ASTM E908-98
(Practice)Standard Practice for Calibrating Gaseous Reference Leaks
Standard Practice for Calibrating Gaseous Reference Leaks
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 the standard. The metric equivalents of inch-pound units may be appropriate.
1.5 This standard does not purport to address the safety problems 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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Designation: E 908 – 98
Standard Practice for
Calibrating Gaseous Reference Leaks
This standard is issued under the fixed designation E 908; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope E 427 Practice for Testing for Leaks Using the Halogen
Leak Detector (Alkali-Ion Diode)
1.1 This practice covers procedures for calibrating leak
E 479 Guide for Preparation of a Leak Testing Specifica-
artifacts of a specified gas, that may be used for determining
tion
the response of leak detectors, or in other situations where a
F 134 Test Methods for Determining Hermeticity of Elec-
known small flow of gas is required. The purpose of this
tron Devices With a Helium Mass Spectrometer Leak
practice is to establish calibration without reference to other
Detector
calibrated leaks in as straightforward a manner as possible
2.2 Other Documents:
using the likeliest available equipment. While the uncertainties
AVS 2.2-1968 Method for Vacuum Leak Calibration
associated with these procedures will most likely be greater
Recommended Practices for the Calibration and Use of
than those obtained via traceable calibration chains (on the
Leaks
order of 10 %), these procedures allow independent means of
establishing or verifying the leakage rate from leak artifacts of
3. Summary of Practice
questionable history, or when traceable leak artifacts are not
3.1 Method A—Accumulation comparison, using a known
available.
volume of tracer gas:
1.2 Two types of leaks are considered:
3.1.1 This method uses a closed chamber of nonreactive
1.2.1 Type I—Pressure to vacuum.
material having a means of removing all tracer gas and a
1.2.2 Type II—Pressure to atmosphere.
connection to the tracer sensor.
1.3 Three calibration methods are described under each type
3.1.2 A small, known quantity of tracer gas is discharged
of reference leak:
into the chamber and the response recorded for a period of time
1.3.1 Method A—Accumulation comparison, using a known
in which it is anticipated the unknown leak will require to reach
volume of gas at specified conditions of temperature and
the same concentration.
pressure as a reference.
3.1.3 The tracer gas is removed from the chamber, and the
1.3.2 Method B—Accumulation comparison, using a leak
unknown leak is allowed to discharge into it until the sensor
artifact calibrated using Method A.
response equals that of 3.1.2.
1.3.3 Method C—Displacement of a liquid slug, by the leak,
3.1.4 The leakage rate in mol/s can be calculated as:
in capillary tube of known dimensions.
1.4 The values stated in inch-pound units are to be regarded Q 5 PV ~t·R·T! (1)
m
as the standard. The metric equivalents of inch-pound units
may be appropriate.
where:
1.5 This standard does not purport to address all of the
P = pressure in known volume in atmospheres (1
safety concerns, if any, associated with its use. It is the
atm = 101 325 Pa),
responsibility of the user of this standard to establish appro- 3
V = the volume of gas in cm introduced in 3.1.2,
priate safety and health practices and determine the applica-
t = the time in seconds required for the concentration in
bility of regulatory limitations prior to use.
3.1.3 to equal that in 3.1.2,
R = gas constant = 82.06 = 1 atm cm /mol/K, and
2. Referenced Documents
T = absolute temperature, K.
2.1 ASTM Standards:
3.1.5 It will be observed that chamber volume and sensor
E 425 Terminology Relating to Leak Testing
linearity are not factors in this equation. However, the chamber
This practice is under the jurisdiction of ASTM Committee E-7 on Nonde-
structive Testing and is the direct responsibility of Subcommittee E07.08 on Leak Annual Book of ASTM Standards, Vol 10.04.
Testing. Available from AVS, American Vacuum Society, 335 E. 45th Street, New York,
Current edition approved May 10, 1998. Published December 1998. Originally N.Y., 10017.
published as E 908 – 82. Last previous edition E 908 – 91. C.D. Ehrlich and J.A. Basford, Journal of Vac. Sci, Technology, A(10), 1992,
Annual Book of ASTM Standards, Vol 03.03. pp. 1–17.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E908–98
volume must be selected to give a concentration within the constant from run to run, and so largely cancel out in final
sensor range. Also, this concentration must also be achieved by result.
the unknown leak discharging into the chamber in a reasonable
4.1.3 Helium Signal Rise—There will usually be a notice-
length of time and must be appropriate so as not to significantly
able increase in helium signal when the chamber is closed, due
affect the equilibrium flow rate from the leak. This is particu-
to outgassing and in-leakage from the atmosphere as indicated
larly true of permeation leaks.
in Fig. 1. Again, this will be a constant which mostly cancels
3.2 Method B—Accumulation comparison using a reference
out.
leak as calibrated in Method A, 3.1:
4.1.4 Spectrometer Sensitivity Drift—This will be noticed as
3.2.1 This method is a means of extending the primary
variations in zero and in reading levels with the same helium
calibration by a factor of up to 10, by comparing with
input. With properly tuned and maintained systems operating at
previously-calibrated leak artifacts for longer periods of time.
least one decade below maximum sensitivity, this should be a
−12
For example, a 5 3 10 mol/s leak that calibrated in Method
minor effect.
−13
A at 300 s can be used for 30 s to calibrate a 5 3 10 mol/s
4.1.5 Leaks—All detectable valve leaks and leaks from the
leak.
atmosphere should be repaired.
3.2.2 When this method is used, it should be realized that
4.1.6 Barometric Variations—(Not applicable to sealed res-
the total possible error will be at least doubled.
ervoir units.) If the gage used to measure the pressure in the
3.3 Method C—Direct measurement of leak rate by timing
known volume is of the gage type, then account must be made
the movement of a liquid slug in a capillary tube of known
of the local barometric pressure when calculating the absolute
dimensions:
pressure. This is probably true for falling pressures of the
3.3.1 The tube is closely coupled to the leak, and has a
known volume near 1 atmosphere or less.
vent/fill valve to allow gas filling or positioning of the slug, or
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 3 10 mol/s (1 μPa·m /s).
pressures. These should be recorded and compensated for
accordingly.
4. Interferences
4.2 Type I Leaks, atmosphere to vacuum, Method C:
4.1 Type I Leaks, atmosphere to vacuum, Methods A and B:
4.2.1 Liquid Slug Friction—This can be appreciable in
4.1.1 For the purposes of this section, it will be assumed that
small capillaries. It should be measured and a correction made
the gas is helium and the detector is the mass spectrometer
for it.
tuned for helium.
4.2.2 Vapor Pressure of Liquid—Water is the recommended
NOTE 1—Other gases or detectors, or both, can be used with little liquid, and has a vapor pressure of about 20 mm Hg (3 kPa) at
difference in procedures or interferences.
room temperature. This gives a theoretical increase in leak
3 −5
indication of 20/760 (3 3 10 /1 3 10 ) or approximately 3 %.
4.1.2 Pressure Rise—There will inevitably be some pres-
This correction should be added to the final result.
sure rise in a closed evacuated chamber, due to outgassing and
small leaks. This may cause a decrease in ionization efficiency 4.2.3 Excess Volume Between Leak and Capillary—This
in the spectrometer tube and thus a steadily declining signal as will cause delayed and jerky movement of the slug, and should
indicated in Fig. 1. However, this effect should be quite be kept to an absolute minimum.
FIG. 1 Typical Detector Curves and Deviation Limits
E908–98
−6
4.2.4 Dirty Capillary—Symptoms similar to 4.2.3. The slug 5.1.3.3 Vacuum pumping to at least 1 3 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 For the purposes of this section, it will be assumed that
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
NOTE 2—Other types of calibrated volumes in this range may be
increase in halogen signal due to outgassing, particularly from
substituted.
elastomers or plastics. With minimum use of these materials in
5.1.3.8 Thermometer.
the chamber, no correction for this will ordinarily be needed.
5.2 Type I Leaks, pressure to vacuum, Method C:
4.3.3 Sensor Sensitivity Drift—This will be noticed as
5.2.1 Glass Capillary Tube with Vent Valve (see Fig. 4).
variations in zero and reading levels with the same halogen
5.2.2 Timer or Stop Watch.
input. With properly maintained systems operating at least one
decade below maximum sensitivity, this should be a minor 5.2.3 Helium Supply.
5.2.4 Indicator Fluid (dyed water).
effect.
4.3.4 Barometric Variations—Substantial variations from 5.2.5 Thermometer.
standard atmosphere pressure should be corrected. 5.3 Type II Leaks, pressure to atmosphere, Methods A and
4.4 Type II Leaks, pressure to atmosphere, Method B:
−13
C—Same as Type I, Method C, in 4.2. 5.3.1 Halogen Detector—Minimum sensitivity 4 3 10
mol/s (1 nPa·m /s).
5. Apparatus 5.3.2 Fluorocarbon Supply with Flowmeter.
5.3.3 Stainless-Steel Chamber (see Fig. 5) with provisions
5.1 Type I Leaks, pressure to vacuum, Methods A and B:
for:
5.1.1 Mass Spectrometer with Remote Tube Tuned for
−15
5.3.3.1 Attachment of sensor sampling tube,
Helium—Minimum resolution (5 3 10 mol/s) helium, when
5.3.3.2 Pure air supply,
operated as a leak detector.
5.3.3.3 Attachm
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