The improvement of the volume ratio measurement method in static expansion vacuum system

Transcription

1 Available olie at Physics Procedia 32 (22 ) th Iteratioal Vacuum Cogress The improvemet of the volume ratio measuremet method i static expasio vacuum system Yu Hogya*, Wag Jiku, Ge Chuxi Natioal Istitute of Metrology, Beijig 3, Chia Abstract Expasio ratio or volume ratio is the most importat parameter i the static expasio system. To determie the volume ratios i such systems, differet methods ca be used. We have improved the methods with the referece volume method, the sequetial icremet method ad the pre-pressurizatio accumulatio method. The deviatios of there results are about Published by Elsevier B.V. Selectio ad/or peer review uder resposibility of Chiese Vacuum Society (CVS). Ope access uder CC BY-NC-ND licese. PACS: 6.2.Dk,6.3.-k Keywords: Static expasio; volume ratio. Itroductio Static expasio systems are used as primary stadards i may atioal metrology istitutes[-5] for geeratig pressures i the high ad medium vacuum rage for vacuum gauge calibratios, ad the relative ucertaity has decreased form -2 magitude to -3 magitude i about 2 years ago. Lower pressures are geerated by gas expasio i the system, ad the expasio coefficiet is called expasio ratio, its iverse, volume ratio. The ucertaity of volume ratio is the mai part of the expasio system. A ew static expasio system, of which the schematic is show i figure, for the geeratio of calibratio pressure i the rage from -4 Pa to 3 Pa has bee established at Natioal Istitute of Metrology (NIM), ad the expad (k=2) relative ucertaities of pressures rage from.4 to.6. A ew method for the measuremet of large volume has bee preseted, ad the preset accumulatio method is bee improved to test the volume ratio more easily. * Correspodig author. Tel.: ; fax: address: yuhy@im.ac.c Published by Elsevier B.V. Selectio ad/or peer review uder resposibility of Chiese Vacuum Society (CVS). Ope access uder CC BY-NC-ND licese. doi:.6/j.phpro

2 Yu Hogya et al. / Physics Procedia 32 ( 22 ) V2 82L.8L v3 V 82L.8L v2 Gas Source.8L Vacuum Pump Vacuum Pump Figure The schematic of the NIM static expasio system 2. Measuremet method of volume ratio The method used for the determiatio of the volume ratio ca be classed ito two types which are the gravimetric method ad pressure ratio method, ad the gravimetric method has more precisio. The gravimetric method is suitable for small volume, but is hard work for large volume. The first geeratio of expad vacuum system, which is called SEVS-, was established at NIM i 978. The gravimetric method was used to determie the two expasio volume. The expasio was iverted to fill water, because all the ilets were at the top ad the pump was used to pump out the air. After the measuremet the mass i the mass laboratory, the two volumes had the followig volumes: 3.2L ad 64.L. The gravimetric method was used to measure the other eight small volumes, which were 8L to 3mL. There is some problem to reproduce the volume ratio, because after the establishig, it is difficult to use gravimetric method agai. It is coveiet to reproduce the volume ratio whe the pressure ratio method is adopted. With the restrict of the measuremet istrumet, the pressure ratio method has larger ucertaity with smaller volume ratio. The pressure ratio method ca be classed ito accumulatio method, liear vacuum gauge method ad referece vacuum gauge, ad may atioal metrology istitutes, such as NPLPTBIMGCUME etc, use the accumulatio method to determie the volume ratio. The advatage of the accumulatio method is that recurrece of volume ratio ca be dow at ay time, the disadvatage of it is that may expad processes is eeded ad may factor will effect it. Some assumptio should be made to use the simplified formula, but make the assumptio is suitable, the coditio is hard to be satisfied. The trouble of expad system is to further improve the ucertaity of volume ratio. I order to get more accuracy volume ratio, o the base of the pressure ratio method ad gravimetric method, the referece volume method, icremetal method ad pre-pressed accumulatio method is bee preseted. 2. Small volumes value were measured with gravimetric method The small volumes value are measured easily with gravimetric, so we obtai the volumes value by this method. The small volume ad two valves are weighed together, ad the seal of valve face the small volume. The small volumes valve ca be obtaied with gravimetric method L v L v L The ucertaity of small volume due to: the mass of small volume; the mass of small volume filled water; the desity of air ad pure water etc, of which the mass of small volume filled water is the mai item. The desity of pure water ca be obtaied through the water desity table, ad the desity of air is measured whe weigh the mass of small volume.

3 494 Yu Hogya et al. / Physics Procedia 32 ( 22 ) Table the ucertaity of small volumes valve Estimated relative ucertaity u r v2 v3 The mass of small volume filled water The mass of small volume The desity of Isotope pure water Temperature The desity of air Total Large volumes valve were measured with referece volume method Because the complex structure of the calibratio volume, it is hard to use the gravimetric method, at the same time, the measuremet ucertaity is ot very small ad the recurrece is very hard. O this coditio, the referece volume method is bee itroduced. This method solved the measuremet ad recurrece of calibratio volume, ad the ucertaity ca reach a ideal level. A referece volume v4, of which the volume is about /4 of the calibratio volume V ad V2, with simple structure ad smooth iteral surface is bee made. The gravimetric method is used to measure the volume v4, which is 2.625L with about -5 relative ucertaity. The referece volume ad the calibratio volume is coected by corer valve. kpa N 2 is ijected ito the two volumes, ad the close the corer valve. Use turbo pump to vacuum the calibratio volume, ope the corer valve. After the expasio, the balace pressure is about 2kPa. The calibratio volume ca be calculated by equatio. P v4 BP T3 T3 V v4 P BP T T i the equatio, P is the pressure before expad, which is about kpa, P is the pressure after expad, which is about 2kPa; v 4 is the referece volume, B is the secod virial coefficiet correspod to pressurewhich is about Pa - for itroge gas at ormal temperature. T ad T 2 is the referece volume temperature (K) before ad after expasio, T 3 is the calibratio temperature (K) after the expasio. The pressure before ad after expad is measured by digital pressure gauge, which is calibrated by gas operated pisto gauge with.2% ucertaity at 2kPa, 8kPa ad kpa. The expasio will make the volume temperature chage withi the rage about K ad temperature distributio ot homogeous. I order to solve this problem, may fas are distributed aroud the system to form the covectio flow up ad dow. The temperature differece betwee the up ad dow will be smaller tha.2kad the temperature differece before ad after expad will be smaller tha.k. May platium resistaces are used to measure the temperature of the referece volume ad calibratio volume, the mea temperature are bee used i the equatio. The referece volume is bee get by gravimetric method, havig high precisio. I the measuremet process of V or V2, oly oe expad process is eeded, which make it simple ad have small ucertaity. The volume ratio ca be get by the, of which the volume value is measured by gravimetric method, divide ad V or V2, of which the volume value is measured by referece volume method. 2.3 The pre-pressurizatio accumulatio method I recet years, may improvemets were made o the accumulatio method. The first tes time data is removed, ad the computatio is start whe the pressure is up to higher rage. This is because i the first times, the pressure is low, i this rage, the digital pressure gauge has poor accuracy [6][7]. The improvemet was made by direct ilet gas with pressure about 2kPa, which make the first decades expad process bee omitted ad ca make the pressure fall ito the rage of digital pressure gauge with good accuracy. This method is called pre-pressure accumulatio method. Accoutig for pre-pressure P p, followig the similar derivatio process of K. Jouste ad F. J. Redgrave, the equatio ca be obtaied. 2

4 Yu Hogya et al. / Physics Procedia 32 ( 22 ) p ( p) p f B p B ( p) f [ ] p p p ( T ) B p B p ( T ) I this equatio, the sigh of is for the small volume at iitial state, the sigh of is for the large volume, the sigh out the bracket preset the expad times, B is the secod virial coefficiet correspod to pressure, is the temperature correctio. 2.4 The sequetial icremet method p 2 I the traditioal accumulatio method, about several tes times expad is eeded. This meas a large amout of work should be doe, ad oly oe measuremet data ca be get. Whe the expad ratio is about., oe expad process will make early kpa pressure icremet. The digital pressure gauge with full rage kpa have eough accuracy whe the pressure is high tha 2kPa, ad the volume ratio ca be get be oe expad process. By about te times of expad, the same times group data ca be get. The same amouts of work ca get more data, ad o the other had, the pre-pressure is ot eeded to keep strict same. The temperature correctio is doe at every expasio, so the temperature coefficiet will ot be itroduced. This is so called sequetial icremet method. The equatio used to compute the volume ratio by sequetial icremet method ca be writte as the followig: P P ( B P ) T3 ( BP ) T 3 V P P P P ( BP ) T2 ( BP ) T4 ( BP ) T3 ( BP ) T i whichp is the pre-pressurewhich is about kpa, P is the pressure after the times expad. The calibrated pressure gauge has good liearity at rage above 2kPa, so it ca be cosider that P ad P - is egative correlatio. The measuremet accuracy will be higher tha.% whe the pressure gauge with.pa resolutio is selected. Accoutig for the effect of temperature, the ucertaity of sigle measuremet is about.%~.5%. By mea of may times measuremet, the ucertaity of sequetial icremet method will be better tha.%. If tes times of measuremet data are used, the stadard deviatio of mea value will be.2%~.5%. 2.5 Result of measuremet The volume V ad V2 was measured three times, ad the result was list i table 2. Table 2 result of volume V ad V2 Date VL V2L / I the measuremet, the differet pressure gauge was used. I 26, leak was foud i valve which is betwee the vacuum system ad the turbo pump. The sealig rig, of which the diameter is mm, was chaged. Some deviatio will be itroduced for the differet moutig place of sealig rig. The referece volume method ad the sequetial icremet method are used for -3 ad -2 expad ratio. The result was listed i table 3. Table 3 result of expad ratio Date v -3 2 v -4 3 V v2 V v3 V (R) (R) 9.623(R) (S) 8.997(A) 9.626(R) (R) (R) /

5 496 Yu Hogya et al. / Physics Procedia 32 ( 22 ) (R) (R) 9.64(R) (S) (A) 9.646(S) I which: R the Referece volume method S the Sequetial icremet method A the pre-pressurizatio Accumulatio method The pre-pressurizatio accumulatio method is also used to measure the volume ratio, ad the deviatio with the referece method is about.5%. This is maily for the effect of ad P. The time of pre-pressurizatio accumulatio method used is log, which make the room temperature chage is large ad the the is big. It is hard to make the P do ot chage i the whole experimet process. The room temperature up ad dow will make the deviatio chage o the coutry sidethe temperature up will make the deviatio large, ad temperature dow will make the deviatio small. So the eed of the room temperature for accumulatio method is eed the high. But for the sequetial icremet method, oly oe expad is couted, there is o accumulatio effect of the room temperature. 3. Aalysis of ucertaity For the ucertaity of calibratio volume is the mai part of the ucertaity of volume ratio, so the emphasis of the ucertaity aalysis is the ucertaity of the calibratio volume. The ucertaity below is the relative stadard ucertaity(k=. Iitial pressure ad fial pressure These two pressure are kpa ad 2kPaad are measured by the crystal resoace pressure gauge. It is calibrated bye the gas operated pisto gauge with -5 relative stadard ucertaityad the amedmet is used i the pressure measuremet. The ucertaity of iitial pressure is2-5, ad the ucertaity of fial pressure is Temperature correctio. The ucertaity of temperature amedmet ad the uiformity of temperature profile of the calibratio volume. After the expasio, the temperature correctio was performed, ad the oliearity of the thermometer is less tha.k. The differece betwee the up ad dow of the calibratio volume is about.2k. Several thermometers were used o the surface of the calibratio volume to measure the gas temperature, ad the mea temperature was used to correct the temperature. Type A ucertaity aalysis are used i this aalysis. The ucertaity comig from temperature is Additioal volume correctio. The additioal volumes iclude the volume of the valve ad the tie-i, which are measured by gravimetric method. The ucertaity of the calibratio volume comig form the additioal volume correctio is Real gas behaviours. The secod virial coefficiet correspod to pressure is cosidered for the correct of o ideal gas, the ucertaity is Repeatability. Seve measuremets are performed, ad the repeatability is.6-5. The ucertaity correspod to the above terms is the relative ucertaity of expad ratio f = v/(v+v) is the Coclusio The referece volume method is preseted for the calibratio volume measuremet, which solved the problem that the gravimetric method is ot suitable for the measuremet of calibratio volume. Based o the research of others, the measuremet of volume ratio is bee improved by usig the high precisio pressure gauge, which iclude sequetial icremet method ad pre-pressurizatio accumulatio method. The validity is bee validated ad the differece betwee the these method is at -4.

6 Yu Hogya et al. / Physics Procedia 32 ( 22 ) Referece [] W Jitschi, J K Migwi ad G Grosse, Pressures i the high ad medium vacuum rage geerated by a series expasio stadard, Vacuum 4(99) [2] P Leggat, P Dugdale, J Greewlld, I Sever, B Waller, New developmets i pressure ad vacuum stadards, IMEKO TC6(23) 26-3 [3] Ceter for Vacuum Techology i Kriss,3th [4] M Bergoglio ad A Calcatelli, Ucertaity evaluatio of the IMGC-CNR static expasio system, Metrologia 4(24) [5] R Kagi, B Ogu ad A Elkatmis, The ew UME primary stadard for pressure geeratio i the rage from 9-4 Pa to 3 Pa, Metrologia 4(24) [6] K. Jouste, P. Rohl, V. Arada Cotreras, Volume ratio determiatio i static expasio systems by meas of a spiig rotor gauge, Vacuum 52(999) [7] F.J. Redgrave, A.B. Forbes, P.M. Harris, A discussio of methods for the estimatio of volumetric ratios determied by multiple expasios, Vacuum 53(999) 59-62