1、Designation: E 296 70 (Reapproved 2004)Standard Practice forIonization Gage Application to Space Simulators1This standard is issued under the fixed designation E 296; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last
2、 revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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 obtainingmean
3、ingful 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 considered infor-matio
4、nal, for it is impossible to specify a means of applying thevacuum-measuring equipment to guarantee accuracy of theobserved vacuum measurement. Therefore, the users judg-ment is essential so that if a problem area is identified, suitablesteps can be taken to either minimize the effect, correct theob
5、served readings as appropriate, or note the possible error inthe 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
6、-cathodedevices 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:2E 297 Methods for Cali
7、brating Ionization Vacuum GageTubes33. 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 application an
8、d 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.3.1.2 controlle
9、d-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 3 4p steradians about the center of the region by an e
10、nvelope 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 the gas in t
11、he 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 nitrogen flux dens
12、itythe 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 nitrogen concent
13、rationby 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 a nudegage,
14、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 nitrogenpressure in newton per square metre is obtained by dividing theion collector current in amperes for a given gas by the
15、 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 Sept. 1, 2004. Published September 2004. Originallyapprove
16、d in 1966. Last previous edition approved in 1999 as E 296 70 (1999).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 Document Summary page onthe ASTM
17、website.3Withdrawn.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.sensitivity of the gage in amperes per newton per square metrefor pure nitrogen under specified operating conditions.3.1.7 gage backgroundthe part of the indicated io
18、n 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 indicationfour times the background.3.1.9 ionization gagea vacuum gage comprising a meansof ionizing the gas molecules and a means of correlating th
19、enumber 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-cathode ionization gagean ionization gage inwhich the ions are produced by a cold-cathode gas discharge
20、,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 molecular flux densitythe number of moleculesincident on a real or imaginary surface per unit area per un
21、ittime. 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 operating conditions and random particle mo-tion.3.1.12 nude ionization gagean ionization gage for whichthe
22、 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 3 4 psteradians (determined by the parts of the gage head). Nostructures shall be within one sensing element diameter of anypart o
23、f 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 at a distance y fromthe disk is given as follows:v52 p1 2 y/y21 r2!1/2# (1)For v = 0.05 3 4p , the
24、 distance y must equal 2.07 r, a valuewhich should be easily attainable for typical ionization gageelectrodes mounted on a circular base of radius r.3.1.13 orifice ionization gagean enclosed gage containinga single orifice or port having a length less than 0.15 of itsdiameter such that molecules fro
25、m the chamber can enter theenvelope directly from within a solid angle nearly equal to 2 psteradians.3.1.14 partial pressure gagean ionization gage that indi-cates the partial pressure of any gas in a mixture irrespective ofthe partial pressure of other gases in the mixture.3.1.15 partially enclosed
26、 ionization gagea gage in whichthe ion formation region is enclosed over less than 0.95 3 4 psteradians but more than 0.05 3 4 p steradians about center byan envelope which has one or more openings such that not allmolecules entering the ion formation region must first cross aplane located outside t
27、his 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 sudden change in theoperating conditions of the gage without appreciable change in
28、the 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 inpressure indication as a result of a specified gas (or vapor)within a gage tube
29、to reach (1 1/e) (or 63 %) of the change insteady-state pressure after a relatively instantaneous change ofthe pressure of that gas in the vacuum chamber. The responsetime may depend on the time of adsorption of the gas (orvapor) on the walls of the gage tube as well as the geometry ofthe tube (incl
30、uding the connecting line to the vacuum cham-ber).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 minimumdiameter of the part of the envelope adjacent to the ion sourceregion and of le
31、ngth 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 equipmentshall consist of those items in which performance is compatiblewith ob
32、taining meaningful measurements. The basic elementsconsist of a power supply, readout, and sensing element. Theseitems must be acceptable for applying the proper calibrationsdescribed in Methods E 297. The electronic power supply andreadout shall have been calibrated either separately or inconjuncti
33、on 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 accounted for inthe calibration activities.4.2 CalibrationThese practices are
34、not concerned withgage calibration criteria except as applicable during test. Teststand calibration criteria is provided by Methods E 297.Recycle of the vacuum-measuring equipment to the calibrationtest stand should not be programmed only on a calendar basis.Periodic recycle can best be determined b
35、y 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, calibration before and after testmay be incorporated as part of major test pro
36、grams.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 geometrically standard mountingmethod (compatible with the calibration test stand)
37、which is aclean assembly free of interfering contamination such as thatproduced by organic or high vapor-pressure sealing materials.5.1.1 Tubulated Ionization Gage (Fig. 1):E 296 70 (2004)25.1.1.1 The flange material shall be stainless steel with aglass-to-metal seal connecting the gage to the flang
38、e 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 directly to chambereliminating flanges and gasketing providing limiting dimen-sion
39、s 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 Internally Mounted Ionization GagesLimitations formounting ionization gages inte
40、rnally 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 Ionization Gages:5.2.1.1 MechanicalThe mechanical support and position-ing of interna
41、lly 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 the envelope, care should be taken to provide means inthe mounting to monitor
42、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 acontrolled temperature enclosed gage.5.2.1.3 ElectricalShielding of the electrical lea
43、ds, 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 hookup, aside from leakage andespecially where long cables may be used, capacitanc
44、e 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 provide equivalent acceptance angles of molecular flux asdefined for the flange-
45、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 attention except in extreme conditions where conduc-tion or radiation paths to nea
46、rby 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-ties. These properties are of at least three kinds: (1) directionalmolecular
47、flux density (directional pressure) as in gas exchangebetween a source 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 anFIG. 1 Tubulated Ionization GageFIG. 2 Flange-Mounted Nude Ionization Gage
48、FIG. 3 Nude Ion Gage (Probe) Mounted Clear of Walls andStructuresE 296 70 (2004)3outgassing body and a cryopump, where the outgassing mate-rial is mainly condensible and the material flowing from thecryopump is mainly noncondensible; (3) directional tempera-ture, as in gas exchange between a warm an
49、d cold surface. Themagnitude of the first two effects is dependent on the fractionof incident molecules captured by the pump; flux densities inopposite directions may differ by a decade or more. Themagnitude of the third effect is of the order of the temperaturedifference. Significant directional gas flow can occur in thenormal operation of large simulators using solar simulationsources, cryopumping, moderate to heavy gas loads arisingfrom test items, and temperature extremes throughout theinternal surfaces.6.2 Response of Gage in Directio
