Fundamentals of Vacuum Technology (1248463), страница 32
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Immediately in front of the pump(as seen from the vessel), a lower pressure is created than in the vessel.Even components having a high conductance may create such a pressuregradient. Finally, the conductance of the connecting line between thevacuum system and the measuring system must not be too small becausethe line will otherwise be evacuated too slowly in the pressure region oflaminar flow so that the indicated pressure is too high.The situation is more complicated in the case of high and ultrahigh vacuum.According to the specific installation features, an excessively high pressureor, in the case of well-degassed measuring tubes, an excessively lowpressure may be recorded due to outgassing of the walls of the vacuumgauge or inadequate degassing of the measuring system. In high andultrahigh vacuum, pressure equalization between the vacuum system andthe measuring tubes may take a long time. If possible, so-called nudegauges are used.
The latter are inserted directly in the vacuum system,flange-mounted, without a connecting line or an envelope. Specialconsideration must always be given to the influence of the measuringprocess itself on the pressure measurement. For example, in ionizationvacuum gauges that work with a hot cathode, gas particles, especiallythose of the higher hydrocarbons, are thermally broken down. This altersthe gas composition.
Such effects play a role in connection with pressuremeasurement in the ultrahigh vacuum range. The same applies to gasclean-up in ionization vacuum gauges, in particular Penning gauges (of theorder of 10-2 to 10-1 l/s). Contamination of the measuring system, interferingelectrical and magnetic fields, insulation errors and inadmissibly highambient temperatures falsify pressure measurement. The consequences ofthese avoidable errors and the necessary remedies are indicated in thediscussion of the individual measuring systems and in summary form insection 8.4.Selection of vacuum gaugesThe desired pressure range is not the only factor considered whenselecting a suitable measuring instrument.
The operating conditions underwhich the gauge works also play an important role. If measurements are tobe carried out under difficult operating conditions, i.e. if there is a high riskof contamination, vibrations in the tubes cannot be ruled out, air bursts canbe expected, etc., then the measuring instrument must be robust. Inindustrial facilities, Bourdon gauges, diaphragm vacuum gauges, thermalconductivity vacuum gauges, hot cathode ionization vacuum gauges andPenning vacuum gauges are used. Some of these measuring instrumentsare sensitive to adverse operating conditions. They should and can only beused successfully if the above mentioned sources of errors are excluded asfar as possible and the operating instructions are followed.3.2Vacuum gauges with pressurereading that is independent ofthe type of gasMechanical vacuum gauges measure the pressure directly by recording theforce which the particles (molecules and atoms) in a gas-filled space exerton a surface by virtue of their thermal velocity.3.2.1 Bourdon vacuum gaugesThe interior of a tube bent into a circular arc (so-called Bourdon tube) (3) isconnected to the vessel to be evacuated (Fig.
3.2). Through the effect ofthe external air pressure the end of the tube is deflected to a greater orlesser extent during evacuation and the attached pointer mechanism (4)and (2) is actuated. Since the pressure reading depends on the externalatmospheric pressure, it is accurate only to approximately 10 mbar,provided that the change in the ambient atmospheric pressure is notcorrected.3.2.2 Diaphragm vacuum gauges3.2.2.1 Capsule vacuum gaugesThe best-known design of a diaphragm vacuum gauge is a barometer withan aneroid capsule as the measuring system.
It contains a hermeticallysealed, evacuated, thin-walled diaphragm capsule made of a copperberyllium alloy. As the pressure drops, the capsule diaphragm expands.This movement is transmitted to a point by a lever system. The capsulevacuum gauge, designed according to this principle, indicates the pressureon a linear scale, independent of the external atmospheric pressure.1 Connecting tube toconnection flange2 PointerFig. 3.23 Bourdon tube4 Lever systemCross-section of a Bourdon gauge77HomeVacuum measurement3.2.2.2DIAVAC diaphragm vacuum gaugeThe most accurate pressure reading possible is frequently required forlevels below 50 mbar.
In this case, a different diaphragm vacuum gauge ismore suitable, i.e. the DIAVAC, whose pressure scale is considerablyextended between 1 and 100 mbar. The section of the interior in which thelever system (2) of the gauge head is located (see Fig. 3.3) is evacuated toa reference pressure pref of less than 10-3 mbar.
The closure to the vesselis in the form of a corrugated diaphragm (4) of special steel. As long as thevessel is not evacuated, this diaphragm is pressed firmly against the wall(1). As evacuation increases, the difference between the pressure to bemeasured px and the reference pressure decreases.
The diaphragm bendsonly slightly at first, but then below 100 mbar to a greater degree. With theDIAVAC the diaphragm deflection is again transmitted to a pointer (9). Inparticular the measuring range between 1 and 20 mbar is considerablyextended so that the pressure can be read quite accurately (to about 0.3mbar).
The sensitivity to vibration of this instrument is somewhat higherthan for the capsule vacuum gauge.3.2.2.3Precision diaphragm vacuum gaugesA significantly higher measuring accuracy than that of the capsule vacuumgauge and the DIAVAC is achieved by the precision diaphragm vacuumgauge. The design of these vacuum gauges resembles that of capsulevacuum gauges. The scale is linear. The obtainable degree of precision isthe maximum possible with present-day state-of-the-art equipment.
Theseinstruments permit measurement of 10-1 mbar with a full-scale deflection of20 mbar. The greater degree of precision also means a higher sensitivity tovibration.Capsule vacuum gauges measure pressure accurately to 10 mbar (due tothe linear scale, they are least accurate at the low pressure end of thescale). If only pressures below 30 mbar are to be measured, the DIAVAC isFig. 3.4Piezoelectric sensor (basic diagram)recommended because its reading (see above) is considerably moreaccurate.
For extremely precise measuring accuracy requirements precisiondiaphragm vacuum gauges should be used. If low pressures have to bemeasured accurately and for this reason a measuring range of, forexample, up to 20 mbar is selected, higher pressures can no longer bemeasured since these gauges have a linear scale. All mechanical vacuumgauges are sensitive to vibration to some extent. Small vibrations, such asthose that arise in the case of direct connection to a backing pump, aregenerally not detrimental.3.2.2.4Capacitance diaphragm gaugesDeflection of a diaphragm can also be electrically measured as ÒstrainÓ oras a change in capacitance. In the past, four strain gauges, which changetheir resistance when the diaphragm is deflected, i.e.
under tensile load,were mounted on a metallic diaphragm in a bridge circuit. At LEYBOLDsuch instruments have been given a special designation, i.e.MEMBRANOVAC. Later, silicon diaphragms that contained four suchÒstrain resistancesÓ directly on their surface were used. The electricalarrangement again consisted of a bridge circuit, and a constant current wasfed in at two opposite corner points while a linear voltage signalproportional to the pressure was picked up at the two other corner points.Fig. 3.4 illustrates the principle of this arrangement. Such instruments weredesignated as PIEZOVAC units and are still in use in many cases.
Todaythe deflection of the diaphragm is measured as the change in capacitanceof a plate capacitor: one electrode is fixed, the other is formed by the12C11 Base plate2 Lever system3 Connecting flange4 Diaphragm5 Reference pressurepref,6 Pinch-off endFig. 3.3789101112Mirror sheetPlexiglass sheetPointerGlass bettMounting plateHousingCross-section of DIAVAC DV 1000 diaphragm vacuum gaugeC2p1Fig. 3.5p2Capacitive sensor (basic diagram)78HomeVacuum measurementC ~ A/dC = capacitanceA = aread = distanceDiaphragm(Inconel)Reference chamber closureGrid + reference chamber closureAmplifierCapacitor plates+ 15 V DC– 15 V DCC ~ A/dC = capacitanceA = aread = distanceDiaphragm(ceramic)Sensor body(ceramic)Signal converter+ amplifier0 – 10 VElectrodeleft: CAPACITRON (Inconel diaphragm)Fig. 3.624 V DC, 4 Ð 20 mAright: MEMBRANOVAC (aluminum oxide diaphragm)Capacitive sensors (basic diagram)diaphragm. When the diaphragm is deflected, the distance between theelectrodes and thus capacitance of the capacitor is altered.
Fig. 3.5illustrates the principle of this arrangement. A distinction is made betweensensors with metallic and those with ceramic diaphragms. The structure ofthe two types is similar and is shown on the basis of two examples in Fig.3.6. Capacitance diaphragm gauges are used from atmospheric pressure to1·10-3 mbar (from 10-4 mbar the measurement uncertainty rises rapidly). Toensure sufficient deflection of the diaphragms at such low pressures,diaphragms of varying thicknesses are used for the various pressure levels.In each case, the pressure can be measured with the sensors to anaccuracy of 3 powers of ten:1013 to 1 mbar100 to 10Ð1 mbar10to 10Ð2 mbar und1to 10Ð3 mbar.If the pressures to be measured exceed these range limits, it isrecommended that a multichannel unit with two or three sensors be used,possibly with automatic channel changeover.
The capacitance diaphragmgauge thus represents, for all practical purposes, the only absolutepressure measuring instrument that is independent of the type of gas anddesigned for pressures under 1 mbar. Today two types of capacitivesensors are available:1) Sensors DI 200 and DI 2000 with aluminum oxide diaphragms, whichare particularly overload-free, with the MEMBRANOVAC DM 11 andDM 12 units.2) Sensors with Inconel diaphragms CM 1, DM 10, CM 100, CM 1000 withextremely high resolution, with the DM 21 and DM 22 units.3.2.3 Liquid-filled (mercury) vacuumgauges3.2.3.1few mbar).
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