ASTM E296-70(2004)

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 101 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.

General Information

• Status: Historical
• Publication Date: 31-Aug-2004
• ICS: 49.140 - Space systems and operations
• Technical Committee: E21 - Space Simulation and Applications of Space Technology
• Drafting Committee: E21.04 - Space Simulation Test Methods

Relations

• Replaces: ASTM E296-70(1999) - Standard Practice for Ionization Gage Application to Space Simulators
• Effective Date: 01-Sep-2004
• Replaced By: ASTM E296-70(2010) - Standard Practice for Ionization Gage Application to Space Simulators
• Effective Date: 01-Apr-2010
• Referred By: ASTM E2900-19 - Standard Practice for Spacecraft Hardware Thermal Vacuum Bakeout
• Effective Date: 01-Sep-2004
• Referred By: ASTM E512-94(2020) - Standard Practice for Combined, Simulated Space Environment Testing of Thermal Control Materials with Electromagnetic and Particulate Radiation
• Effective Date: 01-Sep-2004
• Referred By: ASTM E491-73(2020) - Standard Practice for Solar Simulation for Thermal Balance Testing of Spacecraft
• Effective Date: 01-Sep-2004

Standards Content (Sample)

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:E296–70 (Reapproved 2004)
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 vacuum-measurement devices and are useful in differentiating
between pressure, density, and flux measuring devices for
1.1 This practice provides application criteria, definitions,
proper application and interpretation of low-density molecular
and supplemental information to assist the user in obtaining
−1
measurements.
meaningful vacuum ionization gage measurements below 10
2 −3
3.1.1 Blears effect—the reduction of the partial pressure of
N/m (10 torr) in space-simulation facilities. Since a variety
organic vapors within the envelope of a tubulated ionization
of influences can alter observed vacuum measurements, means
gage below the partial pressure that would prevail in the
of identifying and assessing potential problem areas receive
envelope with a tubulation having infinite conductance.
considerable attention. This practice must be considered infor-
3.1.2 controlled-temperature enclosed gage—an enclosed
mational,foritisimpossibletospecifyameansofapplyingthe
gage in which the envelope is maintained at nearly uniform
vacuum-measuring equipment to guarantee accuracy of the
constant temperature by suitable means.
observed vacuum measurement. Therefore, the user’s judg-
3.1.3 enclosed ionization gage—an ionization gage for
mentisessentialsothatifaproblemareaisidentified,suitable
which the ion source region is enclosed over at least 0.95 34
steps can be taken to either minimize the effect, correct the
p steradians about the center of the region by an envelope at a
observed readings as appropriate, or note the possible error in
known temperature with only a single opening such that all
the observation.
molecules entering the ion source region must have crossed a
1.2 While much of the discussion is concerned with the
plane located outside this region.
application of hot-cathode ionization gages, no exclusion is
3.1.4 equivalent nitrogen concentration—the quantity ob-
made of cold-cathode designs. Since a great deal more expe-
tained when the ion-collector current of a nude gage (in
rience with hot-cathode gages is available and hot-cathode
amperes) for the gas in the system is divided by the concen-
devices are used in the majority of applications, the present
tration sensitivity of the gage for nitrogen. This sensitivity is
emphasis is fully warranted.
defined as the ratio of gage ion collector current in amperes to
1.3 Thevaluesstatedininch-poundunitsaretoberegarded
molecular concentration in molecules per cubic metre of
as the standard. The metric equivalents of inch-pound units
nitrogen under specified operating conditions.
may be approximate.
3.1.5 equivalent nitrogen flux density—the quotient of the
2. Referenced Documents
current output of an enclosed vacuum gage operating under
specified conditions divided by the molecular flux sensitivity
2.1 ASTM Standards:
for nitrogen.
E297 Methods for Calibrating Ionization Vacuum Gage
3.1.6 equivalent nitrogen pressure:
Tubes
3.1.6.1 For a nude gage, equivalent nitrogen pressure is
3. Terminology
obtained by multiplying the equivalent nitrogen concentration
bykT where k is the Boltzmann constant and T is the mean
3.1 Definitions—The following definitions are necessary to
absolute temperature of the walls from which the gas mol-
understanding meaningful application of ionization-type
ecules travel to the ionizing region of the gage, averaged as
nearly as possible on the basis of relative molecular flux.
This practice is under the jurisdiction of ASTM Committee E21 on Space
3.1.6.2 standard equivalent nitrogen pressure—for a nude
Simulation and Applications of Space Technology and are the direct responsibility
gage, the value of the equivalent nitrogen pressure is obtained
of Subcommittee E21.04 on Space Simulation Test Methods.
Current edition approved Sept. 1, 2004. Published September 2004. Originally when T=296K (or standard ambient temperature) is used in
approved in 1966. Last previous edition approved in 1999 as E296–70 (1999).
the factorkT.
DOI: 10.1520/E0296-70R04.
3.1.6.3 For a tubulated gage, the equivalent nitrogen
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
pressureinnewtonpersquaremetreisobtainedbydividingthe
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on ioncollectorcurrentinamperesforagivengasbythepressure
the ASTM website.
Withdrawn.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E296–70 (2004)
sensitivity of the gage in amperes per newton per square metre indications not more than 105% or less than 95% of the final
for pure nitrogen under specified operating conditions. average steady-state value after a sudden change in the
3.1.7 gage background—the part of the indicated ion col- operatingconditionsofthegagewithoutappreciablechangein
lector current produced by phenomena other than ions formed the gas pressure in the vacuum chamber. Pressure changes less
in the gas phase arriving at the collector. than 5% of the initial value shall be regarded as within the
3.1.8 gage limit—a pressure or concentration indication normal fluctuations of pressure indication.
four times the background.
3.1.17 response time—the time required for the change in
3.1.9 ionization gage—a vacuum gage comprising a means pressure indication as a result of a specified gas (or vapor)
of ionizing the gas molecules and a means of correlating the
withinagagetubetoreach(1−1/e)(or63%)ofthechangein
number and type of ions produced with the pressure or steady-state pressure after a relatively instantaneous change of
concentration of the gas. Various types of ionization gages are the pressure of that gas in the vacuum chamber. The response
distinguished according to the method of producing the ion- time may depend on the time of adsorption of the gas (or
ization. vapor)onthewallsofthegagetubeaswellasthegeometryof
3.1.9.1 cold-cathode ionization gage—anionizationgagein the tube (including the connecting line to the vacuum cham-
which the ions are produced by a cold-cathode gas discharge, ber).
usually in the presence of a magnetic field.
3.1.18 tubulated ionization gage—an enclosed ionization
3.1.9.2 hot-cathode ionization gage—an ionization gage in
gage for which the opening in the envelope is determined by a
which ion production is initiated and sustained by electrons
tubulation of diameter equal to or less than the minimum
emitted from a hot cathode.
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
consistofapowersupply,readout,andsensingelement.These
all directions except for a solid angle less than 0.05 34 p
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
4.2 Calibration—These practices are not concerned with
/
v52 p[1 2 y/~y 1 r ! # (1)
gage calibration criteria except as applicable during test. Test
stand calibration criteria is provided by Methods E297. Re-
For v=0.05 34p,thedistance ymustequal2.07 r,avalue
cycle of the vacuum-measuring equipment to the calibration
which should be easily attainable for typical ionization gage
test stand should not be programmed only on a calendar basis.
electrodes mounted on a circular base of radius r.
Periodic recycle can best be determined by the individual
3.1.13 orifice ionization gage—anenclosedgagecontaining
operators compatible with usage requirements. Upon any
a single orifice or port having a length less than 0.15 of its
strong indication that usage in test may have produced an
diameter such that molecules from the chamber can enter the
alteration in gage factor, suspect elements shall be returned to
envelope directly from within a solid angle nearly equal to 2 p
the test stand. Alternatively, calibration before and after test
steradians.
may be incorporated as part of major test programs.
3.1.14 partial pressure gage—an ionization gage that indi-
catesthepartialpressureofanygasinamixtureirrespectiveof
5. Gage Mounting
the partial pressure of other gases in the mixture.
5.1 Flanges and Couplings—Flanging and connections are
3.1.15 partially enclosed ionization gage—a gage in which
the ion formation region is enclosed over less than 0.95 34 p specified in this section both for dimensions and material
between ionization gages and the external walls of high-
steradiansbutmorethan0.05 34 psteradiansaboutcenterby
an envelope which has one or more openings such that not all vacuumsystemstoproduceageometricallystandardmounting
method (compatible with the calibration test stand) which is a
molecules entering the ion formation region must first cross a
plane located outside this region. clean assembly free of interfering contamination such as that
produced by organic or high vapor-pressure sealing materials.
3.1.16 recovery time—the time required for the pressure
indication of a gage to reach and remain within pressure 5.1.1 Tubulated Ionization Gage (Fig. 1):
E296–70 (2004)
FIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls and
Structures
5.2 Internally Mounted Ionization Gages—Limitations for
mounting ionization gages internally are specified in this
FIG. 1 Tubulated Ionization Gage
section to provide mounting considerations applicable to plac-
ing any vacuum-ionization gage within the vacuum volume.
5.1.1.1 The flange material shall be stainless steel with a Measurement considerations are provided in Section 6.
glass-to-metal seal connecting the gage to the flange stub. The
5.2.1 Tubulated Ionization Gages:
flanges shall be welded or high-temperature brazed with 5.2.1.1 Mechanical—The mechanical support and position-
appropriate cleaning to remove residual flux. Gasket material
ing of internally mounted tubulated gages must not influence
shall be metallic: copper, aluminum, indium, and so forth. the distribution of molecules across the tubulation.
5.1.1.2 The gage may be attached directly to chamber
5.2.1.2 Thermal—Since internally mounted tubulated gages
eliminating flanges and gasketing providing limiting dimen- will experience significantly different heat transfer conditions
sions are adhered to.
from the envelope, care should be taken to provide means in
5.1.2 Nude or Partially Enclosed Ionization Gages (Fig. 2 the mounting to monitor or control, or both, the equilibrium
and Fig. 3)—See 5.1.1.1.
temperatureconditionoftheenvelopethatcanbeduplicatedin
5.1.2.1 Intentistogivemaximumsolid-angle(line-of-sight)
a calibration test stand. Temperature control can be by either
exposure of the gage elements to the chamber environments.
active or passive means—an active means representing a
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
gages. Generally, the mechanical support will require no
special attention except in extreme conditions where conduc-
tion or radiation paths to nearby surfaces provide an extreme
temperature differential.
5.2.2.3 Electrical—Same as 5.2.1.3.
6. Gage Orientation
6.1 General—Orientation of gages is significant where the
gas atmosphere in a vacuum chamber has directional proper-
ties.These properties are of at least three kinds: (1) directional
molecularfluxdensity(directionalpressure)asingasexchange
between a source and a pump, where the quantity flowing
toward the pump is greater than that flowing from the pump;
FIG. 2 Flange-Mounted Nude Ionization Gage (2) directional composition, as in gas exchange between an
E296–70 (2004)
outgassing body and a cryopump, where the outgassing mate- 7.1.1.2 Radioactive sources emitting nuclear radiations,
rial is mainly condensible and the material flowing from the
7.1.1.3 Other sources of electromagnetic radiation or par-
cryopump is mainly noncondensible; (3) directional tempera-
ticle radiation being used to simulate the solar spectrum or the
ture,asingasexchangebetweenawarmandcol
...