Designation:D7904−21StandardTestMethodforDeterminationofWaterVapor(MoistureConcentration)inNaturalGasbyTunableDiodeLaserSpectroscopy(TDLAS)1ThisstandardisissuedunderthefixeddesignationD7904;thenumberimmediatelyfollowingthedesignationindicatestheyearoforiginaladoptionor,inthecaseofrevision,theyearoflastrevision.Anumberinparenthesesindicatestheyearoflastreapproval.Asuperscriptepsilon(´)indicatesaneditorialchangesincethelastrevisionorreapproval.1.Scope1.1Thistestmethodcoversonlinedeterminationofvaporphasemoistureconcentrationinnaturalgasusingatunablediodelaserabsorptionspectroscopy(TDLAS)analyzeralsoknownasa“TDLanalyzer.”Theparticularwavelengthformoisturemeasurementvariesbymanufacturer;typicallybe-tween1000and10000nmwithanindividuallaserhavingatunablerangeoflessthan10nm.1.2Processstreampressurescanrangefrom700-mbarto700-bargage.TDLASisperformedatpressuresnearatmo-spheric(700-to2000-mbargage);therefore,pressurereduc-tionistypicallyrequired.TDLAScanbeperformedinvacuumconditionswithgoodresults;however,thesampleconditioningrequirementsaredifferentbecauseofhighercomplexityandatendencyformoistureingressandarenotcoveredbythistestmethod.Generallyspeaking,theventlineofaTDLanalyzeristoleranttosmallpressurechangesontheorderof50to200mbar,butitisimportanttoobservethemanufacturer’spublishedinletpressureandventpressureconstraints.Largespikesorstepsinbackpressuremayaffecttheanalyzerread-ings.1.3Thetypicalsampletemperaturerangeis-20to65°Cintheanalyzercell.Whilesamplesystemdesignisnotcoveredbythisstandard,itiscommonpracticetoheatthesampletransportlinetoaround50°Ctoavoidconcentrationchangesassociatedwithadsorptionanddesorptionofmoisturealongthewallsofthesampletransportline.1.4Themoistureconcentrationrangeis1to10000partspermillionbyvolume(ppmv).Itisunlikelythatonespec-trometercellwillbeusedtomeasurethisentirerange.Forexample,aTDLspectrometermayhaveamaximummeasure-mentof1ppmv,100ppmv,1000ppmv,or10000ppmvwithvaryingdegreesofaccuracyanddifferentlowerdetectionlimits.1.5TDLabsorption...
