ASTM_D_4230_-_02_2012.pdf

Designation:D423002(Reapproved 2012)Standard Test Method ofMeasuring Humidity with Cooled-Surface Condensation(Dew-Point)Hygrometer1This standard is issued under the fixed designation D4230;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 the ther-modynamic dew-or frost-point temperature of ambient air bythe condensation of water vapor on a cooled surface.Forbrevity this is referred to in this method as the condensationtemperature.1.2 This test method is applicable for the range of conden-sation temperatures from 60C to 70C.1.3 This test method includes a general description of theinstrumentation and operational procedures,including siteselection,to be used for obtaining the measurements and adescription of the procedures to be used for calculating theresults.1.4 This test method is applicable for the continuous mea-surement of ambient humidity in the natural atmosphere on astationary platform.1.5 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.For specificprecautionary statements,see Section 8.2.Referenced Documents2.1 ASTM Standards:2D1356 Terminology Relating to Sampling and Analysis ofAtmospheresD3631 Test Methods for Measuring Surface AtmosphericPressure3.Terminology3.1 Definitions:3.1.1 For definitions of other terms in this method,refer toTerminology D1356.3.2 Definitions of Terms Specific to This Standard:3.2.1 nonhygroscopic materialmaterial that neither ab-sorbs nor retains water vapor.3.2.2 mirror(front surface)a polished surface,usually ametallic surface,on which condensates are deposited.3.3 Symbols:e=vapor pressure of water vapor in moist air.ei=saturation pressure of water vapor in equilibriumwith the plane surface of ice.ew=saturation pressure of water vapor in equilibriumwith the plane surface of water.P=ambient pressure.r=mixing ratio.T=ambient air temperature.Td=thermodynamic dew-or frost-point temperature.RHi=relative humidity with respect to ice.RHw=relative humidity with respect to water.4.Summary of Test Method4.1 The ambient humidity is measured with a dew-andfrost-point hygrometer.4.2 The mirror or some other surface on which the conden-sate is deposited is provided with the means for cooling andheating,detection of condensate,and the measurement of thetemperature of the mirror surface.4.3 Calculations of saturation vapor pressure over water andice as functions of temperature are provided.5.Significance and Use5.1 Humidity information is important for the understandingof atmospheric phenomena and industrial processes.Measure-ments of the dew-point and calculations of related vaporpressures are important to quantify the humidity information.6.Interferences6.1 This method is not applicable if other constituents in theatmosphere condense before water vapor.1This test method is under the jurisdiction of ASTM Committee D22 on AirQuality and is the direct responsibility of Subcommittee D22.11 on Meteorology.Current edition approved April 1,2012.Published July 2012.Originallyapproved in 1983.Last previous edition approved in 2007 as D4230-02(2007).DOI:10.1520/D4230-02R12.2For 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 7.Apparatus7.1 Dew-point hygrometers,specifically designed for me-teorological observations are available commercially.A sche-matic arrangement of a typical optical dew-point hygrometer isshown in Fig.1.7.1.1 The sample air flows through a small chamber.7.1.2 Within the chamber is a mirror or surface on which thecondensate can be deposited.7.1.3 A beam of light from an incandescent lamp,lightemitting diode or other suitable light source shines on themirror.7.1.4 Dew or frost is detected with an electro-optic device.7.1.5 The mirror is cooled by a Peltier thermoelectricelement.Peltier cooling is a convenient method for unattendedand automatic instruments.7.1.6 Preferred devices of sensing mirror temperature areresistance thermometers,thermistors,and thermocouples.7.1.6.1 The temperature sensors shall be attached to orembedded in the mirror to measure the temperature of thesurface of the mirror.7.1.7 Suitable control circuitry shall be provided to maintaina constant quantity of condensate on the mirror.7.1.8 Suitable provisions shall be provided to compensatefor the c