An Introduction to Vacuum Science for Non-Scientists

d.w.rowlands [at] gmail.com

The Ceyer Lab at MIT, where I did my graduate research, studies chemistry in "ultra-high vacuum." For us, this means that the base pressures of our vacuum chambers are around 5*10^-11 torr (a couple of orders of magnitude better than Wikipedia thinks: we wouldn't count 5*10^-9 as UHV at all). This is roughly four orders of magnitude higher vacuum than what the International Space Station experiences, a talking point that I like to use when describing my research to people who are unlikely to have a sense of what a milltorr is.

I recently discovered that a lot of people are confused by the idea that there can be multiple "degrees of vacuumness," and are under the impression that outer space is a perfect vacuum. The following is a short explanation I wrote up for one such person, and am posting here in case other people find it useful or interesting.

Vacuum isn't a specific thing, it's just a word that means "very low pressure." If you pump more gas into a container (like a pressurized helium cylinder that you fill balloons from), there are more gas molecules present and they exert more pressure than air at ground level does. If you pump air out of a container---or climb up a mountain---there are less air molecules present and they exert less pressure. But as you go off into space, the number of gas molecules present keeps going down, but it never completely goes to zero. There are some gas molecules escaping from the sun (the "solar wind"), and gas molecules that just float freely in space. The pressure is really low, though: about 10^-9 times the pressure at ground level.

It turns out, however, that that's not nearly low enough for the experiments my lab does. We're firing very controlled beams of gas molecules at a crystal and then measuring what bounces off, and at what angles and speeds. So it's important that we know that the gas molecules we're firing have a really low probability of hitting anything other than the crystal during their trip. To do this, we have to keep the pressure in the chamber at 10^-13 times the pressure at ground level: ten thousand times lower than in space.

This is actually around as low as a pressure as you can make at room temperature, though you can make much lower pressures if you're willing to cool your equipment to very low temperatures. The reason for this is that all materials evaporate a little bit and produce a gas over their surface. At a given temperature, a molecule on the surface of the material has some probability of jumping off the surface and becoming a gas molecule. Gas molecules of the substance also have some probability of sticking to the surface when they hit it. So, if you combine these two probabilities, you find that there's a certain number of collisions (and thus a certain pressure of the gas form of the substance) that has to be present over the surface or else more molecules will come off than stick on and the number of gas molecules over the surface will increase.

This effect is most obvious in an everyday sense with water: it's why having a bowl of water out in your room will increase the humidity, or why it's a bit more humid near a swimming pool in the summer. However, everything has a "vapor pressure": the pressure of its gas form that it will produce over its surface. The lowest pressure you can get from a vacuum chamber is limited by the vapor pressure of whatever is inside it: if you try to get it lower than that, something will evaporate until the pressure has risen. This means that to get to very low vacuum we have to be very careful what materials we use. We can't have any plastic or rubber in our vacuum chamber, so electrical wires have to be insulated with blocks of ceramic. We can't have any grease or lubricants, so our steel screws are plated with gold to keep them from sticking on the steel holes they screw into. (Seriously! We have to use gold-plated screws!) And even stainless steel has a vapor pressure. At 10^-13 times atmospheric pressure, the biggest contaminant in our system is carbon that evaporates out of the surface of the special low-carbon alloy of steel that the walls of the chamber are made out of.