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Vacuum Nomenclature and Definitions
CHAPTER 1.1
Vacuum Nomenclature and Definitions I.U BASIC DEFINITION The term vacuum is generally used to denote a volume or region of space in which the pressure is significantly less than 760 torr. In the traditional measurement system, normal pressure is expressed in millimeters of a column of mercury, and 760 millimeters of mercury is equal to 1 standard atmosphere. The traditional unit of pressure is the torr, which approximately equals 1 millimeter of mercury. A perfect or absolute vacuum, which implies a space that is entirely devoid of matter, is practically unrealizable. For practical purposes, however, and in accordance with the definition proposed by the American Vacuum Society, the term vacuum is generally used to denote a space filled with a gas at less than atmospheric pressure [1]. In the metric, or meter-kilogram-second (MKS), system, the unit of pressure is the pascal. In general, however, the torr still remains one of the most widely used units of pressure. Table 1 lists conversion factors between some of the most generally used vacuum units.
1.1.2 PRESSURE REGIONS OF VACUUM Measuring a system's pressure is the traditional way to classify the degree of vacuum. Nowadays, the general term vacuum refers to a region that consists of about ISBN 0-12-325065-7 525.00
Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
Chapter 1.1: Vacuum Nomenclature and Definitions Table I Conversion Factors for Pressure Units
Unit 1 torr (0°C) 1 pascal (newton ^^IICWIUU m~2) 111 ) ldynecm-2 dyne Ibar 1 atmosphere (standard) 1 pound pound (force) inch-2
Bar
Atmosphere (standard)
1.333 XIO^
1.333 X 10"^
1.3158 X 10"^
1
10
1.0 X 10-5
u.i 0.1
1
1.0 X 10^
9 8692 X 10-^ 9.8692 X 10"^ 0.98692
1.0133
11
6.8948 X lO'^
6.8047 X lO'^
Torr
Pascal
1
1.333X102
7.5006 X 10"^ /.. 7.5006 X 10"^ -7.5006 X 10^
1.0 X 10^
1.0 X 10^
760
1.0133 X 10^
1.0133X10^
6.8948 X 10^
6.8948 X 10^
5.1715 X 10'
Dyne cm
19 orders of magnitude of pressure below 1 atmosphere. For convenience, this extended pressure range is generally divided into several regions that denote the degree of vacuum. This division of the pressure scale below atmosphere is somewhat arbitrary and is a convenient method of denoting the different physical phenomena that occur within the pressure ranges specified for each category. Many industrial applications of vacuum can be also be classified using these categories. Table 2 shows the accepted categories and the corresponding pressure ranges. This table also lists the type of pump generally used to achieve a specified pressure range, as well the typical vacuum gauge used for measurement. To discuss the different physical phenomena associated with the various vacuum categories that are indicated in Table 2, it is useful to introduce other concepts and properties that characterize the degree of vacuum, such as molecular density, mean free path, and time to form a monolayer. These terms are defined as follows: Molecular density
Average number of molecules per unit volume.
Mean free path
Average distance a molecule travels in a gas between two successive collisions with other molecules of the gas.
Time to form a monolayer
Time required for freshly cleaved surface to be covered by a layer of gas of one molecule thickness. This time is given by the ratio between the number of molecules required to form a compact monolayer (about 8 X 10 ^"^ molecules/ cm^) and the molecular incidence rate.
1.1.2 Pressure Regions of Vacuum Table 2 Applications of Vacuum Techniques Physical Situation
Objective
Applications
Low pressure
Achieve pressure difference.
Low molecular density
Remove active atmospheric constituents.
Holding, lifting, transport pneumatic, cleaners,filtering,forming Lamps (incandescent, fluorescent, electric discharge tubes), melting, sintering, packaging, encapsulation, leak detection Drying, dehydration, concentration, freeze-drying, degassing, lyophylization, impregnation Thermal insulation, electrical insulation, vacuum microbalance, space simulation Electron tubes, cathode ray tubes, television tubes, photocells, photomultiplier. X-ray tubes, accelerators, storage rings, mass spectrometers, isotope separators, electron microscopes, electron beam welding, heating, coating (evaporation, sputtering), molecular distillation Fraction, adhesion, emission studies, materials testing for space
Remove occluded or dissolved gas. Decrease energy transfer.
Large mean free path
Avoid collision.
Long monolayer formation time
Clean surfaces.
Figure 1 shows the relationships among these various quantities as a function of pressure. Using the preceding definitions, it is possible to describe the different physical situations that characterize the different vacuum categories.
1.1.2.1 Low and Medium Vacuum In the low-vacuum and medium-vacuum range, the number of molecules in a vacuum vessel in the gas phase are large compared to those covering the surface of the vessel. Thus the pumping of the space serves to remove molecules from the gas phase. This vacuum range extends from atmosphere to about 10 "^ torr. Many industrial processes that need to outgas or dry materials and components use this region.
Chapter 1.1: Vacuum Nomenclature and Definitions Rg.l.
V/Vp Maxwell-Boltzmann molecular velocity distribution curve. I dn
2. v„ 3. V,mean
A ( m \3/2
/
,„v2\
lYT
"average
7= J
/8kT
\ irm
4. Root mean square Vr^s - J
= 1.13v„
3kr
= 1-225 v,
5. Mean energy e = — kT
1.1.2.2 High Vacuum The high-vacuum region corresponds to a state where the gas molecules are located mainly on the surfaces of the enclosure and the mean free path equals or exceeds the dimensions of the vacuum vessel. The particles travel in the vacuum enclosure without colliding with other molecules. Thus, in this vacuum region, under these conditions, the pumping consists of evacuating or capturing mole-
1.1.2
Pressure Regions of Vacuum
7
cules. The molecules leave the surface and individually reach the pump. This region is extensively used in the preparation and application of vacuum coatings, surface treatment, and modification. This region extends from 10 ~'^ to 10 ~^ torr.
1.1.2.3 Ultra-High Vacuum (UHV) Under ultra-high-vacuum (UHV) conditions, time to form a monolayer is equal to or longer than the usual time for most laboratory measurements. Thus clean surfaces can be prepared and their properties determined before an adsorbed gas layer is formed. This vacuum range extends from about 10"^ to 10"^^ torr. An indication of the extensive application of vacuum technology in many key industrial processes in a diverse range of industries is illustrated in Table 2, where common vacuum industrial processes are classified according to the degree of vacuum used.
Vacuum Nomenclature and Definitions I.U BASIC DEFINITION The term vacuum is generally used to denote a volume or region of space in which the pressure is significantly less than 760 torr. In the traditional measurement system, normal pressure is expressed in millimeters of a column of mercury, and 760 millimeters of mercury is equal to 1 standard atmosphere. The traditional unit of pressure is the torr, which approximately equals 1 millimeter of mercury. A perfect or absolute vacuum, which implies a space that is entirely devoid of matter, is practically unrealizable. For practical purposes, however, and in accordance with the definition proposed by the American Vacuum Society, the term vacuum is generally used to denote a space filled with a gas at less than atmospheric pressure [1]. In the metric, or meter-kilogram-second (MKS), system, the unit of pressure is the pascal. In general, however, the torr still remains one of the most widely used units of pressure. Table 1 lists conversion factors between some of the most generally used vacuum units.
1.1.2 PRESSURE REGIONS OF VACUUM Measuring a system's pressure is the traditional way to classify the degree of vacuum. Nowadays, the general term vacuum refers to a region that consists of about ISBN 0-12-325065-7 525.00
Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
Chapter 1.1: Vacuum Nomenclature and Definitions Table I Conversion Factors for Pressure Units
Unit 1 torr (0°C) 1 pascal (newton ^^IICWIUU m~2) 111 ) ldynecm-2 dyne Ibar 1 atmosphere (standard) 1 pound pound (force) inch-2
Bar
Atmosphere (standard)
1.333 XIO^
1.333 X 10"^
1.3158 X 10"^
1
10
1.0 X 10-5
u.i 0.1
1
1.0 X 10^
9 8692 X 10-^ 9.8692 X 10"^ 0.98692
1.0133
11
6.8948 X lO'^
6.8047 X lO'^
Torr
Pascal
1
1.333X102
7.5006 X 10"^ /.. 7.5006 X 10"^ -7.5006 X 10^
1.0 X 10^
1.0 X 10^
760
1.0133 X 10^
1.0133X10^
6.8948 X 10^
6.8948 X 10^
5.1715 X 10'
Dyne cm
19 orders of magnitude of pressure below 1 atmosphere. For convenience, this extended pressure range is generally divided into several regions that denote the degree of vacuum. This division of the pressure scale below atmosphere is somewhat arbitrary and is a convenient method of denoting the different physical phenomena that occur within the pressure ranges specified for each category. Many industrial applications of vacuum can be also be classified using these categories. Table 2 shows the accepted categories and the corresponding pressure ranges. This table also lists the type of pump generally used to achieve a specified pressure range, as well the typical vacuum gauge used for measurement. To discuss the different physical phenomena associated with the various vacuum categories that are indicated in Table 2, it is useful to introduce other concepts and properties that characterize the degree of vacuum, such as molecular density, mean free path, and time to form a monolayer. These terms are defined as follows: Molecular density
Average number of molecules per unit volume.
Mean free path
Average distance a molecule travels in a gas between two successive collisions with other molecules of the gas.
Time to form a monolayer
Time required for freshly cleaved surface to be covered by a layer of gas of one molecule thickness. This time is given by the ratio between the number of molecules required to form a compact monolayer (about 8 X 10 ^"^ molecules/ cm^) and the molecular incidence rate.
1.1.2 Pressure Regions of Vacuum Table 2 Applications of Vacuum Techniques Physical Situation
Objective
Applications
Low pressure
Achieve pressure difference.
Low molecular density
Remove active atmospheric constituents.
Holding, lifting, transport pneumatic, cleaners,filtering,forming Lamps (incandescent, fluorescent, electric discharge tubes), melting, sintering, packaging, encapsulation, leak detection Drying, dehydration, concentration, freeze-drying, degassing, lyophylization, impregnation Thermal insulation, electrical insulation, vacuum microbalance, space simulation Electron tubes, cathode ray tubes, television tubes, photocells, photomultiplier. X-ray tubes, accelerators, storage rings, mass spectrometers, isotope separators, electron microscopes, electron beam welding, heating, coating (evaporation, sputtering), molecular distillation Fraction, adhesion, emission studies, materials testing for space
Remove occluded or dissolved gas. Decrease energy transfer.
Large mean free path
Avoid collision.
Long monolayer formation time
Clean surfaces.
Figure 1 shows the relationships among these various quantities as a function of pressure. Using the preceding definitions, it is possible to describe the different physical situations that characterize the different vacuum categories.
1.1.2.1 Low and Medium Vacuum In the low-vacuum and medium-vacuum range, the number of molecules in a vacuum vessel in the gas phase are large compared to those covering the surface of the vessel. Thus the pumping of the space serves to remove molecules from the gas phase. This vacuum range extends from atmosphere to about 10 "^ torr. Many industrial processes that need to outgas or dry materials and components use this region.
Chapter 1.1: Vacuum Nomenclature and Definitions Rg.l.
V/Vp Maxwell-Boltzmann molecular velocity distribution curve. I dn
2. v„ 3. V,mean
A ( m \3/2
/
,„v2\
lYT
"average
7= J
/8kT
\ irm
4. Root mean square Vr^s - J
= 1.13v„
3kr
= 1-225 v,
5. Mean energy e = — kT
1.1.2.2 High Vacuum The high-vacuum region corresponds to a state where the gas molecules are located mainly on the surfaces of the enclosure and the mean free path equals or exceeds the dimensions of the vacuum vessel. The particles travel in the vacuum enclosure without colliding with other molecules. Thus, in this vacuum region, under these conditions, the pumping consists of evacuating or capturing mole-
1.1.2
Pressure Regions of Vacuum
7
cules. The molecules leave the surface and individually reach the pump. This region is extensively used in the preparation and application of vacuum coatings, surface treatment, and modification. This region extends from 10 ~'^ to 10 ~^ torr.
1.1.2.3 Ultra-High Vacuum (UHV) Under ultra-high-vacuum (UHV) conditions, time to form a monolayer is equal to or longer than the usual time for most laboratory measurements. Thus clean surfaces can be prepared and their properties determined before an adsorbed gas layer is formed. This vacuum range extends from about 10"^ to 10"^^ torr. An indication of the extensive application of vacuum technology in many key industrial processes in a diverse range of industries is illustrated in Table 2, where common vacuum industrial processes are classified according to the degree of vacuum used.