ASTM E908-1998(2018) 0625 Standard Practice for Calibrating Gaseous Reference Leaks.pdf

1、Designation: E908 98 (Reapproved 2018)Standard Practice forCalibrating Gaseous Reference Leaks1This standard is issued under the fixed designation E908; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A n

2、umber in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers procedures for calibrating leakartifacts of a specified gas, that may be used for determiningthe response of leak d

3、etectors, or in other situations where aknown small flow of gas is required. The purpose of thispractice is to establish calibration without reference to othercalibrated leaks in as straightforward a manner as possibleusing the likeliest available equipment. While the uncertaintiesassociated with th

4、ese procedures will most likely be greaterthan those obtained via traceable calibration chains (on theorder of 10 %), these procedures allow independent means ofestablishing or verifying the leakage rate from leak artifacts ofquestionable history, or when traceable leak artifacts are notavailable.1.

5、2 Two types of leaks are considered:1.2.1 Type IPressure to vacuum.1.2.2 Type IIPressure to atmosphere.1.3 Three calibration methods are described under each typeof reference leak:1.3.1 Method AAccumulation comparison, using a knownvolume of gas at specified conditions of temperature andpressure as

6、a reference.1.3.2 Method BAccumulation comparison, using a leakartifact calibrated using Method A.1.3.3 Method CDisplacement of a liquid slug, by the leak,in capillary tube of known dimensions.1.4 The values stated in inch-pound units are to be regardedas standard. The values given in parentheses ar

7、e mathematicalconversions to SI units that are provided for information onlyand are not considered standard.1.5 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety,

8、 health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of Int

9、ernational Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:2E425 Definitions of Terms Relating to Leak Testing (With-drawn 1991)3E427 Practice for Testing for Leaks Using the Halogen Le

10、akDetector Alkali-Ion Diode (Withdrawn 2013)3E479 Guide for Preparation of a Leak Testing Specification(Withdrawn 2014)3F134 Test Methods for Determining Hermeticity of ElectronDevices with a Helium Mass Spectrometer Leak Detector(Withdrawn 1996)32.2 Other Documents:AVS 2.2-1968 Method for Vacuum Le

11、ak Calibration4Recommended Practices for the Calibration and Use ofLeaks53. Summary of Practice3.1 Method AAccumulation comparison, using a knownvolume of tracer gas:3.1.1 This method uses a closed chamber of nonreactivematerial having a means of removing all tracer gas and aconnection to the tracer

12、 sensor.3.1.2 A small, known quantity of tracer gas is dischargedinto the chamber and the response recorded for a period of timein which it is anticipated the unknown leak will require to reachthe same concentration.1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Te

13、sting and is the direct responsibility of Subcommittee E07.08 on LeakTesting Method.Current edition approved June 1, 2018. Published July 2018. Originally approvedin 1982. Last previous edition approved in 2012 as E908 - 98 (2012). DOI:10.1520/E0908-98R18.2For referenced ASTM standards, visit the AS

14、TM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.4Available fromAVS,Am

15、erican Vacuum Society, 335 E. 45th Street, New York,N.Y., 10017.5C.D. Ehrlich and J.A. Basford, Journal of Vac. Sci, Technology, A(10), 1992,pp. 117.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was develo

16、ped in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.13.1.3 The tracer gas

17、 is removed from the chamber, and theunknown leak is allowed to discharge into it until the sensorresponse equals that of 3.1.2.3.1.4 The leakage rate in mol/s can be calculated as:Qm5 PV tRT! (1)where:P = pressure in known volume in atmospheres (1atm = 101 325 Pa),V = the volume of gas in cm3introd

18、uced in 3.1.2,t = the time in seconds required for the concentration in3.1.3 to equal that in 3.1.2,R = gas constant = 82.06 = 1 atm cm3/mol/K, andT = absolute temperature, K.3.1.5 It will be observed that chamber volume and sensorlinearity are not factors in this equation. However, the chambervolum

19、e must be selected to give a concentration within thesensor range.Also, this concentration must also be achieved bythe unknown leak discharging into the chamber in a reasonablelength of time and must be appropriate so as not to significantlyaffect the equilibrium flow rate from the leak. This is par

20、ticu-larly true of permeation leaks.3.2 Method BAccumulation comparison using a referenceleak as calibrated in Method A, 3.1:3.2.1 This method is a means of extending the primarycalibration by a factor of up to 10, by comparing withpreviously-calibrated leak artifacts for longer periods of time.For

21、example, a 5 1012mol/s leak that calibrated in MethodA at 300 s can be used for 30 s to calibrate a 5 1013mol/sleak.3.2.2 When this method is used, it should be realized thatthe total possible error will be at least doubled.3.3 Method CDirect measurement of leak rate by timingthe movement of a liqui

22、d slug in a capillary tube of knowndimensions:3.3.1 The tube is closely coupled to the leak, and has avent/fill valve to allow gas filling or positioning of the slug, orboth, which is then driven by the leakage of the gas.3.3.2 Due to capillary “friction,” this method is limited to aminimum leak siz

23、e of about 4 1010mol/s (1 Pam3/s).4. Interferences4.1 Type I Leaks, atmosphere to vacuum, Methods A and B:4.1.1 For the purposes of this section, it will be assumed thatthe gas is helium and the detector is the mass spectrometertuned for helium.NOTE 1Other gases or detectors, or both, can be used wi

24、th littledifference in procedures or interferences.4.1.2 Pressure RiseThere will inevitably be some pressurerise in a closed evacuated chamber, due to outgassing and smallleaks. This may cause a decrease in ionization efficiency in thespectrometer tube and thus a steadily declining signal asindicate

25、d in Fig. 1. However, this effect should be quiteconstant from run to run, and so largely cancel out in finalresult.4.1.3 Helium Signal RiseThere will usually be a notice-able increase in helium signal when the chamber is closed, dueto outgassing and in-leakage from the atmosphere as indicatedin Fig

26、. 1. Again, this will be a constant which mostly cancelsout.4.1.4 Spectrometer Sensitivity DriftThis will be noticed asvariations in zero and in reading levels with the same heliuminput.With properly tuned and maintained systems operating atleast one decade below maximum sensitivity, this should be

27、aminor effect.4.1.5 LeaksAll detectable valve leaks and leaks from theatmosphere should be repaired.4.1.6 Barometric Variations(Not applicable to sealed res-ervoir units.) If the gage used to measure the pressure in theknown volume is of the gage type, then account must be madeFIG. 1 Typical Detecto

28、r Curves and Deviation LimitsE908 98 (2018)2of the local barometric pressure when calculating the absolutepressure. This is probably true for falling pressures of theknown volume near 1 atmosphere or less.4.1.7 Temperature DriftChanges in temperature betweenmeasurements may result in slight variatio

29、ns in indicatedpressures. These should be recorded and compensated foraccordingly.4.2 Type I Leaks, atmosphere to vacuum, Method C:4.2.1 Liquid Slug FrictionThis can be appreciable insmall capillaries. It should be measured and a correction madefor it.4.2.2 Vapor Pressure of LiquidWater is the recom

30、mendedliquid, and has a vapor pressure of about 20 mm Hg (3 kPa) atroom temperature. This gives a theoretical increase in leakindication of 20/760 (3 103/1105) or approximately 3 %.This correction should be added to the final result.4.2.3 Excess Volume Between Leak and CapillaryThiswill cause delaye

31、d and jerky movement of the slug, and shouldbe kept to an absolute minimum.4.2.4 Dirty CapillarySymptoms similar to 4.2.3. The slugshould move smoothly when capillary tube is held at an angle.4.3 Type II Leaks, pressure to standard atmosphere, Meth-ods A and B:4.3.1 For the purposes of this section,

32、 it will be assumed thatthe gas is fluorocarbon and the detector is the alkali-ionhalogen detector diode. Other gases or detectors, or both, canbe used with little difference in procedures or interferences.4.3.2 Halogen Signal RiseThere will usually be a smallincrease in halogen signal due to outgas

33、sing, particularly fromelastomers or plastics. With minimum use of these materials inthe chamber, no correction for this will ordinarily be needed.4.3.3 Sensor Sensitivity DriftThis will be noticed as varia-tions in zero and reading levels with the same halogen input.With properly maintained systems

34、 operating at least onedecade below maximum sensitivity, this should be a minoreffect.4.3.4 Barometric VariationsSubstantial variations fromstandard atmosphere pressure should be corrected.4.4 Type II Leaks, pressure to atmosphere, MethodCSame as Type I, Method C, in 4.2.5. Apparatus5.1 Type I Leaks

35、, pressure to vacuum, Methods A and B:5.1.1 Mass Spectrometer with Remote Tube Tuned forHeliumMinimum resolution (5 1015mol/s) helium, whenoperated as a leak detector.5.1.2 Helium Supply with Pressure Regulator and Flowme-ter (approximately 10 cm3/s).5.1.3 Stainless-Steel Chamber (see Fig. 2) with p

36、rovisionsfor:5.1.3.1 Attachment of spectrometer tube,5.1.3.2 Liquid nitrogen trap,5.1.3.3 Vacuum pumping to at least 1 106torr (130 Pa)with isolating valve,5.1.3.4 Ionization vacuum gage,5.1.3.5 Attachment of helium leak with isolating valve andseparate rough pumping means,5.1.3.6 Measured helium vo

37、lume device (see Fig. 3) (seeNote 2), and5.1.3.7 Strip chart or flat-bed recorder.NOTE 2Other types of calibrated volumes in this range may besubstituted.FIG. 2 Equipment for Calibrating Helium Leaks, Type I, Methods A and BE908 98 (2018)35.1.3.8 Thermometer.5.2 Type I Leaks, pressure to vacuum, Met

38、hod C:5.2.1 Glass Capillary Tube with Vent Valve (see Fig. 4).5.2.2 Timer or Stop Watch.5.2.3 Helium Supply.5.2.4 Indicator Fluid (dyed water).5.2.5 Thermometer.5.3 Type II Leaks, pressure to atmosphere, Methods A andB:5.3.1 Halogen DetectorMinimum sensitivity 4 1013mol/s (1 nPam3/s).5.3.2 Fluorocar

39、bon Supply with Flowmeter.5.3.3 Stainless-Steel Chamber (see Fig. 5) with provisionsfor:5.3.3.1 Attachment of sensor sampling tube,5.3.3.2 Pure air supply,5.3.3.3 Attachment of halogen leak,5.3.3.4 Measured halogen volume device (see Fig. 3), and5.3.3.5 Strip chart or flat-bed recorder.5.3.3.6 Therm

40、ometer.5.4 Type II Leaks, pressure to atmosphere, Method C:5.4.1 Glass Capillary with Vent Valve (see Fig. 4).5.4.2 Timer or Stop Watch.5.4.3 Halogen Supply.5.4.4 Indicator Fluid (dyed water).5.4.5 Thermometer.6. Procedure6.1 Type I Leaks, atmosphere (or sealed reservoir) tovacuum, Method A:6.1.1 St

41、art vacuum pumps, and pump chamber (see Fig. 2)down to 106torr (130 Pa) or lower, as measured by the iongage. Fill liquid nitrogen trap.6.1.2 Attach measured helium volume device (see Fig. 3),and evacuate to the helium inlet valve.6.1.3 Start mass spectrometer and determine that it isproperly tuned

42、to required sensitivity, and is stable.6.1.4 With the helium outlet valve open, pass helium byhelium inlet valve. No leakage should be observed.6.1.5 Close helium outlet valve and open inlet valve for 5 s.No leakage should be observed.6.1.6 Attach and evacuate leak to be calibrated. Applyhelium if n

43、ot a sealed reservoir type, record the heliumpressure. Allow the system, including the leak itself, sufficienttime to equilibrate.6.1.7 Close vacuum valve and record rate of helium signalrise on the strip chart recorder for several minutes, selecting arange that will stay on scale for this length of

44、 time.6.1.8 Isolate helium leak and pump down the chamber untilchart reads zero. When isolating the helium leak from thechamber, alternate pumping on the leak should be provided, orsufficient time for reequilibration must be left, to attain lowestuncertainties. Reset chart to time zero and close vac

45、uum valve.FIG. 3 Measured Volume DeviceFIG. 4 Calibration Capillary and Vent Valve AssemblyE908 98 (2018)4Record rise for the same period. If the rise exceeds 2 % of thehelium signal, locate the source (such as air in leakage,outgassing of elastomers, or leaky valves) and repair.6.1.9 Note the press

46、ure rise in the system. If it exceeds1104torr (13 mPa) at the end of the time period, repair theleaks responsible.6.1.10 Reevacuate the chamber, close the vacuum valve,and admit the measured helium volume, and note whether ornot the signal is on scale and at least 30 % of full scale.6.1.11 It will b

47、e necessary to have the traces of the unknownleak and the known volume of helium cross in no less than 30s and no longer than it takes: (See Fig. 1.)6.1.11.1 The background helium signal to rise 2 %.6.1.11.2 The helium leak signal to depart 10 % from linear-ity.6.1.11.3 The known helium volume signa

48、l to change 10 %.6.1.11.4 The pressure to rise to more than 1 104torr (13mPa). (With a proper system this time should be in excess of 5min.)6.1.12 If necessary, adjust:6.1.12.1 The accumulation time (within the above limits).6.1.12.2 The helium pressure in the measured volume.6.1.12.3 The size of th

49、e measured volume.6.1.12.4 The sensitivity of the mass spectrometer.6.1.13 Retrace all the above curves. No variation in excessof 2 % should occur.6.1.14 Calculate the unknown leakage rate by:Qm5 PV/tRT! (2)where:Qm= leakage rate, mol/s,P = pressure at known volume in atmospheres (1atm = 101 325 Pa),V = volume of helium, cm3,t = time at which the leak and standard traces cross, s,R = gas constant = 82.06 atm cm3/mol/K, andT = absolute temperature, K.See Fig. 1 and Fig. 6.6.2 Type I Leaks, atmosphere (or sealed chamb