Why Vacuum Engineers Are “Glass Half-Full” Kinda’ Guys
Recently, a customer remarked to us that he had pumped down his chamber to a “perfect vacuum” and we were puzzled as to exactly what he meant by this. He explained that he had pumped the chamber down a full 30″ of mercury, and thus, had a “perfect” vacuum. While we finally figured out what he meant, in our experience, a “perfect vacuum” comes along about as often as seeing the Easter Bunny ride up on a unicorn’s back. Hamlet’s comment to Horatio seems particularly apropos, here. “There are more things in heaven and earth, Horatio, than are dreamt of in our philosophy.” When we pump down a vacuum chamber we do remove particles of atmosphere or air that are within the chamber, but it might be educational to understand exactly what is going on at the molecular level when the pumping is done.
Measuring pressure is an indirect method of measuring the number of molecules that are in an enclosed area. Each molecule in your container is zipping around at an average of 900 miles an hour (at standard temperature and pressure) bouncing off of each other and off of the sides of the container. The sum of all of these bounces off of the sides of the container is measured as pressure and can be used to estimate how many molecules are still inside. At LACO we use the measurement of Torr to describe pressure. Our normal atmosphere at sea level is, as most of us know, 14.7 psia or 1 bar or 29.92 inches of mercury or 760 torr. We divide our vacuum chambers into two groups: those that are designed for “Industrial Vacuum” and those that are designed for “High Vacuum”. Industrial vacuum levels are between 2 torr and 1x 10^-3 or .001 torr. High vacuum chambers and systems are designed to be operated at 10^-3 torr to 10^-6 torr or .000001 torr. A third type of vacuum is “Ultra High Vacuum” which can range all the way down to 10^-12 torr, very low pressure indeed. Even deep space is estimated to have some matter in it which would be measured at 10^-15 torr. These, though are very tiny pressures, you might wonder how much matter is left in a chamber that has been pumped down to 10^-12 torr? Couldn’t be much, could it? Before you read on, take a guess how many molecules of air would be left in a Rubik’s cube sized chamber (2.2″ per side or 10.648 cubic inches or 174.49 cm^3).
To begin with, assuming the test is done at sea level at 20 degrees C, air has a density of 2.65 x 10^19 per cubic centimeter which means that our Rubik’s chamber starts out with 4.62 x 10^21 molecules of air inside of it. That is 4.62 sextillion air molecules! For comparison that is roughly about the number of stars in the known universe. Once we have pumped down our Rubik’s chamber as low as practically possible, say 10^-12 torr, there will still be some air molecules left. At that pressure, the density of air is 2.65 x 10^4 times the volume of our chamber, so even at the lowest practical pressure we can achieve, there will still be 4.62 million air molecules left in our chamber. And that’s why vacuum engineers are “glass half-full” kinda guys. Even when it appears half or completely empty, there are still plenty of air molecules in any glass.
Now I don’t know about you, but my eyes glaze over once I pass a bazillion, so it’s kind of hard for me to envision the difference between a sextillion and a million. I know its huge, but 10^15 just doesn’t speak to me. Let’s imagine that instead of pressure, we are comparing weight or, more precisely, mass. A comparable difference in mass can be demonstrated with carbon aerogel (which is not a new hair product) the lightest substance known to man, currently. A cubic centimeter sized bunch of this stuff weighs .16 milligrams or 1/4 the weight of a human hair. This represents the number of molecules left after pumping the chamber down. Something that is 10^15 times heavier than this is a cruise ship. The Royal Caribean’s “Freedom of the Seas” cruise ship displaces 160,000 tons and is 10^15 times heavier than a cc of carbon aerogel. So you can see that today’s vacuum systems do a remarkable job of removing a lot of air from a chamber, but even a “perfect” vacuum leaves a little to be desired.