Fundamentals of Vacuum Technology (1248463), страница 51
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Ions with masses which are too smallor too great should not be allowed to reach the ion trap (13) at all, but someof these ions will do so in spite of this, either because they are deflected bycollisions with neutral gas particles or because their initial energy deviatestoo far from the required energy level. These ions are then sorted out by thesuppressor (11) so that only ions exhibiting a mass of 4 (helium) can reachthe ion detector (13).
The electron energy at the ion source is 80 eV. It iskept this low so that components with a specific mass of 4 and higher Ðsuch as multi-ionized carbon or quadruply ionized oxygen Ð cannot becreated. The ion sources for the mass spectrometer are simple, rugged andeasy to replace. They are heated continuously during operation and arethus sensitive to contamination. The two selectable yttrium oxide coatediridium cathodes have a long service life.
These cathodes are largelyinsensitive to air ingress, i.e. the quick-acting safety cut-out will keep themfrom burning out even if air enters. However, prolonged use of the ionsource may eventually lead to cathode embrittlement and can cause thecathode to splinter if exposed to vibrations or shock.Depending on the way in which the inlet is connected to the massspectrometer, one can differentiate between two types of MSLD.5.5.2.6Direct-flow and counter-flow leakdetectorsFigure 5.12 shows the vacuum schematic for the two leak detector types. Inboth cases the mass spectrometer is evacuated by the high vacuumpumping system comprising a turbomolecular pump and a rotary vanepump. The diagram on the left shows a direct-flow leak detector. Gasfrom the inlet port is admitted to the spectrometer via a cold trap. It isactually equivalent to a cryopump in which all the vapors and othercontaminants condense. (The cold trap in the past also provided effectiveprotection against the oil vapors of the diffusion pumps used at that time).The auxiliary roughing pump system serves to pre-evacuate thecomponents to be tested or the connector line between the leak detectorand the system to be tested.
Once the relatively low inlet pressure(pumping time!) has been reached, the valve between the auxiliary pumpingsystem and the cold trap will be opened for the measurement. The Seffused in equation 5.4b is the pumping speed of the turbomolecular pump atthe ion source location:QHe = pHe á Seff,turbomolecular pump ion sourceSolution 1: Direct-flow leakdetector(5.5a)Solution 2: Counter-flowleak detectorTest specimenTest specimenTest gas streamTest gas streamIn the case of direct-flow leak detectors, an increase in the sensitivity canbe achieved by reducing the pumping speed, for example by installing athrottle between the turbomolecular pump and the cold trap.
This is alsoemployed to achieve maximum sensitivity. To take an example:The smallest detectable partial pressure for helium ispmin,He = 1 á 10-12 mbar. The pumping speed for helium would beSHe = 10 l/s. Then the smallest detectable leak rate isQmin = 1 á 10-12 mbar á 10 l/s = 1 á 10-11 mbar á l/s. If the pumping speed isnow reduced to 1/s, then one will achieve the smallest detectable leak rateof 1 á 10-12 mbar á l/s.
One must keep in mind, however, that with theincrease in the sensitivity the time constant for achieving a stable test gaspressure in the test specimen will be correspondingly larger (see Section5.5.2.9).In Figure 5.12 the right hand diagram shows the schematic for the counterflow leak detector. The mass spectrometer, the high vacuum system andalso the auxiliary roughing pump system correspond exactly to theconfiguration for the direct-flow arrangement. The feed of the gas to beexamined is however connected between the roughing pump and theturbomolecular pump. Helium which reaches this branch point after thevalve is opened will cause an increase in the helium pressure in the turbomolecular pump and in the mass spectrometer.
The pumping speed Seffinserted in equation 5.4b is the pumping speed for the rotary vane pump atthe branch point. The partial helium pressure established there, reduced bythe helium compression factor for the turbomolecular pump, is measured atthe mass spectrometer. The speed of the turbomolecular pump in thecounter-flow leak detectors is regulated so that pump compression alsoremains constant.
Equation 5.5b is derived from equation 5.5a:QHe = pHe á Seff á K(5.5b)Seff = effective pumping speed at the rotary vane pump at thebranching pointK = Helium compression factor at the turbomolecular pumpThe counter-flow leak detector is a particular benefit for automatic vacuumunits since there is a clearly measurable pressure at which the valve can beopened, namely the roughing vacuum pressure at the turbomolecularpump. Since the turbomolecular pump has a very large compressioncapacity for high masses, heavy molecules in comparison to the light testgas, helium (M = 4), can in practice not reach the mass spectrometer. Theturbomolecular pump thus provides ideal protection for the massspectrometer and thus eliminates the need for an LN2 cold tap, which iscertainly the greatest advantage for the user.
Historically, counter-flow leakdetectors were developed later. This was due in part to inadequatepumping speed stability, which for a long time was not sufficient with therotary vane pumps used here. For both types of leak detector, stationaryunits use a built-in auxiliary pump to assist in the evacuation of the testport. With portable leak detectors, it may be necessary to provide aseparate, external pump, this being for weight reasons.p TOT < 10–4 mbarLN 2MSpHep TOT < 10–4 mbarMSHigh vacuum pumpHigh vacuum pumpAuxiliary pumpRoughing pumpAuxiliary pumpCold trap:2S = 6.1 `/s · cm2Fl ≈ 1000 cmS = 6,100 `/sFig. 5.12 Full-flow and counter-flow leak detectorRoughing pumppHe5.5.2.7Partial flow operationWhere the size of the vacuum vessel or the leak makes it impossible toevacuate the test specimen to the necessary inlet pressure, or where thiswould simply take too long, then supplementary pumps will have to beused.
In this case the helium leak detector is operated in accordance with120HomeLeak detectionthe so-called Òpartial flowÓ concept. This means that usually the larger partof the gas extracted from the test object will be removed by an additional,suitably dimensioned pump system, so that only a part of the gas streamreaches the helium leak detector (see Fig. 5.13). The splitting of the gasflow is effected in accordance with the pumping speed prevailing at thebranching point. The following then applies:QVacuum vessel = γ á DisplayLeak detector(5.6)where g is characterized as the partial flow ratio, i.e.
that fraction of theoverall leak current which is displayed at the detector. Where the partialflow ratio is unknown, g can be determined with a reference leak attachedat the vacuum vessel:Display at the leak detectorγ = ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑQL for the reference leak5.5.2.8(5.7)Connection to vacuum systemsThe partial flow concept is usually used in making the connection of ahelium leak detector to vacuum systems with multi-stage vacuum pumpsets. When considering where to best make the connection, it must be keptin mind that these are usually small, portable units which have only a lowpumping speed at the connection flange (often less than 1 l/s).
This makesit all the more important to estimate Ð based on the partial flow ratio to beexpected vis ˆ vis a diffusion pump with pumping speed of 12000 l/s, forexample Ð which leak rates can be detected at all. In systems with highvacuum- and Roots pumps, the surest option is to connect the leak detectorbetween the rotary vane pump and the roots pump or between the rootspump and the high vacuum pump. If the pressure there is greater than thepermissible inlet pressure for the leak detector, then the leak detector willhave to be connected by way of a metering (variable leak) valve. Naturallyone will have to have a suitable connector flange available. It is alsoadvisable to install a valve at this point from the outset so that, whenneeded, the leak detector can quickly be coupled (with the system running)and leak detection can commence immediately after opening the valve.
Inorder to avoid this valve being opened inadvertently, it should be sealed offwith a blank flange during normal vacuum system operation.A second method for coupling to larger systems, for example, those usedfor removing the air from the turbines in power generating stations, is tocouple at the discharge. A sniffer unit is inserted in the system where itdischarges to atmosphere. One then sniffs the increase in the heliumconcentration in the exhaust. Without a tight coupling to the exhaust,however, the detection limit for this application will be limited to 5 ppm, thenatural helium content in the air. In power plants it is sufficient to insert thetip of the probe at an angle of about 45 ¡ from the top into the dischargeline (usually pointing upward) of the (water ring) pump.5.5.2.9Partial flow principle (example)Time constantsThe time constant for a vacuum system is set by· ` (Leak rate)QHe = 3 · 10 mbars–5V = 150 `SLD = 8 ` Leak detector (LD)sSeff = SPFP + SLD →3m`SPFP = 60 s = 16.66 s Partial flow pump (PFP)A) Signal amplitude:Splitting of the gas flow (also of the testgas!) in accordance with the effective pumpingspeed at the partial flow branch pointτ=VSeff(5.8)τ = Time constantV = Volume of the containerSeff = Effective pumping speed, at the test objectFigure 5.14 shows the course of the signal after spraying a leak in a testspecimen attached to a leak detector, for three different configurations:Overal pumping speed: Seff = SLD + SPFP = 8 + 16.66 = 24.66 `s2H–5Signal to Leak detector: 3 · 10`mbar · `· 8s `s(8 + 16.66)–6= 9.73 · 10s–5Signal to partial flow pump: 3 · 10`mbar · `· 16,668s `s(8 + 16.66)s1QV–5= 2.02 · 10Smbar · `smbar · `sS’QS= 3.00 · 10–5 mbars · `QHe = QLD + QPFPQ = 2pS/2MSLDLDnormalFaster, less sensitiveCheck: Overall signalS/2MSLDLDorγ=SLD1=SSLD + SPFP 1 + nnnSPFPLDHPFPDisplay2,0Slower, more sensitiveeffQ = P/2S + S’sEstimate: Value for S, V and γ are uncertain → certain: calibrate with reference leakFig.
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