|"Vacuum Pumps." McGraw Hill Encyclopedia of Science and Technology. vol.19. 1992: 128.||"A vacuum as high as 10−15 mm Hg has been reached."||1.33 × 10−13 Pa|
|Brunner, W.F., Jr., & T.H. Batzer. Practical Vacuum Techniques. Reinhold Publishing, 1965: 124.||"Ultrahigh vacuum (10−9 torr and below) can also be produced"||1.33 × 10−7 Pa|
|Guthrie, Andrew. Vacuum Technology. New York: Wiley, 1963: 454.||"At the present time various experimenters have claimed pressures in the 10−13 to 10−14 torr region."||1.33 × 10−12 Pa|
|KNF Neuberger Laboport Vacuum Pumps. KNF Neuberger. Trenton, New Jersey.||"Model No.: N842.3FTP
Vacuum: 1.5 Torr
Pressure: 15 psig"
|1.99 × 102 Pa|
|High Vacuum Pumps & Accessories. Girovac (trading as Bewhay Vacuum & Technical Ltd.). High Canons, Well End, Boreham Wood, Hertfordshire.||"Rotary vane pumps … 1 × 10−4 mbar"||9.99 × 10−3 Pa|
|Vacuum Furnaces Pumps Ovens. HK Equipment, Inc. 235 Andover Street, Wilmington, Massachusettes.||"Pressures range from 10−9 Torr"||1.33 × 10−7 Pa|
Through intense research and investigation, the lowest attainable pressure in a laboratory vacuum was found to be 10−13 Pa. Vacuums are broken up into five classes: low, medium, high, very high, and ultrahigh, depending on the region of pressure attainable. After consulting several companies on the internet, I discovered that many vacuum pumps currently produced for laboratory research have pressures ranging anywhere from 1 × 102 Pa to 1 × 10−7 Pa.
There is no theoretical lower pressure limit, save total vacuum itself. However, there are several practical barriers that make extremely low pressures very difficult to reach. Some of these include: leaks in vacuum walls, release of gases and vapors from surface inside the system, inability of pumps to remove gases at very low pressures, ability to measure very low pressures, and permeability of the vacuum wall materials.
Very low pressures are useful mostly in the area of scientific research. In studying plasmas, low pressures are needed to keep the impurity gases to a minimum. In semiconductor research, waste gases are also in need of removing. Also, another major area of research in which low pressures are necessary is space research.
Robert Saronson -- 1998
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