Fundamentals of Vacuum Technology (1248463), страница 46
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In practice, however, an infinite increase of Seff will run up againsteconomic and engineering limitations (such as the space required by thesystem).Whenever it is not possible to achieve the desired ultimate pressure in anapparatus there are usually two causes which can be cited: The presenceof leaks and/or gas being liberated from the container walls and sealants.Partial pressure analysis using a mass spectrometer or the pressure risemethod may be used to differentiate between these two causes. Since thepressure rise method will only prove the presence of a leak withoutindicating its location in the apparatus, it is advisable to use a helium leakdetector with which leaks can, in general, also be located much morequickly.In order to achieve an overview of the correlation between the geometricsize of the hole and the associated leak rate it is possible to operate on thebasis of the following, rough estimate: A circular hole 1 cm in diameter inthe wall of a vacuum vessel is closed with a gate valve.
Atmosphericpressure prevails outside, a vacuum inside. When the valve is suddenlyopened all the air molecules in a cylinder 1 cm in diameter and 330 m highwould within a 1-second period of time Òfall intoÓ the hole at the speed ofsound (330 m/s). The quantity flowing into the vessel each second will be1013 mbar times the cylinder volume (see Fig.
5.1). The result is that for a∆p = 1013 mbar, Hole diameter d = 1 cmmGas speed = Speed of sound = 330 sVolume/second:Quantity/second:3`2+3 cm12 · π330 ms = 25.95 ss · 4 · cm = 25.95 · 10+4+4mbar · `1013 mbar · 25.95 `s = 2.63 · 10 P 10sDiameter cm10–2 m=10–3 m=10–4 m=10–5 m=10–6 m=10–7 m=10–8 m=10–9 m=10–10 m=Fig. 5.11.0 cm1.0 mm0.1 mm0.01 mm1.0 µm0.1 µm0.01 µm1.0 nm1.0 Angstrommbar · `Leak rates+41010+2100 (= 1)10–210–410–610–810–1010–12 (Detection limit, He leak detector)Correlation between leak rate and hole sizehole 1 cm in diameter QL (air) will be 2.6 á 104 mbar á l/s. If all otherconditions are kept identical and helium is allowed to flow into the hole at itsspeed of sound of 970 m/s, then in analogous fashion the QL (helium) willcome to 7.7 á 10+4 mbar á l/s, or a pV leaking gas current which is larger bya factor of 970 / 330 = 2.94.
This greater ÒsensitivityÓ for helium is used inleak detection practice and has resulted in the development and massproduction of highly sensitive helium-based leak detectors (see Section5.5.2).Shown in Figure 5.1 is the correlation between the leak rate and hole sizefor air, with the approximation value of QL (air) of 10+4 mbar á l/s for theÒ1 cm holeÓ. The table shows that when the hole diameter is reduced to1 µm (= 0.001 mm) the leak rate will come to 10-4 mbar á l/s, a value whichin vacuum technology already represents a major leak (see the rule ofthumb above). A leak rate of 10-12 mbar á l/s corresponds to hole diameterof 1 •; this is the lower detection limit for modern helium leak detectors.Since the grid constants for many solids amount to several • and thediameter of smaller molecules and atoms (H2, He) are about 1 •, inherentpermeation by solids can be registered metrologically using helium leakdetectors.
This has led to the development of calibrated reference leakswith very small leak rates (see Section 5.5.2.3). This is a measurable Òlackof tightnessÓ but not a ÒleakÓ in the sense of being a defect in the materialor joint. Estimates or measurements of the sizes of atoms, molecules,viruses, bacteria, etc. have often given rise to everyday terms such asÒwatertightÓ or Òbacteria-tightÓ; see Table 5.1.Compiled in Figure 5.2 are the nature and detection limits of frequentlyused leak detection methods.Concept / criterionCommentQL [mbar á l/s]Water-tight*)DropletsQL < 10Ð2Vapor-tightÒSweatingÓQL < 10Ð3Bacteria-tight*)(cocci)(rod-shaped)QL < 10Ð4Oil-tightQL < 10Ð5Virus-tight*)(vaccines such as pox)(smallest viruses,bacteriophages)(viroids, RNA)QL < 10Ð8QL < 10Ð10Gas-tightQL < 10Ð7ÒAbsolutely tightÓQL < 10Ð6TechnicalRelevant particle sizeAvg.
Å 1 µmAvg. Å 0.5-1 µm, 2Ð10 µm long¯ Å 3 á 10Ð7 m¯ Å 3 á 10Ð8 m¯ »Å 1 á 10Ð9 m (thread-like)QL < 10Ð10*) As opposed to vapor, it is necessary to differentiate between hydrophilic andhydrophobic solids. This also applies to bacteria and viruses since they aretransported primarily in solutions.Table 5.1 Estimating borderline leak rates111HomeLeak detectionHelium leak detector ULTRATEST UL 200 dry/UL 500Helium standard leak rate:p1 = 1 bar, p2 < 1 mbar (∆p = 1 bar)Test gas = HeliumFamiliar leaks:Dripping water faucet4 mm diam., 1 Hz, ∆p = 4 bar➔➔Helium leak detector ULTRATEST UL 200/UL 500 dry/Modul 200/LDS 1000Substance quantity trhough hole per unit of time∆ (p · V)Definition: Q =∆t➔Contura ZVacuum methodLeak <----> HoleQ ...
Leak rate,In short: LeakQuantity escaping:1mg34 s WaterStandard He leak rate:mbar · ø= 6.45Airs20.17mbar · øHe StdsPressure riseHair on a gasket103................100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 mbar · l · s-1Overpressure methodEcotec II / ProtecULTRATEST with helium snifferHalogen sniffer HLD4000ABubble test10 –2mbar · øAirsBicycle tube in water(bubble test)3–3 Ncm2 mm diam., 1 Hz, ∆p = 0.1 bar 4.19 · 10s0.9 · 10 –21 = 4.24 · 10–3mbar · øAirsCar tire loses air25 l, 6 Mo: 1.8 --> 1.6 bar3.18 · 10 –4Small refrigerant cylinderempties in 1 year430 g refrigerant R12, 25°Cmbar · øgF12430 a Frigen = 2.8 · 10 –3s2mbar · øAirs11.88 · 10 –24.3 · 10 –524.33 · 10 –5mbar · øHe Stdsmbar · øHe Stdsmbar · øHe Stdsmbar · øHe StdsPressure drop testFig.
5.2Leak rate ranges for various leak detection processes and devices5.2.1 The standard helium leak rateRequired for unequivocal definition of a leak are, first, specifications for thepressures prevailing on either side of the partition and, secondly, the natureof the medium passing through that partition (viscosity) or its molar mass.The designation Òhelium standard leakÓ (He Std) has become customary todesignate a situation frequently found in practice, where testing is carriedout using helium at 1 bar differential between (external) atmosphericpressure and the vacuum inside a system (internal, p < 1 mbar), thedesignation Òhelium standard leak rateÓ has become customary. In order toindicate the rejection rate for a test using helium under standard heliumconditions it is necessary first to convert the real conditions of use tohelium standard conditions (see Section 5.2.2).
Some examples of suchconversions are shown in Figure 5.3.5.2.2 Conversion equationsWhen calculating pressure relationships and types of gas (viscosity) it isnecessary to keep in mind that different equations are applicable to laminarand molecular flow; the boundary between these areas is very difficult toascertain. As a guideline one may assume that laminar flow is present atleak rates where QL > 10-5 mbar á l/s and molecular flow at leak rateswhere QL < 10-7 mbar á l/s. In the intermediate range the manufacturer(who is liable under the guarantee terms) must assume values on the safeside. The equations are listed in Table 5.2.RangeLaminar()(Molecular)PressureQI ⋅ p12 − p22 II =Q II ⋅ p12 − p22 IGasQgas A á ηgas A = Qgas B á ηgas BQI ⋅ (p1 − p2)II = QII ⋅ (p1 − p2)IQgas A ⋅ Mgas A = Qgas B ⋅ Mgas BTable 5.2 Conversion formulae for changes of pressure and gas typeFig.
5.3Examples for conversion into helium standard leak ratesHere indices ÒIÓ and ÒIIÓ refer to the one or the other pressure ratio andindices Ò1Ó and Ò2Ó reference the inside and outside of the leak point,respectively.5.3Terms and definitionsWhen searching for leaks one will generally have to distinguish betweentwo tasks:1. Locating leaks and2. Measuring the leak rate.In addition, we distinguish, based on the direction of flow for the fluid,between thea.
vacuum method (sometimes known as an Òoutside-in leakÓ), where thedirection of flow is into the test specimen (pressure inside the specimenbeing less than ambient pressure), and theb. positive pressure method (often referred to as the Òinside-out leakÓ),where the fluid passes from inside the test specimen outward (pressureinside the specimen being greater than ambient pressure).The specimens should wherever possible be examined in a configurationcorresponding to their later application Ð components for vacuumapplications using the vacuum method and using the positive pressuremethod for parts which will be pressurized on the inside.When measuring leak rates we differentiate between registeringa.
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