Fundamentals of Vacuum Technology (1248463), страница 33
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Unfortunately their use in technical plants is limited because oftheir size and proneness to breakage (see 3.4.1a).In the evacuated limb of the U-tube vacuum gauge a constant pressure ismaintained equal to the vapor pressure of mercury at room temperature(about 10-3 mbar). The other limb is connected to the volume in which thepressure is to be measured.
From the difference in the levels of the twocolumns, the pressure to be measured can be determined on the mbarscale provided. The reading is independent of the atmospheric pressure.3.2.3.2Compression vacuum gauges (accordingto McLeod)The compression vacuum gauge developed by McLeod in 1874 is a veryrarely used type of vacuum gauge today. In its refined form the instrumentcan be used for absolute pressure measurement in the high vacuum rangedown to 10-5 mbar. In the past it was frequently used as a referenceinstrument for the calibration of medium and sometimes also of highvacuum gauges. For such measurements, however, numerousprecautionary rules had to be taken into account before it was possible toassess the measuring accuracy.
The pressure is measured by compressinga quantity of gas that initially occupies a large volume into a smaller volumeby raising a mercury level. The increased pressure obtained in this mannercan be measured in the same way as in a U-tube manometer and from itthe original pressure is calculated (see equations below).According to the type of scale division, a distinction is made between twoforms of compression vacuum gauges: those with a linear scale (see Fig.3.7) and those with a square-law scale (see Fig.
3.8). In the case of thecompression vacuum gauges of the McLeod linear-scale type, the ratio ofthe enclosed residual volume Vc to the total volume V must be known foreach height of the mercury level in the measurement capillary; this ratio isshown on the scale provided with the instrument. In the case ofcompression vacuum gauges with a square-law scale, the total volume andthe capillary diameter d must be known.U-tube vacuum gaugesU-tube vacuum gauges filled with mercury are the simplest and most exactinstruments for measuring pressure in the rough vacuum range (1013 to aNowadays a ÒshortenedÓ McLeod type compression vacuum gaugeaccording to Kammerer is used to measure the Òpartial final pressureÓ ofmechanically compressing pumps.
Through the high degree of79HomeVacuum measurementUpper limit for:Vcmax. = 1 cm3hmax. = 100 mmtorrUpper limit for:d = 2.5 mmmeasuringrangeUpper limit for:Vcmax. = 0.1 cm3hmax. = 100 mmLower limit for:d = 1 mmVolume V [cm3]Volume V [cm3]Fig.
3.8McLeod compression vacuum gauge with linear scale (equation 3.1b)compression the condensable gas components (vapors) are discharged asliquid (the volume of the same mass is then smaller by a factor of around105 and can be neglected in the measurement) so that only the pressure ofthe permanently gaseous components is measured (this is where theexpression permanent gases comes from).Principle of measurement with compression vacuum gaugesIf h is the difference in the mercury level between the measurementcapillary and the reference capillary (measured in mm), then it follows fromthe Boyle-Mariotte law:p á V = (p + h) á Vcp = h⋅(3.1)VcV − Vc(3.1a)VcV(3.1b)Vc and V must be known, h is read off (linear scale).These relationships remain unchanged if the difference in level is read off ascale with mbar division.
The pressure is then obtained in mbar:p = 4 ⋅ h ⋅ Vc3 Vh in mm(3.1c)If during measurement the mercury level in the measurement capillary isalways set so that the mercury level in the reference capillary correspondsto the upper end of the measurement capillary (see Figs. 3.7 and 3.8), thevolume Vc is always given by:Vc = h ⋅ π ⋅ d 24h ....difference in level, see Fig. 3.5d ....inside diameter of measurement capillaryIf this term is substituted for Vc in equation (3.1b), the result is:McLeod compression vacuum gauge with square-law scale (equation 3.1f)p = h2 ⋅ π ⋅ d4 V2(3.1e)that is, a square-law scale in mm (torr) if d and V are measured in mm ormm3.
If the scale is to be divided into mbar, then the equation is:2p = h2 ⋅ π ⋅ d3 V(3.1f)where h in mmd in mmandV in mm3Compression vacuum gauges ensure a reading of the sum of all partialpressures of the permanent gases, provided that no vapors are present thatcondense during the compression procedure.p measured in mm of mercury (= torr). If Vc << V, then:p = h⋅measuringrangeLower limit for:Vcmin. = 5 á 10Ð3cm3hmin. = 1 mmFig. 3.7Upper limit for:d = 1 mmLower limit for:d = 2.5 mmmeasuringrangePressure pmeasuringrangetorr(3.1d)The measuring range between the top and bottom ends is limited by themaximum and minimum ratios of the capillary volume to the total volume(see Figs.
3.7 and 3.8). The accuracy of the pressure measurementdepends to a great extent on the reading accuracy. By using a vernier andmirror, pressure measurements with an accuracy of ± 2 % can beachieved. In the low pressure range, where h is very small, this accuracy isno longer attainable, chiefly because small geometric deviations have avery noticeable effect at the closed end of the capillary (systematic error).The presence of vapors that may condense during compression influencesthe measurement, often in an indefinite manner. One can easily determinewhether vapors having a pressure that is not negligible are present. Thiscan be done by setting different heights h in the measurement capillaryunder constant pressure while using the linear scale and then calculating paccording to equation 3.1b.
If no vapors are present, or only those whosepressure is negligible at room temperature (such as mercury), then thesame value of p must result for each h.The scale of compression vacuum gauges can be calculated from thegeometric dimensions. This is why they were used in the past by officialcalibration stations as normal pressure (see equation 3.4.1a).80HomeVacuum measurement3.3Vacuum gauges with gasdependent pressure readingThis type of vacuum gauge does not measure the pressure directly as anarea-related force, but indirectly by means of other physical variables thatare proportional to the number density of particles and thus to the pressure.The vacuum gauges with gas-dependent pressure reading include: thedecrement gauge (3.3.1), the thermal conductivity vacuum gauge (3.3.2)and the ionization vacuum gauge having different designs (3.3.3).The instruments consist of the actual sensor (gauge head, sensor) and thecontrol unit required to operate it.
The pressure scales or digital displaysare usually based on nitrogen pressures; if the true pressure pT of a gas (orvapor) has to be determined, the indicated pressure pI must be multipliedby a factor that is characteristic for this gas. These factors differ, dependingon the type of instrument, and are either given in tabular form as factorsindependent of pressure (see Table 3.2) or, if they depend on the pressure,must be determined on the basis of a diagram (see Fig. 3.11).1 Ball2 Measuring tube,closed at one end, weldedintoconnection flange 734567Permanent magnetsStabilization coils4 drive coilsBubble levelConnection flangeIn general, the following applies:Fig. 3.9Cross-section of the gauge head of a VISCOVAC VM 212 spinning rotor gauge (SRG)True pressure pT = indicated pressure pI á correction factorIf the pressure is read off a Ònitrogen scaleÓ but not corrected, one refers toÒnitrogen equivalentÓ values.In all electrical vacuum gauges (they include vacuum gauges that aredependent on the type of gas) the increasing use of computers has led tothe wish to display the pressure directly on the screen, e.g.
to insert it at theappropriate place in a process flow diagram. To be able to use the moststandardized computer interfaces possible, so-called transmitters (signalconverters with standardized current outputs) are built instead of a sensorand display unit (e.g. THERMOVAC transmitter, Penning transmitter,IONIVAC transmitter). Transmitters require a supply voltage (e.g.
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