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For detailed discussion on leaks, how to find them, and what to do about them see the Leak Checking pages
Time to talk numbers (sorry)
We use numbers and units in discussing vacuum systems. The way we use some units can be very counterintuitive (*cough* log scale) This section breaks down the ideas behind the numbers.
Units Used with Pressure
- we use torr to describe the pressure of our vacuum systems
image of vacuum guage used here at SLAC
- we use psi (pounds per square inch) to describe the pressure of our compressed systems
compressed: air, nitrogen, helium, argon, etc...
- we use liters per second (l/s) to describe the rate of gas flowing through something
Log Scales (making numbers lie)
Not this log scale:
The log scale I'm talking about... It's totally 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.
Exponents and powers of 10
The little number above the 10's, called the exponent, is the number of zeros.
you can remember 'exponent' because it exposesthe number of zeros.
A negative exponent is how many zeros past the decimal place.
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.
Common units that use log scales: sound (decibels: dB), earthquakes (Richter magnitude)
UHV: how few molecules
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.
Vacuum Chamber Construction
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