1、Designation: E 908 98 (Reapproved 2004)Standard Practice forCalibrating Gaseous Reference Leaks1This standard is issued under the fixed designation E 908; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A
2、 number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 lea
3、k detectors, 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
4、 these 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
5、.1.2 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
6、as 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 the standard. The metric equivalents of i
7、nch-pound unitsmay be appropriate.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 and health practices and determine the applica-bility of regulatory limita
8、tions prior to use.2. Referenced Documents2.1 ASTM Standards:E 425 Terminology Relating to Leak Testing2E 427 Practice for Testing for Leaks Using the HalogenLeak Detector (Alkali-Ion Diode)3E 479 Guide for Preparation of a Leak Testing Specifica-tion3F 134 Test Methods for Determining Hermeticity o
9、f Elec-tron Devices With a Helium Mass Spectrometer LeakDetector32.2 Other Documents:AVS 2.2-1968 Method for Vacuum Leak 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 metho
10、d uses a closed chamber of nonreactivematerial having a means of removing all tracer gas and aconnection to the tracer 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 requi
11、re to reachthe same concentration.3.1.3 The tracer gas 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
12、(1atm = 101 325 Pa),V = the volume of gas in cm3introduced 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, and1This practice is under the jurisdiction of ASTM Committee E07 on Nonde-structive Testing and is th
13、e direct responsibility of Subcommittee E07.08 on LeakTesting.Current edition approved May 1, 2004. Published June 2004. Originallyapproved in 1982. Last previous edition approved in 1998 as E 908 - 98.2Withdrawn. Replaced by Terminology E 1316.3For referenced ASTM standards, visit the ASTM website,
14、 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.4Available from AVS, American Vacuum Society, 335 E. 45th Street, New York,N.Y., 10017.5C.D. Ehrlich and J.A. Basfor
15、d, Journal of Vac. Sci, Technology, A(10), 1992,pp. 117.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.T = absolute temperature, K.3.1.5 It will be observed that chamber volume and sensorlinearity are not factors in this equation. H
16、owever, the chambervolume 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 f
17、rom the leak. This is particu-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 lo
18、nger periods of time.For example, a 5 3 1012mol/s leak that calibrated in MethodA at 300 s can be used for 30 s to calibrate a 5 3 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
19、timingthe movement of a liquid 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
20、 limited to aminimum leak size of about 4 3 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 det
21、ectors, or both, can be used with littledifference in procedures or interferences.4.1.2 Pressure RiseThere will inevitably be some pres-sure rise in a closed evacuated chamber, due to outgassing andsmall leaks. This may cause a decrease in ionization efficiencyin the spectrometer tube and thus a ste
22、adily declining signal asindicated 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
23、the atmosphere as indicatedin Fig. 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 ma
24、ximum sensitivity, this should be 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
25、must be madeof 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 variations in indicatedpressures. These
26、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 recommendedliquid, and has a vapor pr
27、essure of about 20 mm Hg (3 kPa) atroom temperature. This gives a theoretical increase in leakindication of 20/760 (3 3 103/1 3 105) or approximately 3 %.This correction should be added to the final result.FIG. 1 Typical Detector Curves and Deviation LimitsE 908 98 (2004)24.2.3 Excess Volume Between
28、 Leak and CapillaryThiswill cause delayed 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
29、:4.3.1 For the purposes of this section, 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 small
30、increase in halogen signal due to outgassing, 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 asvariations in zero and reading levels with the same halogeni
31、nput. With properly maintained systems 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
32、C, in 4.2.5. Apparatus5.1 Type I Leaks, pressure to vacuum, Methods A and B:5.1.1 Mass Spectrometer with Remote Tube Tuned forHeliumMinimum resolution (5 3 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 Sta
33、inless-Steel Chamber (see Fig. 2) with provisionsfor: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 3 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 roug
34、h pumping means,5.1.3.6 Measured helium volume 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.5.1.3.8 Thermometer.5.2 Type I Leaks, pressure to vacuum, Method C:5.2.1 Glass Capillary Tube with Vent
35、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 3 1013mol/s (1 nPam3/s).5.3.2 Fluorocarbon Supply with Flowmeter.5.3.3 Stainless
36、-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 Thermometer.5.4 Type II Leaks, pressure to atm
37、osphere, Method C:FIG. 2 Equipment for Calibrating Helium Leaks, Type I, Methods A and BE 908 98 (2004)35.4.1 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.FIG. 3 Measured Volume DeviceFIG. 4 Calibrati
38、on Capillary and Vent Valve AssemblyFIG. 5 Equipment for Calibrating Halogen Leaks, Type II, Methods A and BE 908 98 (2004)46. Procedure6.1 Type I Leaks, atmosphere (or sealed reservoir) tovacuum, Method A:6.1.1 Start vacuum pumps, and pump chamber (see Fig. 2)down to 106torr (130 Pa) or lower, as m
39、easured 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 to required sensitivity, and is stable.6.1.4 With the helium outlet valve open, pass h
40、elium 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 not a sealed reservoir type, record the heliumpressure. Allow the system, including the
41、 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 time.6.1.8 Isolate helium leak and pump down the chamber untilchart reads zero. When
42、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 vacuum valve.Record rise for the same period. If the rise exceeds 2 % of thehelium signal
43、, locate the source (such as air in leakage,outgassing of elastomers, or leaky valves) and repair.6.1.9 Note the pressure rise in the system. If it exceeds1 3 104torr (13 mPa) at the end of the time period, repair theleaks responsible.6.1.10 Reevacuate the chamber, close the vacuum valve,and admit t
44、he measured helium volume, and note whether ornot the signal is on scale and at least 30 % of full scale.6.1.11 It will be 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
45、 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 signal to change 10 %.6.1.11.4 The pressure to rise to more than 1 3 104torr (13mPa). (With a proper system this time should be in excess of 5min.)6.1.12 If necessary, adjust:6.1.12.1
46、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 the 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 l
47、eakage 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
48、.2 Type I Leaks, atmosphere (or sealed chamber) tovacuum, Method B:6.2.1 The purpose of this procedure is to extend the range ofcalibration of a given chamber-measured volume system by afactor of about 10, by comparing with previously calibratedleak artifacts for longer periods of time.6.2.2 The pro
49、cedure is the same as Method A except that aleak that has been calibrated by Method A is used as a transferstandard (with some loss in accuracy) to calibrate an unknownleak.FIG. 6 Typical Run-to-Run Variations and LimitsE 908 98 (2004)56.2.3 Allow the known leak (leak rate Qm1) to leak into thechamber for a measured time T1until a desired level of heliumis achieved, at which time it is shut off and the level recordedfor an additional period (roughly 10 times t1).6.2.4 Reevacuate the chamber, close the vacuum valve,reset the recorder
