1、Designation: D4230 02 (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 o
2、f 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 temperatur
3、e 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 t
4、heinstrumentation 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 o
5、n 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
6、 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 t
7、his 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 = v
8、apor 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- o
9、r 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
10、 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 u
11、nderstandingof atmospheric phenomena and industrial processes. Measure-ments of the dew-point and calculations of related vaporpressures are important to quantify the humidity information.1This test method is under the jurisdiction of ASTM Committee D22 on AirQuality and is the direct responsibility
12、 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 Cus
13、tomer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.6. Interferences6.1 This method is
14、 not applicable if other constituents in theatmosphere condense before water vapor.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
15、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-opti
16、c 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 shal
17、l 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 contamination of the surface
18、 of the mirror.7.2 Auxiliary Equipment:7.2.1 Provision shall be provided for assuring air flow pastthe dewpoint mirror without changing the pressure in themirror chamber more than 0.5 % from the ambient pressuresurrounding the sensor. An air flow of approximately 1.1 litresper minute is recommended
19、for typical chambers.7.2.2 Readout instrumentation is available with the dew-point hygrometer.8. Precautions8.1 Safety Precautions:8.1.1 The hygrometer shall be packaged in a suitableenclosure for application in industrial or outdoor environment.8.1.2 Electrical connectors and cables shall be suitab
20、le forthe industrial or outdoor environment.8.1.3 Appropriate voltage surge protection circuitry must beincorporated.8.2 Technical Precautions:8.2.1 The accuracy of a cooled-surface condensation hy-grometer is degraded by the presence of water-soluble materi-als. A mirror-cleaning schedule, consiste
21、nt with the contami-nation rate, is necessary to maintain the initial calibrationaccuracy. The user must determine the required maintenanceschedule for the specific site, by comparison of calibrationsmade before and after cleaning.8.2.2 Caution in performing this method should be taken ifthe indicat
22、ed mirror temperature is between 0C and 30C.Below freezing, the initial formation of the condensate on thesurface of a mirror may be either dew or frost. In the case ofnonfiltered atmospheric air, the supercooled water usually doesnot persist long on a mirror surface and quickly changes tofrost. The
23、 only positive method for determining the state of thecondensate is by visual observation of the mirror surface.8.2.2.1 The following illustrates the magnitude of the errorinvolved when dew or frost is not differentiated: The saturationvapor pressure of supercooled water at 30C corresponds tosaturat
24、ion vapor pressure of ice at 27.2C; dew pointof 20C corresponds to frost point of 18.0C; 10C dewpoint corresponds to frost point of 8.9C. (The frost pointtemperature is approximately 90 % of the dew-point tempera-ture in degrees Celsius.)8.2.3 A positive method for identifying the state of theconden
25、sate is to visually observe the condensate on the mirrorwith the aid of a microscope or other optical magnifier.8.2.4 Afinite length of time is required for the condensate todeposit on the mirror and for the hygrometer to reach equilib-rium with the ambient humidity. The response of the hygrom-eter
26、depends on the humidity of the ambient air, and on suchfactors as the ventilation rate of the ambient air past the mirror,the sensitivity of the condensate detector, and the maximumcooling rate of the hygrometer. The worst case occurs duringthe initial dew-point reading after clearing the mirror of
27、allcondensates. The time it takes the hygrometer to reach equi-librium after clearing the mirror will vary from instrument toinstrument.As an illustration of the magnitude of this time, thefollowing are approximate times required by a hygrometer toreach equilibrium after clearing the mirror.8.2.4.1
28、For dew points warmer than 0C: 5 min afterclearing.8.2.4.2 For dew points 0C to 20C: 5 to 20 min afterclearing.8.2.4.3 For dew points 20C to 40C: 20 min to 1 h afterclearing.8.2.4.4 For dew points 40C to 60C:1hto2hafterclearing.8.2.4.5 For dew points 60C to 70C:2hto6hafterclearing.8.2.5 The pressure
29、 differential between the mirror chamberand the ambient shall not be greater than 0.5 % of the ambientpressure. For example, the difference shall not exceed 5 hPa atan ambient pressure of 1000 hPa.FIG. 1 Schematic of a Thermoelectric Cooled CondensationHygrometerD4230 02 (2012)28.2.6 The thermometer
30、 must measure the temperature of themirror surface and not be influenced by the ambient airtemperature.8.2.7 All materials, which come into contact with thesample air before it reaches the dew-point mirror, shall benonhygroscopic. Metal, glass, polytetrafluoroethylene, or sta-bilized polypropylene a
31、re examples of suitable materials.Polyvinyl chloride tubing must be avoided.9. Sampling9.1 Automatic dew-point hygrometers provide an outputwhich may be recorded continuously. Modern data loggerssample temperature-sensor output periodically, convert theanalog sensor signal to a digital form, and sto
32、re the data. Theproper sampling interval depends on the data application (see13.2).9.2 Locate a blower or pump, which can be used to movethe air sample through the mirror chamber, downstream of thedew-point mirror. The airflow rate also depends on the dataapplication and sampling environment.9.3 Sel
33、ect the site or location so that the measurement datarepresents the water vapor content of the ambient atmosphereor industrial environment being sampled. Local water vaporsources, including ponds, wet roads, and structures can influ-ence the ambient humidity.Avoid sources of airborne contami-nants t
34、hat can influence to condensation process on the mirror.9.4 The successful application of this method requires thatall the materials which come in contact with the sample airupstream of the dew-point mirror be nonhygroscopic.9.5 The materials which come in contact with the sample airupstream of the
35、dew-point mirror might be wetted by rain, dew,or frost; for example, dew forming on a surface in the earlymorning. Design the sampling system to minimize thesedeleterious effects.10. Calibration10.1 Provide the calibration data for the thermometer, usedfor measuring the condensation temperature with
36、 the hygrom-eter. Consult the manufacturers operating manual for calibrat-ing the thermometer readout instrumentation.10.2 The cooled-surface condensation (dew-point) methodis considered to be an absolute or fundamental method formeasuring humidity. This method requires an accurate mea-surement of t
37、he temperature of the surface of the dew-pointmirror. It is not uncommon for the dew-point temperature to bemore than 35 C colder than the ambient air temperature. Tomeasure this temperature accurately, without being influencedby the warmer ambient and the colder heat-sink temperature,requires caref
38、ul placement of the dew-point thermometer.10.3 Therefore, in addition to the temperature calibration ofthe thermometer, (see 10.1), a humidity calibration must alsobe performed to verify the proper operation of the hygrometer(see Annex A1). The following are additional examples offactors that can af
39、fect the accuracy of the measurement:extraneous thermally-induced voltage (emf), heat leakagethrough the thermometer leads, self-heating of the thermom-eter, poor thermal contact, temperature gradient across themirror, etc.11. Procedure11.1 Selection of Sampling SiteSelect sampling site asindicated
40、in 9.3 and also in 1.3.2 of the World MeteorologicalOrganization, Guide to Meteorological Instrument and Observ-ing Practices (1).311.2 Consult the manufacturers operating manual forstart-up procedures.11.3 Perform necessary calibration as indicated in Section10. The dew-point thermometer will not u
41、ndergo large shifts(0.05C) in calibration unless it is subjected to physical shock.If the thermometer read-out instrumentation is subjected tovarying ambient temperatures, the read-out instrumentationchecks must be over the expected range of ambient tempera-tures. The frequency with which these chec
42、ks are required willbe determined by the stability of the readout instrumentation.11.4 Check and verify that all necessary variables aremeasured and recorded to compute the humidity in the desiredunit(s) see also 12.NOTE 1In general, it is recommended that ambient temperature andpressure (the pressu
43、re in the mirror chamber should not differ from theambient pressure by more than 0.5 %) and the dew-point temperature bemeasured and recorded. The ambient pressure is to be measured accordingto Test Methods D3631. This will enable other users of the data tocalculate in the different units of humidit
44、y.12. Calculations12.1 In the meteorological range of pressure and tempera-ture, the saturation vapor pressure of the pure water phase andof the moist air will be assumed to be equal. This assumptionwill introduce an error of approximately 0.5 % of reading orless.12.2 Calculate the ambient relative
45、humidity with respect towater using the following approximation.RHw!T5eTd!ewT!100 % (1)where:(RHw)T= relative humidity with respect to water, %, attemperature T (C),e(Td) = saturation vapor pressure, Pa, at condensationtemperature Td, C, where Tdis the averagevalue during the sampling period, see No
46、te 2,andew(T) = saturation vapor pressure, Pa, over water atambient temperature T, C, where T is theaverage value during the sampling period.NOTE 2If the condensate on the mirror is water (dew), use thesaturation vapor pressure over water corresponding to the condensationtemperature Td. If the conde
47、nsate is ice (frost), use the saturation vaporpressure over ice corresponding to the condensation temperature Td.Equations for saturation vapor pressure are provided in 12.5.12.3 Calculate the relative humidity with respect to ice asfollows:RHi!T5eTd!eiT!100 % (2)3The boldface numbers in parentheses
48、 refer to the references at the end of thismethod.D4230 02 (2012)3where:(RHi)T= relative humidity with respect to ice, %, at tem-perature T (C),ei(T) = saturation vapor pressure, Pa, over ice at ambienttemperature T, C, where T is the average valueduring the sampling period, ande(Td) = see 12.2.12.4
49、 Saturation vapor pressure calculations are provided inthis section. The equations provided in 12.4.1-12.4.4 are recentformulations (2) of the Magnus form of exponential expres-sions for saturation vapor pressure. Using similar constantsfrom other reliable resources can yield reasonably accurateresults as well. Using the appropriate equation depending if thecondensate is water or ice will improve the accuracy of thecalculation. The calculations for moist air rather than the purewater vapor equations could improve t
