Fundamentals of Vacuum Technology (1248463), страница 34
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+24 volts)and deliver a pressure-dependent current signal that is linear over theentire measuring range from 4 to 20 mA or 0 Ð 10 V. The pressure readingis not provided until after supply of this signal to the computer andprocessing by the appropriate software and is then displayed directly on thescreen.3.3.1 Spinning rotor gauge (SRG)(VISCOVAC)Pressure-dependent gas friction at low gas pressures can be utilized tomeasure pressures in the medium and high vacuum range. In technicalinstruments of this kind a steel ball that has a diameter of severalmillimeters and is suspended without contact in a magnetic field (see Fig.3.9) is used as the measuring element. The ball is set into rotation throughan electromagnetic rotating field: after reaching a starting speed (around425 Hz), the ball is left to itself.
The speed then declines at a rate thatdepends on the prevailing pressure under the influence of the pressuredependent gas friction. The gas pressure is derived from the relativedecline of the speed f (slowing down) using the following equation:−f ⋅df 10 p ⋅ σ= ⋅dt π c ⋅ r ⋅ ρ(3.2)p = gas pressurer = radius of the ballρ = density of the ball material-c = mean speed of the gas particles,dependent on type of gasσ = coefficient of friction of the ball, independent of thetype of gas, nearly 1.As long as a measurement uncertainty of 3 % is sufficient, which is usuallythe case, one can apply σ = 1 so that the sensitivity of the spinning rotorgauge (SRG) with rotating steel ball is given by the calculable physical sizeof the ball, i.e. the product radius x density r á ρ (see equation 3.2).
Once aball has been ÒcalibratedÓ, it is suitable for use as a Òtransfer standardÓ, i.e.as a reference device for calibrating another vacuum gauge throughcomparison, and is characterized by high long-term stability. Measurementswith the VISCOVAC are not limited to measurement of the pressure,however. Other variables involved in the kinetic theory of gases, such asmean free path, monolayer time, particle number density and impingementrate, can also be measured.
The instrument permits storage of 10 programsand easy changeover between these programs. The measuring time perslowing-down operation is between 5 seconds for high pressures and about40 seconds for lower pressures. The measurement sequence of theinstrument is controlled fully automatically by a microprocessor so that anew value is displayed after every measurement (slowing-down procedure).The programs additionally enable calculation of a number of statisticalvariables (arithmetic mean, standard deviation) after a previously specifiednumber of measurements.While in the case of the kinetic theory of gases with VISCOVAC thecounting of particles directly represents the measuring principle (transferringthe particle pulses to the rotating ball, which is thus slowed down).
With81HomeVacuum measurementother electrical measuring methods that are dependent on the type of gas,the particle number density is measured indirectly by means of the amountof heat lost through the particles (thermal conductivity vacuum gauge) or bymeans of the number of ions formed (ionization vacuum gauge).3.3.2 Thermal conductivity vacuumgaugesClassical physics teaches and provides experimental confirmation that thethermal conductivity of a static gas is independent of the pressure at higherpressures (particle number density), p > 1 mbar. At lower pressures,p < 1 mbar, however, the thermal conductivity is pressure-dependent(approximately proportional 1 / ADM). It decreases in the medium vacuumrange starting from approx.
1 mbar proportionally to the pressure andreaches a value of zero in the high vacuum range. This pressuredependence is utilized in the thermal conductivity vacuum gauge andenables precise measurement (dependent on the type of gas) of pressuresin the medium vacuum range.The most widespread measuring instrument of this kind is the Piranivacuum gauge. A current-carrying filament with a radius of r1 heated up toaround 100 to 150 ¡C (Fig. 3.10) gives off the heat generated in it to thegas surrounding it through radiation and thermal conduction (as well as, ofcourse, to the supports at the filament ends). In the rough vacuum rangethe thermal conduction through gas convection is virtually independent ofpressure (see Fig.
3.10). If, however, at a few mbar, the mean free path ofthe gas is of the same order of magnitude as the filament diameter, thistype of heat transfer declines more and more, becoming dependent on thedensity and thus on the pressure. Below 10-3 mbar the mean free path of agas roughly corresponds to the size of radius r2 of the measuring tubes.The sensing filament in the gauge head forms a branch of a Wheatstonebridge. In the THERMOTRON thermal conductivity gauges with variableresistance which were commonly used in the past, the sensing filamentwas heated with a constant current. As gas pressure increases, thetemperature of the filament decreases because of the greater thermalIIAs in all vacuum gauges dependent on the type of gas, the scales of theindicating instruments and digital displays in the case of thermalconductivity vacuum gauges also apply to nitrogen and air.
Within the limitsof error, the pressure of gases with similar molecular masses, i.e. O2, COand others, can be read off directly. Calibration curves for a series of gasesare shown in Fig. 3.11.An extreme example of the discrepancy between true pressure pT andindicated pressure pI in pressure measurement is the admission of air to avacuum system with argon from a pressure cylinder to avoid moisture(pumping time). According to Fig. 3.11, one would obtain a pI reading ofN2, O2 air Ð pI =IIIr2r1l # r2l ‹‹ r – r- 2 1l ‹‹ r1True pressure [mbar] pWHeat lossAbgeführterWärmeflußIconduction through the gas so that the bridge becomes out of balance.
Thebridge current serves as a measure for the gas pressure, which is indicatedon a measuring scale. In the THERMOVAC thermal conductivity gaugeswith constant resistance which are almost exclusively built today, thesensing filament is also a branch of a Wheatstone bridge. The heatingvoltage applied to this bridge is regulated so that the resistance andtherefore the temperature of the filament remain constant, regardless of theheat loss.
This means that the bridge is always balanced. This mode ofregulation involves a time constant of a few milliseconds so that suchinstruments, in contrast to those with variable resistance, respond veryquickly to pressure changes. The voltage applied to the bridge is ameasure of the pressure.
The measuring voltage is corrected electronicallysuch that an approximately logarithmic scale is obtained over the entiremeasuring range. Thermal conductivity vacuum gauges with constantresistance have a measuring range from 10-4 to 1013 mbar. Due to the veryshort response time, they are particularly suitable for controlling andpressure monitoring applications (see section 3.5).
The measurementuncertainty varies in the different pressure ranges. The maximum error atfull-scale deflection is about 1 to 2 %. In the most sensitive range, i.e.between 10-3 and 1 mbar, this corresponds to around 10 % of the pressurereading. The measurement uncertainty is significantly greater outside thisrange.l ›› r1pI > pT–510–410I Thermal dissipation dueto radiation and conductionin the metallic endsII Thermal dissipation due–310–2–110101Druck[mbar][mbar]Pressure10100to the gas, pressuredependentIII Thermal dissipation dueto radiation and convectionIndicated pressure [mbar] pIFig. 3.10 Dependence of the amount heat dissipated by a heated filament (radius r1) in a tube(radius r2) at a constant temperature difference on the gas pressure (schematicdiagram).Fig. 3.11 Calibration curves of THERMOVAC gauges for various gases, based on nitrogenequivalent reading82HomeVacuum measurementonly 40 mbar on reaching an ÒAr atmospheric pressureÓ pT with aTHERMOVAC as a pressure measuring instrument.
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