Fundamentals of Vacuum Technology (1248463), страница 31
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However, a special case isthe pendulum-type gate valve. Despite its grease-covered seal, the leakrate between vacuum and external atmosphere is virtually the same as forbellows-sealed valves because when the valve is in operation the shaftcarries out only a rotary motion so that no gas molecules are transferredinto the vacuum. Pendulum-type gate valves are not manufactured byLEYBOLD.For working pressures down to 10-7 mbar, valves of standard design sufficebecause their seals and the housing materials are such that permeationand outgassing are insignificant to the actual process. If pressures down to10-9 mbar are required, baking up to 200 ¡C is usually necessary, whichrequires heat resistant sealing materials (e.g., VITILANh) and materials ofhigh mechanical strength, with prepared (inner) surfaces and a lowoutgassing rate.
Such valves are usually made of stainless steel. Flangeconnections are sealed with aluminum gaskets, so permeation problems ofelastomer seals are avoided. In the UHV range these issues are of specialsignificance so that mainly metallic seals must be used. The gas moleculesbonded to the surface of the materials have, at pressures below 10-9 mbar,a very great influence. They can only be pumped away within a reasonableperiod of time by simultaneous degassing. Degassing temperatures up to500 ¡C required in UHV systems, pose special requirements on the sealingmaterials and the entire sealing geometry. Gaskets made of gold or coppermust be used.74HomeVacuum generation2.3.8 Gas locks and seal-off fittingsIn many cases it is desirable not only to be able to seal off gas-filled orevacuated vessels, but also to be in a position to check the pressure or thevacuum in these vessels at some later time and to post-evacuate orsupplement or exchange the gas filling.This can be done quite easily with a seal-off fitting from LEYBOLD which isactuated via a corresponding gas lock.
The small flange connection of theevacuated or gas-filled vessel is hermetically sealed off within the tube by asmall closure piece which forms the actual valve. The gas lock required foractuation is removed after evacuation or filling with gas. Thus one gas lockwill do to actuate any number of seal-off fittings. Shown in Fig. 2.81 is asectional view of such an arrangement. Gas locks and seal-off fittings aremanufactured by LEYBOLD having a nominal width of DN 16 KF, DN 25 KFand DN 40 KF.
They are made of stainless steel. The leak rate of the sealoff fittings is less than 1 · 10-9 mbar l/s. They can sustain overpressures upto 2.5 bar, are temperature resistant up to 150 ¡C and may be protectedagainst dirt by a standard blank flange.Typical application examples are double-walled vessels with an insulatingvacuum, like Dewar vessels, liquid gas vessels (tanks) or long distanceenergy pipelines and many more. They are also used for evacuation orpost-evacuation of reference and support vacua in scientific instrumentsseal-off fittings with gas locks are often used. Previously it was necessaryto have a pump permanently connected in order to post-evacuate asrequired. Through the use of gas locks with seal-off fittings a vacuum-tightseal is provided for the vessel and the pump is only required from time totime for checking or post-evacuation.hDNaah1h2dFig.
2.81 Gas lock with centering ring and seal-off fitting, sectional view75HomeVacuum measurement3. Vacuummeasurement,monitoring, controland regulation3.1The pressures measured in vacuum technology today cover a range from1013 mbar to 10-12 mbar, i.e. over 15 orders of magnitude. The enormousdynamics involved here can be shown through an analogy analysis ofvacuum pressure measurement and length measurement, as depicted inTable 3.1.Analogy analysisDetermination bymeans ofempirical worldof human beingssimple measuringmethodsmechanicalmeasuringmethodsindirectmethodsextreme indirectmethods>AbsolutepressureLength1 bar1m> 1 mbar> 1 mm10Ð3mbar10Ð9 mbar10Ð12 mbar1. Instruments that by definition measure the pressure as the force whichacts on an area, the so-called direct or absolute vacuum gauges.According to the kinetic theory of gases, this force, which the particlesexert through their impact on the wall, depends only on the number ofgas molecules per unit volume (number density of molecules n) andtheir temperature, but not on their molar mass.
The reading of themeasuring instrument is independent of the type of gas. Such unitsinclude liquid-filled vacuum gauges and mechanical vacuum gauges.> 1 mm≈ 1/100 atom∅≈ 0.18electron∅Measuring instruments designated as vacuum gauges are used formeasurement in this broad pressure range. Since it is impossible forphysical reasons to build a vacuum gauge which can carry out quantitativemeasurements in the entire vacuum range, a series of vacuum gauges isavailable, each of which has a characteristic measuring range that usuallyextends over several orders of magnitude (see Fig.
9.16a). In order to beable to allocate the largest possible measuring ranges to the individualtypes of vacuum gauges, one accepts the fact that the measurementuncertainty rises very rapidly, by up to 100 % in some cases, at the upperand lower range limits. This interrelationship is shown in Fig. 3.1 using theexample of the VISCOVAC.
Therefore, a distinction must be made betweenthe measuring range as stated in the catalogue and the measuring rangefor ÒpreciseÓ measurement. The measuring ranges of the individual vacuumgauges are limited in the upper and lower range by physical effects.Relative measurement uncertainty (%)Vacuum gauges are devices for measuring gas pressures belowatmospheric pressure (DIN 28 400, Part 3, 1992 issue). In many cases thepressure indication depends on the nature of the gas. With compressionvacuum gauges it should be noted that if vapors are present, condensationmay occur due to the compression, as a result of which the pressureindication is falsified. Compression vacuum gauges measure the sum of thepartial pressures of all gas components that do not condense during themeasurement procedure.
In the case of mechanically compressing pumps,the final partial pressure can be measured in this way (see 1.1). Anotherway of measuring this pressure, is to freeze out the condensablecomponents in an LN2 cold trap. Exact measurement of partial pressures ofcertain gases or vapors is carried out with the aid of partial pressuremeasuring instruments which operate on the mass spectrometer principle(see section 4).Dependence of the pressure indication on the type of gasA distinction must be made between the following vacuum gauges:Table 3.12.
Instruments with indirect pressure measurement. In this case, thepressure is determined as a function of a pressure-dependent (or moreaccurately, density-dependent) property (thermal conductivity, ionizationprobability, electrical conductivity) of the gas. These properties aredependent on the molar mass as well as on the pressure. The pressurereading of the measuring instrument depends on the type of gas.The scales of these pressure measuring instruments are always based onair or nitrogen as the test gas. For other gases or vapors correction factors,usually based on air or nitrogen, must be given (see Table 3.2). For precisepressure measurement with indirectly measuring vacuum gauges thatdetermine the number density through the application of electrical energy(indirect pressure measurement), it is important to know the gascomposition.
In practice, the gas composition is known only as a roughapproximation. In many cases, however, it is sufficient to know whether lightor heavy molecules predominate in the gas mixture whose pressure is to bemeasured (e.g. hydrogen or pump fluid vapor molecules).20Example: If the pressure of a gas essentially consisting of pump fluidmolecules is measured with an ionization vacuum gauge, then the pressurereading (applying to air or N2), as shown in Table 3.2, is too high by a factorof about 10.“favorable measuring range”15(relative measurement uncertainty < 5%)1051–610Fig.
3.1Fundamentals of low-pressuremeasurement10–510–410–3Pressure (mbar)10–210–1Measurement uncertainty distribution over the measuring range: VISCOVAC1Measurement of pressures in the rough vacuum range can be carried outrelatively precisely by means of vacuum gauges with direct pressuremeasurement. Measurement of lower pressures, on the other hand, isalmost always subject to a number of fundamental errors that limit themeasuring accuracy right from the start so that it is not comparable at all to76HomeVacuum measurementthe degree of accuracy usually achieved with measuring instruments. Inorder to measure pressure in the medium and high vacuum ranges with ameasurement uncertainty of less than 50 %, the person conducting theexperiment must proceed with extreme care.
Pressure measure-ments thatneed to be accurate to a few percent require great effort and, in general,the deployment of special measuring instruments. This applies particularlyto all pressure measurements in the ultrahigh vacuum range(p < 10-7 mbar).To be able to make a meaningful statement about a pressure indicated by avacuum gauge, one first has to take into account at what location and inwhat way the measuring system is connected. In all pressure areas wherelaminar flows prevail (1013 > p > 10-1 mbar), note must be taken ofpressure gradients caused by pumping.
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