teeny tiny
what's the difference between 1x103 and 1x107? the same difference between 1,000 and 10,000,000: over 9 million.
What's the difference between 107 and 103? the same difference between 10,000,000 and 1,000: over 9 million.
What's the difference between 10-7 and 10-3? the same difference between 0.00000001 and 0.0001: less (much less) than 1
Different, but in both cases the difference is 4 orders of magnitude. look at the exponents: 7 - 3 = 4, tiny or huge, doesn't matter.
Linear vs Logarithmic
In linear counting the numbers increase one at a time. It's what we're used to. t's how we normally count: 1, 2, 3, 4, 5 etc...
In logarithmic (i.e.: log) counting the numbers increase by bigger amounts each time. Log counting is strange, counterintuitive and, unfortunately, useful.
the log scale and the linear scale both start around 1 go up to 100,000. But what's different is how they get from 1 to 100,000. The second number in linear scale is 20,000, but in log scale the second number is only 10. Also in log scale the second to last number is 10,000, still much less than the second number in the linear scale: 20,000.
in the linear scale the distance between 0 and 500 is a small portion of the vertical aspect of the graph, but in the log scale that same distance takes up over half the graph; this is the stretching.
common units that use log scales: sound (decibels: dB), earthquakes (Richter magnitude)
UHV: how teeny tiny
Ultra-High Vacuum (UHV) is an official term for really really fancy vacuum of a certain (extremely low) range of pressures. And it's the type of vacuum we care about the most.
For molecules in UHV the mean free path is at least 200 miles. Any molecule in UHV is going to see the chamber walls thousands of times before it will see another molecule. what's the difference between 1x10-3 and 1x10-7? the same difference between 0.0001 and 0.00000001: less (much less) than 1
Vacuum Chambers
it's just a box for holding nothing.
Sadly the details of vacuum chambers and their construction And this topic is really beyond what this document is about (introduction to theory), but keep exploring!
For more details on Vacuum System parts and how they actually work, see the confluence pages on vacuum components
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