Published: 19 December, 2007

The ins and out of man-made and natural vacuums

What is the lowest recorded manmade vacuum? What is the lowest recorded natural vacuum? With known physical restraints of the universe, what is the lowest vacuum pressure in theory? Is is possible for low pressures to break...chemical, atomic, particle bonds/interactions? If so, can you give examples at each level?

In scientific/engineering terms, a vacuum is referred to as a space in which gaseous pressure is less than standard atmospheric pressure on the Earth's surface. A commonly used unit in the United States is torr, or an equivalent pressure exerted by a 1-mm tall mercury cylinder. The standard atmospheric pressure is about 760 torr. One can also measure the level of a vacuum in a space by the gaseous density, which is about 3x10¹⁹ atoms per cubic centimeter (cm^-3) at standard atmospheric pressure.

Technically, vacuum levels are broadly categorized as:

Rough vacuum: 760 to 1 torr

Low Vacuum: 1 torr to 10^-3 torr

Medium vacuum: 10^-3 to 10^-6 torr

High Vacuum: 10^-6 to 10^-9 torr

Ultra-high vacuum (UHV): 10^-9 to 10^-12 torr

Extreme-High vacuum (XHV): <10^-12 torr

Vacuum pumps are used to create vacuums in closed spaces by removing the gaseous molecules. Pressures in the evacuated enclosures can be measured by various types of vacuum gauges. It is routine to generate rough to high vacuums by using off-the-shelf equipment. To achieve UHV and XHV, however, is everything but routine. Only certain types of clean vacuum pumps are suitable in these pressure ranges. Not only does one have to ensure that the vacuum enclosure is absolutely leak-free, but it is also vital to have everything in the interior of the vacuum enclosure clean. Any contaminants, such as machining oil residual or grease from a finger print, on an in-vacuum part will compromise its ability to reach UHV.

The ultimate pressure in a 'perfect' vacuum enclosure is often limited by gaseous molecules (such as hydrogen, carbon monoxide, methane) slowly releasing from surfaces, often referred to as 'outgassing'. This is why a finger print on the in-vacuum surface will compromise the ability to reach UHV; the grease from the finger print will slowly, but continuously outgas. One source of outgassing comes from molecules (such as water) that get 'stuck' on the inner surfaces of a vacuum chamber and are exposed to the environment. Another type of outgassing occurs when molecules, such as hydrogen, are 'trapped' in the material. In many research fields (such as surface sciences, particle accelerators, nano-technologies, etc) where UHV and XHV environment is essential, vacuum scientists and engineers rely on specialized material and surface treatments (such as electro-polishing, coating, in-vacuum high temperature baking, etc.) to reduce the outgassing.

The ironic fact about obtaining extremely low pressure (i.e. extremely high vacuum) is that it is very difficult, if not possible, to measure a pressure lower than 10^-13 torr without disturbing the vacuum condition.

To date, the best man-made vacuum on the Earth surface is reported to be <1x10^-13 torr (that is, less that 1000 molecules/cm³), achieved by the Vacuum Scientists at CERN, a European Particle Accelerator Facilities, measured by a custom-made gauge. To achieve the reported XHV, the materials (a type of stainless steel) used for the vacuum chamber were treated by vacuum melting to reduce the hydrogen contents of the chamber, thus minimizing the amount of hydrogen outgassing. In addition, the interior surfaces were then coated with a thin (1~2 micro-meter in thickness) film called non-evaporable getters (NEGs). The NEG thin film, when activated by heating under vacuum, improves vacuum in two ways. First, it forms a barrier which blocks outgassing from the substrate. Furthermore, it serves as a very effective vacuum pump. The activated NEG thin film is so chemically reactive that molecules land on its surface will be permanently bonded (thus removed from the vacuum space) in a way similar to fly-paper catching insects.

Nearly 'perfect' vacuum naturally exists in the out space, due to extremely low gravity. It is estimated the gaseous density between stars in the Milky Way to be ~0.1 to 1 atom/cm³ (10^-17 to 10^-16 torr). For intergalactic voids, the density drop further to ~0.001 atom/cm³ (10^-19 torr!), the lowest density (or highest vacuum) ever measured.