Leak Calibration Service

1 Page 1 NISTMeasurementServices: . . T . ^^^^^^^^^^^^^^^^^^^^^^^^^^ ResearchInformationCenter NationalInstituteofStandards andTechnology Gaitbersburg,Maryland203*) NISTLeakCalibration Service CharlesD.EhrlichandStuartA.Tison U.S... Open at page → 3 Page 3 NiSTMEASUREMENTSERVICES: NISTLeakCalibrationService CharlesD.EhrlichandStuartA.Tison Thermophysics Division ChemicalScienceandTechnologyLaboratory NationalInstituteofStandardsandTechnology Gaithersburg,MD20899 January199... Open at page → 4 Page 4 NationalInstituteofStandardsandTechnologySpecialPublication250-38 Natl.Inst.Stand.Technol.Spec.Publ.250-38,89pages(Jan.1992) CODEN:NSPUE2 U.S.GOVERNMENTPRINTINGOFFICE WASHINGTON: 1992 ForsalebytheSuperintendent ofDocumen... Open at page → 5 Page 5 PREFACE Calibrations andrelatedmeasurement services oftheNational InstituteofStandardsandTechnologyprovidethemeansformakers andusersofmeasuringtoolstoachievelevelsofmeasurement accuracythatarenecessarytoattainquality,pro... Open at page → 6 Page 6 ServicesUsersGuide1989,pages49to51.Inquiriesconcerning thecontentsofthesedocumentsmaybedirectedtotheauthor(s)or tooneofthetechnicalcontactpersonsidentifiedintheUsers Guide. Suggestionsforimprovingtheeffectiveness andusef... Open at page → 7 Page 7 TABLEOFCONTENTS Page Preface iii 1.Introduction 1 2.DescriptionoftheNISTLeakCalibrationService 2 3.DesignPhilosophyandTheory 3 DesignPhilosophy: Introduction 3 DesignPhilosophy:NISTPrimaryLeakStandard 3 FlowintoUpperCham... Open at page → 8 Page 8 TABLEOFCONTENTS(CONTINUED) Page 6.QualityControl 30 ControlCharts 30 Closed-Reservoir HeliumPermeationLeaks 30 Upper-to-Lower ChamberFlowRatio 31 ComparisonofPrimaryStandards 31 FlowmeterIntercomparisons 31 DomesticInter... Open at page → 9 Page 9 LISTOFTABLES Page 1 1. ConversionofUnitsforUniversalbasLionscant\&) 4 2. BellowsFlowmeterUncertainties 21 3. SummaryofUncertainties forFlowintotheUpperChamber 25 4. SummaryofUncertainties forFlowintotheLowerChamber 27 A-... Open at page → 10 Page 10 LISTOFFIGURES Page 1. SimpleSchematicoftheNISTPrimaryLeakStandard 40 2. SchematicoftheNISTPrimaryLeakStandard 41 3. ValvingSequenceforLeakCalibrationThroughUpperChamber 42 4. ValvingSequenceforLeakCalibrationThroughLower... Open at page → 11 Page 11 1 .INTRODUCTION Theindustrialneedfordemonstratedaccuracyoflow-rangegasflowrateor leakratemeasurements usedtocalibrate leakdetectorsandgasanalyzers, traceabletoarecognizedstandardsauthority,promptedtheNationalInstitute of... Open at page → 12 Page 12 2.DESCRIPTIONOFTHENISTLEAKCALIBRATIONSERVICE TheThermophysics Division oftheNational Institute ofStandards and Technology provides aCalibration Service* forheliumpermeation leak artifactsintherange 10" 8 to 10"11 mol/s(2... Open at page → 13 Page 13 3.DESIGNPHILOSOPHYANDTHEORY DESIGNPHILOSOPHY: INTRODUCTION ThePrimaryLeakStandard thathasbeendeveloped atNIST isshown schematically infigure 1.Thistypeofapparatuswaschosenoverseveral alternatives becauseitbenefitsfromcon... Open at page → 14 Page 14 FlowintoUpperChamber AfterStep 2ofthecalibration process thesystem isallowedtimeto equilibrate, atwhichpointthevalvingintheflowmeterisconfiguredandthe flowmeterelectronicsareactivatedsothattheflowrateofthegasemanating fr... Open at page → 15 Page 15 (]?U1 -P L1 ) .C ol Ni=N oi = (2) R •T o1 whereP L1 isthepressureinthelowerchamber.Butthismustalsoequalthe flowrateofgasthroughthepump,whichfromeq(B12)is N sl = *L1 ' S l R•T o1 (3) EquatingNjwithN s x thengives Pui=P L... Open at page → 16 Page 16 Whencomparingeq(8)andthecorresponding parameters ineq(1),notethat P r 3 replacesP w3 andthatAV/AtreplacesdV/dt. Thisisduetothefactthat intheflowmeter, itisthereference volumepressure (P r )andnotthe workingvolumepressure... Open at page → 17 Page 17 equalto 1fortheslightvariations inroomtemperature, itisroutinely retainedinthedataanalysisroutine. Adifferenceof1°Cinthechamber temperature resultsinacorrection factorof1.002ineq(10). Further discussionconcerningsourceso... Open at page → 18 Page 18 FromStep1,conservation offlowrate(N x )attheflowmeter, orificeand pumpgives : Pli 'S i N x = (11) and Si Pui=Pli ' ( 1+ ) • (12) Col InStep2,underequilibriumconditions, therecanbenonetflowacrossthe orifice,sothat P U2=P... Open at page → 19 Page 19 P r2 .(AV/At) 2 •T rl <Rl/u>» = <16 > P rl •(AV/At), •T r2 wherethesubscript"m"referstothemeasuredquantity. Substituting eq(16) intoeq(15)andsolvingfor(R L/u)t (referencedtoS x andT Qx )gives: P r2 •(AV/At) 2 •T rl P ux... Open at page → 20 Page 20 FlowintoLowerChamber Withtheupper-to-lowerchamberflowrationowdetermined, itispossibleto usethetechniqueshowninfigure4tomeasureflowratesbelow10"12 mol/s withtheflowmeter,ortomeasurehigherflowrateswithsmalleruncertainties... Open at page → 21 Page 21 . . P U3 * S 3 *T ol S l N 3 /N L = • (1+ ). (25) P U1 *S l 'T o3 C ol Fromthediscussion intheprevioussection,thelastfactorontherightin eq(25), 1 4-S 1 /C ol ,isthetheoretical flowrateratio(R L/U)t forpump speed S x ando... Open at page → 22 Page 22 leakorstandardleak,orbothleaks,canbevalvedintotheleakdetector. Whengasfromaleakisnotroutedtotheleakdetector,thenitisroutedto theturbopump,sothatbothleaksarecontinuously pumped. Theunknownleak isencased inatemperature-con... Open at page → 23 Page 23 andeq(30)intheform a [L(T)/LJ -1 (T -T Q) (31) B= T •T, (T-T D) Jin L(T) •T (32) Notethatinbotheqs(31)and(32)theratio(L(T)/L Q)appears,andnotL Q alone,sothatitisnotnecessarytoknowtheabsolutemagnitudeoftheleak rate,butonl... Open at page → 24 Page 24 4.DESCRIPTIONOFSYSTEMANDOPERATIONALPROCEDURES PRIMARYLEAKSTANDARDSYSTEMDESCRIPTION TheNISTPrimaryLeakStandard(PLS)iscomposedofthreemaincomponents; themainvacuumchamber, theleakmanifold,andaflowmeter. ThePLSmaybe usedwith... Open at page → 25 Page 25 pressuresrangingfrom 1x 10" 4 to 1x10" 8 Pa. Thisrequiresthatthe turbomolecular pumphavethecapacityandstabilityoverthecorresponding flowratestoachievepumpingspeedstabilities oflessthan0.1%perhour. Itisalsonecessarythatth... Open at page → 26 Page 26 closingvalveL30andopeningvalveL24iftheunknownleakisinstalledon portPi.ThegasthenflowsthroughvalvesL21andL36andenterstheupper chamberofthemainvacuumsystem,andisultimatelyevacuatedthroughthe orificeintoaturbomolecular pump... Open at page → 27 Page 27 Theflowfromtheflowmeterismeasuredinthefollowingmanner. The 1Torr differential CDG(D5)iszeroedandthevariableandreservoirvolumesare isolatedbyclosingvalvesBlandB2 .Thepistonisthensetinmotionby pushingtheenablebuttonontheco... Open at page → 28 Page 28 TheLCScontrolsystemusesanIBM-compatible computertocontrolthedata acquisition andanalysis ofthedata. Itsmainfunctions include the following:controlofthetemperaturebathtowithin0.02°Cofthesetpoint temperature, controlofthev... Open at page → 29 Page 29 zerodrifts. Thisprocessisrepeatedforthedatapointsateachtemperature inthedatacollectionsequence. Attheconclusionofthedataacquisition theprogramperformsaleastsquarefittothedatausingthefunctionalform ofeq(33)insection 3.The... Open at page → 30 Page 30 5.ASSESSMENTOFUNCERTAINTIES PRIMARYLEAKSTANDARD Flowmeters Thefundamentalequationsrelatingthemeasuredleakagerateofaleak(ata giventemperature) tothemeasuredparameters ofthePrimaryLeakStandard werederivedinsection 3(eqs(10... Open at page → 31 Page 31 LO CO LU 1 — CO LU LJ 21 ZD C CD CO CD QQ <3 < O -4— > 1 II CD 1 1 --r-1m O0 1 1X1 --r-| 0 inO 1 COLOrn OOOO0O cnO en -«— 1 -r-l 1 1 0 •f— 1 1 j 1 --r-1m 1— 1 CVJ -r-lOO0 -i-t00 **sT CVJOOJ -= «-— 1 inOO0O cnO 0 --r-l -r... Open at page → 32 Page 32 Table 2givesestimateduncertainties oftheindividual termsforthe "bellows"flowmeterforflowrateswhicharelower(10" 12 mol/s)thanthose presented inAppendix Dforthe"piston" flowmeter. Thisinformation is presentedgraphicallyinf... Open at page → 33 Page 33 courseofaworkday,particularly intheafternoon,werenotuncommon, sothat corrections certainlyneededtobetakenintoaccount. Theassumption thatthepumpingspeed isdirectlyproportional tothe rotational frequency isdifficult toveri... Open at page → 34 Page 34 foundtodependstronglyontheparticularoperatingrangeofthequadrupole instruments, andatthelowestpressures (10~ 7 Paandbelow),correctingfor thedifferential hydrogenoutgassing rendered thetotalpressure gageof marginaluse. Itw... Open at page → 35 Page 35 discussanoveralluncertaintyassociatedwithanindividualmeasurement of flowratebyflowintotheupperchamber. Theequationwhichdescribesthis flowrateisreproducedhereforconvenience: P r3 (AV/At) Pul To 3 Si C o3 +S 3 R •T r3 _ P... Open at page → 36 Page 36 flowratioiseq(17).Aslightlymodifiedform(thereciprocalvalue)is: P rl •(AV/At) 1 •T r2 P U2 •S 2 •T 0l (R u/L)t(Si,Tol )= — • . (17a) P r2 •(AV/At) 2 •Tr x P 01 •S x •T o2 Thefirstsetoffactorsinvolvingtheflowmeter isagaina... Open at page → 37 Page 37 whichcontainselementssimilartothoseinbothofthesectionsabove. The uncertainties associatedwiththeparticularfactorsareagainafunctionof theflowrateoftheflowmeterandaresummarizedintable4. TABLE4.SummaryofUncertainties forFlo... Open at page → 38 Page 38 reportispresentedinAppendixE. LEAKCOMPARATORSYSTEM Asdiscussed earlier, theexponential temperature coefficient ofhelium permeationleaksisdeterminedusingtheLeakComparatorSystem(LCS) .This isdonebyfittingtherelativeleakrat... Open at page → 39 Page 39 Thisuncertainty inBmustthenbetranslated intoanuncertainty inleak rate,whichisexpressed asanadditionaluncertainty per°Cawayfromthe 23°Creferencetemperatureatwhichtheleakwasmeasuredontheprimaryleak standard. Rewritingeq(30... Open at page → 40 Page 40 6.QUALITYCONTROL Oncetheoperatingcharacteristics anduncertainties ofaprimarystandard havebeenestablished, subsequent useoftheprimarystandardatthesame levelofuncertaintyrequiresperiodicverification toensurethatnoneofthe u... Open at page → 41 Page 41 Asanexample,foraleakrateof 10" 9 mol/s,afillpressureof145psi(1 MPa) ,andareservoirvolumeof200cm 3 ,thetimeconstant risapproximately 3yr(1yr=3.1536xl0 7 sec). In1yr,theleakratewillhavedecayedby(1- e" 1/3 ),or28%. Mostleak... Open at page → 42 Page 42 frombothflowmeterscombined. TheNISTserialnumberfortheleakusedata givenflowrateisindicatedalongthetopaxisofthefigure. Themeasured randomuncertainties areseentoslightlyincreasewithdecreasingflowrate. Figure18presentsthedif... Open at page → 43 Page 43 International Intercomparisons Whiletherehavenotbeenanyinternational roundrobinsforNISTto participate insincetheleakprogrambeganin1984,aroundrobinsponsored bytheCommissionoftheEuropeanCommunities, CommunityBureauofRefere... Open at page → 44 Page 44 7.FUTURE Thereareseveralareasinwhichtoconcentrate futureresearchactivities associated withtheNISTLeakCalibration Service. First, there is considerable industryinterest inhavingNISTextendthelowerendofthe serviceatleastano... Open at page → 45 Page 45 servicecanbeperformedonthenewLCS. Thiswillmaketheprocessmore efficientinseveralways,reducingthecostpercalibration ofanindividual leak. 8.ACKNOWLEDGMENTS Theauthorswouldliketoacknowledge themajorcontributions ofDrs .K.E.... Open at page → 47 Page 47 9.REFERENCES [1] S.Dittmann, "HighVacuumStandardandItsUse,"U.S.Government PrintingOffice,NISTSpecialPublication250-34 .31(1989) [2]R.W.Hyland,CD.Ehrlich, C.R.TilfordandS.Thornberg, "Transfer LeakStudiesandComparisons ofP... Open at page → 48 Page 48 [15]J.H.DymondandE.B.Smith,"TheVirialCoefficients ofGases,"Oxford UniversityPress,copyright1969 [16]J.F.O'Hanlon, "AUser'sGuidetoVacuumTechnology," J.Wiley&Sons, copyright1980 38 Open at page → 49 Page 49 10.BIBLIOGRAPHY Inadditiontothereferencescitedabove,theinterestedreadermaywishto consultthefollowing. Units : "Standardization andTemperatureCorrectionofCalibratedLeaks,"CM. Soloman,J.Vac.Sci.Technol.A4,327(1986) Leaks :... Open at page → 50 Page 50 Flowmeter Nfm Vacuum Chamber Upper Chamber P,T Lower Chamber Turbo Pump LeakManifold Leak ,Partial Lj—UrrPressureGage Orifice Figure 1. SchematicdrawingoftheNISTPrimaryLeakStandard,showingthe majorcomponents. Thepressure... Open at page → 51 Page 51 41 Open at page → 52 Page 52 LEAKCALIBRATIONTHROUGHUPPERCHAMBER Z5£ 35 Step3 Tch3 38= O Pu3 Turbo S3 Unknown Leak Figure3.Valvingsequenceformeasuringleakrateofunknownleakbyflowing gasfromflowmeterintotheupperchamberofthemainvacuumchamber. 42 Open at page → 53 Page 53 LEAKCALIBRATIONTHROUGHLOWERCHAMBER 331 Figure4.Valvingsequenceformeasuringleakrateofunknownleakbyflowing gasfromflowmeterintothelowerchamberofthemainvacuumchamber. 43 Open at page → 54 Page 54 Figure5.Valvingsequenceformeasuringflowrateratio. 44 Open at page → 55 Page 55 LeakComparator Ballasto Gas Cylinder I.G. S^Jv9 SV8 rv s = L* a I I r SV6 Computer Control System ~jTemperature Control System I L_ S.V. 6-9 1 Electrometer l: LeakDetector Figure6.SchematicdiagramoftheLeakComparatorSyste... Open at page → 56 Page 56 o 0 Q 0 —9 co 0 Q. E _^ CO 0 O 6 a; w 00 5g * ?o <u e C og*t a) ^ a) $ uo 4J*d •HC 64-1 <u o ex ^ rn 2! •X3 ,0 Q)O o^a. 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Open at page → 70 Page 70 t C o (0 o o JL c o V) Q_ CM CM a. \\\\\\\\\\\ Eo 0) 4JW co a) co.h (0 Cfl4J S-io X> IP Io CO CO 0) CO CO o 4-> P o o CD co 0) M Pu P C cfl P CO P O O •H CO™ o o P 29 CD cfl s cu x: CJ CO CM ctj a CO p o v . CO dj •H 0 P... Open at page → 71 Page 71 11.APPENDICES APPENDIX A USEANDCONVERSIONOFUNITSOFFLOWRATEANDTHROUGHPUT Todiscussthedifferencesbetweenflowrateandthroughputunits,consider thesimpleexperimental apparatusinfigure20,whereacylindricalvolumeis dividedbyaplat... Open at page → 72 Page 72 Q=P(dV/dt)=(1atm)(1cm 3 s" 1 )=1atmcm 3 s" 1 , (A4) independentofthetemperatureofthegas. Ifthegastemperature is273.15K (0°C),themolarflowrateofgascanbecomputedusingeq(A3)as Qm =Q/(RT) =(1atmcm 3 s" 1 )/[(82 .057atmcm 3 m... Open at page → 73 Page 73 thisflowis1atmcm 3 s" 1 (323°C. ItisimportanttonotethatPVunitsor throughputunitsarenotreallyunitsatall,inthesensethatontheirown theydonothavecleardimensions: theyaremerelyexpressions ofconvenience thatare,infact,definedb... Open at page → 74 Page 74 quadrant,astheTableAlcaptionindicates isintended, thethroughputunits canthenberegardedasflowrateunits,andconversionofflowrateunits acrosstheheavyblacklineisthenallowed. Substituting eq(All)intoeq (AlO)thengives A 3 {Pam... Open at page → 75 Page 75 1x10" 7 atmcm 3 s"1@0°C=A 3 Torr1s" 1 @23°C. (A19) Then,ineq(A16) ,A A =1x10" 7 ,QA =atmcm 3 s" 1 ,andT A =0°C=273.15K,Q3 =Torr1s" 1 ,andT 3 =23°C=296.15K.FromTableAl ,theconversionfactor ineq(A17)is1atmcm 3 s" 1 =(0.760... Open at page → 76 Page 76 A-6 Open at page → 77 Page 77 APPENDIXB GENERALTHEORYOFGASFLOWINVACUUMSYSTEMS Theflowofhighlyrarefiedgasesthroughvariouscomponents ofavacuum apparatushasbeenatopicofseriousinterestsincetheearly1900's.The worksofKnudsen[11],Smoluchowski [12],Dushman [... Open at page → 78 Page 78 mol" 1 ),andthecoefficients B t arethe"virialcoefficients", whichare availableintheliteratureformanygases(e.g.,[15]). Asanexample,for heliumatroomtemperature andatmospheric pressure, thecorrection term corresponding toth... Open at page → 79 Page 79 3)A/D~1 certaingeometries otherthananaperture it ispossible toderiveexplicitexpressions, suchasPoiseuilleflowthrougha'long'pipe. transitionflowThenetflowrateisingeneralafunctionof severalparameters, andadequatetheoriesar... Open at page → 80 Page 80 Therateatwhichgasleavesvolume 1(orentersvolume2)isthen: dNj dN 2 N^t-0)C -e' t/T . (BIO) dt dt V x Theseresultsdemonstrate thatevenforsuchaverysimplegasflowsystemthe flowrateofthegasthroughtheorificeisconstantlychanging,... Open at page → 81 Page 81 APPENDIX C Anoteonflowrateandleakrateunits CharlesD.Ehrlich CenterforBasicStandards.NationalBureauofStandards.Gaithersburg.Maryland20899 (Received31October1985;accepted28April1986) Theconfusionintheliteratureandinthelabo... Open at page → 82 Page 82 CharlesD.Ehrllch:Anoteonflowrateandleakrateunits ityforconfusion isgreatlyreduced. Anothercommonpracticewithleakrateunitsistoex- presstheleakrateintermsof"standard"conditions,suchas "standardcmVs"or"cmVsSTP."Unfortunatel... Open at page → 83 Page 83 APPENDIX I) Low-rangeflowmetersforusewithvacuumandleakstandards K.E.McCulloh,C.R.Tilford,C.D.Ehrlich,andF.G.Long NationalBureauofStandards.CenterforBasicStandards.Gaithersburji.Maryland20X99 (Received20October1986;accept... Open at page → 84 Page 84 377 McCullohelat.:Low-rangeflowmeters(orusewithvacuumandleakstandards 377 Fro.1.Schematicoftheconstantpressure pistonflowmeter.Twodifferentsizepis- tonsareincludedtoextendtherange.The turbomolccularpumpprovidesalowref- e... Open at page → 85 Page 85 378 McCullohetal. :Low-rangeflowmeters(orusewithvacuumandleakstandards 378 ballasttank,andreferencevolume.Theflowmetershave beenoperatedwithhydrogen,helium,neon,nitrogen,and argon.Thesystempressure isbroughtuptothevaluer... Open at page → 86 Page 86 379 slightlyaboveatmosphericwillberequiredtocompressthe bellows.Thiswillminimizeleaks,inparticular,leakageof airintotheoil,whichwouldcausesignificanterrors,and maintainpositivecontactbetweenthepistonandthedrive mechanism... Open at page → 87 Page 87 380 McCullohelal.:Low-rangeflowmeterslorusewithvacuumandleakstandards 380 D.Timeinterval:6(A/)/Af Theelapsedtimeisrecordedforeach Iurnasihemicro- meterscrewadvances.Timeintervalsarecalculatedfrom thedifferencesofsuccessi... Open at page → 88 Page 88 381 A.Randomerrors Themeasuredrandomerrorsduringgaugeandleakcali- brationsdifferwithflowrateandtypeofdevicecalibrated. Thepooledstandarddeviationaboutthemeanforsixmolec- ulardraggaugesrepeatedlycalibratedoveratwo-monthpe... Open at page → 89 Page 89 UNITEDSTATESDEPARTMENTOFCOMMERCE NationalBureauofStandards Gaithersburg.Maryland2QB99 SAMPLECALIBRATIONREPORT ReportDate:February 1,1988 LEAKARTIFACT Submittedby NameofCompany,Inc. Anywhere,USA TestperformedJanuary1988 N... Open at page → 90 Page 90 REPORTOFTEST(continued) Page2 PROCEDURES Thisleakartifactwascalibratednear23°Cbycomparisonwith Flowmeter#2oftheNBSPrimaryLeakStandard,usingtheuppertolower chamberflowdivisiontechnique. Thetemperaturedependencewasdetermin... Open at page → 91 Page 91 REPORTOFTEST(continued) Page3 "REPORTOFTEST" NBSTest#L12/240840 ReportDate :February 1,1988 + LEAKRATEvs.TEMPERATURE LeakRate LeakRate erap.(°C) mol/s stdcc/s Temp.(°C) mol/s L * stdcc/s 0 2.16E-12 4.84E-08 1 2.25E-12 5.... Open at page → 93 Page 93 NIST-114A U.S.DEPARTMENTOFCOMMERCE 1.PUBLICATIONORREPORTNUMBER (REV.3-90) NATIONALINSTITUTEOFSTANDARDSANDTECHNOLOGY NIST/SP-250/38 2.PERFORMINGORGANIZATIONREPORTNUMBER BIBLIOGRAPHICDATASHEET 3.PUBUCATIONDATE January1992... Open at page → 95 Page 95 THESP250SERIESONNISTMEASUREMENTSERVICES* SP250-1 SpectralRadianceCalibrations PB87179883 SP250-2 FarUltravioletDetectorStandards PB87227609 SP250-3 RadiometricStandards intheVacuum Ultraviolet PB87227625 SP250-4 FrickeDo... Open at page → 96 Page 96 >> D) O O c sz o <D E - Oc£ CO05 OC0§ ^h-CM tu w« § a>-=wn10u LJ C<D •0£ "Tim 3ZO 3 COCD cg wQ- cqo To oCO OQ- Open at page →