In physics, a vacuum is the absence of matter in a volume of space. A perfect vacuum is an ideal state that cannot really exist, but is best approximated by outer space. Physicists use the term partial vacuum to describe real-life non-ideal vacuum. A complete characterization of the physical state would require further parameters, such as temperature. The antithesis of a vacuum, which is also an ideal unachievable state, is called a plenum.
In engineering, a vacuum is any region where the gas pressure is less than atmospheric pressure. Engineers measure the degree of vacuum in units of pressure. The SI unit of pressure is the Pascal (abbreviation Pa), but vacuum is usually measured using the Torr, which equals 133.3223684 Pascals. It is often also measured using the barometer scale, or as a percentage of atmospheric pressure using the bar. For commercial purposes, vacuum is often measured in inches of Mercury(Hg). This means that the pressure in vacuum, when specified in inches of Hg, is equal to the specified inches of Hg subtracted from 29.92. Thus a vacuum of 26 inches of Hg is equivalent to a pressure of (29.92 - 26) or 3.92 inches of Hg. Here, 29.92 inches of Hg means perfect vacuum.
Degrees of Vacuum
Atmospheric pressure = 101.3 kPa (760 Torr)
Mechanical vacuum pump = approximately 100 Pa to 100 μPa (1 Torr to 10−6 Torr)
As gas pressure decreases, the mean free path (MFP) of the gas molecules increases. When the MFP is greater than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics.
In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure. Astrophysicists prefer to use density to describe these environments, in units of particles per cubic metre.
Creating a vacuum
The easiest way to create an artificial vacuum is to expand the volume of a container. For example, your muscles expand your lungs to create a partial vacuum inside them, and air rushes in to fill the vacuum. By repeatedly closing off a compartment of the vacuum and exhausting it, it is possible to pump air out of a chamber of fixed size in a manner analogous to pumping a milkshake out of a glass. This is the principle behind most mechanical vacuum pumps. Inside the pump, a mechanism expands a small sealed cavity to create a deep vacuum. Because of the pressure differential, some air from the chamber is pushed into the pump's small cavity. The pump's cavity is then sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.
A mechanical vacuum pump moves the same volume of gas with each cycle, but as the chamber's pressure drops, this volume contains less and less mass. So although the pumping speed remains constant when measured in litres/second, it drops exponentially when measured in kilograms/second. Meanwhile, the leakage rates, evaporation rates, and sublimation rates produce a constant mass flow into the system. When the pump's mass flow drops to the same level as the mass flows into the chamber, the system asymptotically approaches a constant pressure called the base pressure. Evaporation and sublimation into a vacuum is called outgassing, and the most common source is water absorbed by materials in the chamber. Outgassing can be reduced by desiccation prior to vacuum pumping. The base pressure of a rubber- and plastic-sealed piston pump system is typically 1 to 50 kPa, while a scroll pump might reach 10 Pa and a rotary vane oil pump with a clean and empty metallic chamber can easily achieve 0.1 Pa.
If the dominant mass flow into the vacuum system is chamber leakage or outgassing of materials under vacuum, then the vacuum can be improved simply by installing bigger pumps. However, there is a point where backstream leakage through the pump and outgassing of the pump oils become the dominant mass flows into the chamber. In this situation, the vacuum will approach the pump's ultimate pressure - the best vacuum that this type of pump can achieve under ideal conditions. Adding more pumps in parallel or bigger pumps of the same type can still improve the pump-down speed, but they will not reduce the base pressure below ultimate. Better pumping technologies must be used to go beyond this barrier.
High vacuum
Fortunately, once the pressure has dropped below 1 kPa or so, another vacuum pumping technique becomes possible. Matter flows differently at different pressures based on the laws of fluid dynamics. At atmospheric pressure and mild vacuums, molecules interact with each other and push on their neighboring molecules in what is known as viscous flow. When the distance between the molecules increases, the molecules interact with the walls of the chamber more often than the other molecules, and molecular pumping becomes more effective than compression pumping. This regime is generally called high vacuum.
Molecular pumps sweep out a larger area than mechanical pumps, and do so more frequently, making them capable of much higher pumping speeds as measured in volume per time. They do this at the expense of the seal between the vacuum and their exhaust. Since there is no seal, a small pressure at the exhaust can easily force flow backstream through the pump; this is called stall. In high vacuum, however, pressure gradients have little effect on fluid flows, and molecular pumps can attain their full potential.
The two main types of molecular pumps are the diffusion pump and the turbomolecular pump. Both types of pumps blow out gas molecules that diffuse into the pump. Diffusion pumps blow out molecules with jets of oil, while turbomolecular pumps use high speed fans. Both of these pumps will stall and fail to pump if exhausted directly to atmospheric pressure, so they must be exhausted to a lower grade vacuum created by a mechanical pump.
As with mechanical pumps, the base pressure will be reached when leakage, outgassing, and backstreaming equal the pump speed, but now minimizing leakage and outgassing to a level comparable to backstreaming becomes much more difficult. High vacuum systems generally require metal chambers with O-ring seals such as Klein flanges or ISO flanges. The system must be clean and free of organic matter to minimize outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. As a result, many materials that work well in low vacuums, such as epoxy, will become a problematic source of outgassing when attempting to achieve high vacuums.
With these standard precautions, vacuums of 1 mPa are easily achieved with off-the-shelf molecular pumps. With careful design and operation, 1μPa is possible.
Source: Wikipedia - en.wikipedia.org
