1、Designation:D202997(Reapproved 2008)Standard Test Methods forWater Vapor Content of Electrical Insulating Gases byMeasurement of Dew Point1This standard is issued under the fixed designation D2029;the number immediately following the designation indicates the year oforiginal adoption or,in the case
2、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.This standard has been approved for use by agencies of the Department of Defense.1.Scope1.1 These test methods
3、 describe the determination of thewater vapor content of electrical insulating gases by direct orindirect measurement of the dew point and the calculation ofthe water vapor content.1.2 The following four test methods are provided:1.2.1 Method A describes the automatic chilled mirrormethod for measur
4、ement of dew point as low as 73C(99F).1.2.2 Method B describes the manual chilled mirror or dewcup method for measurement of dew point as low as 73C(99F).1.2.3 Method C describes the adiabatic expansion methodfor measurement of dew point as low as 62C(80F).1.2.4 Method D describes the capacitance me
5、thod for mea-surement of dew point as low as 110C(166F).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 o
6、f regulatory limitations prior to use.For specificwarnings,see 8.1.1,9.2,10.1.2 and 10.2.5.2.Referenced Documents2.1 ASTM Standards:2D1933 Specification for Nitrogen Gas as an Electrical Insu-lating MaterialD2472 Specification for Sulfur HexafluorideD3283 Specification for Air as an Electrical Insul
7、ating Ma-terial3.Terminology3.1 Definitions:3.1.1 dew point,nthe temperature to which a gas must becooled at constant pressure and constant water vapor content inorder for saturation to occur.Any further cooling usuallyresults in formation of the first drop of dew.3.1.2 hygroscopic,adjreadily taking
8、 up and retainingmoisture.4.Summary of Test Methods4.1 Method AThe automatic chilled mirror method usesthe chilled mirror dew point condensation principle to deter-mine the water vapor content in gas mixtures.An internalmirror,which is in the path of the test gas,is automaticallycooled.Internal elec
9、tronics sense the presence of moisture onthe mirror.The device then automatically brings itself toequilibrium and provides a direct reading of dew point tem-perature.4.2 Method BThis method uses the same basic condensa-tion principle in 4.1;however,the manual chilled mirrormethod uses a mixture of a
10、cetone and ice or other coolingmedia to manually chill the dew cup polished surface whichacts as the mirror.4.3 Method CAdiabatic expansion uses a process in whichthe test gas is cooled rapidly to determine dew point tempera-ture.This rapid exhausting of the test gas to atmosphere resultsin an expan
11、sion and cooling of the gas.If the cooling issufficient to reduce the temperature of the gas to or below thedew point,water vapor will condense out in the form of a finemist or fog.Successive trials will determine the minimuminitial pressure that will produce a fog.From this,the dew pointtemperature
12、 can be calculated.4.3.1 The relationship between pressure and temperatureduring adiabatic expansion is as follows:TF5 TIPF/PI#K21/K#where:K=ratio of specific heats for a given gas,TF=final temperature,TI=initial temperature,1These test methods are under the jurisdiction of ASTM Committee D27 onElec
13、trical Insulating Liquids and Gasesand are the direct responsibility of Subcom-mittee D27.07 on Physical Test.Current edition approved Dec.1,2008.Published December 2008.Originallyapproved in 1964.Last previous edition approved in 2003 as D2029 97(2003).DOI:10.1520/D2029-97R08.2For referenced ASTM s
14、tandards,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 1942
15、8-2959.United States1 PF=final pressure,andPI=initial pressure.4.4 Method DThe capacitance method uses a moisturesensor,typically aluminum oxide or silicon oxide,whichchanges its electrical output with the amount of water vapor towhich it is exposed.5.Significance and Use5.1 Certain gases have excel
16、lent dielectric and electric arcinterruption characteristics which make their use in electricalinstallations very desirable.5.2 Water content,as the test parameter,is of great impor-tance in determining the dielectric effectiveness of the gas.Under certain conditions,water may condense and become aconducting liquid resulting in a catastrophic dielectric break-down of the insulation.The water content of these insulatinggases as expressed by dew point is listed in SpecificationsD1933,D2472,and D32
