this post was submitted on 27 Jul 2025
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Aluminum foil is very common in physics labs. And a main use for it is "baking"! To get ultra high vacuum (UHV)* you generally need to "bake out" your chamber while you pump down. Foil is used same as with baking food
keep the heat in and evenly distributed on the chamber.
Sadly, it's usually not food grade aluminum foil, as that can contain oils, and oils and vacuum are generally a big no-no.
*Just how good is UHV? Roughly: I live in San Francisco, which is ~7 miles by ~7 miles (~11km). Imagine you raise that by another 7 miles to make a cube. Now, evacuate every last molecule of gas out of it. Now take a family sedan's trunk, fill it with 1 atmosphere of gas, and release that into the 7 mile cube. That's roughly UHV pressure.
How can a metal contain oil?
Aluminum foil that's food grade will have coatings on it to assist with food release. I assume that's part of why one side is more shiny than the other.
I have always understood the rough side to simply be an outcome of the rolling process.
If I'm wrong I'd love to know!
edit: I looked it up, yeah that's the reason one side is shinier... the rolling process, nothing else
Ah, it's coated.
It'd be coated, but it's from processing, cold rolling metal generates a lot of heat, especially going that thin (thinnest I was around often was ~0.2mm), we'd often temper the material after processing, mainly for surface finish, mill rolls would be sprayed with lubricating coolant really close to what you'd see in use on a milling machine. This was with steel but same principle applies, pretty sure the lubricant we used is also labeled for use on aluminum mills, but you'd use food safe stuff for kitchen foil.
Thanks for the analogy, that really helps to put it in perspective. I was trying to work out the number of molecules per metre that would leave you with, but either my sense of scales is off kilter or I've got it wrong.
From what I can find, there are approximately 2.5e25 molecules per m^3^ at 1atm. Given an 11km cube has a volume of 1.3e12 m^3^, that gives around 2e13 molecules per m^3^ per m^3^ released. That sounds high, have I got the figures wrong somewhere?
Your numbers seem reasonable
more intuitive for me to work in terms of pressure. Atmosphere is (roughly) 1e3 Torr, good UHV can be around 1e-10, so that's 13 orders of magnitude, which is (roughly) the same difference that you calculated.