Fundamentals of Vacuum Technology (1248463), страница 50
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Here the problem of gas being dissolved in oil does not occur butsimilar purging techniques will nonetheless be employed.5.5.2.3Calibrating leak detectors; test leaksCalibrating a leak detector is to be understood as matching the display at aleak detector unit, to which a test leak is attached, with the value shown onthe ÒlabelÓ or calibration certificate. The prerequisite for this is correctadjustment of the ion paths in the spectrometer, also known as tuning.Often the distinction is not made quite so carefully and both procedurestogether are referred to as calibration.In the calibration process proper the straight-line curve representing thenumerically correct, linear correlation between the gas flow per unit of timeand the leak rate is defined by two points: the zero point (no display whereno emissions are detected) and the value shown with the test leak (correctdisplay for a known leak).differentiate between two types of calibration: with an internal or externaltest leak.
When using a test leak built into the leak detector the unit canitself be calibrated but it can only calibrate itself. When using an externaltest leak not just the device but also a complete configuration, such as apartial flow arrangement, can be included. Internal test leaks arepermanently installed and cannot be misplaced. At present all the leakdetectors being distributed by INFICON are fitted with an automaticcalibration routine.Sniffer units or configurations will as a rule have to be calibrated withspecial, external test leaks in which there is a guarantee that on the onehand all the test gas issuing from the test leak reaches the tip of the probeand on the other hand that the gas flow in the sniffer unit is not hindered bycalibration.
When making measurements using the sniffer technique (seeSection 5.7.2) it is also necessary to take into account the distance from theprobe tip to the surface of the specimen and the scanning speed; thesemust be included as a part of the calibration. In the special case wherehelium concentration is being measured, calibration can be made using thehelium content in the air, which is a uniform 5 ppm world-wide.Test leaks (also known as standard leaks or reference leaks) normallycomprise a gas supply, a choke with a defined conductance value, and avalve.
The configuration will be in accordance with the test leak raterequired. Figure 5.9 shows various test leaks. Permeation leaks are usuallyused for leak rates of 10-10 < QL < 10-7, capillaries, between 10-8 and 10-4and, for very large leak rates in a range from 10 to 1000 mbar á l/s, pipesections or orifice plates with exactly defined conductance values(dimensions).Test leaks used with a refrigerant charge represent a special situation sincethe refrigerants are liquid at room temperature. Such test leaks have asupply space for liquid from which, through a shut-off valve, the space filledonly with the refrigerant vapor (saturation vapor pressure) can be reached,ahead of the capillary leak.
One technological problem which is difficult tosolve is posed by the fact that all refrigerants are also very good solventsfor oil and grease and thus are often seriously contaminated so that it isdifficult to fill the test leaks with pure refrigerant. Decisive here is not onlythe chemical composition but above all dissolved particles which canrepeatedly clog the fine capillaries.In vacuum operations (spray technique, see Section 5.7.1) one must10Ð6cautio10Ð710Ð8fine10Ð9prec10Ð1010Ð11Equipment background level:Leak:Display:Fig.
5.8a< 2 á 10Ð102 á 10Ð82 á 10Ð8Example of zero-point suppression1 á 10Ð82 á 10Ð83 á 10Ð81 á 10Ð10 (suppressed)2 á 10Ð82 á 10Ð8bca Reference leak without gas supply, TL4, TL6b Reference leak for sniffer and vacuumapplications, TL4-6c (Internal) capillary test leak TL7Fig. 5.9ded Permeation (diffusion) reference leak, TL8e Refrigerant calibrated leakExamples for the construction of test leaks118HomeLeak detection5.5.2.4Leak detectors with quadrupole massspectrometer (ECOTEC II)INFICON builds leak detectors with quadrupole mass spectrometers toregister masses greater than helium. Apart from special cases, these will berefrigerants.
These devices thus serve to examine the tightness ofrefrigeration units, particularly those for refrigerators and air conditioningequipment.Figure 4.2 shows a functional diagram for a quadrupole mass spectrometer.Of the four rods in the separation system, the two pairs of opposing rodswill have identical potential and excite the ions passing through along thecenter line so that they oscillate transversely.
Only when the amplitude ofthese oscillations remains smaller than the distance between the rods canthe appropriate ion pass through the system of rods and ultimately reachthe ion trap, where it will discharge and thus be counted. The flow ofelectrons thus created in the line forms the measurement signal proper.
Theother ions come into contact with one of the rods and will be neutralizedthere.Figure 5.10 shows the vacuum schematic for an ECOTEC II. The massspectrometer (4) only operates under high vacuum conditions, i.e. thepressure here must always remain below 10-4 mbar. This vacuum isgenerated by the turbomolecular pump (3) with the support of thediaphragm pump (1). The pressure PV between the two pumps ismeasured with a piezo resistive measuring system (2) and this pressurelies in the range between 1 to 4 mbar while in the measurement mode.
Thispressure must not exceed a value of 10 mbar as otherwise theturbomolecular pump will not be capable of maintaining the vacuum in themass spectrometer. The unit can easily be switched over at the control unitfrom helium to any of various refrigerants, some of which may be selectedas desired. Naturally the unit must be calibrated separately for each ofthese masses. Once set, however, the values remain available in storageso that after calibration has been effected for all the gases (and a separatereference leak is required for each gas!) it is possible to switch directly fromone gas to another.5internalflowlimiter 1 particlefilterexternalparticlefilter6flow divider 1QMA 200flowlimiter 21 Diaphragm pump2 Piezo-resistivepressure sensor3 Turbomolecularpump4 Quadrupole massspectrometer5 Sniffer line6 Gas flow limiter7 Gas flow limiter8 Gas flow meter5.5.2.5Helium leak detectors with 180° sectormass spectrometer (UL 200, UL 500)These units are the most sensitive and also provide the greatest degree ofcertainty.
Here ÒcertainÓ is intended to mean that there is no other methodwith which one can, with greater reliability and better stability, locate leaksand measure them quantitatively. For this reason helium leak detectors,even though the purchase price is relatively high, are often far moreeconomical in the long run since much less time is required for the leakdetection procedure itself.A helium leak detector comprises basically two sub-systems in portableunits and three in stationary units.
These are:1. the mass spectrometer2. the high vacuum pump and3. the auxiliary roughing pump system in stationary units.The mass spectrometer (see Fig. 5.11) comprises the ion source (1Ð4) andthe deflection system (5Ð9). The ion beam is extracted through the orificeplate (5) and enters the magnetic field (8) at a certain energy level.
Insidethe magnetic field the ions move along circular paths whereby the radius fora low mass is smaller than that for higher masses. With the correct settingof the acceleration voltage during tuning one can achieve a situation inwhich the ions describe a circular arc with a defined curvature radius.Where mass 4 (helium) is involved, they pass thorough the aperture (9) tothe ion trap (13).
In some devices the discharge current for the ionsimpinging upon the total pressure electrodes will be measured and141132345121164flow divider 2737flow meterpv210flowlimiter 39811Fig. 5.10 Vacuum schematic for the ECOTEC II21 Ion source flange2 Cathode(2 cathodes, Ir + Y2O3)3 Anode4 Shielding of the ionsource with dischargeorifice856789ExtractorIon traces for M > 4Total pressure electrodeIon traces for M = 4Intermediate orificeplate10 Magnetic field11121314SuppressorShielding of the ion trapIon trapFlange for ion trap withpreamplifierFig. 5.11 Configuration of the 180¡ sector mass spectrometer119HomeLeak detectionevaluated as a total pressure signal.
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