1、Designation: E296 70 (Reapproved 2015)Standard Practice forIonization Gage Application to Space Simulators1This standard is issued under the fixed designation E296; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last r
2、evision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice provides application criteria, definitions,and supplemental information to assist the user in obtainingmeaning
3、ful vacuum ionization gage measurements below 101N/m2(103torr) in space-simulation facilities. Since a varietyof influences can alter observed vacuum measurements, meansof identifying and assessing potential problem areas receiveconsiderable attention. This practice must be consideredinformational,
4、for it is impossible to specify a means ofapplying the vacuum-measuring equipment to guarantee accu-racy of the observed vacuum measurement. Therefore, theusers judgment is essential so that if a problem area isidentified, suitable steps can be taken to either minimize theeffect, correct the observe
5、d readings as appropriate, or note thepossible error in the observation.1.2 While much of the discussion is concerned with theapplication of hot-cathode ionization gages, no exclusion ismade of cold-cathode designs. Since a great deal more expe-rience with hot-cathode gages is available and hot-cath
6、odedevices are used in the majority of applications, the presentemphasis is fully warranted.1.3 The values stated in inch-pound units are to be regardedas the standard. The metric equivalents of inch-pound unitsmay be approximate.2. Referenced Documents2.1 ASTM Standards:2E297 Test Method for Calibr
7、ating Ionization Vacuum GageTubes (Withdrawn 1983)33. Terminology3.1 DefinitionsThe following definitions are necessary tounderstanding meaningful application of ionization-typevacuum-measurement devices and are useful in differentiatingbetween pressure, density, and flux measuring devices forproper
8、 application and interpretation of low-density molecularmeasurements.3.1.1 Blears effectthe reduction of the partial pressure oforganic vapors within the envelope of a tubulated ionizationgage below the partial pressure that would prevail in theenvelope with a tubulation having infinite conductance.
9、3.1.2 controlled-temperature enclosed gagean enclosedgage in which the envelope is maintained at nearly uniformconstant temperature by suitable means.3.1.3 enclosed ionization gagean ionization gage forwhich the ion source region is enclosed over at least 0.95 4 steradians about the center of the re
10、gion by an envelope at aknown temperature with only a single opening such that allmolecules entering the ion source region must have crossed aplane located outside this region.3.1.4 equivalent nitrogen concentrationthe quantity ob-tained when the ion-collector current of a nude gage (inamperes) for
11、the gas in the system is divided by the concen-tration sensitivity of the gage for nitrogen. This sensitivity isdefined as the ratio of gage ion collector current in amperes tomolecular concentration in molecules per cubic metre ofnitrogen under specified operating conditions.3.1.5 equivalent nitrog
12、en flux densitythe quotient of thecurrent output of an enclosed vacuum gage operating underspecified conditions divided by the molecular flux sensitivityfor nitrogen.3.1.6 equivalent nitrogen pressure:3.1.6.1 For a nude gage, equivalent nitrogen pressure isobtained by multiplying the equivalent nitr
13、ogen concentrationby kTwhere k is the Boltzmann constant and T is the meanabsolute temperature of the walls from which the gas mol-ecules travel to the ionizing region of the gage, averaged asnearly as possible on the basis of relative molecular flux.3.1.6.2 standard equivalent nitrogen pressurefor
14、a nudegage, the value of the equivalent nitrogen pressure is obtainedwhen T = 296K (or standard ambient temperature) is used inthe factor kT.3.1.6.3 For a tubulated gage, the equivalent nitrogen pres-sure in newton per square metre is obtained by dividing the ioncollector current in amperes for a gi
15、ven gas by the pressure1This practice is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and are the direct responsibilityof Subcommittee E21.04 on Space Simulation Test Methods.Current edition approved Oct. 1, 2015. Published May 2010. Originally
16、 approvedin 1966. Last previous edition approved in 2010 as E296 70(2010). DOI:10.1520/E0296-70R15.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Doc
17、ument Summary page onthe ASTM website.3The last approved version of this historical standard is referenced onwww.astm.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1sensitivity of the gage in amperes per newton per square metrefo
18、r pure nitrogen under specified operating conditions.3.1.7 gage backgroundthe part of the indicated ion col-lector current produced by phenomena other than ions formedin the gas phase arriving at the collector.3.1.8 gage limita pressure or concentration indication fourtimes the background.3.1.9 ioni
19、zation gagea vacuum gage comprising a meansof ionizing the gas molecules and a means of correlating thenumber and type of ions produced with the pressure orconcentration of the gas. Various types of ionization gages aredistinguished according to the method of producing the ion-ization.3.1.9.1 cold-c
20、athode ionization gagean ionization gage inwhich the ions are produced by a cold-cathode gas discharge,usually in the presence of a magnetic field.3.1.9.2 hot-cathode ionization gagean ionization gage inwhich ion production is initiated and sustained by electronsemitted from a hot cathode.3.1.10 mol
21、ecular flux densitythe number of moleculesincident on a real or imaginary surface per unit area per unittime. The unit is molecules per second per square centimetre.3.1.11 molecular flux sensitivitythe output current of anenclosed vacuum gage per unit molecular flux density underspecified gage opera
22、ting conditions and random particle mo-tion.3.1.12 nude ionization gagean ionization gage for whichthe center of the ion source region is exposed to directmolecular flux (from surfaces not forming part of the gage) inall directions except for a solid angle less than 0.05 4 steradians (determined by
23、the parts of the gage head). Nostructures shall be within one sensing element diameter of anypart of the sensing element unless similar structures are presentduring calibration.NOTE 1The solid angle subtended by a circular disk of radius r withaxis passing through the center point of the solid angle
24、 at a distance y fromthe disk is given as follows: 5 2 1 2 y/y21r2!1/2# (1)For =0.054 , the distance y must equal 2.07 r,avalue which should be easily attainable for typical ionizationgage electrodes mounted on a circular base of radius r.3.1.13 orifice ionization gagean enclosed gage containinga si
25、ngle orifice or port having a length less than 0.15 of itsdiameter such that molecules from the chamber can enter theenvelope directly from within a solid angle nearly equal to 2 steradians.3.1.14 partial pressure gagean ionization gage that indi-cates the partial pressure of any gas in a mixture ir
26、respective ofthe partial pressure of other gases in the mixture.3.1.15 partially enclosed ionization gagea gage in whichthe ion formation region is enclosed over less than 0.95 4 steradians but more than 0.05 4 steradians about center byan envelope which has one or more openings such that not allmol
27、ecules entering the ion formation region must first cross aplane located outside this region.3.1.16 recovery timethe time required for the pressureindication of a gage to reach and remain within pressureindications not more than 105 % or less than 95 % of the finalaverage steady-state value after a
28、sudden change in theoperating conditions of the gage without appreciable change inthe gas pressure in the vacuum chamber. Pressure changes lessthan 5 % of the initial value shall be regarded as within thenormal fluctuations of pressure indication.3.1.17 response timethe time required for the change
29、inpressure indication as a result of a specified gas (or vapor)within a gage tube to reach (1 1e) (or 63 %) of the changein steady-state pressure after a relatively instantaneous changeof the pressure of that gas in the vacuum chamber. Theresponse time may depend on the time of adsorption of the gas
30、(or vapor) on the walls of the gage tube as well as the geometryof the tube (including the connecting line to the vacuumchamber).3.1.18 tubulated ionization gagean enclosed ionizationgage for which the opening in the envelope is determined by atubulation of diameter equal to or less than the minimum
31、diameter of the part of the envelope adjacent to the ion sourceregion and of length at least equal to the diameter of thetubulation.3.1.19 vacuum gas analyzera device capable of indicatingthe relative composition of a gas mixture at low pressures.4. Apparatus4.1 EquipmentAcceptable vacuum-measuring
32、equipmentshall consist of those items in which performance is compatiblewith obtaining meaningful measurements. The basic elementsconsist of a power supply, readout, and sensing element. Theseitems must be acceptable for applying the proper calibrationsdescribed in Methods E297. The electronic power
33、 supply andreadout shall have been calibrated either separately or inconjunction with the test stand calibration of the gage sensor.Special attention must be given to cabling, especially wherecabling runs are long (as in large vacuum systems) in order thatimpedance or resistance errors are properly
34、accounted for inthe calibration activities.4.2 CalibrationThese practices are not concerned withgage calibration criteria except as applicable during test. Teststand calibration criteria is provided by Methods E297. Re-cycle of the vacuum-measuring equipment to the calibrationtest stand should not b
35、e programmed only on a calendar basis.Periodic recycle can best be determined by the individualoperators compatible with usage requirements. Upon anystrong indication that usage in test may have produced analteration in gage factor, suspect elements shall be returned tothe test stand. Alternatively,
36、 calibration before and after testmay be incorporated as part of major test programs.5. Gage Mounting5.1 Flanges and CouplingsFlanging and connections arespecified in this section both for dimensions and materialbetween ionization gages and the external walls of high-vacuum systems to produce a geom
37、etrically standard mountingmethod (compatible with the calibration test stand) which is aE296 70 (2015)2clean assembly free of interfering contamination such as thatproduced by organic or high vapor-pressure sealing materials.5.1.1 Tubulated Ionization Gage (Fig. 1):5.1.1.1 The flange material shall
38、 be stainless steel with aglass-to-metal seal connecting the gage to the flange stub. Theflanges shall be welded or high-temperature brazed withappropriate cleaning to remove residual flux. Gasket materialshall be metallic: copper, aluminum, indium, and so forth.5.1.1.2 The gage may be attached dire
39、ctly to chambereliminating flanges and gasketing providing limiting dimen-sions are adhered to.5.1.2 Nude or Partially Enclosed Ionization Gages (Fig. 2and Fig. 3)See 5.1.1.1.5.1.2.1 Intent is to give maximum solid-angle (line-of-sight)exposure of the gage elements to the chamber environments.5.2 In
40、ternally Mounted Ionization GagesLimitations formounting ionization gages internally are specified in thissection 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 Ionizat
41、ion Gages:5.2.1.1 MechanicalThe mechanical support and position-ing of internally mounted tubulated gages must not influencethe distribution of molecules across the tubulation.5.2.1.2 ThermalSince internally mounted tubulated gageswill experience significantly different heat transfer conditionsfrom
42、the envelope, care should be taken to provide means inthe mounting to monitor or control, or both, the equilibriumtemperature condition of the envelope that can be duplicated ina calibration test stand. Temperature control can be by eitheractive or passive meansan active means representing acontroll
43、ed temperature enclosed gage.5.2.1.3 ElectricalShielding of the electrical leads, espe-cially the collector, poses somewhat more of a problem thanwith externally mounted gages. Care must be taken in the useof unshielded wires that external pickup does not compromisethe collector current. In any hook
44、up, aside from leakage andespecially where long cables may be used, capacitance andresistance 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 MechanicalThe mechanical support shall be suchas to pr
45、ovide equivalent acceptance angles of molecular flux asdefined for the flange-mounted condition (Fig. 2 and Fig. 3).5.2.2.2 ThermalThermal considerations with nude andpartially enclosed gages are less significant than with tubulatedgages. Generally, the mechanical support will require nospecial atte
46、ntion except in extreme conditions where conduc-tion or radiation paths to nearby surfaces provide an extremetemperature differential.5.2.2.3 ElectricalSame as 5.2.1.3.6. Gage Orientation6.1 GeneralOrientation of gages is significant where thegas atmosphere in a vacuum chamber has directional proper
47、-ties. These properties are of at least three kinds: (1) directionalmolecular flux density (directional pressure) as in gas exchangeFIG. 1 Tubulated Ionization GageFIG. 2 Flange-Mounted Nude Ionization GageFIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls and Struc-turesE296 70 (2015)3between a so
48、urce and a pump, where the quantity flowingtoward the pump is greater than that flowing from the pump;(2) directional composition, as in gas exchange between anoutgassing body and a cryopump, where the outgassing mate-rial is mainly condensible and the material flowing from thecryopump is mainly non
49、condensible; (3) directionaltemperature, as in gas exchange between a warm and coldsurface. The magnitude of the first two effects is dependent onthe fraction of incident molecules captured by the pump; fluxdensities in opposite directions may differ by a decade or more.The magnitude of the third effect is of the order of thetemperature difference. Significant directional gas flow canoccur in the normal operation of large simulators using solarsimulation sources, cryopumping, moderate to heavy gas loadsarising from te
