ASTM E296-70(1999)

Standard Practice for Ionization Gage Application to Space Simulators

SCOPE
1.1 The purpose of this practice is to provide application criteria, definitions, and supplemental information to assist the user in obtaining meaningful vacuum ionization gage measurements below 10   N/m  (10   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: 09-Oct-1999
• 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

• Replaced By: ASTM E296-70(2004) - Standard Practice for Ionization Gage Application to Space Simulators
• Effective Date: 01-Sep-2004
• Referred By: ASTM E2900-19 - Standard Practice for Spacecraft Hardware Thermal Vacuum Bakeout
• Effective Date: 10-Oct-1999
• 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: 10-Oct-1999
• Referred By: ASTM E491-73(2020) - Standard Practice for Solar Simulation for Thermal Balance Testing of Spacecraft
• Effective Date: 10-Oct-1999

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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.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: E 296 – 70 (Reapproved 1999)
Standard Practice for
Ionization Gage Application to Space Simulators
This standard is issued under the fixed designation E 296; 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 3.1.1 Blears effect—the reduction of the partial pressure of
organic vapors within the envelope of a tubulated ionization
1.1 The purpose of this practice is to provide application
gage below the partial pressure that would prevail in the
criteria, definitions, and supplemental information to assist the
envelope with a tubulation having infinite conductance.
user in obtaining meaningful vacuum ionization gage measure-
−1 2 −3
3.1.2 controlled-temperature enclosed gage—an enclosed
ments below 10 N/m (10 torr) in space-simulation facili-
gage in which the envelope is maintained at nearly uniform
ties. Since a variety of influences can alter observed vacuum
constant temperature by suitable means.
measurements, means of identifying and assessing potential
3.1.3 enclosed ionization gage—an ionization gage for
problem areas receive considerable attention. This practice
which the ion source region is enclosed over at least 0.95 3 4
must be considered informational, for it is impossible to
p steradians about the center of the region by an envelope at a
specify a means of applying the vacuum-measuring equipment
known temperature with only a single opening such that all
to guarantee accuracy of the observed vacuum measurement.
molecules entering the ion source region must have crossed a
Therefore, the user’s judgment is essential so that if a problem
plane located outside this region.
area is identified, suitable steps can be taken to either minimize
3.1.4 equivalent nitrogen concentration—the quantity ob-
the effect, correct the observed readings as appropriate, or note
tained when the ion-collector current of a nude gage (in
the possible error in the observation.
amperes) for the gas in the system is divided by the concen-
1.2 While much of the discussion is concerned with the
tration sensitivity of the gage for nitrogen. This sensitivity is
application of hot-cathode ionization gages, no exclusion is
defined as the ratio of gage ion collector current in amperes to
made of cold-cathode designs. Since a great deal more expe-
molecular concentration in molecules per cubic metre of
rience with hot-cathode gages is available and hot-cathode
nitrogen under specified operating conditions.
devices are used in the majority of applications, the present
3.1.5 equivalent nitrogen flux density—the quotient of the
emphasis is fully warranted.
current output of an enclosed vacuum gage operating under
1.3 The values stated in inch-pound units are to be regarded
specified conditions divided by the molecular flux sensitivity
as the standard. The metric equivalents of inch-pound units
for nitrogen.
may be approximate.
3.1.6 equivalent nitrogen pressure:
2. Referenced Documents
3.1.6.1 For a nude gage equivalent nitrogen pressure is
obtained by multiplying the equivalent nitrogen concentration
2.1 ASTM Standards:
by kT where k is the Boltzmann constant and T is the mean
E 297 Methods for Calibrating Ionization Vacuum Gage
absolute temperature of the walls from which the gas mol-
Tubes
ecules travel to the ionizing region of the gage, averaged as
3. Terminology
nearly as possible on the basis of relative molecular flux.
3.1.6.2 standard equivalent nitrogen pressure—for a nude
3.1 Definitions—The following definitions are necessary to
gage the value of the equivalent nitrogen pressure is obtained
understanding meaningful application of ionization-type
when T 5 296K (or standard ambient temperature) is used in
vacuum-measurement devices and are useful in differentiating
the factor kT.
between pressure, density, and flux measuring devices for
3.1.6.3 For a tubulated gage, the equivalent nitrogen
proper application and interpretation of low-density molecular
pressure in newton per square metre is obtained by dividing the
measurements.
ion collector current in amperes for a given gas by the pressure
sensitivity of the gage in amperes per newton per square metre
These practices are under the jurisdiction of ASTM Committee E-21 on Space
for pure nitrogen under specified operating conditions.
Simulation and Applications of Space Technology and are the direct responsibility
3.1.7 gage background—the part of the indicated ion col-
of Subcommittee E21.04 on Space Simulation Test Methods.
lector current produced by phenomena other than ions formed
Current edition effective Feb. 27, 1970. Originally issued 1966. Replaces E 296
–66 T. in the gas phase arriving at the collector.
Discontinued, see 1984 Annual Book of ASTM Standards, Vol 15.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E 296
3.1.8 gage limit—a pressure or concentration indication 3.1.17 response time—the time required for the change in
four times the background. pressure indication as a result of a specified gas (or vapor)
3.1.9 ionization gage—a vacuum gage comprising a means within a gage tube to reach (1 − 1/e) (or 63 %) of the change in
of ionizing the gas molecules and a means of correlating the steady-state pressure after a relatively instantaneous change of
number and type of ions produced with the pressure or
the pressure of that gas in the vacuum chamber. The response
concentration of the gas. Various types of ionization gages are time may depend on the time of adsorption of the gas (or
distinguished according to the method of producing the ion- vapor) on the walls of the gage tube as well as the geometry of
ization. the tube (including the connecting line to the vacuum cham-
3.1.9.1 cold-cathode ionization gage—an ionization gage in ber).
which the ions are produced by a cold-cathode gas discharge,
3.1.18 tubulated ionization gage—an enclosed ionization
usually in the presence of a magnetic field.
gage for which the opening in the envelope is determined by a
3.1.9.2 hot-cathode ionization gage—an ionization gage in
tubulation of diameter equal to or less than the minimum
which ion production is initiated and sustained by electrons
diameter of the part of the envelope adjacent to the ion source
emitted from a hot cathode.
region and of length at least equal to the diameter of the
3.1.10 molecular flux density—the number of molecules
tubulation.
incident on a real or imaginary surface per unit area per unit
3.1.19 vacuum gas analyzer—a device capable of indicating
time. The unit is molecules per second per square centimetre.
the relative composition of a gas mixture at low pressures.
3.1.11 molecular flux sensitivity—the output current of an
enclosed vacuum gage per unit molecular flux density under
4. Apparatus
specified gage operating conditions and random particle mo-
4.1 Equipment—Acceptable vacuum-measuring equipment
tion.
shall consist of those items in which performance is compatible
3.1.12 nude ionization gage—an ionization gage for which
with obtaining meaningful measurements. The basic elements
the center of the ion source region is exposed to direct
consist of a power supply, readout, and sensing element. These
molecular flux (from surfaces not forming part of the gage) in
items must be acceptable for applying the proper calibrations
all directions except for a solid angle less than 0.05 3 4 p
described in Methods E 297. The electronic power supply and
steradians (determined by the parts of the gage head). No
readout shall have been calibrated either separately or in
structures shall be within one sensing element diameter of any
conjunction with the test stand calibration of the gage sensor.
part of the sensing element unless similar structures are present
Special attention must be given to cabling, especially where
during calibration.
cabling runs are long (as in large vacuum systems) in order that
NOTE 1—The solid angle subtended by a circular disk of radius r with
impedance or resistance errors are properly accounted for in
axis passing through the center point of the solid angle at a distance y from
the calibration activities.
the disk is given as follows:
4.2 Calibration—These practices are not concerned with
2 2 1 2
/
v5 2 p@1 2 y/~y 1 r ! # (1)
gage calibration criteria except as applicable during test. Test
For v5 0.05 3 4p , the distance y must equal 2.07 r,a
stand calibration criteria is provided by Methods E 297.
value which should be easily attainable for typical ionization
Recycle of the vacuum-measuring equipment to the calibration
gage electrodes mounted on a circular base of radius r.
test stand should not be programmed only on a calendar basis.
3.1.13 orifice ionization gage—an enclosed gage containing
Periodic recycle can best be determined by the individual
a single orifice or port having a length less than 0.15 of its
operators compatible with usage requirements. Upon any
diameter such that molecules from the chamber can enter the
strong indication that usage in test may have produced an
envelope directly from within a solid angle nearly equal to 2 p
alteration in gage factor, suspect elements shall be returned to
steradians.
the test stand. Alternatively, calibration before and after test
3.1.14 partial pressure gage—an ionization gage that indi-
may be incorporated as part of major test programs.
cates the partial pressure of any gas in a mixture irrespective of
the partial pressure of other gases in the mixture.
5. Gage Mounting
3.1.15 partially enclosed ionization gage—a gage in which
5.1 Flanges and Couplings—Flanging and connections are
the ion formation region is enclosed over less than 0.95 3 4 p
specified in this section both for dimensions and material
steradians but more than 0.05 3 4 p steradians about center by
between ionization gages and the external walls of high-
an envelope which has one or more openings such that not all
vacuum systems to produce a geometrically standard mounting
molecules entering the ion formation region must first cross a
method (compatible with the calibration test stand) which is a
plane located outside this region.
clean assembly free of interfering contamination such as that
3.1.16 recovery time—the time required for the pressure
produced by organic or high vapor-pressure sealing materials.
indication of a gage to reach and remain within pressure
5.1.1 Tubulated Ionization Gage (Fig. 1):
indications not more than 105 % or less than 95 % of the final
average steady-state value after a sudden change in the 5.1.1.1 The flange material shall be stainless steel with a
operating conditions of the gage without appreciable change in glass-to-metal seal connecting the gage to the flange stub. The
the gas pressure in the vacuum chamber. Pressure changes less flanges shall be welded or high-temperature brazed with
than 5 % of the initial value shall be regarded as within the appropriate cleaning to remove residual flux. Gasket material
normal fluctuations of pressure indication. shall be metallic: copper, aluminum, indium, and so forth.
E 296
FIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls and
Structures
5.2.1.1 Mechanical—The mechanical support and position-
ing of internally mounted tubulated gages must not influence
FIG. 1 Tubulated Ionization Gage
the distribution of molecules across the tubulation.
5.2.1.2 Thermal—Since internally mounted tubulated gages
will experience significantly different heat transfer conditions
5.1.1.2 The gage may be attached directly to chamber
from the envelope, care should be taken to provide means in
eliminating flanges and gasketing providing limiting dimen-
the mounting to monitor or control, or both, the equilibrium
sions are adhered to.
temperature condition of the envelope that can be duplicated in
5.1.2 Nude or Partially Enclosed Ionization Gages (Fig. 2
a calibration test stand. Temperature control can be by either
and Fig. 3)—See 5.1.1.1.
active or passive means—an active means representing a
5.1.2.1 Intent is to give maximum solid-angle (line-of-sight)
controlled temperature enclosed gage.
exposure of the gage elements to the chamber environments.
5.2.1.3 Electrical—Shielding of the electrical leads, espe-
5.2 Internally Mounted Ionization Gages—Limitations for
cially the collector, poses somewhat more of a problem than
mounting ionization gages internally are specified in this
with externally mounted gages. Care must be taken in the use
section to provide mounting considerations applicable to plac-
of unshielded wires that external pickup does not compromise
ing any vacuum-ionization gage within the vacuum volume.
the collector current. In any hookup, aside from leakage and
Measurement considerations are provided in Section 6.
especially where long cables may be used, capacitance and
5.2.1 Tubulated Ionization Gages:
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
as to provide equivalent acceptance angles of molecular flux as
defined for the flange-mounted condition (Fig. 2 and Fig. 3).
5.2.2.2 Thermal—Thermal considerations with nude and
partially enclosed gages are less significant than with tubulated
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
molecular flux density (directional pressure) as in gas exchange
between a source and a pump, where the quantity flowing
toward the pump is greater than that flowing from the pump;
(2) directional composition, as in gas exchange between an
outgassing body and a cryopump, where the outgassing mate-
rial is mainly condensible and the material flowing from the
cryopump is mainly noncondensible; (3) directional tempera-
ture, as in gas exchange between a warm and cold surface. The
magnitude of the first two effects is dependent on the fraction
of incident molecules captured by the pump; flux densities in
FIG. 2 Flange-Mounted Nude Ionization Gage opposite directions may differ by a decade or more. The
E 296
magnitude of the third
...