Fundamentals of Vacuum Technology (1248463), страница 37
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Hence:pM = α á pg + pγwith(3.5)Calibration refers to comparison with a standard in accordance with certainstatutory regulations by specially authorized personnel (calibration facility).The result of this procedure is a calibration certificate which contains thedeviations of the readings of the instrument being calibrated from thestandard.86HomeVacuum measurementCalibration facilities carry out this calibration work. One problem that arisesis the question of how good the standards are and where they arecalibrated.
Such standards are calibrated in calibration facilities of theGerman Calibration Service (DKD). The German Calibration Service ismanaged by the Federal Physical-Technical Institute (PTB). Its function is toensure that measuring and testing equipment used for industrialmeasurement purposes is subjected to official standards. Calibration ofvacuum gauges and test leaks within the framework of the DKD has beenassigned to LEYBOLD, as well as other companies, by the PTB. Therequired calibration pump bench was set up in accordance with DIN 28 418(see Table 11.1) and then inspected and accepted by the PTB.
Thestandards of the DKD facilities, so-called transfer standards (referencevacuum gauges), are calibrated directly by the PTB at regular intervals.Vacuum gauges of all makes are calibrated on an impartial basis byLEYBOLD in Cologne according to customer order. A DKD calibrationcertificate is issued with all characteristic data on the calibration.
Thestandards of the Federal Physical-Technical Institute are the so-callednational standards. To be able to guarantee adequate measuring accuracyor as little measurement uncertainty as possible in its calibrations, the PTBlargely carries out its measurements through the application offundamental methods. This means, for example, that one attempts todescribe the calibration pressures through the measurement of force andarea or by thinning the gases in strict accordance with physical laws. Thechain of the recalibration of standard instruments carried out once a year atthe next higher qualified calibration facility up to the PTB is called Òresettingto national standardsÓ.
In other countries as well, similar methods arecarried out by the national standards institutes as those applied by theFederal Physical-Technical Institute (PTB) in Germany. Fig. 3.17 shows thepressure scale of the PTB. Calibration guidelines are specified in DINstandards (DIN 28 416) and ISO proposals.3.4.1 Examples of fundamental pressuremeasurement methods (as standardmethods for calibrating vacuumgauges)a) Measuring pressure with a reference gaugeAn example of such an instrument is the U-tube vacuum gauge, with whichthe measurement of the pressure in the measurement capillary is based ona measurement of the weight over the length of the mercury column.In the past the McLeod vacuum gauge was also used for calibrationpurposes. With a precision-made McLeod and carefully executedmeasurements, taking into account all possible sources of error, pressuresdown to 10-4 mbar can be measured with considerable accuracy by meansof such an instrument.Another reference gauge is the VISCOVAC decrement gauge with rotatingball (see 3.3.1) as well as the capacitance diaphragm gauge (see 3.2.2.4).b) Generation of a known pressure; static expansion methodOn the basis of a certain quantity of gas whose parameters p, V and T areknown exactly Ð p lies within the measuring range of a reference gaugesuch as a U-tube or McLeod vacuum gauge Ð a lower pressure within theworking range of ionization gauges is reached via expansion in severalstages.If the gas having volume V1 is expanded to a volume (V1 + V2), and fromV2 to (V2 + V3), etc., one obtains, after n stages of expansion:pn = p ⋅1V1V2⋅V1 + V2 V2 + V3⋅⋅ ⋅⋅Vn −1(3.7)Vn −1 + Vnp1 = initial pressure measured directly in mbarpn = calibration pressureThe volumes here must be known as precisely as possible (see Fig.
3.18)and the temperature has to remain constant. This method requires that theapparatus used be kept very clean and reaches its limit at pressures wherethe gas quantity can be altered by desorption or adsorption effects beyondDynamicexpansionIM10+p1V2 = 1000 cm3p3Static expansion1V1 = 25 cm3+p2+Molecularbeam++3+V3 = 25 cm3IMV4 =313000 cmp4U-Tube+Relative uncertainly of the pressure determination [%]300.30.110–1210–910–610–3100103Pressure [mbar]Fig.
3.17 Pressure scale of Federal Physical-Technical Institute (PTB), Berlin, (status as atAugust 1984) for inert gases, nitrogen and methaneFig. 3.18 Generation of low pressures through static expansion87HomeVacuum measurementand thusp2 = p ⋅1L1S=p⋅1L1L2⋅ (1 +L2Sp)(3.9)This method has the advantage that, after reaching a state of equilibrium,sorption effects can be ignored and this procedure can therefore be usedfor calibrating gauges at very low pressures.p2 = p112345678Volume 1Volume 2Inlet valve (conductance L1)Aperture with conductance L2Valveto pump systemValveto gas reservoir9101112131415L1S3.5(Sp >> L2)ValveLN2 cold trapto pump systemU-tube vacuum gaugeMcLeod vacuum gaugeValveCalibrated ionization gaugetube16 to pump(pumping speed PSp)17 Gas inlet18 Mass spectrometer19,20 Gauges to be calibrated21 Nude gauge to be calibrated22 Bake-out furnaceFig.
3.19 Apparatus for calibration according to the dynamic expansion methodthe permissible limits of error. According to experience, this lower limit isaround 5 á 10-7 mbar. This method is called the static expansion methodbecause the pressure and volume of the gas at rest are the decisivevariables.c) Dynamic expansion method (see Fig. 3.19)According to this method, the calibration pressure p is produced byadmitting gas at a constant throughput rate Q into a vacuum chamber whilegas is simultaneously pumped out of the chamber by a pump unit with aconstant pumping speed S.
At equilibrium the following applies according toequation 1.10 a:Pressure monitoring, controland regulation in vacuumsystems3.5.1 Fundamentals of pressuremonitoring and controlIn all vacuum processes the pressure in the system must be constantlychecked and, if necessary, regulated. Modern plant control additionallyrequires that all measured values which are important for monitoring a plantare transmitted to central stations, monitoring and control centers andcompiled in a clear manner. Pressure changes are frequently recorded overtime by recording equipment. This means that additional demands areplaced on vacuum gauges:a) continuous indication of measured values, analog and digital as far aspossibleb) clear and convenient reading of the measured valuesc) recorder output to connect a recording instrument or control orregulation equipmentd) built-in computer interface (e.g.
RS 232)p = Q/Se) facility for triggering switching operations through built-in trigger pointsQ is obtained either from the quantity of gas that flows into the calibrationchamber from a supply vessel in which constant pressure prevails or fromthe quantity of gas flowing into the calibration chamber at a measuredpressure through a known conductance.
The pressure in front of the inletvalve must be high enough so that it can be measured with a referencegauge. The inlet apertures of the valve (small capillaries, sintered bodies)must be so small that the condition d << λ is met, i.e. a molecular flow andhence a constant conductance of the inlet valve are obtained (see Section1.5). The quantity of gas is then defined by p1 á L1, where p1 = pressure infront of the inlet valve and L1 = conductance of the valve. The pumpingsystem consists of a precisely measured aperture with the conductance L2in a wall that is as thin as possible (screen conductance) and a pump witha pumping speed of PSp:S=L 2 ⋅ SpL + Sp2=L21+L2SpThese demands are generally met by all vacuum gauges that have anelectric measured value display, with the exception of Bourdon, precisiondiaphragm and liquid-filled vacuum gauges.
The respective control units areequipped with recorder outputs that supply continuous voltages between 0and 10 V, depending on the pressure reading on the meter scale, so thatthe pressure values can be recorded over time by means of a recordinginstrument. If a pressure switching unit is connected to the recorder outputof the gauge, switching operations can be triggered when the values goover or below specified setpoints. The setpoints or switch threshold valuesfor triggering switching operations directly in the gauges are called triggervalues.
Apart from vacuum gauges, there are diaphragm pressure switchesthat trigger a switching operation (without display of a measured value) viaa contact amplifier when a certain pressure is reached. Valves, for example,can also be controlled through such switching operations.(3.8)88HomeVacuum measurement3.5.2 Automatic protection, monitoringand control of vacuum systemsg) pressure rise above a maximum backing pressure (critical forepressureof the diffusion pump)Protection of a vacuum system against malfunctions is extremely important.In the event of failure, very high material values may be at risk, whetherthrough loss of the entire system or major components of it, due to loss ofthe batch of material to be processed or due to further production downtime.
Adequate operational control and protection should therefore beprovided for, particularly in the case of large production plants. Theindividual factors to be taken into account in this connection are bestillustrated on the basis of an example: Fig. 3.20 shows the schematicdiagram of a high vacuum pump system.
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