ASTM_D_2685_-_11.pdf

Designation:D268511Standard Test Method forAir and Carbon Tetrafluoride in Sulfur Hexafluoride by GasChromatography1This standard is issued under the fixed designation D2685;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.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 test method covers the determination of air(Note1)and carbon tetrafluoride as impurities in sulfur hexafluoride.NOTE1Nitrogen,oxygen,or any of their mixtures is considered to beair.Commercial grade air or nitrogen is used for standardization.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2D2472 Specification for Sulfur HexafluorideE260 Practice for Packed Column Gas ChromatographyE355 Practice for Gas Chromatography Terms and Relation-ships3.Summary of Test Method3.1 Air and carbon tetrafluoride are separated physically bygas chromatography and compared to corresponding compo-nents separated under similar conditions from a referencestandard mixture of known composition.The individual com-pounds of air are not separated.The composition of the sampleis calculated from its chromatogram by comparing the area ofthe peak of each component with the area of the peak of thecorresponding component on the reference standard chromato-gram.4.Significance and Use4.1 Air and carbon tetrafluoride(CF4)are two contaminantsof interest in sulfur hexafluoride(SF6).Both of these contami-nants adversely affect the performance of SF6when used as anelectrical insulating gas.Specification for maximum levels ofthese contaminants are given in Specification D2472.4.2 Gas chromatography is used to separate these contami-nants from a sample of SF6and to determine their concentra-tion.5.Apparatus5.1 Gas Chromatograph,consisting of a sample inlet sys-tem,adsorption column,flow meter,detector,and data han-dling system.Ensure that the column material of constructionand sample components are compatible.The apparatus mustcompletely separate air,carbon tetrafluoride,and sulfurhexafluoride as indicated by return of the recorded peak to thebase line between each successive peak.Chromatograms mustbe reproducible so that successive runs of a reference standardagree on each component peak area or height within 5%.Foradditional information on gas chromatography see PracticesE260 and E355.6.Reagents and Materials6.1 Cylinder of Helium Gas.6.2 Reference Standard MixtureA gas mixture that con-tains known percentages of air and carbon tetrafluoride inhelium or air and carbon tetrafluoride in sulfur hexafluoride isrequired.The concentration of a component in the referencesample should not be less than 50%nor more than 300%ofthe concentration of the corresponding component in theunknown.7.Calibration and Standardization7.1 Apparatus PreparationPrepare the gas chromatographfor use as directed by the manufacturer.The following operat-ing conditions have been found satisfactory.However,anycombination of conditions that result in complete separationsas indicated in the apparatus section will be satisfactory.1This test method is under the jurisdiction of ASTM Committee D27 ElectricalInsulating Liquids and Gases and is the direct responsibility of SubcommitteeD27.03 on Analytical Tests.Current edition approved May 1,2011.Published May 2011.Originallyapproved in 1968.Last previous edition approved in 2003 as D2685 95(2003).DOI:10.1520/D2685-11.2For 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 website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 Carrier gashelium,40 to 50 mL/minColumnPorapak Q-80/100 mesh or Porapak R 50/80 meshColumn size6 to 10 Ft(23.5 m)by in.(6.4 mm)nominalColumn temperature40 to 50CDetector temperature 70 to 80CSample volume2 to 5 mL approximatelyAttenuationlowest which will keep peaks on a scale may be variedduring runDetectorthermal conductivity7.2 Example of Chromatography ConditionsSpecific con-ditions used by one laboratory which produces acceptablechromatograms are as follows:Carrier gashelium,50 mL/minColumn packingPorapak R 50/80 meshColumn size and material0.25 in.(6.4 mm)nominal ID,10 ft(3.5 m)long,copperColumn temperature48CDetectorthermal conductivityDetector temperature75CSample volume5 mL7.3 Reference Standard Introduction and