Total Pressure Measurements in Vacuum Technology (eBook)

A. Berman (Autor)

eBook Download: PDF | EPUB

2014 | 1. Auflage
412 Seiten
Elsevier Science (Verlag)
978-1-4832-7379-2 (ISBN)

Infos

Total Pressure Measurements in Vacuum Technology focuses on the measurement of low total pressure in hostile environments or in the presence of magnetic fields. This book emphasizes the general processes and problems involved in measurement techniques and physical principles on which vacuum gauges operate, rather than on the detailed description of the gauges. The design and techniques involved in the use of special instruments that determine 'pressure or gas density, such as pressure converters or radioactive gauges, are also described. This publication is mainly intended for graduate students and research scientists who have a good general background in physics and engineering.

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Total Pressure Measurements in Vacuum Technology focuses on the measurement of low total pressure in hostile environments or in the presence of magnetic fields. This book emphasizes the general processes and problems involved in measurement techniques and physical principles on which vacuum gauges operate, rather than on the detailed description of the gauges. The design and techniques involved in the use of special instruments that determine "e;pressure or gas density, such as pressure converters or radioactive gauges, are also described. This publication is mainly intended for graduate students and research scientists who have a good general background in physics and engineering.

Textauszug

CHAPTER 1

Units and Terminology in Vacuum Technology

Publisher Summary

This chapter discusses the units and terminology in vacuum technology. The root of the word vacuum is the Latin word vacuus, which means a space devoid of matter. Vacuum developed from the state of an art to a precise science in the past four decades when scientists throughout the world began the systematic study of the physics and chemistry of vacuum techniques. As a result of the evolution of vacuum science and technology, an increasing number of physical quantities had to be considered and measured and a correct terminology had to be established. The chapter discusses the presentation of units for pressure, throughput, and conductance to gas flow. It presents the nomenclature in vacuum technique and in standardization of measuring methods. The pressure units in use at present in vacuum science and technology fall into two categories. In the first, pressure units are grouped within coherent unit systems and the second contains units that do not belong to such systems.

1.1. Pressure Units in Different Systems

1.1.1. Introduction

1.1.2. Expressions for Pressure

1.1.3. Pressure in Coherent Systems of Units

1.1.4. Pressure Units Not Belonging to Coherent Systems

1.2. The Logarithmic Representation of Pressure

1.2.1. Pressure Scales

1.2.2. Pressure Scale Analogous to the Bel Concept

1.2.3. Pressure Scale Analogous to the Decibel Concept

1.2.4. Pressure Scale Analogous to the pH Concept

1.3. Throughput and Conductance in the SI System

1.4. Terminology and Standardization in Vacuum Technology

1.4.1. Terms Related to Specific Vacuum Applications and Graphic Symbols

1.4.2. Standardization of Interchangeable Parts, Measuring Methods, and Equipment Performance

References

1.1 Pressure Units in Different Systems

1.1.1 Introduction

The root of the word vacuum is the Latin word vacuus (pl. vacua), which means a space devoid of matter. Such a state, however, can never be practically attained in a laboratory, nor even in outer space, where there are a few hydrogen atoms per cubic centimeter at 10−14 Pa (10−16 Torr). In modern usage vacuum is considered to exist in an enclosed space when the pressure of the gaseous environment is lower than atmospheric pressure or has been reduced as much as necessary to prevent the influence of some gas on a process being carried out in that space.

Vacuum developed from the state of an art to a precise science in the past four decades when scientists throughout the world began the systematic study of the physics and chemistry of vacuum techniques. As a result of these studies, the physical phenomena which occur in vacuum could be explained and quantified by the kinetic theory of gases, the theory of gas flow through impedances, and the theory of elementary gas transport. These theories are treated in detail in many excellent reference books, and it is assumed that the reader is familiar with them.

As a result of the evolution of vacuum science and technology, an increasing number of physical quantities had to be considered and measured, and a correct terminology had to be established. Pressure, used to measure the degree of rarefaction (even though not correct for all situations) and to calculate gas throughput and conductance of ducts and orifices to gas flow, was being reported in no fewer than 15 units in 1945. The decision taken to use the pascal (newtons per square meter) belonging to the International System of Units (SI) as a unit for pressure seemed to make order in the chaos created by the use of so many units, but it was not until the past decade that countries all over the world started using the pascal or the bar (also belonging to SI).

This section is restricted primarily to a short presentation of units for pressure, throughput, and conductance to gas flow. Nomenclature in vacuum technique and in standardization of measuring methods and interchangeable parts is also presented.

1.1.2 Expressions for Pressure

The pressure units presently in use in vacuum science and technology fall into two categories. In the first, pressure units are grouped within coherent unit systems; the second contains units that do not belong to such systems.

Pressure P is defined as the force F which a gas or vapor exerts perpendicularly on an area A and is expressed by:

=F/A (1.1)

(1.1)

Pressure can also be written

=hρg (1.2)

(1.2)

This expression is obtained by applying the general theorem of the states of perfect liquids to the equilibrium of a liquid column under the conditions of Torricelli’s experiment. Here h is the height of the liquid column, ρ the density of the liquid in the column, and g the acceleration due to gravity at the location of the measurement.

The derived unit of pressure, expressed by Eq. (1.1), is in either of the forms

=[m][l]−1[x]−2 (1.3)

(1.3)

=[F][l]−2 (1.4)

(1.4)

depending on which of the base units length l, mass m, time l, or length l, force F of a coherent system of units have been used to express P.

The derived unit of pressure in Eq. (1.2) expressed in the same units as Eq. (1.3) does not belong to a coherent system.

Figure 1.1 illustrates the derived pressure units in coherent as well as other systems. Some of the pressure units shown, such as pascal, Torr, millimeters of mercury, and bar, are frequently used, others rarely, and some (vac, gaede) never. The vac was proposed to supersede the millimeter of mercury, which does not belong to a coherent system of units (Florescu, 1960, 1961). The unit, having a value 1 vac = 103 dyn/cm2, was not accepted, since it represents another name for the millibar which was in use at the time (Volet, 1960; Bigg, 1960, 1961). The gaede was intended to provide a smaller unit than the picotorr, supposed not to suffice for the measurement of pressures less than 10−12 Torr (Thomas et al., 1959).

Fig. 1.1 Various units of pressure P. * French literature; ** German literature.

The use of units other than pressure in order to characterize the degree of gas rarefaction in the ranges of high and ultrahigh vacuum has been proposed. Worell (1963), Chutbert (1964), and Chuan (1965) proposed the measurement of the molecular density of a gas, and Bobenrieth (1959) suggested the measurement of the “main” spacing between gas molecules. This spacing varies as the third root of the inverse of the number density of gas molecules.

1.1.3 Pressure in Coherent Systems of Units

A coherent system of units is one in which any quantities other than base units are derived by simple multiplication or division, without the use of a numerical factor. The base units of such a system, presumably suited to practical requirements, have to be adaptable to all scientific disciplines. Thus, for instance, in the CGS system arbitrary constants of proportionality between derived and base units have been eliminated, except for electromagnetic and electrostatic quantities. The magnitudes of the units are not practical for all cases. This drawback of the CGS system is eliminated in the Systeme Internationale (SI) units adopted in 1954 by the Conference Generale des Poids et Mesures (CGPM). The SI system, based on the MKSA system, is obviously coherent, but the pressure unit pascal is not matched to practical requirements. Therefore the use of a unit outside the SI, the bar, with submultiples millibar and microbar, is permitted. The bar is recognized by the Comite International des Poids et Mesures (CIPM) as having to be retained either because of its importance in metrology or because of its use in other specialized fields (BS 5555, 1976). Different expressions for pressure in different coherent systems of units are given in Table 1.1. The graph in Fig. 1.2 permits the conversion of torr and millibar to pascals in the range 1.013 × 105 Pa (1 atm) to 10−16 Pa.

Table 1.1

PRESSURE IN COHERENT SYSTEMS OF UNITS

SI...

Produkt-Details

• Erscheint lt. Verlag: 28.6.2014
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Mechanik
• Themenwelt: Technik ► Bauwesen
• ISBN-10: 1-4832-7379-2 / 1483273792
• ISBN-13: 978-1-4832-7379-2 / 9781483273792