Fundamentals of Vacuum Technology (1248463), страница 48
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Thus it is possible to brush or spray suspectedleaks with alcohol. The alcohol vapors which flow into the device Ð thethermal conductivity and ionizablity of which will vary greatly from the sameproperties for air Ð will affect and change pressure indication to a greater orlesser extent. The availability of more precise, easy-to-use helium leakdetectors has, however, rendered this method almost completely obsolete.If the container is evacuated with a TURBOVAC 50 turbomolecular pump,for example (S = 50 l/s), which is attached to the vacuum vessel by way of114HomeLeak detection5.4.4 Bubble immersion test5.4.7 Krypton 85 testThe pressurized test specimen is submerged in a liquid bath. Rising gasbubbles indicate the leak. Leak detection will depend greatly on theattentiveness of the person conducting the test and involves the temptationto increase the ÒsensitivityÓ by using ever higher temperatures, wherein theapplicable safety regulations are sometimes disregarded.
This method isvery time-consuming for smaller leaks, as Table 5.3 shows. It referencesleak testing on a refrigeration system using type R12 refrigerant. Here theleak rate is specified in grams of refrigerant loss per year (g/a). Water isused as a test liquid (which may be heated or to which a surfactant may beadded) or petroleum-based oils. The surface tension should not exceed75 dyn/cm (1 dyn = 10-5 N).When dealing with small, hermetically sealed parts where the enclosure isleaky, krypton 85, a gaseous, radioactive isotope, can first be forced intothe device by applying pressure from the outside. Once an exactlymeasured holding period has elapsed the pressure will be relieved, thecomponent flushed and the activity of the Ògas chargeÓ will be measured.
Inthe same way it is also possible to use helium as the test gas (see Section5.7.4, bombing test).Freon F12 loss Time taken to form Equivalent Detection time usingper yeara gas bubbleleak ratehelium leak detector(g/a)(s)(cm3[STP]/s)(s)2808428142.813.34014529024 min1.8 á 10Ð3a few seconds5.4 á10Ð4a few seconds1.8 á10Ð4a few seconds9.0á10Ð5a few seconds1.8 á10Ð5a few seconds0.28 *6h1.8 á 10Ð6a few seconds*) This leak rate represents the detection limit for good halogen leakdetectors (Å 0,1 g/a).Table 5.3 Comparison of bubble test method (immersion technique) wit helium leak5.4.5 Foam-spray testIn many cases pressurized containers or gas lines (including the gas supplylines for vacuum systems) can be checked quite conveniently for leaks bybrushing or spraying a surfactant solution on them.
Corresponding leakdetection sprays are also available commercially. Escaping gas forms ÒsoapbubblesÓ at the leak points. Here, again, the detection of smaller leaks istime-consuming and will depend greatly on the attentiveness of theinspector. The hydrogen gas cooling systems used in power plantgenerators represent a special case. These are indeed sometimes tested inthe fashion described above but they can be examined much better and atmuch higher sensitivity by ÒsniffingÓ the hydrogen escaping at leaks using ahelium leak detector which has been adjusted to respond to H2 (seeSection 5.7.2).5.4.6 Vacuum box check bubbleAs a variation on the spray technique mentioned above, in which theescaping gas causes the bubbles, it is possible to place a so-calledÒvacuum boxÓ with a seal (something like a diverÕs goggles) on the surfacebeing examined once it has been sprayed with a soap solution.
This box isthen evacuated with a vacuum pump. Air entering from the outside throughleaks will cause bubbles inside the box, which can be observed through aglass window in the box. In this way it is also possible, for example, toexamine flat sheet metal plates for leaks. Vacuum boxes are available for avariety of applications, made to suit a wide range of surface contours.5.4.8 High-frequency vacuum testThe so-called high-frequency vacuum tester can be used not only to checkthe pressure in glass equipment but also to locate porous areas in plastic orpaint coatings on metals.
This comprises a hand-held unit with a brush-likehigh-frequency electrode and a power pack. The shape and color of theelectrical gas discharge can serve as a rough indicator for the pressureprevailing inside glass equipment. In the case of the vacuum tester Ð whichcomprises primarily a tesla transformer (which delivers a high-voltage, highfrequency AC current) Ð the corona electrode approaching the apparatuswill trigger an electrode-free discharge inside the apparatus.
The intensityand color of this discharge will depend on the pressure and the type of gas.The luminous discharge phenomenon allows us to draw conclusionsregarding the approximate value for the pressure prevailing inside theapparatus. The discharge luminosity will disappear at high and lowpressures.When searching for leaks in glass equipment the suspect sections will bescanned or traced with the high-frequency vacuum tester electrode. Wherethere is a leak an arc will strike through to the pore in the glass wall, tracinga brightly lit discharge trail.
Small pores can be enlarged by these sparks!The corona discharge of the vacuum tester can also penetrate thin areas inthe glass particularly at weld points and transitional areas betweenintermediate components. Equipment which was originally leak-free canbecome leaky in this fashion! In contrast to the actual leak detector units,the high-frequency vacuum tester is highly limited in its functioning.5.4.9 Test with chemical reactions and dyepenetrationOccasionally leaks can also be located or detected by means of chemicalreactions which result in a discoloration or by penetration of a dye solutioninto fine openings. The discoloration of a flame due to halogen gasescaping through leaks was used earlier to locate leaks in solder joints forrefrigeration units.A less frequently employed example of a chemical effect would be that ofescaping ammonia when it makes contact with ozalid paper (blueprintpaper) or with other materials suitably prepared and wrapped around theoutside of the specimen.
Leaks are then detected based on thediscoloration of the paper.An example of a dye penetration test is the inspection of the tightness ofrubber plugs or plungers in glass tubes, used for example in testingmaterials suitability for disposable syringes or pharmaceutical packages.When evaluating tiny leaks for liquids it will be necessary to consider the115HomeLeak detectionwetability of the surface of the solid and the capillary action; see also Table5.1. Some widely used leak detection methods are shown Ð together withthe test gas, application range and their particular features Ð in Table 5.4.MethodTest gasSmallest detectableleak rateFoamingliquidsAir and others10UltrasonicmicrophoneAir and others10Thermal conductivity leak detectorGases otherthan air10 – 10Halogenleak detectionSubstancescontaininghalogens10–5(10 )Universalsnifferleak detectorRefrigerants,helium andother gases10Heliumleak detectionHelium10–710Bubble testAir and othergases10Water pressuretestWater10Pressuredrop testAir and othergases10Pressurerise testAir10mbar `/sQuantitativemeasurement7 · 10Positive pressureNo70Positive pressureNo10 – 7–1Positive pressureand vacuumNo–3Positive pressure(vacuum)Withlimitations–3Positive pressureYes–9g/a R 134 a–4–1–2–3Pressure range–5–67 · 10–1(10 )–57 · 10–127 · 10–47 · 10Vacuum,positive pressureYes–37Positive pressureNo–270Positive pressureNo–1Positive pressureYes–1VacuumYes–47 · 10–47 · 10Table 5.4 Comparison of leak detection methods5.5Leak detectors and how theyworkMost leak testing today is carried out using special leak detection devices.These can detect far smaller leak rates than techniques which do not usespecial equipment.
These methods are all based on using specific gasesfor testing purposes. The differences in the physical properties of these testgases and the gases used in real-life applications or those surrounding thetest configuration will be measured by the leak detectors. This could, forexample, be the differing thermal conductivity of the test gas andsurrounding air.
The most widely used method today, however, is thedetection of helium used as the test gas.The function of most leak detectors is based on the fact that testing isconducted with a special test gas, i.e. with a medium other than the oneused in normal operation. The leak test may, for example, be carried outusing helium, which is detected using a mass spectrometer, even thoughthe component being tested might, for example, be a cardiac pacemakerwhose interior components are to be protected against the ingress of bodilyfluids during normal operation. This example alone makes it clear that thevarying flow properties of the test and the working media need to be takeninto consideration.5.5.1 Halogen leak detectors(HLD 4000, D-Tek)Gaseous chemical compounds whose molecules contain chlorine and/orfluorine Ð such as refrigerants R12, R22 and R134a Ð will influence theemissions of alkali ions from a surface impregnated with a mixture of KOHand Iron(III)hydroxide and maintained at 800 ¡C to 900 ¡C by an externalPt heater.
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