ASTM E908-98(2012)
(Practice)Standard Practice for Calibrating Gaseous Reference Leaks
Standard Practice for Calibrating Gaseous Reference Leaks
ABSTRACT
This practice establishes the standard procedures for calibrating leak artifacts of a specified gas, that may be used for determining the response of leak detectors, or in other situations where a known small flow of gas is required. The purpose of this practice is to establish calibration without reference to other calibrated leaks in as straightforward a manner as possible using the likeliest available equipment. The two types of leaks considered here are Type I, which is pressure to vacuum, and Type II, which is pressure to atmosphere. Three calibration methods are described under each type of reference leak, as follows: Method A—accumulation comparison using a known volume of tracer gas at specified conditions of temperature and pressure as a reference; Method B—accumulation comparison using a reference leak artifact calibrated using Method A; and Method C—direct measurement of leak rate by timing the movement (displacement) of a liquid slug, by the leak, in a capillary tube of known dimensions.
SCOPE
1.1 This practice covers procedures for calibrating leak artifacts of a specified gas, that may be used for determining the response of leak detectors, or in other situations where a known small flow of gas is required. The purpose of this practice is to establish calibration without reference to other calibrated leaks in as straightforward a manner as possible using the likeliest available equipment. While the uncertainties associated with these procedures will most likely be greater than those obtained via traceable calibration chains (on the order of 10 %), these procedures allow independent means of establishing or verifying the leakage rate from leak artifacts of questionable history, or when traceable leak artifacts are not available.
1.2 Two types of leaks are considered:
1.2.1 Type I—Pressure to vacuum.
1.2.2 Type II—Pressure to atmosphere.
1.3 Three calibration methods are described under each type of reference leak:
1.3.1 Method A—Accumulation comparison, using a known volume of gas at specified conditions of temperature and pressure as a reference.
1.3.2 Method B—Accumulation comparison, using a leak artifact calibrated using Method A.
1.3.3 Method C—Displacement 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 regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: E908 − 98 (Reapproved 2012)
Standard Practice for
Calibrating Gaseous Reference Leaks
This standard is issued under the fixed designation E908; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This practice covers procedures for calibrating leak
2.1 ASTM Standards:
artifacts of a specified gas, that may be used for determining
E425Definitions of Terms Relating to Leak Testing (With-
the response of leak detectors, or in other situations where a
drawn 1991)
known small flow of gas is required. The purpose of this
E427PracticeforTestingforLeaksUsingtheHalogenLeak
practice is to establish calibration without reference to other
Detector Alkali-Ion Diode (Withdrawn 2013)
calibrated leaks in as straightforward a manner as possible
E479Guide for Preparation of a Leak Testing Specification
using the likeliest available equipment.While the uncertainties
(Withdrawn 2014)
associated with these procedures will most likely be greater
F134Test Methods for Determining Hermeticity of Electron
than those obtained via traceable calibration chains (on the
Devices with a Helium Mass Spectrometer Leak Detector
order of 10%), these procedures allow independent means of 3
(Withdrawn 1996)
establishing or verifying the leakage rate from leak artifacts of
2.2 Other Documents:
questionable history, or when traceable leak artifacts are not
AVS 2.2-1968Method for Vacuum Leak Calibration
available.
Recommended Practices for the Calibration and Use of
1.2 Two types of leaks are considered: 5
Leaks
1.2.1 Type I—Pressure to vacuum.
1.2.2 Type II—Pressure to atmosphere.
3. Summary of Practice
1.3 Threecalibrationmethodsaredescribedundereachtype
3.1 Method A—Accumulation comparison, using a known
of reference leak:
volume of tracer gas:
1.3.1 MethodA—Accumulation comparison, using a known
3.1.1 This method uses a closed chamber of nonreactive
volume of gas at specified conditions of temperature and
material having a means of removing all tracer gas and a
pressure as a reference.
connection to the tracer sensor.
1.3.2 Method B—Accumulation comparison, using a leak
3.1.2 A small, known quantity of tracer gas is discharged
artifact calibrated using Method A.
intothechamberandtheresponserecordedforaperiodoftime
1.3.3 MethodC—Displacementofaliquidslug,bytheleak,
inwhichitisanticipatedtheunknownleakwillrequiretoreach
in capillary tube of known dimensions.
the same concentration.
1.4 The values stated in inch-pound units are to be regarded
3.1.3 The tracer gas is removed from the chamber, and the
as standard. The values given in parentheses are mathematical
unknown leak is allowed to discharge into it until the sensor
conversions to SI units that are provided for information only
response equals that of 3.1.2.
and are not considered standard.
3.1.4 The leakage rate in mol/s can be calculated as:
1.5 This standard does not purport to address all of the
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safety concerns, if any, associated with its use. It is the
m
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
1 3
This practice is under the jurisdiction of ASTM Committee E07 on Nonde- The last approved version of this historical standard is referenced on
structive Testing and is the direct responsibility of Subcommittee E07.08 on Leak www.astm.org.
Testing Method. AvailablefromAVS,AmericanVacuumSociety,335E.45thStreet,NewYork,
Current edition approved Nov. 15, 2012. Published November 2012. Originally N.Y., 10017.
approvedin1982.Lastpreviouseditionapprovedin2004asE908-98(2004).DOI: C.D. Ehrlich and J.A. Basford, Journal of Vac. Sci, Technology, A(10), 1992,
10.1520/E0908-98R12. pp. 1–17.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E908 − 98 (2012)
where: 4.1.1 Forthepurposesofthissection,itwillbeassumedthat
the gas is helium and the detector is the mass spectrometer
P = pressure in known volume in atmospheres (1
tuned for helium.
atm=101 325 Pa),
V = the volume of gas in cm introduced in 3.1.2,
NOTE 1—Other gases or detectors, or both, can be used with little
t = the time in seconds required for the concentration in
difference in procedures or interferences.
3.1.3 to equal that in 3.1.2,
R = gas constant=82.06=1 atm cm /mol/K, and 4.1.2 PressureRise—Therewillinevitablybesomepressure
T = absolute temperature, K.
riseinaclosedevacuatedchamber,duetooutgassingandsmall
leaks.This may cause a decrease in ionization efficiency in the
3.1.5 It will be observed that chamber volume and sensor
spectrometer tube and thus a steadily declining signal as
linearityarenotfactorsinthisequation.However,thechamber
indicated in Fig. 1. However, this effect should be quite
volume must be selected to give a concentration within the
constant from run to run, and so largely cancel out in final
sensorrange.Also,thisconcentrationmustalsobeachievedby
result.
theunknownleakdischargingintothechamberinareasonable
lengthoftimeandmustbeappropriatesoasnottosignificantly
4.1.3 Helium Signal Rise—There will usually be a notice-
affect the equilibrium flow rate from the leak. This is particu-
ableincreaseinheliumsignalwhenthechamberisclosed,due
larly true of permeation leaks.
to outgassing and in-leakage from the atmosphere as indicated
in Fig. 1. Again, this will be a constant which mostly cancels
3.2 MethodB—Accumulationcomparisonusingareference
out.
leak as calibrated in Method A, 3.1:
3.2.1 This method is a means of extending the primary 4.1.4 SpectrometerSensitivityDrift—Thiswillbenoticedas
calibration by a factor of up to 10, by comparing with variations in zero and in reading levels with the same helium
previously-calibrated leak artifacts for longer periods of time.
input.Withproperlytunedandmaintainedsystemsoperatingat
−12
For example, a 5×10 mol/s leak that calibrated in Method
least one decade below maximum sensitivity, this should be a
−13
A at 300 s can be used for 30 s to calibrate a 5×10 mol/s
minor effect.
leak.
4.1.5 Leaks—All detectable valve leaks and leaks from the
3.2.2 When this method is used, it should be realized that
atmosphere should be repaired.
the total possible error will be at least doubled.
4.1.6 Barometric Variations—(Not applicable to sealed res-
3.3 Method C—Direct measurement of leak rate by timing
ervoir units.) If the gage used to measure the pressure in the
the movement of a liquid slug in a capillary tube of known
known volume is of the gage type, then account must be made
dimensions:
of the local barometric pressure when calculating the absolute
3.3.1 The tube is closely coupled to the leak, and has a
pressure. This is probably true for falling pressures of the
vent/fill valve to allow gas filling or positioning of the slug, or
known volume near 1 atmosphere or less.
both, which is then driven by the leakage of the gas.
4.1.7 Temperature Drift—Changes in temperature between
3.3.2 Due to capillary “friction,” this method is limited to a
measurements may result in slight variations in indicated
−10 3
minimum leak size of about 4×10 mol/s (1 µPa·m /s).
pressures. These should be recorded and compensated for
accordingly.
4. Interferences
4.1 Type I Leaks, atmosphere to vacuum, MethodsAand B: 4.2 Type I Leaks, atmosphere to vacuum, Method C:
FIG. 1 Typical Detector Curves and Deviation Limits
E908 − 98 (2012)
4.2.1 Liquid Slug Friction—This can be appreciable in 5. Apparatus
small capillaries. It should be measured and a correction made
5.1 Type I Leaks, pressure to vacuum, Methods A and B:
for it.
5.1.1 Mass Spectrometer with Remote Tube Tuned for
4.2.2 Vapor Pressure of Liquid—Water is the recommended
−15
Helium—Minimum resolution (5×10 mol/s) helium, when
liquid, and has a vapor pressure of about 20 mm Hg (3 kPa) at
operated as a leak detector.
room temperature. This gives a theoretical increase in leak
5.1.2 Helium Supply with Pressure Regulator and Flowme-
3 −5
indication of 20/760 (3×10/1×10 ) or approximately 3%.
ter (approximately 10 cm /s).
This correction should be added to the final result.
5.1.3 Stainless-Steel Chamber (see Fig. 2) with provisions
4.2.3 Excess Volume Between Leak and Capillary—This
for:
willcausedelayedandjerkymovementoftheslug,andshould
5.1.3.1 Attachment of spectrometer tube,
be kept to an absolute minimum.
5.1.3.2 Liquid nitrogen trap,
4.2.4 Dirty Capillary—Symptoms similar to 4.2.3.The slug
−6
5.1.3.3 Vacuum pumping to at least 1×10 torr (130 µPa)
should move smoothly when capillary tube is held at an angle.
with isolating valve,
4.3 Type II Leaks, pressure to standard atmosphere, Meth-
5.1.3.4 Ionization vacuum gage,
ods A and B:
5.1.3.5 Attachment of helium leak with isolating valve and
4.3.1 Forthepurposesofthissection,itwillbeassumedthat
separate rough pumping means,
the gas is fluorocarbon and the detector is the alkali-ion
5.1.3.6 Measured helium volume device (see Fig. 3) (see
halogen detector diode. Other gases or detectors, or both, can
Note 2), and
be used with little difference in procedures or interferences.
5.1.3.7 Strip chart or flat-bed recorder.
4.3.2 Halogen Signal Rise—There will usually be a small
increase in halogen signal due to outgassing, particularly from
NOTE 2—Other types of calibrated volumes in this range may be
substituted.
elastomers or plastics.With minimum use of these materials in
the chamber, no correction for this will ordinarily be needed.
5.1.3.8 Thermometer.
4.3.3 SensorSensitivityDrift—Thiswillbenoticedasvaria-
5.2 Type I Leaks, pressure to vacuum, Method C:
tions in zero and reading levels with the same halogen input.
5.2.1 Glass Capillary Tube with Vent Valve (see Fig. 4).
With properly maintained systems operating at least one
5.2.2 Timer or Stop Watch.
decade below maximum sensitivity, this should be a minor
5.2.3 Helium Supply.
effect.
5.2.4 Indicator Fluid (dyed water).
4.3.4 Barometric Variations—Substantial variations from
5.2.5 Thermometer.
standard atmosphere pressure should be corrected.
4.4 Type II Leaks, pressure to atmosphere, Method 5.3 Type II Leaks, pressure to atmosphere, Methods A and
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