Fundamentals of Vacuum Technology (1248463), страница 49
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The released ions flow to a cathode where the ion current ismeasured and then amplified (halogen diode principle). This effect is sogreat that partial pressures for halogens can be measured down to10-7 mbar.Whereas such devices were used in the past for leak testing in accordancewith the vacuum method, today Ð because of the problems associated withthe CFCs Ð more sniffer units are being built. The attainable detection limitis about 1 á 10-6 mbar á l/s for all the devisces. Equipment operating inaccordance with the halogen diode principle can also detect SF6.Consequently these sniffer units are used to determine whether refrigerantsare escaping from a refrigeration unit or from an SF6 type switch box (filledwith arc suppression gas).5.5.2 Leak detectors with massspectrometers (MSLD)The detection of a test gas using mass spectrometers is far and away themost sensitive leak detection method and the one most widely used inindustry.
The MS leak detectors developed for this purpose make possiblequantitative measurement of leak rates in a range extending across manypowers of ten (see Section 5.2) whereby the lower limit Å 10-12 mbar á l/s,thus making it possible to demonstrate the inherent gas permeability ofsolids where helium is used as the test gas. It is actually possible inprinciple to detect all gases using mass spectrometry. Of all the availableoptions, the use of helium as a tracer gas has proved to be especiallypractical.
The detection of helium using the mass spectrometer is absolutely(!) unequivocal. Helium is chemically inert, non-explosive, non-toxic, ispresent in normal air in a concentration of only 5 ppm and is quiteeconomical. Two types of mass spectrometer are used in commerciallyavailable MSLDÕs:a) The quadrupole mass spectrometer, although this is used lessfrequently due to the more elaborate and complex design (above all dueto the electrical supply for the sensor), orb) the 180¡ magnetic sector field mass spectrometer, primarily due to therelatively simple design.Regardless of the functional principle employed, every mass spectrometercomprises three physically important sub-systems: the ion source,separation system and ion trap.
The ions must be able to travel along thepath from the ion source and through the separation system to the ion trap,to the greatest possible extent without colliding with gas molecules. Thispath amounts to about 15 cm for all types of spectrometers and thusrequires a medium free path length of at least 60 cm, corresponding topressure of about 1 á 10-4 mbar; in other words, a mass spectrometer willoperate only in a vacuum. Due to the minimum vacuum level of 1 á 10-4mbar, a high vacuum will be required.
Turbomolecular pumps and suitableroughing pumps are used in modern leak detectors. Associated with the116HomeLeak detectionindividual component groups are the required electrical- and electronicsupply systems and software which, via a microprocessor, allow for thegreatest possible degree of automation in the operating sequence, includingall adjustment and calibration routines and measured value display.microprocessor to multiply the signal arriving from the mass spectrometerby a numerical constant and to have the leak rate displayed direct.5.5.2.25.5.2.1The operating principle for a MSLDThe basic function of a leak detector and the difference between a leakdetector and mass spectrometer can be explained using Figure 5.6.
Thissketch shows the most commonly found configuration for leak detectionusing the helium spray method (see Section 5.7.1) at a vacuum component.When the sprayed helium is drawn into the component through a leak it ispumped thorough the interior of the leak detector to the exhaust, where itagain leaves the detector. Assuming that the detector itself is free of leaks,the amount of gas flowing through each pipe section (at any desired point)per unit of time will remain constant regardless of the cross section and therouting of the piping.
The following applies for the entry into the pumpingport at the vacuum pump:Q=páS(5.4)At all other pointsQ = p á Seff(5.4a)applies, taking the line losses into account.The equation applies to all gases which are pumped through the piping andthus also for helium.QHe = pHe á Seff, He(5.4b)In this case the gas quantity per unit of time is the leak rate being sought;the total pressure may not be used, but only the share for helium or thepartial pressure for helium. This signal is delivered by the massspectrometer when it is set for atomic number 4 (helium).
The value for Seffis a constant for every series of leak detectors, making it possible to use aTest gase.g. HeDetection limit, background, gas storage inoil (gas ballast), floating zero-pointsuppressionThe smallest detectable leak rate is dictated by the natural backgroundlevel for the gas to be detected. Even with the test connector at the leakdetector closed, every gas will pass Ð counter to the pumping direction Ðthrough the exhaust and through the pumps (but will be reducedaccordingly by their compression) through to the spectrometer and will bedetected there if the electronic means are adequate to do so. The signalgenerated represents the detection limit. The high vacuum system used toevacuate the mass spectrometer will normally comprise a turbomolecularpump and an oil-sealed rotary vane pump.
(Diffusion pumps were usedearlier instead of the turbomolecular pumps.) Like every liquid, the sealingoil in the rotary vane pump has the capability of dissolving gases untilequilibrium is reached between the gas dissolved in the oil and the gasoutside the oil. When the pump is warm (operating temperature) thisequilibrium state represents the detection limit for the leak detector.
Thehelium stored in the oil thus influences the detection limit for the leakdetector. It is possible for test gas to enter not only through the testconnection and into the leak detector; improper installation or inept handlingof the test gas can allow test gas to enter through the exhaust and theairing or gas ballast valve and into the interior of the detector, to increasethe helium level in the oil and the elastomer seals there and thus to inducea background signal in the mass spectrometer which is well above thenormal detection limit. When the device is properly installed (see Fig. 5.7)the gas ballast valve and the airing valve will be connected to fresh air andthe discharge line (oil filter!) should at least be routed to outside the roomwhere the leak test takes place.An increased test gas (helium) background level can be lowered byopening the gas ballast valve and introducing gas which is free of the testgas (helium-free gas, fresh air).
The dissolved helium will be flushed out, soto speak. Since the effect always affects only the part of the oil present inthe pump body at the particular moment, the flushing procedure will have toTest connectionTest specimenTest connectionMass spectrometerTurbomolecularpumpMSVentingvalveLeak detectorQHe = pHe · SeffHeRoughing pumpExhaustExhaustFig. 5.6Gas ballastvalveBasic operating principle for a leak detectorFig. 5.7Correct set-up for a MSLD117HomeLeak detectionbe continued until all the oil from the pumpÕs oil pan has been recirculatedseveral times.
This period of time will usually be 20 to 30 minutes.In order to spare the user the trouble of always having to keep an eye onthe background level, what has been dubbed floating zero-pointsuppression has been integrated into the automatic operating concepts ofall INFICON leak detectors (Section 5.5.2.5). Here the background levelmeasured after the inlet valve has been closed is placed in storage; whenthe valve is then opened again this value will automatically be deductedfrom subsequent measurements. Only at a relatively high threshold levelwill the display panel show a warning indicating that the background noiselevel is too high.
Figure 5.8 is provided to illustrate the process followed inzero point suppression. Chart on the left. The signal is clearly larger thanthe background. Center chart: the background has risen considerably; thesignal can hardly be discerned. Chart on the right: the background issuppressed electrically; the signal can again be clearly identified.Independent of this floating zero-point suppression, all the leak detectorsoffer the capability for manual zero point shifting. Here the display for theleak detector at the particular moment will be Òreset to zeroÓ so that onlyrises in the leak rate from that point on will be shown. This serves only tofacilitate the evaluation of a display but can, of course, not influence itsaccuracy.Modern leak detectors are being more frequently equipped with oil-freevacuum systems, the so-called Òdry leak detectorsÓ (UL 200 dry, UL 500dry).
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