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Vacuum Technology and Space Simulation

Engineering Fluids Design

Vacuum Technology and Space Simulation
NASA
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Vaccum Technology and Space Simulation

Preface

The first steps into any new venture generally require a guide to insure that the correct path is being followed. Thus, an engineer assuming new responsibilities in activities concerned with vacuum technology and space simulation will need aid in becoming acclimatized to the language, terminology, and units peculiar to vacuum. It is the intent of chapter 1 to serve as a map to the charted courses of vacuum technology. Terms and units defined in this chapter will be elaborated upon in later chapters. All symbols used in the text, their descriptions and units have been alphabetically listed at the conclusion
cf this chapter.

Since popular usage has established atmospheric pressure as the upper limit of vacuum, a vacuum may be regarded as any pressure less than standard atmospheric pressure. When scientists first became interested in vacuum-level measurements, a value of 28 or 29 mm Hg below atmospheric pressure was considered a respectable vacuum. As research improved vacuum levels, it became necessary to convert to an absolute scale to give meaning to engineering equations, in much the same manner as an absolute scale is used in temperature calculations. Under the traditional measurement system, normal atmospheric pressure is expressed in millimeters of mercury; that is, 760 mm Hg equals atmospheric pressure.

In recent years the traditional pressure units have given way to the torr which is defined as 1/760 of a standard atmosphere, and is now the measure of vacuum level accepted by those working with vacuum. It should be noted, however, that the standard atmosphere is not quite 760 mm Hg. There is a discrepancy of approximately 1 part in 7 million between mm Hg and the torr; but, for practical considerations, the terms "torr" and "mm Hg" may be interchanged. The following table lists the conversion factors between the torr and other pressure units popularly used today:

TOC

1 VACUUM NOMENCLATURE ........................... 1
Molecular Fundamentals ............................ 2
Gas Flow ......................................... 6
Directional Effects .................................. 7
Glossary .......................................... 8
Symbols .......................................... 15
2 PRESSURE ............................................ 19
Definitions of Pressure .............................. 19
Kinetic Theory of Gases ............................. 23
Molecular Kinetics .................................. 28
Application to Space Simulation ..................... 33
3 PRESSURE MEASUREMENT 41
Types of Gages ..................................... 41
Ionization-Gage Characteristics ...................... 52
Vacuum-Gage Calibration .......................... 55
Space-Simulation Gages ............................. 63
4 PUMPING SPEED ...................................... 71
Steady-State Case .................................. 71
Transient Case, Pumpdown, and Repressurization ...... 77
5 GAS FLOW IN COMPONENTS AND SYSTEMS .......... 83
Molecular Flow Through an Orifice .................. 84
Molecular Flow Through a Tube of Circular
Cross Section .................................... 85
Laminar Viscous Flow in a Long Circular Tube ........ 88
Flow Characteristics of Complex Shapes--Molecular
Flow ............................................ 94
Flow Characteristics of Complex Shapes---Transition
Flow ............................................ 105
Flow Characteristics of Complex Shapes--Laminar
Viscous Flow ................. .:.................. 105
Flow Characteristics of Complex Shapes--Choked or
Critical Flow ..................................... 106
Flow of Gas Through Cylindrical Tubes ............... 108
Traps and Baffles .................................. 116
6 PUMPING SYSTEMS ................................... 123
Rotary Oil-Sealed Pumps ........................... 124
Positive Displacement Blowers ....................... 134
Diffusion Pumps ................................... 135
Sputter Ion Pumps .................................. 142
Sorption Pumping .................................. 143
7 CRYOGENIC PUMPING ................................ 151
Perfect Cryogenic Pumping Surfaces ................. 151
Extended Cryogenic Pumping Surfaces .............. 154
Liquid Nitrogen Traps .............................. 166
8 GAS LOAD ........................................... 173
Definition of QL,Gas Load on Systems ................ 173
Sources of the Gas Load ............................ 175
9 OUTGASSING OF MATERIALS......................... 197
Metals ............................................ 197
Cleaning Chemicals ................................ 208
High-Temperature Degas ............................ 210
Low Temperature .................................. 213
Exposure .......................................... 213
Polymers .......................................... 213
10 SYSTEMS ............................................. 223
Space Simulation .................................. 223
Types of VacuUm Systems .......................... 229
High-Vacuum Systems--General Considerations ....... 234
System Design ..................................... 248
Troubleshooting Diffusion-Pumped Systems ........... 267
11 LEAKDETECTION ..................................... 275
Gas Flow Through Leaks ........................... 277
Basic Equations of Steady-State Leak Detection ........ 279
Basic Equations of Transient Leak Detection ........... 283
Other Aspects of Leak Detection ...................... 288
Vacuum-Gage Methods OfLeak Detection ............. 291
Leak-Detection Techniques for Systems ............... 300

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