ASTM E296-70(2020)
(Practice)Standard Practice for Ionization Gage Application to Space Simulators
Standard Practice for Ionization Gage Application to Space Simulators
ABSTRACT
This practice provides application criteria, definitions, and supplemental information to assist the user in obtaining meaningful vacuum ionization gage measurements in space-simulation facilities. Acceptable vacuum-measuring equipment shall consist of those items in which performance is compatible with obtaining meaningful measurements. The gage mounting, gage orientation, gage operational error, and gage correction for gas composition are presented in details. The gas composition determination, operating criteria, heavy molecular weight contamination effects, apparent X-ray limit for hot-cathode gages, and cold cathode gages are presented in details.
SCOPE
1.1 This practice provides application criteria, definitions, and supplemental information to assist the user in obtaining meaningful vacuum ionization gage measurements below 10−1 N/m2 (10−3 torr) in space-simulation facilities. Since a variety of influences can alter observed vacuum measurements, means of identifying and assessing potential problem areas receive considerable attention. This practice must be considered informational, for it is impossible to specify a means of applying the vacuum-measuring equipment to guarantee accuracy of the observed vacuum measurement. Therefore, the user's judgment is essential so that if a problem area is identified, suitable steps can be taken to either minimize the effect, correct the observed readings as appropriate, or note the possible error in the observation.
1.2 While much of the discussion is concerned with the application of hot-cathode ionization gages, no exclusion is made of cold-cathode designs. Since a great deal more experience with hot-cathode gages is available and hot-cathode devices are used in the majority of applications, the present emphasis is fully warranted.
1.3 The values stated in inch-pound units are to be regarded as the standard. The metric equivalents of inch-pound units may be approximate.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E296 − 70 (Reapproved 2020)
Standard Practice for
Ionization Gage Application to Space Simulators
This standard is issued under the fixed designation E296; 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 E297Test Method for Calibrating Ionization Vacuum Gage
Tubes (Withdrawn 1983)
1.1 This practice provides application criteria, definitions,
and supplemental information to assist the user in obtaining
−1
3. Terminology
meaningful vacuum ionization gage measurements below 10
2 −3
N/m (10 torr) in space-simulation facilities. Since a variety 3.1 Definitions—The following definitions are necessary to
of influences can alter observed vacuum measurements, means
understanding meaningful application of ionization-type
of identifying and assessing potential problem areas receive vacuum-measurement devices and are useful in differentiating
considerable attention. This practice must be considered
between pressure, density, and flux measuring devices for
informational, for it is impossible to specify a means of proper application and interpretation of low-density molecular
applying the vacuum-measuring equipment to guarantee accu-
measurements.
racy of the observed vacuum measurement. Therefore, the
3.1.1 Blears effect—the reduction of the partial pressure of
user’s judgment is essential so that if a problem area is
organic vapors within the envelope of a tubulated ionization
identified, suitable steps can be taken to either minimize the
gage below the partial pressure that would prevail in the
effect, correct the observed readings as appropriate, or note the
envelope with a tubulation having infinite conductance.
possible error in the observation.
3.1.2 controlled-temperature enclosed gage—an enclosed
1.2 While much of the discussion is concerned with the
gage in which the envelope is maintained at nearly uniform
application of hot-cathode ionization gages, no exclusion is constant temperature by suitable means.
made of cold-cathode designs. Since a great deal more expe-
3.1.3 enclosed ionization gage—an ionization gage for
rience with hot-cathode gages is available and hot-cathode
which the ion source region is enclosed over at least 0.95×4
devices are used in the majority of applications, the present
π steradians about the center of the region by an envelope at a
emphasis is fully warranted.
known temperature with only a single opening such that all
1.3 The values stated in inch-pound units are to be regarded molecules entering the ion source region must have crossed a
as the standard. The metric equivalents of inch-pound units
plane located outside this region.
may be approximate.
3.1.4 equivalent nitrogen concentration—the quantity ob-
1.4 This international standard was developed in accor-
tained when the ion-collector current of a nude gage (in
dance with internationally recognized principles on standard-
amperes) for the gas in the system is divided by the concen-
ization established in the Decision on Principles for the
tration sensitivity of the gage for nitrogen. This sensitivity is
Development of International Standards, Guides and Recom-
defined as the ratio of gage ion collector current in amperes to
mendations issued by the World Trade Organization Technical
molecular concentration in molecules per cubic metre of
Barriers to Trade (TBT) Committee.
nitrogen under specified operating conditions.
3.1.5 equivalent nitrogen flux density—the quotient of the
2. Referenced Documents
current output of an enclosed vacuum gage operating under
2.1 ASTM Standards:
specified conditions divided by the molecular flux sensitivity
for nitrogen.
1 3.1.6 equivalent nitrogen pressure:
This practice is under the jurisdiction of ASTM Committee E21 on Space
Simulation andApplications of SpaceTechnology and is the direct responsibility of
3.1.6.1 For a nude gage, equivalent nitrogen pressure is
Subcommittee E21.04 on Space Simulation Test Methods.
obtained by multiplying the equivalent nitrogen concentration
Current edition approved Nov. 1, 2020. Published December 2020. Originally
bykT where k is the Boltzmann constant and T is the mean
approvedin1966.Lastpreviouseditionapprovedin2015asE296–70(2015).DOI:
10.1520/E0296-70R20.
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 last approved version of this historical standard is referenced on
the ASTM website. www.astm.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E296 − 70 (2020)
absolute temperature of the walls from which the gas mol- 3.1.14 partial pressure gage—an ionization gage that indi-
ecules travel to the ionizing region of the gage, averaged as catesthepartialpressureofanygasinamixtureirrespectiveof
nearly as possible on the basis of relative molecular flux. the partial pressure of other gases in the mixture.
3.1.6.2 standard equivalent nitrogen pressure—for a nude
3.1.15 partially enclosed ionization gage—a gage in which
gage, the value of the equivalent nitrogen pressure is obtained
the ion formation region is enclosed over less than 0.95×4 π
when T=296K (or standard ambient temperature) is used in
steradians but more than 0.05×4 π steradians about center by
the factorkT.
an envelope which has one or more openings such that not all
3.1.6.3 For a tubulated gage, the equivalent nitrogen pres-
molecules entering the ion formation region must first cross a
sureinnewtonpersquaremetreisobtainedbydividingtheion
plane located outside this region.
collector current in amperes for a given gas by the pressure
3.1.16 recovery time—the time required for the pressure
sensitivity of the gage in amperes per newton per square metre
indication of a gage to reach and remain within pressure
for pure nitrogen under specified operating conditions.
indications not more than 105% or less than 95% of the final
3.1.7 gage background—the part of the indicated ion col-
average steady-state value after a sudden change in the
lector current produced by phenomena other than ions formed
operatingconditionsofthegagewithoutappreciablechangein
in the gas phase arriving at the collector.
the gas pressure in the vacuum chamber. Pressure changes less
3.1.8 gage limit—apressureorconcentrationindicationfour
than 5% of the initial value shall be regarded as within the
times the background.
normal fluctuations of pressure indication.
3.1.9 ionization gage—a vacuum gage comprising a means
3.1.17 response time—the time required for the change in
of ionizing the gas molecules and a means of correlating the
pressure indication as a result of a specified gas (or vapor)
number and type of ions produced with the pressure or
within a gage tube to reach (1−1⁄e) (or 63%) of the change
concentration of the gas. Various types of ionization gages are
in steady-state pressure after a relatively instantaneous change
distinguished according to the method of producing the ion-
of the pressure of that gas in the vacuum chamber. The
ization.
response time may depend on the time of adsorption of the gas
3.1.9.1 cold-cathode ionization gage—an ionization gage in (orvapor)onthewallsofthegagetubeaswellasthegeometry
which the ions are produced by a cold-cathode gas discharge,
of the tube (including the connecting line to the vacuum
usually in the presence of a magnetic field. chamber).
3.1.9.2 hot-cathode ionization gage—an ionization gage in
3.1.18 tubulated ionization gage—an enclosed ionization
which ion production is initiated and sustained by electrons
gage for which the opening in the envelope is determined by a
emitted from a hot cathode.
tubulation of diameter equal to or less than the minimum
diameter of the part of the envelope adjacent to the ion source
3.1.10 molecular flux density—the number of molecules
region and of length at least equal to the diameter of the
incident on a real or imaginary surface per unit area per unit
tubulation.
time. The unit is molecules per second per square centimetre.
3.1.19 vacuum gas analyzer—adevicecapableofindicating
3.1.11 molecular flux sensitivity—the output current of an
the relative composition of a gas mixture at low pressures.
enclosed vacuum gage per unit molecular flux density under
specified gage operating conditions and random particle mo-
4. Apparatus
tion.
4.1 Equipment—Acceptable vacuum-measuring equipment
3.1.12 nude ionization gage—an ionization gage for which
shallconsistofthoseitemsinwhichperformanceiscompatible
the center of the ion source region is exposed to direct
with obtaining meaningful measurements. The basic elements
molecular flux (from surfaces not forming part of the gage) in
consist of a power supply, readout, and sensing element.These
all directions except for a solid angle less than 0.05×4 π
items must be acceptable for applying the proper calibrations
steradians (determined by the parts of the gage head). No
described in Methods E297. The electronic power supply and
structures shall be within one sensing element diameter of any
readout shall have been calibrated either separately or in
partofthesensingelementunlesssimilarstructuresarepresent
conjunction with the test stand calibration of the gage sensor.
during calibration.
Special attention must be given to cabling, especially where
NOTE 1—The solid angle subtended by a circular disk of radius r with
cablingrunsarelong(asinlargevacuumsystems)inorderthat
axispassingthroughthecenterpointofthesolidangleatadistance yfrom
impedance or resistance errors are properly accounted for in
the disk is given as follows:
the calibration activities.
2 2 1/2
ω 5 2 π 1 2 y/ y 1r (1)
@ ~ ! #
4.2 Calibration—These practices are not concerned with
For ω=0.05×4π , the distance y must equal 2.07 r,a
gage calibration criteria except as applicable during test. Test
value which should be easily attainable for typical ionization
stand calibration criteria is provided by Methods E297. Re-
gage electrodes mounted on a circular base of radius r.
cycle of the vacuum-measuring equipment to the calibration
3.1.13 orifice ionization gage—an enclosed gage containing test stand should not be programmed only on a calendar basis.
a single orifice or port having a length less than 0.15 of its Periodic recycle can best be determined by the individual
diameter such that molecules from the chamber can enter the operators compatible with usage requirements. Upon any
envelope directly from within a solid angle nearly equal to 2π strong indication that usage in test may have produced an
steradians. alteration in gage factor, suspect elements shall be returned to
E296 − 70 (2020)
the test stand. Alternatively, calibration before and after test
may be incorporated as part of major test programs.
5. Gage Mounting
5.1 Flanges and Couplings—Flanging and connections are
specified in this section both for dimensions and material
between ionization gages and the external walls of high-
vacuumsystemstoproduceageometricallystandardmounting
method (compatible with the calibration test stand) which is a
clean assembly free of interfering contamination such as that
produced by organic or high vapor-pressure sealing materials.
5.1.1 Tubulated Ionization Gage (Fig. 1):
5.1.1.1 The flange material shall be stainless steel with a
glass-to-metal seal connecting the gage to the flange stub. The
flanges shall be welded or high-temperature brazed with
appropriate cleaning to remove residual flux. Gasket material
shall be metallic: copper, aluminum, indium, and so forth.
5.1.1.2 The gage may be attached directly to chamber
eliminating flanges and gasketing providing limiting dimen-
sions are adhered to.
5.1.2 Nude or Partially Enclosed Ionization Gages (Fig. 2
and Fig. 3)—See 5.1.1.1.
5.1.2.1 Intentistogivemaximumsolid-angle(line-of-sight)
exposure of the gage elements to the chamber environments.
5.2 Internally Mounted Ionization Gages—Limitations for
mounting ionization gages internally are specified in this
section to provide mounting considerations applicable to plac-
FIG. 2 Flange-Mounted Nude Ionization Gage
ing any vacuum-ionization gage within the vacuum volume.
Measurement considerations are provided in Section 6.
5.2.1 Tubulated Ionization Gages:
5.2.1.1 Mechanical—The mechanical support and position-
ing of internally mounted tubulated gages must not influence
the distribution of molecules across the tubulation.
5.2.1.2 Thermal—Since internally mounted tubulated gages
will experience significantly different heat transfer conditions
from the envelope, care should be taken to provide means in
the mounting to monitor or control, or both, the equilibrium
temperatureconditionoftheenvelopethatcanbeduplicatedin
a calibration test stand. Temperature control can be by either
FIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls and Struc-
active or passive means—an active means representing a
tures
controlled temperature enclosed gage.
5.2.1.3 Electrical—Shielding of the electrical leads, espe-
cially the collector, poses somewhat more of a problem than
with externally mounted gages. Care must be taken in the use
of unshielded wires that external pickup does not compromise
the collector current. In any hookup, aside from leakage and
especially where long cables may be used, capacitance and
resistance losses may contribute significant errors unless cor-
rected or suitably accounted for during calibration.
5.2.2 Nude and Partially Enclosed Gages:
5.2.2.1 Mechanical—The mechanical support shall be such
astoprovideequivalentacceptanceanglesofmolecularfluxas
defined for the flange-mounted condition (Fig. 2 and Fig. 3).
5.2.2.2 Thermal—Thermal considerations with nude and
partiallyenclosedgagesarelesssignificantthanwithtubulated
FIG. 1 Tubulated Ionization Gage gages. Generally, the mechanical support will require no
E296 − 70 (2020)
special attention except in extreme conditions where conduc- 6.5 Reporting of Data—The gage readings should be re-
tion or radiation paths to nearby surfaces provide an extreme ported in terms that clearly indicate the location, orientation,
temperature differential. and type of enclosure of the gage when substantial directional
5.2.2.3 Electrical—Same as 5.2.1.3. effe
...
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