ASTM E296-70(2010)

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.

General Information

• Status: Historical
• Publication Date: 31-Mar-2010
• 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(2004) - Standard Practice for Ionization Gage Application to Space Simulators
• Effective Date: 01-Apr-2010
• Referred By: ASTM E491-73(2020) - Standard Practice for Solar Simulation for Thermal Balance Testing of Spacecraft
• Effective Date: 01-Apr-2010
• Referred By: ASTM E2900-19 - Standard Practice for Spacecraft Hardware Thermal Vacuum Bakeout
• Effective Date: 01-Apr-2010
• 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-Apr-2010

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 2010)
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
informational, for it is impossible to specify a means of
3.1.2 controlled-temperature enclosed gage—an enclosed
applying the vacuum-measuring equipment to guarantee accu-
gage in which the envelope is maintained at nearly uniform
racy of the observed vacuum measurement. Therefore, the
constant temperature by suitable means.
user’s judgment is essential so that if a problem area is
3.1.3 enclosed ionization gage—an ionization gage for
identified, suitable steps can be taken to either minimize the
which the ion source region is enclosed over at least 0.95×4
effect, correct the observed readings as appropriate, or note the
π steradians about the center of the region by an envelope at a
possible error in the observation.
known temperature with only a single opening such that all
1.2 While much of the discussion is concerned with the
molecules entering the ion source region must have crossed a
application of hot-cathode ionization gages, no exclusion is
plane located outside this region.
made of cold-cathode designs. Since a great deal more expe-
3.1.4 equivalent nitrogen concentration—the quantity ob-
rience with hot-cathode gages is available and hot-cathode
tained when the ion-collector current of a nude gage (in
devices are used in the majority of applications, the present
amperes) for the gas in the system is divided by the concen-
emphasis is fully warranted.
tration sensitivity of the gage for nitrogen. This sensitivity is
1.3 The values stated in inch-pound units are to be regarded
defined as the ratio of gage ion collector current in amperes to
as the standard. The metric equivalents of inch-pound units
molecular concentration in molecules per cubic metre of
may be approximate.
nitrogen under specified operating conditions.
2. Referenced Documents 3.1.5 equivalent nitrogen flux density—the quotient of the
current output of an enclosed vacuum gage operating under
2.1 ASTM Standards:
specified conditions divided by the molecular flux sensitivity
E297Test Method for Calibrating Ionization Vacuum Gage
for nitrogen.
Tubes (Withdrawn 1983)
3.1.6 equivalent nitrogen pressure:
3. Terminology
3.1.6.1 For a nude gage, equivalent nitrogen pressure is
3.1 Definitions—The following definitions are necessary to
obtained by multiplying the equivalent nitrogen concentration
understanding meaningful application of ionization-type
bykT where k is the Boltzmann constant and T is the mean
absolute temperature of the walls from which the gas mol-
This practice is under the jurisdiction of ASTM Committee E21 on Space
ecules travel to the ionizing region of the gage, averaged as
Simulation and Applications of Space Technology and are the direct responsibility
nearly as possible on the basis of relative molecular flux.
of Subcommittee E21.04 on Space Simulation Test Methods.
Current edition approved April 1, 2010. Published May 2010. Originally
3.1.6.2 standard equivalent nitrogen pressure—for a nude
approved in 1966. Last previous edition approved in 2004 as E296–70 (2004).
gage, the value of the equivalent nitrogen pressure is obtained
DOI: 10.1520/E0296-70R10.
2 when T=296K (or standard ambient temperature) is used in
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
the factorkT.
Standards volume information, refer to the standard’s Document Summary page on
3.1.6.3 For a tubulated gage, the equivalent nitrogen pres-
the ASTM website.
sureinnewtonpersquaremetreisobtainedbydividingtheion
The last approved version of this historical standard is referenced on
www.astm.org. collector current in amperes for a given gas by the pressure
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E296 − 70 (2010)
sensitivity of the gage in amperes per newton per square metre 3.1.16 recovery time—the time required for the pressure
for pure nitrogen under specified operating conditions. indication of a gage to reach and remain within pressure
3.1.7 gage background—the part of the indicated ion col- indications not more than 105% or less than 95% of the final
lector current produced by phenomena other than ions formed average steady-state value after a sudden change in the
in the gas phase arriving at the collector. operatingconditionsofthegagewithoutappreciablechangein
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
3.1.10 molecular flux density—the number of molecules
diameter of the part of the envelope adjacent to the ion source
incident on a real or imaginary surface per unit area per unit
region and of length at least equal to the diameter of the
time. The unit is molecules per second per square centimetre.
tubulation.
3.1.11 molecular flux sensitivity—the output current of an
3.1.19 vacuum gas analyzer—adevicecapableofindicating
enclosed vacuum gage per unit molecular flux density under
the relative composition of a gas mixture at low pressures.
specified gage operating conditions and random particle mo-
tion.
4. Apparatus
3.1.12 nude ionization gage—an ionization gage for which
4.1 Equipment—Acceptable vacuum-measuring equipment
the center of the ion source region is exposed to direct
shallconsistofthoseitemsinwhichperformanceiscompatible
molecular flux (from surfaces not forming part of the gage) in
with obtaining meaningful measurements. The basic elements
all directions except for a solid angle less than 0.05×4 π
consist of a power supply, readout, and sensing element.These
steradians (determined by the parts of the gage head). No
items must be acceptable for applying the proper calibrations
structures shall be within one sensing element diameter of any
described in Methods E297. The electronic power supply and
partofthesensingelementunlesssimilarstructuresarepresent
readout shall have been calibrated either separately or in
during calibration.
conjunction with the test stand calibration of the gage sensor.
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
the disk is given as follows:
impedance or resistance errors are properly accounted for in
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
the test stand. Alternatively, calibration before and after test
3.1.14 partial pressure gage—an ionization gage that indi-
catesthepartialpressureofanygasinamixtureirrespectiveof may be incorporated as part of major test programs.
the partial pressure of other gases in the mixture.
5. Gage Mounting
3.1.15 partially enclosed ionization gage—a gage in which
the ion formation region is enclosed over less than 0.95×4 π 5.1 Flanges and Couplings—Flanging and connections are
steradians but more than 0.05×4 π steradians about center by specified in this section both for dimensions and material
an envelope which has one or more openings such that not all between ionization gages and the external walls of high-
molecules entering the ion formation region must first cross a vacuumsystemstoproduceageometricallystandardmounting
plane located outside this region. method (compatible with the calibration test stand) which is a
E296 − 70 (2010)
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-
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
FIG. 2 Flange-Mounted Nude Ionization Gage
the mounting to monitor or control, or both, the equilibrium
temperatureconditionoftheenvelopethatcanbeduplicatedin
a calibration test stand. Temperature control can be by either
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-
FIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls and Struc-
rected or suitably accounted for during calibration.
tures
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
FIG. 1 Tubulated Ionization Gage molecularfluxdensity(directionalpressure)asingasexchange
E296 − 70 (2010)
between a source and a pump, where the quantity flowing simplest check that can be made is the isolation of the gage by
toward the pump is greater than that flowing from the pump; intermittent operation of possible sources of ion coupling.
(2) directional composition, as in gas exchange between an
7.1.1 Sources:
outgassing body and a cryopump, where the outgassing mate-
7.1.1.1 Other vacuum ionization meas
...