ASTM E296-70(2020)

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Tubes (Withdrawn 1983)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. effects are present.
7. Operational Errors
6. Gage Orientation
7.1 Ion Coupling—Errors may be introduced into the ob-
6.1 General—Orientation of gages is significant where the
served readings through ion coupling. Such coupling may
gas atmosphere in a vacuum chamber has directional proper-
resultineitherlossorgainoftheobservedcurrentreading.The
ties.These properties are of at least three kinds: (1) directional
simplest check that can be made is the isolation of the gage by
molecularfluxdensity(directionalpressure)asingasexchange
intermittent operation of possible sources of ion coupling.
between a source and a pump, where the quantity flowing
toward the pump is greater than that flowing from the pump; 7.1.1 Sources:
7.1.1.1 Other vacuum ionization measurement devices such
(2) directional composition, as in gas exchange between an
outgassing body and a cryopump, where the outgassing mate- as multiple ionization gages or vacuum gas analyzers (VGA)
being run in close proximity,
rial is mainly condensible and the material flowing from the
cryopump is mainly noncondensible; (3) directional 7.1.1.2 Radioactive sources emitting nuclear radiations,
temperature, as in gas exchange between a warm and cold 7.1.1.3 Other sources of electromagnetic radiation or par-
surface. The magnitude of the first two effects is dependent on ticle radiation being used to simulate the solar spectrum or the
the fraction of incident molecules captured by the pump; flux particulate radiation of space and,
densitiesinoppositedirectionsmaydifferbyadecadeormore.
7.1.1.4 Other hot-filament or high-voltage test elements
The magnitude of the third effect is of the order of the capable of producing ions.
temperature difference. Significant directional gas flow can
7.1.2 Magnetic Field Effects:
occur in the normal operation of large simulators using solar
7.1.2.1 Operational characteristics of hot- or cold-cathode
simulation sources, cryopumping, moderate to heavy gas loads
ionization gages may be influenced by the presence of disturb-
arising from test items, and temperature extremes throughout
ingmagneticorelectricalfields.Publisheddataonexperiments
the internal surfaces.
relatingthealterationofaparticulargageoperationversusfield
strengths and the gage-field orientation are limited.
6.2 Response of Gage in Directional Environments—Nude
7.1.2.2 To minimize introducing such errors, gages should
gages indicate primarily the local gas concentration, but they
not be placed in close proximity to known sources of strong
aresensitivetothedirectionoftheincidentgasbecauseoftheir
magnetic or electrical fields. In the event it is necessary to
nonsymmetrical construction. The response of enclosed gages
operate a gage in the presence of such fields, intermittent
is dependent on the flux density and the direction of gas
operation of the field source should be made to note possible
incidence. The directional effect is, to a first approximation,
influence. Under extreme cases, special shielding may be used
proportionaltothecosineoftheanglebetweenthedirectionof
toreduceexternalfieldeffects.Undersuchcases,acheckmust
gas flow and the normal to the plane of the gage mount, but
be made to ascertain that the shielding itself does not alter the
also depends on the geometry of the tubulation.
gage sensitivity.
6.3 Ideal Gage Orientation:
7.2 Temperature Effects—Operational temperature correc-
6.3.1 Ideal simulation of space vacuum would be achieved
tions cannot be neglected when tubulated gages are used at, or
if outgassing products from the test item were pumped with a
areattachedto,chamberswhichoperateoverwidetemperature
100% capture coefficient, while the test item was bombarded
ranges. Such errors are not to be confused with temperature
withastreamofions,molecules,andotherparticleshavingthe
effects on contaminant sorption, desorption, or condensation.
flux density, composition, and energy present in space. Gages
At the gage calibration temperature conditions, the sensor has
placed to measure these characteristics of the gas environment
known operational constants. The determination of a pressure
incidentonthetestiteminapracticalsimulatorwouldindicate
atsignificantlydifferenttemperaturesotherthanthecalibration
the degree of departure from space conditions.
conditions requires appropriate correction. Examples of tem-
6.3.2 The flux density of particles incident on the test item
perature corrections are presented in Appendix X2.
may be measured near the surface of the test item or near the
source (the wall) with an appropriately placed, enclosed gage
8. Gage Correction for Gas Composition
facing the wall. The latter arrangement is mechanically
simpler, but the interpretation of readings is more complex if 8.1 Since an ionization vacuum gage is a device that
the gas load leaving the wall is not uniform. provides an ion current derived from all the species present
with different gage factors for each, it is possible the resulting
6.4 Departures from Ideal Orientation—In cases in which
ion current may be derived substantially from species other
theincidentgasfluxisknowntobeadecadeormorebelowthe
than nitrogen, the most typical calibrating gas. Provided both
level at which significant effects occur, the directional proper-
the percentage of other gases and the appropriate gage factors
ties of the environment may be safely ignored. Likewise,
are known, the indicated gage reading can be normalized to
phenomena such as electrical breakdown may be known to
provide corrected concentration or pressure values.
depend on gas concentration rather than flux, and directional
gages are not appropriate. 8.2 Data Reduction:
E296 − 70 (2020)
8.2.1 By applying known gage sensitivity factors the con- setting should be periodically checked throughout the test as
tributionofeachgasbeingionizedcanbeaccountedforwithin should the meter zero(s) before and during the gage operation.
the total ion current. Generally the sensitivity of the gage for
10.2 Continuous Operating Criteria—Once the gage has
other gases is presented as the ratio of the sensitivity based on
been turned on (both hot- and cold-cathode gages) the gage
nitrogen being 1. The change in sensitivities as a first approxi-
should remain on continuously throughout the test. Exception
mation is proportional to the difference in ionization probabil-
may have to be made for short periods to check meter zeros.
ity for the gases involved. It should be remembered nitrogen
10.3 Degas—Degas of the gage should be performed in a
simply serves as a convenient reference gas for pressure or
manner that has previously been checked while the gage is on
concentration calibrations.
the calibration test stand. For reestablishment of equilibrium
8.2.2 Theprecisionofthecorrectionisdirectlyrelatedtothe
conditions, comparable with the calibration conditions, it is
validity of the sensitivity factors used for the particular gage
essentialthewaitingperiodfollowingdegascorrespondstothe
involved. See Appendix X1 for a method for measuring or
gage recovery time or longer before using the indicated
estimating sensitivity factors.
readings. Degas should be used when sorption of high or low
NOTE 2—When specific sensitivity factors are otherwise unavailable,
molecular weight materials may have occurred. It is not
one may use values such as those given in Refs (1) and (2). If corrected
recommended degassing of the gage be periodically or auto-
pressures or concentrations are reported, the user should clearly indicate
matically used since the recovery time may extend for many
thesensitivityfactorsusedforperformingthecorrection.Anillustrationof
the method of calculating true total pressure from the ion current reading
hours under certain circumstances.
and the relative gas composition as given by a vacuum gas analyzer
10.4 Method of Degas—Available methods of degas consist
(VGA) is given as follows:
−8
Ion current from gage=1.0×10 A (from test) of electron bombardment heating, resistive heating of the grid
−1 2
Gage sensitivity for nitrogen=10 A·N/m = K
N2 structures,orbakeoutofthegagewithorwithoutsimultaneous
Gas composition in chamber=80% He, 10% A, 10% N (relative
bakeout of the system, or a combination of these. The pro-
composition from VGA).
cesses should be in accordance with the recommended proce-
Relative gage sensitivity factors:
dures of the equipment manufacturer, if specified, or should be
K /K =0.2,K /K =1.3,K /K =1.0
H N A N N N
e 2 2 2 2
P + P + P = P or (n + n + n ) developed in conjunction with the calibration activity and
H A N T H A N
e 2 e 2
–8
= P K + P K + P K = P K =1.0x10 A
H He A A N N T T
e 2 2 determination of the gage recovery time. In general, note that
P =true total pressure, P =true partial pressure of nitrogen
T N
typical recovery times for nude ionization gages are generally
P K P quite short (minutes) as compared to those of enclosed gages.
P K K
H H N
e e A A 2 T
1 1 5 (2)
P K P K P K
T N T N T N
2 2 2
11. Heavy Molecular Weight Contamination Effects
−1
0.8×0.2+0.1×1.3+0.1= K /10
T
−1 2
11.1 Transient Effects:
K = (0.16+0.13+0.1) 10 A·N/m
T
−1 2
K = 3.9×10 A·N/m 11.1.1 Discrepancy between readings of nude and tubulated
T
−8 −2 2 −7 2
P = (1.0 × 10 )/3.9 × 10 N/m =2.6×10 N/m = true total
T gages on the same system has been shown to be particularly
pressure
high where the residual atmosphere is largely of high molecu-
−8 −1 2
P = (equivalent)=(1.0×10 )/10 N/m
N
−7 2 larweight—asoutgasproductsfromtestspecimens,lubricants,
= 1.0×10 N/m
or vapors of vacuum pump fluids. Partial pressure analysis of
= equivalent nitrogen pressure
the chamber atmosphere can warn that the situation is likely,
9. Determination of Gas Composition but can do nothing to allow quantitative correction for it,
becausethetubulatediongageistoalargeextentgeneratingits
9.1 Determination of the gas composition in a vacuum
own atmosphere (Blears effect).
chamber is obtained by use of special simplified mass spec-
11.1.2 Decontamination—Where operation of test system,
trometers vacuum gas analyzer (VGA) to separate and deter-
specimen, and gage have permitted a film of condensable
mine the amount of the various molecular species present.
material to coat the gage envelope or supporting structure, the
Thesedevicescanbecalibratedtoindicatetheabsoluteamount
gage should be vacuum baked mildly to evaporate such
of a given species being detected and thus serve as a partial
material before any outgassing by electron bombardment.This
pressuregage.Forgagecorrectiononlytherelativedistribution
will avoid deposit of carbon or resinous films.
is needed.
11.1.3 Reading the Gage Following Degas—In ultrahigh
NOTE 3—Methods of calibration and application of vacuum gas
vacuum systems, and again where heavy molecules make up a
analyzers are now in preparation by Committee E21.
large proportion of the residual atmosphere, whether outgas
products or diffusion pump fluids in untrapped or poorly
10. Operating Criteria
trapped systems, the time required for an outgassed gage to
10.1 Electronic Adjustment—Upon achievement of a high
reach an equilibrium with the chamber to which it is attached
vacuum condition in the working volume, the gage shall be
may be hours or days, depending on geometry. For example, a
turned on and appropriate adjustment of the emission current
gage attached by a 2-in. (50-mm) diameter tube, 8 in. (200
made to correspond with the calibration conditions. This
mm) long, to a test dome at equilibrium with silicone oil vapor
atroomtemperature,requires6to8hafteroutgassingtoreach
−7 2 −99
a final reading of 3×10 N/m (2×10 torr). Immediately
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
−8 2 −10
this practice. after outgassing, the reading may be 7×10 N/m (5×10
E296 − 70 (2020)
torr). The delay is, of course, due to adsorption of fluid vapor manner). If however, the observed ion current is seen to
on the clean walls of the tubulation. The ion gage reading increasetovaluesmuchlargerthanthefactorbywhichthegrid
levelsonlyaftertheadsorptioncapacityofthewallissatisfied. current was increased, the grid is probably dirty and requires
11.1.4 Temperature Effects—Where heavy vapors are per- further outgassing. Note that in tubulated gages the increase in
mitted in the vicinity of the ion gage at partial pressures near fil
...