Fundamentals of Vacuum Technology (1248463), страница 4
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. . . . . . . . . . . . . .164Fig. 9.10 Determination of pump-down time in the medium vacuumrange taking into account the evolution of gasfrom the walls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .165Fig.9.11 Saturation vapor pressure of various substances . . .
. . . . . . .166Fig. 9.12 Saturation vapor pressure of pump fluids for oil andmercury fluid entrainment pumps . . . . . . . . . . . . . . . . . . . . .166Fig. 9.13 Saturation vapor pressure of major metals used invacuum technology . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . .166Fig. 9.14 Vapor pressure of nonmetallic sealing materials(the vapor pressure curve for fluoro rubber lies betweenthe curves for silicone rubber and Teflon). . . . . . . . . . . . . . . .167Fig. 9.15 Saturation vapor pressure ps of various substancesrelevant for cryogenic technology in atemperaturerange of T = 2 Ð 80 K. . . . . . . . . .
. . . . . . . . . . .167Fig. 9.16 Common working ranges of vacuum pumps . . . . . . . . . . . . .167Fig. 9.16a Measurement ranges of common vacuum gauges . . . . . . . .168Fig. 9.17 Specific volume of saturated water vapor . . . . . . . . . . . . . . .169Fig.
9.18 Breakdown voltage between electrodes for air(Paschen curve) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .169Fig 9.19 Phase diagram of water . . . . . . . . . . . . . . . . . . . . . . . . . . . .17010.10.110.210.3The statutory units used in vacuum technology . . . . . . . .171Introduction .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171Alphabetical list of variables, symbols and units frequentlyused in vacuum technology and its applications . . . . . . . . . .171Remarks on alphabetical list in Section 10.2 . . . . . . . .
. . . . .1757HomeTable of Contents10.410.4.110.4.210.4.310.4.4Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .176Basic SI units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .176Derived coherent SI units with special names andsymbols . .177Atomic units . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . .177Derived noncoherent SI units with special namesand symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17711.National and international standards andrecommendations particularly relevantto vacuum technology . . . . . . . .
. . . . . . . . . . . . . . . . . . . .178National and international standards andrecommendations of special relevance tovacuum technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17811.112.References . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . .18213.Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1948HomeVacuum physics1. Quantities, theirsymbols, units ofmeasure anddefinitions1.1(cf. DIN 28 400, Part 1, 1990,DIN 1314 and DIN 28 402)Ultimate pressure pendThe lowest pressure which can be achieved in a vacuum vessel. The socalled ultimate pressure pend depends not only on the pumpÕs suction speedbut also upon the vapor pressure pd for the lubricants, sealants andpropellants used in the pump.
If a container is evacuated simply with an oilsealed rotary (positive displacement) vacuum pump, then the ultimatepressure which can be attained will be determined primarily by the vaporpressure of the pump oil being used and, depending on the cleanliness ofthe vessel, also on the vapors released from the vessel walls and, ofcourse, on the leak tightness of the vacuum vessel itself.Basic terms and concepts invacuum technologyAmbient pressure pambor (absolute) atmospheric pressurePressure p (mbar)of fluids (gases and liquids). (Quantity: pressure; symbol: p; unit ofmeasure: millibar; abbreviation: mbar.) Pressure is defined in DIN Standard1314 as the quotient of standardized force applied to a surface and theextent of this surface (force referenced to the surface area).
Even thoughthe Torr is no longer used as a unit for measuring pressure (see Section10), it is nonetheless useful in the interest of ÒtransparencyÓ to mention thispressure unit: 1 Torr is that gas pressure which is able to raise a column ofmercury by 1 mm at 0 ¡C. (Standard atmospheric pressure is 760 Torr or760 mm Hg.) Pressure p can be more closely defined by way of subscripts:Absolute pressure pabsAbsolute pressure is always specified in vacuum technology so that theÒabsÓ index can normally be omitted.Total pressure ptThe total pressure in a vessel is the sum of the partial pressures for all thegases and vapors within the vessel.Partial pressure piThe partial pressure of a certain gas or vapor is the pressure which that gasor vapor would exert if it alone were present in the vessel.Important note: Particularly in rough vacuum technology, partial pressure ina mix of gas and vapor is often understood to be the sum of the partialpressures for all the non-condensable components present in the mix Ð incase of the Òpartial ultimate pressureÓ at a rotary vane pump, for example.Saturation vapor pressure psThe pressure of the saturated vapor is referred to as saturation vaporpressure ps.
ps will be a function of temperature for any given substance.Vapor pressure pdPartial pressure of those vapors which can be liquefied at the temperatureof liquid nitrogen (LN2).Overpressure pe or gauge pressure(Index symbol from ÒexcessÓ)pe = pabs Ð pambHere positive values for pe will indicate overpressure or gauge pressure;negative values will characterize a vacuum.Working pressure pWDuring evacuation the gases and/or vapors are removed from a vessel.Gases are understood to be matter in a gaseous state which will not,however, condense at working or operating temperature.
Vapor is alsomatter in a gaseous state but it may be liquefied at prevailing temperaturesby increasing pressure. Finally, saturated vapor is matter which at theprevailing temperature is gas in equilibrium with the liquid phase of thesame substance. A strict differentiation between gases and vapors will bemade in the comments which follow only where necessary for completeunderstanding.Particle number density n (cm-3)According to the kinetic gas theory the number n of the gas molecules,referenced to the volume, is dependent on pressure p and thermodynamictemperature T as expressed in the following:p=nákáTn = particle number densityk = BoltzmannÕs constantAt a certain temperature, therefore, the pressure exerted by a gas dependsonly on the particle number density and not on the nature of the gas. Thenature of a gaseous particle is characterized, among other factors, by itsmass mT.Gas density ρ (kg á m-3, g á cm-3)The product of the particle number density n and the particle mass mT isthe gas density ρ:ρ = n á mTStandard pressure pnStandard pressure pn is defined in DIN 1343 as a pressure ofpn = 1013.25 mbar.(1.1)(1.2)The ideal gas lawThe relationship between the mass mT of a gas molecule and the molarmass M of this gas is as follows:M = NA á mT(1.3)9HomeVacuum physicsAvogadroÕs number (or constant) NA indicates how many gas particles willbe contained in a mole of gas.
In addition to this, it is the proportionalityfactor between the gas constant R and BoltzmannÕs constant k:R = NA á k(1.4)Derivable directly from the above equations (1.1) to (1.4) is the correlationbetween the pressure p and the gas density ρ of an ideal gas.p=ρ⋅ RáT(1.5)MIn practice we will often consider a certain enclosed volume V in which thegas is present at a certain pressure p. If m is the mass of the gas presentwithin that volume, thenmρ = −−−V(1.6)The ideal gas law then follows directly from equation (1.5):mp ⋅ V = −−− ⋅ R ⋅ T = ν ⋅ R ⋅ TM(1.7)Here the quotient m / M is the number of moles υ present in volume V.The simpler form applies for m / M = 1, i.e.
for 1 mole:páV=RáT(1.7a)The following numerical example is intended to illustrate the correlationbetween the mass of the gas and pressure for gases with differing molarmasses, drawing here on the numerical values in Table IV (Chapter 9).Contained in a 10-liter volume, at 20 ¡C, will bea) 1g of heliumb) 1g of nitrogenWhen using the equation (1.7) there results then at V = 10 l , m = 1 g,R = 83.14 mbar á l á molÐ1 á KÐ1, T = 293 K (20 ¡C)In case a) where M = 4 g á mole-1 (monatomic gas):p=1·g · 83.14 · mbar · ` · mol – 1· K– 1 · 293 · K10 · `· K · 4 · g · mol –1= 609 mbar1·g · 83.14 · mbar · ` · mol – 1· K– 1 · 293 · K10 · `· K · 28 · g · mol –1The following important terms and concepts are often used in vacuumtechnology:Volume V (l, m3, cm3)The term volume is used to designatea) the purely geometric, usually predetermined, volumetric content of avacuum chamber or a complete vacuum system including all the pipingand connecting spaces (this volume can be calculated);b) the pressure-dependent volume of a gas or vapor which, for example, ismoved by a pump or absorbed by an adsorption agent.Volumetric flow (flow volume) qv(l/s, m3/h, cm3/s )The term Òflow volumeÓ designates the volume of the gas which flowsthrough a piping element within a unit of time, at the pressure andtemperature prevailing at the particular moment.
Here one must realizethat, although volumetric flow may be identical, the number of moleculesmoved may differ, depending on the pressure and temperature.Pumping speed S (l/s, m3/h, cm3/s )The pumping speed is the volumetric flow through the pumpÕs intake port.dVS = −−−dt(1.8a)If S remains constant during the pumping process, then one can use thedifference quotient instead of the differential quotient:=In case b), with M = 28 ≠ g mole-1 (diatomic gas):p=The main task of vacuum technology is to reduce the particle numberdensity n inside a given volume V. At constant temperature this is alwaysequivalent to reducing the gas pressure p.
Explicit attention must at thispoint be drawn to the fact that a reduction in pressure (maintaining thevolume) can be achieved not only by reducing the particle number densityn but also (in accordance with Equation 1.5) by reducing temperature T atconstant gas density. This important phenomenon will always have to betaken into account where the temperature is not uniform throughoutvolume V.ÆVS = −−−Æt(1.8b)(A conversion table for the various units of measure used in conjunctionwith pumping speed is provided in Section 9, Table VI).== 87 mbarThe result, though appearing to be paradoxical, is that a certain mass of alight gas exerts a greater pressure than the same mass of a heavier gas.
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